AP Environmental Science Unit 2: The Living World: Biodiversity — Free Review Games.

Biodiversity encompasses three levels: genetic diversity (variation within species), species diversity (variety of species), and ecosystem diversity (variety of habitats and ecological processes).

Invasive species are non-native organisms that spread aggressively, often lacking natural predators or diseases in their new environment. They outcompete native species for food, habitat, and other resources.

In commensalism, one organism benefits while the other is neither helped nor harmed. An example is barnacles attaching to whales for transport.

A keystone species plays a critical role in maintaining ecosystem structure. Removing it causes significant changes, even though it may not be the most abundant species. Sea otters and wolves are classic examples.

Primary succession begins on surfaces without soil, such as bare rock, lava flows, or newly exposed land. Pioneer species like lichens and mosses are the first colonizers.

Gause's competitive exclusion principle states that two species occupying the same niche cannot coexist long-term. One will eventually outcompete and displace the other.

Resource partitioning reduces competition by species using different portions of a resource. For example, warblers feeding at different heights in the same tree species.

MacArthur and Wilson's theory predicts that larger islands closer to the mainland have more species due to higher immigration rates and lower extinction rates.

Indicator species are sensitive to environmental changes. Their health or population status signals the condition of their ecosystem. Amphibians and lichens are common indicator species.

Secondary succession occurs in areas where soil remains after a disturbance (fire, flood, farming). Since soil, seeds, and root systems may persist, recovery is faster than primary succession.

When a small number of individuals colonize an island, they carry only a subset of the source population's genetic diversity. This limited gene pool affects the island population's ability to adapt.

Habitat fragmentation creates more edge area with altered conditions (more light, wind, temperature variation). Edge-adapted species thrive while interior specialists that need undisturbed core habitat decline.

Coevolution drives an arms race: as prey evolve better defenses (camouflage, toxins, speed), predators evolve countermeasures (better vision, toxin resistance, speed). Neither side achieves permanent advantage.

Ecosystem engineers physically modify their environment. Beavers build dams creating wetlands, and corals build reefs providing habitat. These modifications affect many other species.

Biodiversity generally peaks near the equator and decreases toward the poles. This is attributed to greater solar energy, stable climate, larger habitat area, and longer evolutionary time in tropical regions.

Provisioning services are direct goods obtained from ecosystems — food, fresh water, timber, and fiber. Timber harvested for construction is a provisioning service. Pollination is a supporting service, and both flood regulation and carbon sequestration are regulating services. The key distinction is that provisioning services yield tangible products people extract directly.

Genetic diversity provides the raw material for natural selection. When conditions shift — due to disease, drought, or temperature change — individuals carrying certain genetic variants may survive and reproduce while others perish. Without genetic diversity, a population may lack the variants needed to adapt, increasing extinction risk. Genetic diversity does not directly increase species richness or reduce interspecific competition.

Endemic species are native to and restricted to a specific geographic area. Because their entire global range is confined to one location, habitat destruction in that area threatens the entire species with extinction. This is why island ecosystems and biodiversity hotspots — which harbor many endemics — are priority conservation targets. Species introduced from elsewhere are called exotic or non-native, not endemic.

Commensalism is a (+/0) relationship: one organism benefits and the other is unaffected. A classic example is an epiphytic plant growing on a tree branch — the epiphyte gains structural support while the tree experiences no significant harm or benefit. Mutualism is (+/+), parasitism is (+/−), and competition is (−/−). Distinguishing these interaction types requires evaluating the fitness outcome for each partner.

Habitat corridors are strips of suitable habitat connecting fragmented patches. They allow animals to move between patches for feeding, mating, and recolonization after local extinction events. This maintains genetic exchange among subpopulations and enables species to shift their ranges in response to climate change. Without corridors, isolated fragments can become too small to support viable populations, accelerating local extinctions.

Biodiversity hotspots, as defined by Conservation International, are regions containing at least 1,500 endemic vascular plant species that have also lost at least 70% of their original habitat extent. The combination of irreplaceable biodiversity and severe ongoing threat makes these regions the highest conservation priorities. Examples include the tropical Andes, the Cape Floristic Region, and the Indo-Burma hotspot.

A climax community is the relatively stable, mature community that develops after a series of successional stages. It is dominated by species well-adapted to local abiotic conditions and tends to persist unless disturbed. Pioneer species are the early colonizers that initiate succession, not the endpoint. Climax communities can develop following many types of disturbance and on diverse substrates, not only volcanic surfaces.

Both plots have identical species richness (3 species). However, species evenness — how equally individuals are distributed among species — is far higher in Plot Y, where each species contributes equally. Biodiversity indices such as the Shannon index incorporate both richness and evenness, so Plot Y is more diverse overall. Dominance by a single species (as in Plot X) reduces evenness and lowers measured biodiversity even when richness is equal.

A trophic cascade occurs when a change at one trophic level — here, the reintroduction of wolves — propagates effects through multiple lower trophic levels. Wolves caused elk to alter their behavior (avoiding riverbeds), which released plant communities from overgrazing, stabilized stream banks, and benefited beavers and fish. This is a classic top-down cascade because the driving change originated at the apex predator level and cascaded downward.

Island biogeography theory balances immigration and extinction rates. Large islands have lower extinction rates because larger area supports larger populations that are less prone to stochastic extinction and can sustain more niches. Islands near the mainland have higher immigration rates because dispersal distances are shorter. A large, nearby island therefore achieves the highest equilibrium species richness by combining low extinction with high immigration.

An umbrella species has large area requirements or specific habitat needs such that conservation efforts designed to protect it also shelter many other species sharing that landscape. The grizzly bear is a classic example: protecting enough wilderness for viable grizzly populations shelters hundreds of co-occurring species. Keystone species have disproportionate community effects relative to their abundance; indicator species signal environmental health; and ecosystem engineers physically alter habitat.

Regulating services are benefits derived from ecosystem processes that regulate environmental conditions, such as water purification, climate regulation, flood control, and disease regulation. Wetlands filtering agricultural pollutants is a textbook regulating service. Harvesting salmon and collecting medicinal plants are provisioning services. Ecotourism revenue is a cultural service. The key distinction is that regulating services maintain conditions rather than provide extractable goods or experiential benefits.

Species requiring large contiguous territories — such as wide-ranging carnivores — are most vulnerable to fragmentation because individual habitat patches may fall below the minimum area needed to support a viable population. Low population density means fewer individuals per patch, increasing extinction risk from demographic and environmental stochasticity. Generalists, rapid reproducers, and strong dispersers are buffered against fragmentation because they can exploit remnant patches or move through the matrix between fragments.

Specialist species are adapted to a narrow range of habitats, food sources, or environmental conditions. If those specific resources decline — through habitat loss, prey extinction, or climate shift — specialists cannot easily substitute alternatives, placing them at high risk. Generalists, by contrast, can exploit many resource types and adapt behaviorally to change. The ecological breadth of specialists, not their reproductive rate or metabolic rate alone, is the primary driver of their elevated extinction risk.

Ecological resilience is the capacity of an ecosystem to recover and reorganize after disturbance, returning to essentially the same composition, structure, and function. It is distinct from resistance, which refers to how much a system changes during a disturbance. A resilient ecosystem may change substantially during a disturbance but recovers quickly; a resistant ecosystem resists change in the first place. Neither concept involves preventing invasive species establishment or maximizing species packing.

The intermediate disturbance hypothesis (IDH) proposes that diversity peaks at intermediate levels of disturbance frequency and intensity. At low disturbance, competitive exclusion allows dominant species to eliminate weaker competitors, reducing diversity. At very high disturbance, only disturbance-tolerant pioneer species can persist. At intermediate levels, both competitive dominants and early-successional species coexist across patches in different recovery stages, maximizing local diversity.

Character displacement is the evolutionary divergence of morphological or behavioral traits in species that co-occur and compete, reducing niche overlap and the intensity of competition. In Darwin's finches, species that live together on the same island show greater beak-size differences than when the same species occur alone on separate islands — evidence that interspecific competition drove evolutionary divergence in resource use. This is a competitive, not predatory or random, evolutionary process.

Cultural ecosystem services are the non-material benefits humans obtain from ecosystems, including recreational, aesthetic, spiritual, and educational values. Recreational diving and reef tourism represent cultural services. Forests absorbing \(CO_2\) and mangroves reducing storm surge are regulating services. Decomposition and nutrient cycling by soil microbes are supporting services that underpin all other ecosystem service categories. Cultural services are often overlooked in economic analyses but have significant human wellbeing value.

A metapopulation is a set of spatially separated local populations connected by dispersal. Individual local populations may go extinct due to stochastic events, but the regional metapopulation persists because dispersers from surviving patches recolonize empty habitat. This 'rescue effect' is the defining functional feature of metapopulation dynamics. Hybridization, identical environments, and synchronized reproduction are not the defining mechanism, though dispersal can incidentally facilitate gene flow.

When one species dominates completely, \(p_i = 1\) for that species and \(p_i = 0\) for all others. Because \(1 \times \ln(1) = 0\), the sum equals zero and \(H = 0\). Maximum \(H\) occurs when all species are equally abundant (maximum evenness), yielding \(H = \ln(S)\) where \(S\) is species richness. \(H\) captures both richness and evenness; it does not measure successional stage. A value of \(H = 0\) represents the minimum possible diversity — complete monoculture.

Extinction debt is the delayed loss of species following habitat destruction. After habitat is fragmented or reduced below a critical threshold, remnant populations may persist for decades before going extinct because current population sizes still exceed zero. However, these populations are below minimum viable size and will eventually disappear. This means observed extinction rates underestimate the true biodiversity loss already caused by past land-use change. The debt 'comes due' over time as relict populations dwindle.

The Allee effect occurs when individuals in small or sparse populations experience reduced per capita fitness — due to difficulty locating mates, reduced cooperative anti-predator behavior, inbreeding depression, or demographic stochasticity. Below a critical population threshold, per capita growth rate can become negative, making extinction nearly inevitable without intervention even in the absence of additional habitat loss. This contrasts with standard logistic growth theory, which predicts maximum per capita growth at low density.

MVP estimates decrease when extinction threats are minimized. High dispersal ability allows recolonization of empty patches after local crashes, buffering against regional extinction. Access to large, stable habitat reduces both environmental stochasticity and catastrophic events. High reproductive rate enables rapid recovery from population dips. Options A and C describe factors that elevate extinction risk, increasing MVP estimates. Option D describes ecological pressures that increase vulnerability without the compensating factors that would lower MVP.

Coextinction is the secondary extinction of a species following the loss of another species it depends on. The risk is greatest in obligate mutualisms — for example, a parasitoid insect that can complete its life cycle only on one host species, or a plant pollinated solely by a single bee species. When the partner goes extinct, the dependent specialist loses its only viable ecological option and faces extinction as well. Generalist predators can switch prey, and competitors or newly overlapping species may adjust without extinction cascades.

Matrix quality determines how easily — or how dangerously — individuals can move between habitat patches. A high-quality matrix contains some vegetation, reduced road density, or other features that lower mortality risk for dispersing animals and facilitate inter-patch movement. This connectivity supports the metapopulation rescue effect. Monocultures and roads (option A) constitute a hostile, low-quality matrix that isolates populations. Options C and D describe patch geometry rather than the ecological character of the intervening land.

Species richness counts only the number of distinct species in a community, without regard to how many individuals belong to each. The distractor "The relative abundance of individuals across species in a community" describes species evenness, a separate component of diversity. Students should remember that overall species diversity combines both richness and evenness.

Species evenness measures how equally individuals are distributed among species, and the first community's balanced distribution gives it higher evenness and thus higher overall diversity. "Species richness" is wrong here because both communities have the same richness (four species); the difference lies in distribution, not count. On the AP exam, remember that two communities can have identical richness yet different diversity due to evenness.

Provisioning services are direct material goods extracted from ecosystems, and timber harvested for construction is a tangible product taken from nature. "Pollination of crops by native bees" is a regulating service because it involves a natural process that supports production rather than being the product itself. Students should sort ecosystem services into provisioning, regulating, supporting, and cultural categories to answer these questions quickly.

Keystone species exert effects on community structure and biodiversity far greater than their numbers would suggest, often by controlling prey populations or engineering habitat. "Has the largest biomass in an ecosystem" describes a dominant species, not a keystone species, since dominant species influence communities primarily through sheer abundance. The exam expects students to distinguish keystone species from dominant, indicator, and umbrella species based on this disproportionate-impact rule.

The species-area relationship shows that larger islands provide more habitat diversity and resource availability, which supports a greater number of species. "Fewer species than smaller islands due to increased competition" contradicts the well-documented positive correlation between area and richness observed across island systems. Students should recognize that island size is one of the two central variables, along with isolation, in the equilibrium theory of island biogeography.

Biodiversity hotspots are defined by containing at least 1,500 endemic plant species while having lost 70 percent or more of their original habitat, combining exceptional uniqueness with severe threat. "Large total area and stable climate" is incorrect because hotspots are often relatively small regions under intense environmental pressure, not large stable ones. Students should associate hotspots with the pairing of high endemism and high threat level, not simply high biodiversity alone.

MacArthur and Wilson's model plots immigration rate declining and extinction rate rising as species number increases, and equilibrium species richness occurs where these two curves cross, representing a dynamic turnover rather than a static endpoint. "The point where immigration rate reaches zero" is wrong because immigration never truly reaches zero as long as colonization from the mainland continues. Students should picture the intersecting-curves graph whenever equilibrium island richness is discussed on the exam.

Distance from the mainland reduces the rate at which new colonizing species successfully reach an island, lowering the immigration curve and shifting the equilibrium to a smaller number of species. "A higher immigration rate because fewer competitors reach it" misapplies the model, since fewer competitors do not increase colonization rate but rather reflect the low colonization rate itself. Remember the general rule: near islands have higher immigration, far islands have lower immigration, and this shapes equilibrium richness independent of extinction dynamics.

Smaller islands support smaller populations that are more vulnerable to local extinction from random events and resource limitation, raising the extinction curve and lowering equilibrium richness. "A lower extinction rate because populations face less competition" is incorrect because smaller area generally means smaller, more vulnerable populations, not more stable ones. Students should pair island size with the extinction curve and distance with the immigration curve when reasoning through island biogeography problems.

Genetic diversity provides the raw variation that natural selection acts on, so a population with low genetic diversity has fewer heritable traits available to respond successfully to new stressors like pathogens or shifting climate conditions. "Reproduce at all under any environmental condition" overstates the effect, since low genetic diversity reduces adaptive capacity rather than eliminating reproduction outright. This principle explains why conservation programs prioritize maintaining genetic diversity, not just population numbers, in endangered species management.

Removing sea otters, a keystone predator, released sea urchin populations from predation control, allowing urchins to overgraze kelp forests and cascade effects through multiple trophic levels. "Character displacement between competing species" is wrong because this scenario involves a predator-prey-producer chain, not two competing species evolving different traits. This example illustrates how a single keystone species removal can restructure an entire ecosystem, a concept frequently tested through real-world case studies.

Habitat edges experience different light, wind, temperature, and moisture conditions than forest interiors, and these altered microclimates favor generalist, invasive, and edge-adapted predator species over specialist interior species. "Decreased sunlight penetration compared to the forest interior" is incorrect because edges actually receive more sunlight due to canopy openness, not less. Students should link edge effects to increased fragmentation impacts, since smaller fragments have proportionally more edge relative to interior habitat.

The founder effect occurs when a small colonizing group carries only a subset of the source population's genetic variation, causing the new island population's gene pool to differ from the mainland by chance. "The Allee effect" is incorrect because it describes reduced fitness at low population density, not the genetic consequences of colonization by a small founding group. This concept explains why many island species show unusual traits and reduced genetic diversity compared to their mainland relatives.

Supporting services are the underlying ecological processes, such as nutrient cycling, soil formation, and primary production, that enable all other ecosystem services to occur. "Regulating service" is incorrect because regulating services involve processes like climate or disease control that moderate environmental conditions rather than forming the foundational biogeochemical processes themselves. Students should remember that supporting services underpin the other three categories rather than being consumed directly by humans.

The insurance hypothesis holds that greater species diversity increases functional redundancy, so if environmental conditions change and some species decline, other species with similar roles can maintain overall ecosystem processes like productivity and nutrient cycling. "Guarantees that no species will ever go extinct locally" overstates the hypothesis, which addresses functional stability, not the prevention of any single species' extinction. This principle underlies why conservation biologists argue that biodiversity loss threatens ecosystem stability even before any given species is critical to that function.

A near island has a higher immigration curve because colonizers arrive more easily, and a large island has a lower extinction curve because it supports bigger, more stable populations, so their intersection occurs at a higher equilibrium species number than for a far, small island. "Lower on the species richness axis because large islands experience higher extinction rates" reverses the actual relationship, since larger area reduces extinction risk rather than increasing it. Synthesizing both curves simultaneously, rather than considering size or distance alone, is essential for correctly predicting equilibrium richness on the exam.

The rescue effect describes how continued immigration of individuals from mainland or other island populations can bolster small, vulnerable populations and reduce their probability of local extinction. "Extinction rates decline as island area increases regardless of immigration" describes the area effect on extinction, a separate mechanism that does not involve ongoing immigration replenishing populations. Distinguishing the rescue effect, driven by immigration, from the area effect, driven by population size and habitat, is a common higher-order distinction tested on the AP exam.

Nested subset patterns arise because some species are better dispersers or more resistant to extinction on small or isolated islands, so species-poor islands tend to retain only the subset of species capable of persisting or colonizing under those harsher conditions. "Random distribution of species with no relationship to island area or isolation" contradicts the very existence of a predictable nested pattern, which implies non-random processes are at work. Recognizing that nestedness reflects systematic differences in colonization and extinction probabilities, not chance, helps students interpret biogeographic data sets on the exam.

Island species frequently evolved in isolation without exposure to mainland predators, parasites, or competitors, so they often lack behavioral or physical defenses when invasive species arrive, making them highly vulnerable to predation and competition. "Islands generally have higher species richness than continents, making them more resistant to invasion" is incorrect because islands typically have lower species richness than comparable mainland areas due to limited colonization opportunities. This vulnerability explains why a large proportion of documented extinctions attributed to invasive species have occurred on islands.

Because \(z\) is the exponent in \(S = cA^z\), a larger \(z\) produces a steeper slope on a log-log species-area plot, meaning richness rises more rapidly with increasing area on Island X than on Island Y. "Island X will always have more total species than Island Y regardless of area" is wrong because a steeper slope only guarantees a faster rate of increase, not a higher absolute richness at every area value, especially at small areas where the curves may cross. Students should interpret \(z\) as describing sensitivity of richness to area changes, a concept applicable to comparing habitat fragmentation impacts across different ecosystems.