AP Biology Unit 8: Ecology — Free Review Games.

A population consists of all individuals of a single species inhabiting the same geographic area at the same time.

Primary producers (mainly plants and algae) convert solar energy to chemical energy through photosynthesis, forming the base of most food webs.

Competition for food intensifies as population density increases, making it a density-dependent factor that limits population growth.

Mutualism benefits both species: the bee obtains nectar for food while the flower achieves pollination and reproduction.

The phosphorus cycle moves primarily through rocks, soil, water, and organisms. Unlike carbon and nitrogen, phosphorus does not have a significant gaseous phase.

Only about 10% of energy is transferred between trophic levels; the rest is lost as heat through metabolic processes.

Logistic growth incorporates carrying capacity (K), causing the growth rate to slow and level off as the population approaches environmental limits, producing an S-shaped curve.

A keystone species exerts a large influence on community structure and composition despite being relatively low in abundance; its removal causes major changes.

Primary succession begins on new surfaces like bare rock or lava flows where no soil or organisms previously existed, starting with pioneer species like lichens.

Invasive species typically lack natural predators in the new environment, allowing them to outcompete native species for resources and disrupt established ecological relationships.

Growth rate = (birth rate - death rate) x N = (0.3 - 0.1) x 500 = 0.2 x 500 = 100 individuals per year.

NPP = GPP - respiration of producers. It represents the energy stored in producer biomass that is available to consumers and decomposers.

The competitive exclusion principle (Gause's principle) states that two species occupying the same niche cannot stably coexist; one will outcompete and displace the other.

Biological magnification concentrates persistent toxins at higher trophic levels because predators accumulate the toxins from all the prey they consume over their lifetimes.

When CO2 dissolves in ocean water, it reacts to form carbonic acid (H2CO3), which dissociates and releases H+ ions, lowering the ocean pH.

Carrying capacity is the maximum number of individuals a given environment can support indefinitely, determined by limiting resources such as food, water, and space. It is not a growth rate (that describes intrinsic rate of increase, r) nor a minimum threshold (that relates to minimum viable population size).

Abiotic factors are nonliving physical and chemical components of the environment. Soil pH is a chemical property of the nonliving environment. Fungal decomposers and herbivores are living organisms (biotic factors). Leaf litter, while derived from organisms, is often debated, but soil pH is the clearest purely abiotic choice here.

Decomposers (bacteria and fungi) break down dead organic matter through decomposition, releasing inorganic nutrients such as nitrogen, phosphorus, and carbon back into the soil and water where they can be taken up by producers again. Nitrogen fixation is performed by specialized bacteria, not decomposers broadly. Energy transfer from producers to consumers is the role of herbivores.

A food chain represents a single linear sequence of who eats whom, while a food web is a more realistic depiction showing many interconnected feeding relationships among organisms in a community. Both food webs and food chains represent energy flow and apply to all ecosystem types.

r-selected species maximize reproductive rate (r) by producing large numbers of offspring with minimal parental investment and typically have short lifespans. K-selected species do the opposite: fewer offspring, high parental investment, and longer lifespans near carrying capacity. Type I survivorship curves describe organisms (like humans) with high early survival and late-life mortality.

Commensalism is a relationship where one organism benefits and the other is neither helped nor harmed. Barnacles benefit from transportation and feeding opportunities while the whale experiences no measurable benefit or harm. Clownfish and anemones are mutualistic (both benefit). Tapeworms are parasites (one benefits, one is harmed). Mycorrhizal fungi and plants are mutualistic.

Tropical rainforests have the highest species richness and biodiversity of any terrestrial biome due to year-round warm temperatures, high rainfall, and stable climate that has allowed species to accumulate and specialize over millions of years. Species diversity generally decreases with increasing latitude, so boreal forests and arctic tundra have far fewer species.

Type III survivorship curves show very high mortality early in life, with few individuals surviving to adulthood but those that do living relatively long lives. This pattern is typical of organisms like oysters or trees that produce vast numbers of offspring with little parental care. Type I describes humans and large mammals (low early mortality, high late mortality). Type II describes birds with constant mortality.

A trophic cascade occurs when a top predator indirectly affects primary producers by controlling herbivore populations. The wolves (top predator) reduced elk (herbivore) grazing pressure, allowing willows and other streamside plants (producers) to recover. This is a classic example of a top-down trophic cascade. Competitive exclusion involves two species competing for the same resource.

Although N2 makes up about 78% of the atmosphere, most organisms lack the enzyme nitrogenase needed to break the triple bond and use it. Nitrogen-fixing bacteria (such as Rhizobium) convert N2 into ammonia (NH3), making nitrogen biologically available. Converting ammonia to nitrate is nitrification, a separate step. Breaking down organic nitrogen is ammonification, performed by decomposers.

Habitat fragmentation divides continuous habitat into smaller, isolated patches. This reduces population sizes (increasing extinction risk), limits gene flow between subpopulations (reducing genetic diversity), and creates more edge habitat relative to interior habitat. Edge effects favor generalist species and often harm interior specialists. Fragmentation reduces, not increases, suitable habitat for most species.

In Lotka-Volterra dynamics, prey populations increase first (when predator numbers are low), which then supports predator population growth. As predators increase, they reduce prey numbers, which subsequently causes predator populations to decline. This creates characteristic out-of-phase oscillating cycles. The populations are never exactly synchronized because there is a lag between changes in prey abundance and predator responses.

Secondary succession occurs after a disturbance (fire, flood, farming) in an area that already has soil and may retain seed banks, root systems, and soil microbes from the previous community. Primary succession begins on bare substrate (lava, glacial till) with no soil or biological legacy, requiring pioneer organisms like lichens to slowly build soil over centuries. The existing soil and biological legacy of secondary succession dramatically accelerates recovery.

Eutrophication is the process by which excessive nutrients — primarily nitrogen and phosphorus from agricultural runoff and sewage — cause explosive algal growth. When the algae die, bacterial decomposition consumes dissolved oxygen, creating hypoxic (low-oxygen) dead zones that kill fish and other aerobic organisms. Acidification from SO2 is a separate phenomenon called acid rain and does not directly cause eutrophication.

The theory of island biogeography (MacArthur and Wilson) predicts that species richness is a balance between immigration and extinction. Large islands support larger populations (lower extinction risk) and closer islands receive more immigrants (higher immigration rate). Therefore, a large, close island has both the lowest extinction rate and highest immigration rate, predicting the highest species richness. Size and proximity have additive, not canceling, effects.

In Batesian mimicry, a palatable (harmless) species gains protection by resembling an unpalatable (harmful) species, effectively deceiving predators. In Mullerian mimicry, two or more genuinely unpalatable species evolve similar warning coloration, which reinforces predator avoidance learning and benefits all species involved. Batesian mimicry is a form of deception; Mullerian mimicry is honest signaling between defended species.

Energy is lost at each trophic level primarily as heat through cellular respiration, with roughly 10% transferred to the next level (the 10% rule). This means a community needs a very large base of producers to support each successive consumer level. The shape of the pyramid reflects energy availability, not organism size or reproductive rate. Producers do not store energy more efficiently — they simply represent the entry point for all solar energy into the ecosystem.

The Allee effect occurs when individual fitness decreases at low population densities, often because sparse populations struggle to find mates, cooperate in defense, or locate food. This creates a minimum viable population threshold — below a critical density, populations may spiral toward extinction rather than recover. The Allee effect is an exception to the simple logistic model, which predicts that growth rate always increases as density decreases.

The logistic growth equation is dN/dt = r * N * (K - N) / K, where r = b - d = 0.4 - 0.15 = 0.25. With N = 200 and K = 800: dN/dt = 0.25 * 200 * (800 - 200) / 800 = 0.25 * 200 * 600/800 = 0.25 * 200 * 0.75 = 0.25 * 150 = 37.5 individuals per year. The unrestricted exponential rate (r * N = 0.25 * 200 = 50) is reduced by the (K-N)/K term (0.75) because the population is at 25% of carrying capacity.

Resistance is an ecosystem's ability to avoid being disturbed, while resilience is its ability to return to its original state after disturbance. These properties often trade off: coral reefs can resist minor temperature changes but have low resilience after severe bleaching (slow recovery). Temperate forests may be heavily damaged by windstorms (low resistance) but recover through succession relatively quickly (higher resilience). Species richness can contribute to resilience through functional redundancy but is not the sole determinant.

A metapopulation is a set of spatially separated subpopulations of the same species that are linked by occasional dispersal. Key features are: discrete patches, local extinction risk, and recolonization from neighboring patches. The butterfly scenario captures all these elements. Migratory birds move as a whole population rather than occupying discrete patches with extinction-recolonization dynamics. A continuous breeding population is a single population, not a metapopulation.

Gross primary productivity (GPP) is all carbon fixed by photosynthesis. NPP = GPP minus autotroph respiration, representing organic matter available to consumers. NEP = NPP minus heterotroph respiration (consumers and decomposers), representing whether the ecosystem as a whole is a net carbon source or sink. A positive NEP means the ecosystem stores carbon; negative NEP means it releases more carbon than it fixes. This distinction is critical for understanding ecosystem carbon budgets and climate change.

At low disturbance levels, competitive exclusion allows dominant species to eliminate others, reducing diversity. At high disturbance levels, only highly tolerant pioneer species survive, also reducing diversity. At intermediate disturbance, competitively superior species cannot fully exclude others before the next disturbance resets the community, and enough time passes between events to allow moderate recovery — maximizing coexistence and diversity. This produces a hump-shaped relationship between disturbance and diversity.

Coevolution occurs when two species exert reciprocal selective pressure on each other, with each species' adaptation driving further adaptation in the other. A plant-herbivore arms race is a textbook coevolutionary scenario: plant toxins select for insect detoxification alleles, and resistant insects exert pressure for stronger plant defenses. This reciprocal feedback loop distinguishes coevolution from independent evolution or single-species selection. Predator-driven selection on the insect alone would not constitute coevolution with the plant.

Unlike carbon and nitrogen, phosphorus has no significant atmospheric reservoir. It enters ecosystems almost exclusively through the slow weathering of phosphate rocks, making it naturally scarce. Nitrogen, by contrast, can be replenished from the vast atmospheric N2 pool through nitrogen fixation, partially compensating for losses. Because phosphorus cannot be replenished this way, it is commonly the limiting nutrient in freshwater systems (and sometimes marine systems). Phosphorus is not toxic to algae — in fact, phosphorus additions stimulate algal blooms (eutrophication).

Carrying capacity is the population size at which births equal deaths, determined by available resources such as food, water, and space. It is NOT a growth rate — exponential (unlimited) growth occurs when populations are far below K. At K, net population growth rate equals zero.

Commensalism involves one species benefiting (+) while the other is neither helped nor harmed (0). The barnacles benefit from transport and feeding opportunities; the whale gains nothing but loses nothing either. Mutualism requires both species to benefit, and parasitism requires one to be harmed.

Decomposers (bacteria and fungi) perform decomposition, mineralizing organic compounds back into inorganic forms like nitrates and phosphates that producers can absorb. This closes nutrient cycles. Nitrogen fixation is performed by specific prokaryotes like Rhizobium, not decomposers generally.

Abiotic factors are non-living physical or chemical components of the environment. Rainfall is abiotic because it is a physical condition, not a living organism or its interaction. Competition, predation, and parasitism all involve living organisms interacting with one another, making them biotic factors.

r-selected species maximize reproductive rate (r) by producing large numbers of offspring quickly, with little parental care. This strategy is advantageous in unpredictable or disturbed environments. K-selected species, by contrast, invest heavily in fewer offspring and maintain populations near carrying capacity (K).

Nitrogen fixation — converting atmospheric N2 into ammonia (NH3) — is performed by nitrogen-fixing prokaryotes. Rhizobium forms mutualistic associations in legume root nodules; free-living bacteria like Azotobacter do so in soil. Plants cannot fix N2 directly. Nitrifying bacteria convert ammonia to nitrite/nitrate, which is a different step in the nitrogen cycle.

Habitat loss and fragmentation is the leading direct threat to biodiversity worldwide. Converting habitat to agriculture reduces both the area available and connectivity between patches, shrinking population sizes and restricting gene flow. Inbreeding and genetic drift become concerns after populations are already small, making habitat loss the primary driver here.

The niche is a multidimensional concept that includes what resources an organism uses, how it interacts with other species, and what conditions it can tolerate. Habitat refers only to the physical location. Trophic level is just one dimension of the niche. Abiotic tolerances alone describe the fundamental niche but exclude biotic interactions that define the realized niche.

Population change = (Births - Deaths) + (Immigration - Emigration) = (40 - 10) + (5 - 15) = 30 + (-10) = 20. Many students forget to include immigration and emigration, or subtract them in the wrong order. The population grows from 200 to 220, a net increase of 20.

Island biogeography predicts that species richness reflects a balance between immigration and extinction rates. Small islands have higher extinction rates (smaller populations, fewer microhabitats) and distant islands have lower immigration rates. A small, remote island therefore has the lowest equilibrium species richness of the four options.

Secondary succession occurs after a disturbance (fire, farming, logging) removes an existing community but leaves the soil intact. Because soil, seeds, and vegetative roots often remain, secondary succession proceeds faster than primary succession. Primary succession, not secondary, begins in areas lacking soil entirely — such as bare rock or lava flows.

Nutrient runoff (especially nitrogen and phosphorus) causes explosive algal blooms. When algae die, decomposing bacteria experience a population surge and consume most of the dissolved oxygen, creating hypoxic or anoxic dead zones. Aquatic animals suffocate. While algae do respire at night, bacterial decomposition of massive algal die-offs is the primary cause of oxygen depletion.

A trophic cascade occurs when a top predator indirectly affects lower trophic levels. Wolf predation changed elk behavior (not just numbers), which released plants from overgrazing and altered the physical habitat. This is a classic example of a behaviorally mediated trophic cascade. Competitive exclusion involves two species competing for the same resource, which is not the mechanism here.

The Lincoln-Petersen mark-recapture formula is: N = (M x C) / R, where M = marked individuals released (60), C = total individuals in second capture (45), and R = recaptured marked individuals (9). N = (60 x 45) / 9 = 2700 / 9 = 300. A common error is dividing incorrectly or confusing which values go where in the formula.

Species evenness measures how equally individuals are distributed across species — it is independent of species richness. The first community has perfect evenness (all species equally abundant). The second has low evenness: one species dominates at 91%, and the rest are rare. Both communities have equal species richness (4 species), but evenness differs dramatically.

Mycorrhizae are mutualistic: the fungal hyphae dramatically increase absorptive surface area, giving plants access to phosphorus, water, and micronutrients over a much larger soil volume. In return, plants supply the fungi with photosynthetically produced sugars. Nitrogen fixation is done by specific bacteria (e.g., Rhizobium), not mycorrhizal fungi.

Density-independent factors affect the same proportion of a population regardless of density — a severe frost kills vulnerable individuals whether the population is 50 or 5,000. Density-dependent factors (competition, disease, predation) intensify as population density increases. Temperature is abiotic, not biotic, even though it affects living organisms.

A pyramid-shaped age structure with a large base indicates many pre-reproductive individuals who will enter reproductive age in the near future, producing a population growth surge called population momentum. The population is not currently declining — it is growing or about to accelerate growth. Resource depletion is a possible long-term consequence but is not the most direct prediction from age structure alone.

NPP = GPP - Respiration by producers = 9,000 - 3,500 = 5,500 kcal/m²/yr. NPP represents the energy actually stored in plant biomass and available to consumers. The 400 kcal/m²/yr assimilated by primary consumers is a subset of NPP consumed, not a value used in calculating NPP itself. A common error is subtracting consumer assimilation from GPP, which conflates NPP with secondary productivity.

At low disturbance, competitive exclusion allows dominant species to drive others to extinction. At high disturbance, only disturbance-tolerant specialists persist. At intermediate disturbance, no species dominates long enough to exclude others, and diverse successional stages coexist across the landscape. Frequent disturbances do NOT maximize diversity — they favor only the most disturbance-adapted species.

Metapopulation theory (Levins model) describes a 'population of populations' in fragmented habitat patches. Key features include local extinction risk, recolonization from surviving patches, and the importance of patch connectivity. Without dispersal among patches, the entire metapopulation risks collapse even if individual patches seem viable. A continuous single population is simply a population, not a metapopulation.

In the Lotka-Volterra predator-prey model, prey decline lags behind the predator peak. At peak predator density, predation is still occurring at a high rate, causing prey numbers to fall further. Only after prey become sufficiently scarce do predator numbers begin falling (due to food limitation), and only then does reduced predation allow prey to recover. The prey trough follows the predator peak with a time lag.

Resistance is the ability to avoid disruption; resilience is the ability to recover after disruption. A grassland that is severely damaged (low resistance — it was affected) but recovers quickly (high resilience) matches high resilience / low resistance. The old-growth forest example shows high resistance. The collapsed reef shows low resilience. The stable desert shows high resistance.

The Allee effect is a positive relationship between population density and individual fitness at low densities — the opposite of typical density-dependent regulation. Mechanisms include difficulty finding mates, loss of group hunting or defense, and reduced cooperative rearing. This creates a minimum viable population concept: below a threshold density, the population growth rate becomes negative and extinction risk rises sharply.

Alpha diversity measures species richness within a single community; beta diversity measures the turnover in species composition between communities. High alpha + low beta means each local patch is species-rich, but the same species keep appearing across patches (low turnover). This pattern often results from generalist species with broad habitat tolerances dominating all patches. High alpha combined with high beta would maximize gamma (landscape-level) diversity.

Carrying capacity (K) is the maximum population size that a given environment can sustain indefinitely, given available resources. When a population reaches K, per capita growth rate approaches zero. Choice A describes the intrinsic rate of increase (r) under exponential growth, not K. Choice C describes minimum viable population, a conservation concept distinct from K.

Decomposers (bacteria and fungi) break down dead organisms and waste products, releasing inorganic nutrients such as nitrogen and phosphorus back into the soil and water, where producers can reuse them. This completes biogeochemical cycles. Choice A describes producers, not decomposers. Choice B incorrectly skips trophic levels — energy moves sequentially, not directly from producers to tertiary consumers.

Commensalism is a symbiotic relationship in which one species benefits and the other is neither helped nor harmed. The epiphyte gains a physical support structure and access to light, while the host plant is unaffected. Parasitism would require harm to the host. Mutualism requires both species to benefit. Amensalism involves one species being harmed while the other is unaffected — the reverse of commensalism.

A density-independent factor affects the same proportion of a population regardless of its size. A severe frost kills plants based on environmental conditions, not population density. Choices A, B, and D are all density-dependent: their impact becomes stronger or more severe as population density increases, creating a negative feedback that regulates population size.

A community is the biotic component of an ecosystem — all populations of different species that coexist and interact in the same area. Choice A describes a population (one species). Choice C describes an ecosystem, which includes both biotic and abiotic components. Choice D describes the abiotic environment alone, with no organisms included.

Secondary succession occurs after a disturbance (such as a fire, flood, or abandoned farmland) removes the existing community but leaves the soil intact. Surviving seeds, roots, and soil organisms allow faster recovery. Primary succession, by contrast, starts on bare substrate (such as new volcanic rock) with no soil, which is why it proceeds more slowly — not secondary succession as stated in choice D.

Abiotic factors are nonliving physical and chemical components of the environment, including temperature, light, water, pH, and soil minerals. Choices A, B, and D all describe living organisms (bacteria, fungi, and earthworms), which are biotic components. Temperature and solar radiation are classic examples of abiotic factors that strongly influence species distribution and ecosystem function.

r-selected species thrive in unpredictable environments by reproducing rapidly and producing many offspring, offsetting high juvenile mortality through sheer numbers rather than parental investment. Choices A, B, and D describe K-selected life history traits: few offspring with high parental investment, populations near K, and long lifespans. K-selected strategies are favored in stable, competitive environments where offspring survival depends on quality over quantity.

A trophic cascade occurs when changes at one trophic level ripple through the food web to affect other levels indirectly. Removing wolves (top predator) released elk from predation pressure, leading to overgrazing that altered plant communities and affected other species. This is top-down regulation. Bottom-up regulation (choice D) would involve changes at the producer level driving changes up through the food web.

Island biogeography predicts species richness is determined by immigration rate and extinction rate. A large island close to the mainland has the highest immigration rate (short distance from the species source) and the lowest extinction rate (large area supports more species with larger populations). Small islands have higher extinction rates due to smaller habitat area, and distant islands have lower immigration rates. Both factors favor the large, near island.

Resource partitioning occurs when competing species divide a resource — in this case, foraging zones within a tree — reducing direct competition and allowing coexistence. Character displacement (choice A) refers to morphological divergence in allopatric populations that become sympatric; it results from past competition but is not the same as resource partitioning itself. Competitive exclusion (choice C) predicts one species eliminates the other — the opposite outcome of what is observed here.

Nitrogen fixation — converting inert N2 gas to biologically available ammonia — is carried out by prokaryotes including free-living Azotobacter and symbiotic Rhizobium in legume root nodules. Nitrifying bacteria (choice B) convert ammonia to nitrate, a different step in the cycle. Denitrifying bacteria (choice D) return nitrogen to the atmosphere. Mycorrhizal fungi (choice A) help plants absorb phosphorus and other nutrients, not nitrogen gas.

The intrinsic rate of increase is r = birth rate minus death rate = 0.40 minus 0.15 = 0.25. The number of individuals added is r times N = 0.25 times 200 = 50 individuals. Choice A incorrectly adds birth and death rates instead of subtracting. Choice C uses the correct r but multiplies by the wrong value. Choice D uses only the birth rate, ignoring mortality.

Predator population growth is limited by generation time and reproductive rates. Even when prey are abundant, it takes time for predators to reproduce and raise offspring to reproductive age, creating a predictable lag. This delay drives the oscillating cycles seen in classic Lotka-Volterra models (and observed in systems like the Canadian lynx and snowshoe hare). Chemical deterrents (choice C) are not part of classic predator-prey theory.

Fragmentation isolates populations into small patches, reducing gene flow and increasing inbreeding depression. It also increases the proportion of edge habitat relative to interior habitat; edge environments differ in temperature, humidity, and predation pressure, harming interior-specialist species. Fragmentation typically harms specialists more than generalists (opposite of choice D), and does not inherently increase productivity or introduce invasives.

NPP = GPP minus plant respiration = 8,000 minus 3,000 = 5,000 kcal/m2/year. NPP represents the organic matter actually stored in plant biomass — the energy available to herbivores, detritivores, and decomposers. Choice D is incorrect because only about 10% of NPP is typically transferred to the next trophic level due to metabolic losses; 100% transfer does not occur.

This process is called eutrophication. Excess nutrients cause explosive algal blooms (algal mats). When algae die, bacterial decomposers consume them using aerobic respiration, drastically depleting dissolved oxygen in a hypoxic or anoxic zone. Fish and other aerobic organisms suffocate. Choice C has the sequence reversed — fish kills come after oxygen depletion, not before the algal bloom.

At intermediate disturbance levels, no single competitive dominant species has time to exclude others, yet disturbances are not so severe that only disturbance-tolerant pioneers can survive. This creates openings for a mix of early- and late-successional species to coexist. At low disturbance (choice A), competitive exclusion reduces diversity. At high disturbance (choice B), only stress-tolerant generalists persist, also reducing diversity.

The Lincoln-Petersen estimator is: N = (M times C) divided by R, where M = number initially marked (80), C = size of second capture (100), and R = recaptured marked individuals (20). N = (80 times 100) divided by 20 = 8,000 divided by 20 = 400. Choice B (200) results from dividing M by the recapture proportion incorrectly. This method assumes no births, deaths, immigration, or emigration between captures, and that marked individuals mix randomly.

Biodiversity has two components: species richness (number of species) and species evenness (how equally individuals are distributed among species). Plot A has maximum evenness — neither species dominates. Plot B is dominated by one species, making it functionally less diverse and more vulnerable if species X declines. Shannon diversity indices would assign Plot A a higher value than Plot B. Choice D is incorrect because evenness is a well-established and ecologically meaningful component of diversity.

In source-sink metapopulation dynamics, source habitats have high-quality conditions where births exceed deaths, generating emigrating individuals. Sink habitats have deaths exceeding births and would go locally extinct without immigration from sources — a phenomenon called the rescue effect. Choice A describes a sink, not a source. Choice C reverses the birth rate logic; sinks have net negative growth, not high birth rates.

Resilience is the ability of an ecosystem to return to its original state after a disturbance. The coral reef demonstrates high resilience by recovering quickly. The kelp forest shifted to a fundamentally different alternate stable state (urchin barren) that is self-reinforcing and resistant to recovery — it lacks resilience. Resistance (choice A) refers to how much disturbance an ecosystem can absorb without changing state, which is not what the coral scenario describes.

An inverted biomass pyramid can occur when producers have very short generation times and high productivity relative to their standing biomass. Phytoplankton reproduce in hours to days, so their biomass at any snapshot is low even though annual productivity is high. Zooplankton live longer and accumulate biomass. Energy transfer rules still apply (choice A is false), and more trophic levels would reduce biomass at higher levels, not invert the pyramid.

A coevolutionary arms race describes reciprocal evolutionary responses between two species — here, plant defenses and herbivore counter-defenses driving each other forward continuously. The Red Queen hypothesis (named after Lewis Carroll's character who must keep running to stay in place) captures the idea that species must constantly evolve just to maintain their fitness relative to coevolving antagonists. Character displacement (choice A) involves morphological divergence between competing species in the same location, not a host-parasite or plant-herbivore chemical arms race.

A positive feedback loop amplifies the original change rather than counteracting it. Warming thaws permafrost, which releases greenhouse gases, which cause more warming, which thaws more permafrost — each step reinforces the next. This is a major concern in climate science. Choice A describes a negative feedback (carbon uptake counteracting release), which does occur but is not the dominant dynamic of permafrost thaw. Density-dependent regulation (choice C) applies to population biology, not atmospheric chemistry.

Carrying capacity (K) is the maximum population size that a given environment can sustainably support given available resources. It is not the maximum growth rate (that would be r-max), nor is it specific to exponential growth — carrying capacity is the concept central to logistic growth models.

An omnivore consumes organisms from multiple trophic levels, including both producers (plants) and consumers (herbivores). A detritivore feeds on dead organic matter. A parasite lives on or in a host and harms it. Keystone predator is a functional role, not a feeding category based on trophic level.

Density-independent factors affect populations regardless of their size or density. A wildfire destroys habitat without regard to how many organisms are present. Competition, density-dependent predation, and disease transmission that increases with crowding are all density-dependent because their impact scales with population density.

Species richness is simply the count of distinct species in a defined area or community. It does not account for how abundant each species is — that dimension of diversity is captured by species evenness. Together, richness and evenness contribute to species diversity indices such as the Shannon index.

Commensalism is a relationship in which one organism benefits and the other is unaffected (neither harmed nor helped). Mutualism involves benefit to both. Parasitism involves benefit to one at the expense of the other. Competition is not a type of symbiosis but rather an antagonistic interaction over shared resources.

The trophic hierarchy begins with producers (autotrophs) that fix energy, followed by primary consumers (herbivores), then secondary consumers (carnivores that eat herbivores), and finally tertiary consumers. Decomposers recycle nutrients at all levels but are not a sequential trophic level above producers.

A biome is a large-scale geographic region defined by its climate (especially temperature and precipitation) and the characteristic organisms adapted to those conditions, such as tropical rainforest or temperate grassland. The term for all organisms plus their physical environment is 'ecosystem,' which can exist at any scale.

Decomposers (fungi and bacteria) break down dead organic material and release inorganic nutrients back into the soil and water, making them available to producers again. Converting inorganic to organic using sunlight describes photosynthesis (autotrophs). Nitrogen fixation is performed by specific prokaryotes, not decomposers broadly.

In the logistic growth equation dN/dt = rN(1 - N/K), the growth rate is maximized when N = K/2. At this point the term N(1 - N/K) reaches its peak value of K/4. When N is very small the growth is slow due to few individuals, and when N approaches K growth slows due to resource limitation, so the peak is at the midpoint.

At low disturbance levels, competitive exclusion reduces diversity as dominant species outcompete others. At high disturbance levels, only disturbance-tolerant species survive. At intermediate levels, competitive dominants are kept in check but conditions are stable enough for many species to coexist, maximizing diversity.

Denitrification is performed by anaerobic bacteria that reduce nitrates (NO3-) to N2 gas, which is released into the atmosphere. This completes the nitrogen cycle by returning nitrogen to the atmosphere. Nitrogen fixation converts N2 to ammonia; nitrification converts ammonia to nitrites and nitrates; ammonification converts organic N to ammonia.

Resource partitioning occurs when species that would otherwise compete divide up a niche — in this case, vertical foraging zones in the canopy — allowing coexistence by reducing direct competition. This is distinct from competitive exclusion, which predicts elimination of one competitor when niches overlap completely. The MacArthur warblers are a classic example.

Secondary succession occurs after a disturbance — such as a fire, flood, or agricultural abandonment — that removes the existing community but leaves the soil and seed bank in place. Because soil is already present, secondary succession proceeds much faster than primary succession, which starts on bare, lifeless substrate.

NPP is the energy available to consumers after producers use some of the energy they fix for their own cellular respiration. GPP is the total energy fixed by photosynthesis; subtracting the producers' own respiratory costs (R) gives NPP, which represents the actual organic matter available to the rest of the ecosystem.

The MacArthur-Wilson equilibrium model predicts that larger islands have lower extinction rates because they can sustain larger individual population sizes (reducing local extinction risk) and often have greater habitat heterogeneity. Immigration rate is primarily determined by distance from the mainland, not island size.

This is a classic trophic cascade. Removing the top predator allows herbivorous zooplankton or fish to increase in abundance. With more herbivores present, one might expect less algae — but if the herbivores preferentially eat algae's competitors (e.g., larger zooplankton eat rotifers that graze algae), algae can bloom. More commonly in lakes, removing piscivorous fish allows planktivores to increase, reducing large zooplankton that graze algae, causing blooms. The top-down effect propagates through the food web.

Tropical regions receive consistent solar energy year-round and have experienced climatic stability over long evolutionary timescales. This combination promotes high rates of speciation (through habitat complexity and niche diversity) and lower extinction rates compared to regions with seasonal or historically glaciated climates. Nutrient-rich soil is not a general feature of tropical rainforests — many tropical soils are actually nutrient-poor.

The logistic growth equation dN/dt = rN(1 - N/K) is maximized when N = K/2, because at this point the product of N and (1 - N/K) is greatest. When N is very small, the population is large enough to grow rapidly in theory, but few individuals means absolute numbers added are small. When N = K, growth stops entirely because (1 - N/K) = 0.

A single large continuous habitat patch (SLOSS debate: Single Large Or Several Small) supports larger population sizes that are less vulnerable to extinction from stochastic events, maintains interior habitat that edge-sensitive species require, and reduces the negative edge effects (altered microclimate, increased predation, invasive species) that fragment boundaries create. While fragmentation may reduce disease spread, this benefit is generally outweighed by the population viability costs.

Biological magnification (biomagnification) occurs with lipophilic (fat-soluble), persistent compounds such as DDT, PCBs, and mercury. Because these compounds are stored in fat rather than excreted, each predator accumulates the total pollutant burden of all prey consumed over its lifetime. Top predators therefore achieve tissue concentrations orders of magnitude higher than those in the water or at lower trophic levels, despite consuming relatively few individuals.

The classic Lotka-Volterra predator-prey model predicts neutrally stable oscillations in which predator and prey cycle continuously. Prey populations peak first; increased prey supports predator population growth, whose peak lags behind; elevated predation then reduces prey, which subsequently causes predator decline. The cycles are out of phase — this pattern is supported by empirical data such as the Canadian lynx and snowshoe hare fur records.

Functional redundancy means that when multiple species perform similar ecological roles, the loss of one species can be compensated by others, maintaining ecosystem function. In a diverse forest, if drought eliminates drought-sensitive species, drought-tolerant species with similar functional roles maintain productivity. The grassland, with fewer species, has less redundancy and thus greater functional collapse. This is a key argument for biodiversity conservation — diversity confers ecosystem resilience.

Unlike carbon (CO2, CH4) and nitrogen (N2), phosphorus has no significant atmospheric gaseous form. It enters ecosystems primarily through weathering of phosphate-containing rocks and is cycled through biological uptake, decomposition, and sedimentation back into rock over geological timescales. This absence of an atmospheric reservoir makes phosphorus a common limiting nutrient, particularly in freshwater systems, and means its cycle is fundamentally geologic rather than atmospheric.

Genetic drift — random changes in allele frequency — has the greatest effect in small populations. In a small island population, chance events (which individuals survive and reproduce) can rapidly shift allele frequencies away from those of the source population, even when selection pressures are identical. This is distinct from inbreeding depression (which reduces overall fitness) and gene flow (which would introduce mainland alleles, not diverge from them).

Ecological efficiency is calculated as the energy at one trophic level divided by energy at the previous trophic level. NPP = GPP - producer respiration = 8,000 - 3,200 = 4,800 kcal/m²/year. Herbivore assimilation is not given directly, but using herbivore consumption (400 kcal) as the input to the next level: efficiency = 400 / 8,000 = 5% (using GPP) or 400/4,800 ≈ 8.3% (using NPP). If using the producer trophic level energy as NPP (4,800), efficiency ≈ 8.3%. However, ecological efficiency conventionally uses production-to-production ratios; herbivore production = consumption - respiration = 400 - 300 = 100 kcal; efficiency = 100/4,800 ≈ 2.1%. Using the standard 'energy available at next level / energy available at current level' with consumption as proxy: 400/8,000 = 5%, which is the answer consistent with the 10% rule approximation being an upper bound. The answer 5% reflects energy consumed by herbivores relative to GPP.

Abiotic factors are non-living physical and chemical components of an environment, such as soil pH, temperature, salinity, and light intensity. Fungi, bacteria, and competing plants are biotic (living) factors.

A niche encompasses everything about how a species interacts with its environment: what it eats, where it lives, when it is active, and how it interacts with other species. It is broader than habitat, which refers only to physical location.

Decomposers break down dead organic matter, releasing inorganic nutrients like phosphorus and nitrogen back into the soil and water where producers can absorb them. Without decomposers, nutrients would remain locked in dead biomass and nutrient cycles would halt. Nitrogen fixation is performed by specific nitrogen-fixing bacteria, not decomposers in general.

K-selected species (such as elephants and humans) produce few offspring but invest heavily in parental care, live long lives, and are adapted to compete in stable, resource-limited environments near carrying capacity (K). r-selected species show the opposite traits, such as many offspring with little parental care.

A food chain shows a single linear sequence of feeding relationships, while a food web represents the complex, interconnected network of all feeding relationships within a community. Food webs are more realistic because most organisms consume and are consumed by multiple species.

Interspecific competition occurs between individuals of different species that rely on the same limited resources, such as food, space, or light. Intraspecific competition, by contrast, occurs among individuals of the same species. Both reduce the fitness of the competing individuals.

Density-independent factors affect populations regardless of their size or density. A volcanic eruption destroys habitat whether 10 or 10,000 individuals are present. Disease transmission, predation efficiency, and food competition all intensify as population density increases, making them density-dependent factors.

The mark-recapture formula is: N = (M x C) / R, where M = number initially marked (80), C = second sample size (100), and R = recaptured marked individuals (20). N = (80 x 100) / 20 = 400. This method assumes the population is closed, marks do not affect survival, and marked individuals mix randomly with the population.

Biodiversity measures both species richness (number of species) and species evenness (relative abundance). Although both plots have the same richness, Plot A has much higher evenness because individuals are distributed more equally among species. Plot B's low evenness reduces its overall diversity. Indices like the Shannon diversity index incorporate both components.

Nitrification is a two-step aerobic process: first, Nitrosomonas bacteria oxidize ammonia (NH3) to nitrite (NO2-), then Nitrobacter bacteria oxidize nitrite to nitrate (NO3-). This is distinct from nitrogen fixation (N2 to NH3), denitrification (NO3- to N2), and assimilation (plant uptake of nitrates).

At low disturbance levels, competitive exclusion reduces diversity as dominant species outcompete others. At very high disturbance levels, only disturbance-tolerant species survive. At intermediate levels, disturbance prevents monopolization by dominant competitors while still allowing diverse species to persist, maximizing overall diversity.

Commensalism describes an interaction in which one species benefits and the other neither benefits nor is harmed. The orchid gains a structural attachment and light exposure without providing anything in return, and the tree experiences no measurable cost or gain. Mutualism requires both species to benefit; parasitism requires one species to be harmed.

A trophic cascade occurs when changes to a top predator ripple down through the food web. Wolves suppressed elk behavior and numbers, indirectly protecting riparian vegetation. Removing wolves cascaded down through elk to plants to soil and stream structure — a classic top-down trophic cascade demonstrating the indirect ecosystem role of apex predators.

A pyramid-shaped age structure with a wide base reflects many young, pre-reproductive individuals. As these individuals mature and reproduce, the population will grow substantially. A stable population would show a more rectangular distribution, and a declining population would show a narrow base (few young individuals).

MacArthur and Wilson's theory predicts that species richness is determined by the balance between immigration and extinction rates. Larger islands have lower extinction rates (more resources, larger habitat), and islands closer to a mainland source have higher immigration rates. A large, near island therefore supports the most species.

Excess nutrients (especially phosphorus and nitrogen) trigger explosive algal growth. When the algae die, bacterial decomposers consume the organic matter and deplete dissolved oxygen in a process called hypoxia. The resulting low-oxygen conditions cause fish and invertebrate die-offs. The problem is indirect — driven by decomposer respiration, not direct toxicity.

The most widely supported explanation for the latitudinal diversity gradient involves energy: tropical regions receive more consistent solar radiation, supporting higher net primary productivity. Greater productivity sustains more biomass, more trophic levels, and finer resource partitioning — all of which allow more species to coexist. Multiple hypotheses contribute, but energy availability is central.

Substituting into dN/dt = rN((K-N)/K): dN/dt = 0.4 x 250 x ((1000-250)/1000) = 0.4 x 250 x 0.75 = 75 individuals per year. The term (K-N)/K equals 0.75 because the population is only 25% of K, so 75% of growth potential remains. Maximum growth rate occurs at N = K/2 = 500.

Net primary productivity (NPP) = GPP - plant respiration = 8,000 - 3,500 = 4,500 kcal/m²/year. Herbivores consume 60% of NPP: 0.60 x 4,500 = 2,700 kcal/m²/year. The remaining 40% accumulates in plant biomass or decomposes. Note that NPP, not GPP, represents the energy available to consumers.

The Allee effect describes a positive relationship between population density and individual fitness at low densities — the opposite of typical density-dependent regulation. When populations fall too small, individuals struggle to find mates, lose cooperative anti-predator behaviors, or suffer reduced foraging efficiency. This can create a minimum viable population below which extinction becomes likely.

Character displacement occurs when two species living in sympatry (together) evolve more divergent traits than when in allopatry (apart), because natural selection favors individuals that use different resources and thus avoid direct competition. This is strong evidence that interspecific competition shapes phenotypic evolution and promotes resource partitioning within communities.

Metapopulation persistence depends on the balance between local extinction and recolonization. If dispersal corridors are destroyed, locally extinct patches cannot be recolonized, causing patches to blink out one by one until total extinction occurs. High dispersal and asynchronous dynamics are stabilizing because they allow rescue effects. Large total size distributed across many patches also buffers against total extinction.

Resilience is the ability of an ecosystem to recover after a disturbance; Community A recovers quickly, showing high resilience. Resistance is the ability to withstand or avoid disturbance without changing; Community B resists burning altogether, showing high resistance. These two properties often trade off — highly resistant systems may be brittle and slow to recover if disturbed.

Two mechanisms explain the biodiversity-productivity relationship. The sampling effect means that high-diversity plots are statistically more likely to contain a highly productive species. Complementarity means that species with different functional traits (root depths, phenologies, nutrient requirements) collectively use available resources more completely. Together, these mechanisms explain both higher productivity and greater stability in diverse communities.

Lotka-Volterra equations predict perpetual, coupled oscillations. Prey increase when predators are scarce, then abundant prey support predator population growth, which then drives prey numbers down, followed by predator decline — creating a lag. The predator peak follows the prey peak because predators require time to numerically respond to increased food availability. In reality, these cycles are damped by additional ecological factors.

Carrying capacity (K) is the maximum population size that a given environment can support given its limiting resources such as food, water, and space. It is not the maximum growth rate (that would be r-max) nor is it tied to birth rate equaling zero.

Interspecific competition occurs between individuals of different species. A lion and a cheetah are different species competing for the same resource (prey). The other options involve individuals of the same species (intraspecific competition) or a single species affecting its own offspring.

A habitat is the physical place or environment where an organism lives, including abiotic factors like temperature, moisture, and substrate. The functional role an organism plays is its niche. Biotic factors alone do not define a habitat.

Density-independent factors affect populations regardless of their size or density. A volcanic eruption destroys habitat no matter how many individuals are present. Predation, disease spread, and intraspecific competition all become stronger as population density increases, making them density-dependent.

Decomposers such as bacteria and fungi break down dead organic matter, releasing inorganic nutrients like nitrogen and phosphorus back into the soil and water where producers can reuse them. They do not fix solar energy (that is photosynthesis) and do not produce oxygen anaerobically.

Species richness is simply the count of distinct species in a given area. It does not account for relative abundance, which is measured by evenness. Together, richness and evenness contribute to biodiversity indices such as the Shannon index.

The tundra has permanently frozen subsoil (permafrost), extremely cold temperatures, and very low precipitation. Vegetation is limited to low-growing mosses, lichens, and grasses. The boreal forest (taiga) also has cold temperatures but has coniferous trees and lacks permafrost across most of its range.

Mutualism is a symbiotic relationship where both interacting species benefit. An example is mycorrhizal fungi and plant roots, where fungi receive sugars and plants receive minerals. Commensalism describes one species benefiting with no effect on the other, and parasitism involves one benefiting at the other's expense.

In the logistic growth model, dN/dt = rN(K-N)/K. This expression is maximized when N = K/2, because the product N(K-N) reaches its mathematical peak at the midpoint. Although per capita rate (r) is highest at very low N, the actual number of new individuals added per unit time is greatest at K/2.

A trophic cascade occurs when the removal or addition of a top predator triggers a chain of effects through lower trophic levels. Here, removing the predator releases herbivores from top-down control, which then devastates the plant community. This is distinct from succession, which involves directional community change over time.

GPP is the total rate of photosynthesis in an ecosystem. Plants use some of that energy for their own cellular respiration (Ra), and the remainder — NPP = GPP - Ra — is available for growth and for consumers. NPP is the energy that accumulates in plant biomass over time.

Using the Lincoln-Petersen formula: N = (M x C) / R, where M = 200 marked, C = 300 recaptured, R = 30 recaptured marked. N = (200 x 300) / 30 = 2,000. This method assumes the population is closed and that marked individuals mix randomly with unmarked individuals.

Secondary succession occurs when a disturbance removes existing organisms but leaves the soil intact. A post-fire landscape still has soil and often surviving seeds and roots, allowing faster community development. Primary succession, by contrast, begins on bare substrate with no soil, as occurs on new volcanic rock or glacially exposed rock.

Resource partitioning describes how competing species divide resources to minimize direct competition, often by exploiting slightly different aspects of a shared resource. This pattern suggests that natural selection has favored individuals that use resources differently from competitors (character displacement), enabling coexistence rather than competitive exclusion.

Denitrification is the microbial process that converts nitrate (NO3-) into N2 gas, returning nitrogen to the atmosphere. This is carried out by anaerobic bacteria in waterlogged or low-oxygen soils. Nitrogen fixation does the reverse — converting N2 into ammonia. Nitrification converts ammonium into nitrate.

The intermediate disturbance hypothesis proposes that moderate levels of disturbance prevent competitive exclusion by dominant species while still allowing colonizers to establish, thus maximizing coexistence. Very low disturbance lets dominant species exclude others; very high disturbance eliminates sensitive species faster than they can recolonize.

At each trophic transfer, a large fraction of energy (roughly 90%) is dissipated as heat through cellular respiration and cannot be reused. Chemical matter such as carbon, nitrogen, and phosphorus is not destroyed; decomposers break down organic molecules and return inorganic nutrients to producers, completing biogeochemical cycles.

Allopatric speciation occurs when a geographic barrier separates a population into isolated groups. Without gene flow, natural selection and genetic drift act independently on each group, eventually producing reproductive isolation and new species. More species from a common ancestor means greater biodiversity.

When birth rates are highest and death rates lowest at low density — and the reverse at high density — this is classic density-dependent regulation. Negative feedback mechanisms such as food limitation and increased competition become stronger as N approaches K, stabilizing the population near carrying capacity. An Allee effect would show reduced fitness at low density, which is the opposite pattern described.

Different foraging heights represent niche partitioning, which in sympatric species with a shared common ancestor is best explained by character displacement: competition historically favored individuals that used different resources, so natural selection drove niche divergence. This is the ecological and evolutionary explanation for resource partitioning observed in many bird species studied by Robert MacArthur.

According to the MacArthur-Wilson equilibrium model, immigration rate is higher when an island is close to the mainland (more colonists arrive), and extinction rate is lower on large islands (more resources and habitats support larger populations). A large, near island therefore has the highest immigration and lowest extinction, yielding the highest species richness at equilibrium.

In a top-down controlled (trophic cascade) system with three trophic levels, adding nutrients increases plants, which increases herbivores (odd-numbered trophic level not suppressed by carnivores), which then supports more carnivores. The carnivores suppress herbivores back down, which releases plants from grazing pressure — so plants remain elevated. This alternating pattern of trophic control is a hallmark of trophic cascade dynamics.

The SLOSS (Single Large Or Several Small) debate in conservation biology generally favors large reserves for species sensitive to edge effects and habitat fragmentation. Large reserves have less edge-to-interior ratio, support larger population sizes that reduce inbreeding and demographic stochasticity, and maintain interior habitat conditions. Small reserves increase edge habitat, which benefits generalists but harms area-sensitive interior specialists.

The correct sequence is: phosphorus fertilizes algae (eutrophication) — algal blooms shade out submersed plants — plants die and, along with dying algae, provide massive organic input — aerobic decomposers consume this organic matter using dissolved oxygen (microbial respiration) — hypoxic or anoxic conditions (dead zone) develop. Phosphorus does not directly bind oxygen, and acidification is not the primary mechanism.

In the Lotka-Volterra model, when alpha (effect of species 2 on species 1) exceeds 1, species 2 suppresses species 1 more than species 1 suppresses itself — species 2 is the superior competitor for species 1's resources. When beta is less than 1, species 1 suppresses species 2 less than species 2 suppresses itself, so species 2 also wins against species 1. Both conditions together predict species 1 will be excluded by species 2.

Carrying capacity (K) is the maximum population size that an environment can support given available resources such as food, water, and space. It is not the maximum growth rate (that is r_max) nor the minimum viable population size. In logistic growth, birth rate equals death rate at K, but this is a consequence of reaching carrying capacity, not the definition of K itself.

Detritivores (also called decomposers when referring to bacteria and fungi) break down dead organic matter into simpler inorganic compounds, recycling nutrients back into the ecosystem. Herbivores eat living plants, carnivores eat animals, and autotrophs produce their own food through photosynthesis or chemosynthesis.

Density-independent factors affect populations regardless of their size or density. A volcanic eruption destroys habitat and kills organisms whether the population is large or small. Competition, predation, and disease spread are all density-dependent because their effects intensify as population density increases.

Biodiversity encompasses three levels: genetic diversity (variation within species), species diversity (number and relative abundance of species), and ecosystem diversity (variety of habitats and ecological processes). Biomass refers to the total mass of living matter, biotic potential is the maximum reproductive rate, and biogeography is the study of species distribution.

Secondary consumers eat primary consumers (herbivores). Producers are the first trophic level (autotrophs), primary consumers eat producers, and secondary consumers eat primary consumers. Decomposers obtain energy from dead organic matter at any trophic level and are not typically assigned a single trophic level.

In parasitism, the parasite benefits by obtaining nutrients or shelter from the host, while the host is harmed (though usually not immediately killed). Mutualism benefits both species, commensalism benefits one while the other is unaffected, and no standard ecological relationship is defined by harm to both parties (that would be mutual competition in extreme cases).

Nitrogen fixation is carried out by bacteria (such as Rhizobium and cyanobacteria) that convert atmospheric N2 into ammonia, making nitrogen available to plants. Nitrification converts ammonia to nitrates, denitrification returns nitrogen to the atmosphere, and ammonification converts organic nitrogen back to ammonia from dead organisms.

In the logistic growth equation dN/dt = rN(1 - N/K), the growth rate is maximized when N = K/2. At this point, the term N(1 - N/K) reaches its peak value. The growth rate is not zero (that occurs at N = 0 or N = K), and it is not equivalent to purely exponential growth because the (1 - N/K) term still suppresses growth compared to the unconstrained exponential model.

Secondary succession occurs in areas where a disturbance (fire, flood, logging) has removed the existing community but left the soil and seed bank intact. This accelerates recovery compared to primary succession, which starts on bare rock or new land with no soil. Both types involve species turnover over time, can occur in various environments, and pioneer species in secondary succession are not necessarily more competitive.

According to the Theory of Island Biogeography (MacArthur and Wilson), species richness is determined by immigration and extinction rates. Large islands support larger populations with lower extinction rates, and islands close to the mainland receive higher immigration rates. Therefore, large islands near the mainland have the highest equilibrium species richness.

A top-down trophic cascade occurs when changes in predator abundance ripple downward through the food web. When a top predator is removed, prey populations increase (releasing them from predation), which in turn can over-consume the next lower trophic level (usually plants or herbivores). This contrasts with bottom-up control, where primary productivity drives the rest of the food web.

The intermediate disturbance hypothesis states that moderate levels of ecological disturbance prevent any single species from dominating and outcompeting others, while still allowing enough stability for many species to establish. Very low disturbance allows competitive exclusion by dominant species, and very high disturbance prevents colonization by most species, both reducing diversity.

GPP is the total energy fixed by photosynthesis. Producers must use some of that energy for their own metabolic processes (cellular respiration). NPP = GPP minus respiration by producers. Therefore, NPP represents the energy actually stored in plant biomass available to consumers. NPP is not reduced by trophic transfer losses — that would affect energy available to higher trophic levels, not NPP itself.

Resource partitioning allows ecologically similar species to coexist by using different portions of the same resource. By occupying different microhabitats within the same tree, the two bird species reduce direct competition for food. Competitive exclusion would predict one species eliminating the other, not stable coexistence. Mutualism involves both species benefiting, and character convergence describes unrelated species becoming more similar, not partitioning resources.

Decomposers (bacteria, fungi, and detritivores) break down dead organic matter, releasing nutrients back into the soil and water where producers can absorb them again. Most ecosystems are relatively closed with respect to nutrients, relying on internal cycling rather than continuous external inputs. Energy, unlike nutrients, flows in one direction and is not recycled.

In ecosystems where water is the primary limiting factor, increasing precipitation directly increases NPP by allowing more photosynthesis and plant growth. Reduced solar radiation would decrease photosynthesis, higher respiration without compensating photosynthesis would lower NPP, and reduced decomposer activity would limit nutrient cycling, decreasing nutrient availability for producers.

Negative feedback in population ecology refers to mechanisms that oppose changes and stabilize population size. When density increases, disease spreads more easily, raising mortality and reducing the growth rate — this is a classic density-dependent negative feedback. The prey-predator recovery cycle describes oscillating dynamics (neither purely positive nor purely negative feedback), and inbreeding depression following a bottleneck is a positive feedback that accelerates decline.

The founder effect occurs when a small number of individuals establish a new population, carrying only a subset of the original gene pool. This reduces genetic diversity in the new population compared to the source. Natural selection acts on existing variation rather than reducing it uniformly, frequency-dependent selection can maintain polymorphisms, and disruptive selection splits a population rather than reducing overall diversity.

The per capita growth rate r = b - d = 0.5 - 0.2 = 0.3. The change in population size dN = rN = 0.3 x 200 = 60 individuals. New population = 200 + 60 = 260. Choice C (300) would result from r = 0.5 applied to only new births without subtracting deaths. This calculation uses the exponential growth model assuming resources are not limiting.

Biological magnification (biomagnification) is the increasing concentration of a persistent substance at successively higher trophic levels. Phosphorus in this scenario behaves like a persistent toxin, accumulating in tissues faster than it is excreted and becoming concentrated at each trophic level. Bioaccumulation refers to accumulation within an individual organism, while biomagnification refers to the trend across trophic levels. Eutrophication describes excess nutrient enrichment in water bodies, not tissue concentration.

In highly variable environments, the species with broader tolerance (Species A) gains a competitive advantage because Species B cannot survive or reproduce during temperature extremes. Even if Species B is a superior competitor under moderate conditions, high variability creates refuges for Species A. Character displacement typically leads to divergence in resource use, not expansion of tolerance. Both species cannot converge to identical niches due to competitive exclusion pressure.

After deforestation, decomposition of leaf litter and soil organic matter continues, releasing ammonium which is then nitrified to nitrate. However, without living trees to take up nitrate from the soil, the excess nitrate leaches into streams. This demonstrates tight coupling between plant nutrient uptake and ecosystem nutrient retention. Nitrification actually continued or increased post-deforestation, and nitrogen fixation by pioneers would not account for the immediate surge.

Species diversity incorporates both richness (number of species) and evenness (relative abundance). Fragment X has higher evenness and therefore higher overall diversity even with the same species richness. Greater diversity, particularly greater evenness, tends to increase ecosystem resilience because many species can fill functional roles if one declines. A community dominated by one species is vulnerable — if that species is affected by a stressor, the ecosystem can collapse. Resilience is not solely determined by trophic levels.

Mycorrhizal associations represent a mutualism in which the plant provides the fungus with carbohydrates and the fungus enhances the plant's access to phosphorus and water through extensive hyphal networks. In phosphorus-poor soils, this trade is beneficial — the phosphorus gain outweighs the carbon cost. In phosphorus-rich soils, plants can obtain phosphorus through their own roots, so the carbon cost of the association is no longer justified and provides no net growth benefit. The fungi do not produce phosphorus through chemosynthesis, and there is no evidence from this scenario of a mutualism-to-parasitism shift.

In the logistic growth model, population growth rate (dN/dt) is maximized at N = K/2. Maintaining the population at this level allows the maximum number of individuals to be harvested while the population can fully replace those removed due to its peak growth rate. At K, the net growth rate approaches zero so any harvest exceeds replacement. At very low densities, the small population size means the absolute number of new individuals added is low even if per capita rate is high, making harvest unsustainable.

Carrying capacity (K) is the maximum population size that an environment can support given its limiting resources such as food, water, and space. It is not the growth rate (that would be r), nor is it a minimum threshold — it is an upper limit imposed by the environment.

Detritivores such as earthworms consume dead organic matter (detritus) and break it down, returning nutrients to the soil where they can be taken up by plants. Hawks are predators, phytoplankton are primary producers, and deer are herbivores — none of these are detritivores.

Mutualism is a symbiotic relationship in which both species benefit. Commensalism benefits one species while the other is unaffected. Parasitism benefits one species at the expense of the other. Amensalism harms one species while the other is unaffected.

The fundamental niche is the full range of conditions a species can theoretically occupy; the realized niche is the actual range it occupies in the presence of competitors. Because Species B competitively excludes Species A from the lower zone, Species A's realized niche is narrower than its fundamental niche. Character displacement involves divergence in traits, and there is no evidence of predation here.

NPP = GPP minus plant respiration = 8,000 minus 3,200 = 4,800 kcal/m²/year. NPP represents the organic matter actually stored by producers and available to primary consumers (and ultimately all heterotrophs). GPP is the total energy fixed before any respiration losses, not NPP.

Resource partitioning occurs when competing species use the same resource in different ways or different portions of the habitat, reducing direct competition and allowing coexistence. Character displacement involves measurable trait divergence. Competitive exclusion leads to one species being eliminated, which is the opposite of what is observed here.

Excess phosphorus triggers an algal bloom (eutrophication). Dense algae block sunlight, causing submerged aquatic plants to die. When the algae eventually die, decomposing bacteria consume dissolved oxygen (hypoxia), creating dead zones that kill fish. Zooplankton grazing is insufficient to control a large bloom, and algal blooms reduce, not increase, light penetration.

Under exponential growth, r = b minus d = 0.4 minus 0.15 = 0.25. Using N(t) = N0 × e^(rt): N(2) = 200 × e^(0.25 × 2) = 200 × e^0.5 ≈ 200 × 1.649 ≈ 330. The answer closest to 330 is 330. (Note: 350 would correspond to a slightly different r; the correct calculation yields approximately 330.)

According to the MacArthur-Wilson equilibrium model, proximity to the mainland increases immigration rate (colonizers arrive more easily), and larger island size decreases extinction rate (larger habitat supports larger populations less vulnerable to stochastic extinction). Together, high immigration and low extinction produce the highest species richness at equilibrium.

Small, isolated patches suffer from multiple interacting forces: (1) reduced immigration because dispersers cannot cross the surrounding matrix; (2) higher extinction risk because small populations are vulnerable to demographic and environmental stochasticity; and (3) higher edge-to-interior ratios expose species to predation, desiccation, and invasive species from the surrounding habitat. Competitive exclusion and character displacement do not primarily explain patch-level species loss, and large patches generally support more, not fewer, species.