Ecology and Ecosystems — Free Biology Review Games.

An ecosystem includes all living organisms in an area plus the nonliving components they interact with.

Producers (autotrophs) make their own food, typically through photosynthesis, forming the base of food chains.

Decomposers like bacteria and fungi break down dead organisms and waste, recycling nutrients back into the ecosystem.

A food web shows the complex, interconnected feeding relationships among organisms in an ecosystem.

A population consists of all individuals of the same species living in a particular area at a given time.

Approximately 10% of energy at each trophic level is passed to the next; the rest is lost as heat through metabolism.

Carrying capacity is the maximum number of individuals an environment can support with available resources over time.

A habitat is the physical place where an organism lives, while a niche describes its ecological role including diet, behavior, and interactions.

Parasitism is a symbiotic relationship where one organism (the parasite) benefits at the expense of the host.

Tropical rainforests have the highest biodiversity of any biome due to warm temperatures, abundant rainfall, and stable conditions.

Primary succession begins in barren areas with no soil (like bare rock or new lava), starting with pioneer species like lichens.

Bioaccumulation causes toxins to concentrate at progressively higher levels in organisms up the food chain, most severely affecting top predators.

A keystone species has a disproportionately large effect on its ecosystem relative to its abundance; its removal causes significant changes.

The competitive exclusion principle states that two species competing for identical resources cannot coexist; one will outcompete the other.

The nitrogen cycle requires nitrogen-fixing bacteria to convert atmospheric N2 into usable forms, whereas carbon enters ecosystems mainly through photosynthesis.

Primary consumers sit at the second trophic level because they feed directly on producers, which occupy the first level. The distractor "The third trophic level; they eat primary consumers" actually describes secondary consumers, not primary consumers. Remembering that trophic level numbering starts with producers at level one helps students correctly trace energy flow through any food chain.

Biomass is defined as the total mass of living or once-living organic matter in a specified area, usually measured per unit area. The distractor "The total number of species present in a habitat" describes species richness, a completely different ecological measurement. Distinguishing biomass from species diversity is essential because ecological pyramids often measure biomass rather than organism count.

The tundra biome has permanently frozen subsoil called permafrost and supports few species due to its harsh cold climate and short growing season. The distractor "Taiga" refers to the boreal forest biome south of the tundra, which has coniferous trees and is not defined by permafrost. Knowing key defining abiotic features of each biome, like permafrost for tundra, helps students quickly identify biomes on the exam.

Autotrophs, such as plants and certain bacteria, synthesize organic molecules from inorganic sources through photosynthesis or chemosynthesis. The distractor "Heterotrophs" is incorrect because heterotrophs must consume other organisms for energy rather than making their own food. This distinction underlies the base of every food web, since autotrophs are always the producers.

A limiting factor is any resource, such as food, water, or space, whose scarcity restrains how large a population can grow. The distractor "A genetic mutation that changes species traits" refers to a source of evolutionary variation, not an ecological constraint on population size. Recognizing limiting factors is key to understanding why populations stabilize near carrying capacity rather than growing indefinitely.

A pyramid of energy always narrows toward the top because the second law of thermodynamics guarantees energy loss as heat at every trophic transfer, so it can never be inverted. The distractor "Pyramid of biomass" can sometimes appear inverted, such as in aquatic ecosystems where phytoplankton reproduce faster than they are consumed. Students should remember that energy pyramids are the most reliable and consistent of the three pyramid types.

Detritivores, like earthworms, physically consume and digest decaying matter inside their bodies, whereas decomposers such as fungi and bacteria secrete enzymes to break down organic material externally before absorbing the nutrients. The distractor stating they are "simply two names for the exact same process" ignores this real mechanistic difference in how each group processes dead matter. Both groups are essential recyclers of nutrients back into ecosystems, but their methods of breakdown differ.

Exponential growth occurs when resources are unlimited, allowing a population's growth rate to increase continuously and produce a J-shaped curve on a graph. The distractor describing an "S-shaped curve" actually refers to logistic growth, which accounts for environmental resistance and carrying capacity. Recognizing the shape of these growth curves helps students quickly identify which population model a graph represents.

Deserts are defined primarily by extremely low annual precipitation, often less than 25 centimeters per year, which shapes their sparse vegetation and specialized organisms. The distractor "Tropical rainforest" is incorrect because rainforests receive some of the highest annual precipitation of any biome on Earth. Precipitation and temperature together are the two main abiotic factors used to classify terrestrial biomes.

Immigration specifically refers to individuals entering a population from another location, which increases population size independent of birth rate. The distractor describing "the movement of individuals out of a population" is emigration, the opposite process that decreases population size. Population change is calculated using births, deaths, immigration, and emigration together, so distinguishing these terms is essential for solving population math problems.

Temperature is a nonliving, physical component of the environment, making it a classic abiotic factor that influences species distribution and metabolic rates. The distractor "Predation" is incorrect because it describes an interaction between two living organisms and is therefore a biotic factor. Sorting environmental influences into biotic and abiotic categories is a foundational skill for analyzing any ecosystem.

Scavengers, such as vultures, obtain energy by consuming carrion that other organisms have already killed, distinguishing them from active predators. The distractor "An animal that hunts and kills only live prey" describes a predator, which is a functionally different feeding role in the food web. Recognizing scavengers as a distinct feeding strategy helps students accurately map energy pathways in complex food webs.

Solar energy captured by photosynthetic producers is the ultimate energy source powering the vast majority of Earth's food webs. The distractor "Geothermal heat from Earth's core" only fuels a small number of chemosynthetic ecosystems, such as deep-sea hydrothermal vent communities, rather than most ecosystems globally. Understanding the sun as the primary energy input explains why energy flow diagrams always begin with producers converting solar energy into chemical energy.

Applying the 10 percent rule twice, \(10{,}000\) kcal at the producer level yields \(1{,}000\) kcal for primary consumers and then \(100\) kcal for secondary consumers. The distractor "\(1{,}000\) kcal" mistakenly stops the calculation after only one trophic transfer instead of two. Students must always apply the 10 percent reduction once for each trophic level crossed, not just once total.

Type III survivorship curves, seen in species like fish and insects, show a steep early decline because most offspring die young while a few survivors live long lives. The distractor "Type I" describes the opposite pattern, where most individuals survive to old age and mortality is concentrated later in life, as seen in large mammals like humans. There is no standard "Type IV" curve, so recognizing that only three types exist helps avoid confusion on the exam.

As prey populations peak, predators have abundant food, so their numbers rise shortly afterward with a time lag, which then drives prey numbers back down as predation pressure increases. The distractor "Predator populations crash immediately before prey populations peak" reverses the actual lagged relationship documented in classic predator-prey data like the lynx and snowshoe hare cycle. This lag between prey and predator peaks is a hallmark feature students should recognize on cyclical population graphs.

Parasitism occurs when one organism, the tick, benefits by extracting nutrients while harming the host, the dog, which loses blood and may contract disease. The distractor "Commensalism" is incorrect because commensalism requires the host to be unaffected, whereas the dog is actively harmed by the tick's feeding. Identifying whether a relationship helps, harms, or has no effect on each participant is the key to correctly classifying symbiotic interactions.

A Type II survivorship curve appears as a fairly straight diagonal line because individuals, such as many birds and rodents, face a relatively constant probability of death regardless of age. The distractor "Mortality is concentrated almost entirely in old age" instead describes a Type I curve, which is typical of species with extensive parental care and few natural predators. Recognizing the linear shape of a Type II curve on a graph immediately signals constant risk of death throughout life.

Disease transmission is density-dependent because its impact intensifies as population density rises, since crowded individuals have more contact and spread pathogens more efficiently. The distractor "A sudden volcanic eruption that destroys habitat" is density-independent because its destructive effect occurs regardless of how many individuals are present. Distinguishing density-dependent from density-independent factors helps predict how limiting factors behave differently as populations grow or shrink.

Resource partitioning occurs when species divide a limited resource, such as feeding height, along different niche dimensions, allowing them to coexist rather than directly compete. The distractor "Competitive exclusion resulting in extinction" is incorrect because both species persist in this scenario rather than one being driven to local extinction. Resource partitioning is a common evolutionary outcome that reduces niche overlap and permits similar species to share the same habitat.

Climographs combine a bar graph of monthly precipitation with a line graph of monthly average temperature so that both key abiotic variables defining a biome can be compared simultaneously. The distractor "Only precipitation data is included on the graph" ignores that temperature is equally essential for classifying which biome a location belongs to. Reading climographs correctly allows students to predict biome type from combined patterns of rainfall and temperature.

A trophic cascade describes how changes in the abundance of one trophic level, such as removing top predators, indirectly cause population shifts in the levels below through altered predation and competition pressures. The distractor describing "the consistent 10 percent efficiency of energy transfer" refers to a completely separate concept, the energy transfer rule, rather than the indirect multi-level effects of a cascade. Understanding trophic cascades explains why removing a single species can dramatically reshape an entire ecosystem's structure.

In the logistic model, growth rate decreases as \(N\) approaches carrying capacity \(K\) because resources become scarcer and environmental resistance increases, eventually leveling the population at \(K\). The distractor "Zero" is incorrect because growth actually slows as the population increases toward \(K\), not as it decreases toward zero. This resource-limited slowing near carrying capacity is what produces the characteristic S-shaped logistic growth curve.

Decomposers break down organic matter and release carbon dioxide as a byproduct of cellular respiration, returning carbon from dead organisms back into the atmosphere for producers to reuse. The distractor "Oxygen" is incorrect because oxygen is consumed, not released, during aerobic respiration, since it acts as the final electron acceptor. This decomposer-driven release of carbon dioxide is a critical step that closes the carbon cycle's loop.

A wide base indicates a large proportion of young, pre-reproductive individuals, which signals that the population will continue growing rapidly as those individuals mature and reproduce. The distractor "Declining in size" is incorrect because a declining population typically shows an age structure diagram with a narrower base and more individuals in older age classes. Age structure diagrams are a powerful tool for predicting future population trends based on current age distribution.

The savanna biome experiences pronounced wet and dry seasons that support a mix of grasses and widely spaced trees adapted to periodic drought and fire. The distractor "Chaparral" instead describes a biome with mild, wet winters and hot, dry summers dominated by shrubby vegetation, not scattered trees among grasses. Matching seasonal precipitation patterns and dominant vegetation types is essential for correctly identifying terrestrial biomes.

Because persistent toxins like DDT are fat-soluble and poorly excreted, they accumulate in each consumer's tissues and become increasingly concentrated as biomass shrinks moving up the food chain, resulting in the highest concentrations in top predators. The distractor "Toxins are diluted more and more as they move up each trophic level" describes the opposite of what actually happens during biomagnification. This concept explains why apex predators, including humans in some cases, face the greatest health risks from persistent environmental pollutants.

r-selected species, such as insects and weeds, invest in producing many offspring quickly with little parental care, an adaptation that favors exploiting unstable or newly available environments. The distractor describing "low reproductive rate with extensive parental care and long lifespan" instead characterizes K-selected species, which invest heavily in fewer offspring. Distinguishing r-selected from K-selected strategies helps predict how a species will respond to environmental disturbance.

Invasive species without natural predators or population controls can rapidly increase in number, outcompeting native species for food and habitat and disrupting established predator-prey and competitive relationships throughout the food web. The distractor "It always goes locally extinct within a few generations" contradicts the well-documented pattern of successful invasive species persisting and spreading precisely because they escape their native population controls. Understanding this dynamic explains why invasive species are a major driver of biodiversity loss worldwide.

A hurricane affects a population's survival and resources without regard to how many individuals are present, making its impact density-independent. The distractor "Disease transmission within a crowded population" is incorrect because disease spread is density-dependent, intensifying as population density increases. Density-independent factors like natural disasters can cause dramatic population crashes irrespective of a population's current size.

Because roughly 90 percent of energy is lost as heat during metabolic processes at each trophic transfer, only a small fraction of producer energy can support consumer biomass, resulting in a much smaller biomass at higher levels. The distractor "Producers simply reproduce more slowly than consumers" is factually backwards, since producers like algae often reproduce very quickly compared to many consumers. This energy loss principle is the fundamental reason ecological pyramids narrow as they rise through trophic levels.

Commensalism occurs when one species benefits, in this case barnacles gaining mobility and feeding opportunities, while the other species experiences essentially no measurable cost or benefit. The distractor "Both the barnacle and the whale gain significant survival benefits" instead describes mutualism, a different symbiotic relationship where both partners clearly benefit. Correctly classifying symbiotic relationships requires carefully evaluating the outcome for each participating organism.

Applying the 10 percent rule across three transfers, \(1{,}000{,}000\) kcal becomes \(100{,}000\) kcal for primary consumers, \(10{,}000\) kcal for secondary consumers, and finally \(1{,}000\) kcal for tertiary consumers. The distractor "\(10{,}000\) kcal" mistakenly represents the secondary consumer level rather than completing the full three-step transfer to tertiary consumers. This multi-step application of the 10 percent rule is a common exam calculation that requires tracking each trophic transfer carefully.

The maximum growth rate occurs at \(N = K/2\) because this is the point where the product of population size and remaining available resources, represented by the term \(\frac{K-N}{K}\), is maximized. The distractor "\(N = K\)" is incorrect because at carrying capacity the term \(\frac{K-N}{K}\) equals zero, meaning growth rate has actually dropped to zero, not its peak. Recognizing that the steepest point of the S-shaped logistic curve occurs at half of carrying capacity is a key quantitative concept for population ecology questions.

Classic studies of keystone predators like sea stars show that removing them allows a competitively dominant prey species, such as mussels, to monopolize space and outcompete other organisms, sharply reducing overall species diversity. The distractor "Biodiversity always increases immediately after the predator is removed" contradicts observed real-world outcomes, where diversity typically declines without the predator controlling the dominant competitor. This cascading loss of biodiversity illustrates why keystone species have effects on community structure disproportionate to their own abundance.

The Allee effect describes how very small populations can experience reduced growth rates because low density makes it harder to find mates, defend against predators, or cooperate socially, creating a positive feedback toward further decline. The distractor describing "a large population's growth slows purely due to increased resource competition" instead describes standard density-dependent regulation near carrying capacity, which is unrelated to the Allee effect's focus on small population vulnerability. Understanding the Allee effect is important for conservation biology, since it explains why some endangered populations struggle to recover even when threats are removed.

Secondary succession begins on soil that already contains organic matter and seeds, allowing fast-growing grasses and shrubs to establish quickly, followed over time by shade-intolerant trees and eventually a stable, shade-tolerant climax community. The distractor "Lichens colonize bare rock first, followed by mosses and then grasses" instead describes primary succession, which starts on bare rock without existing soil, a fundamentally different starting condition. Distinguishing the starting conditions and pioneer species of primary versus secondary succession is essential for correctly sequencing ecological recovery after disturbances.

Desertification occurs when reduced precipitation, an abiotic factor, shifts the water balance so that drought-tolerant desert vegetation replaces grassland species that can no longer survive the drier conditions. The distractor "A trophic cascade triggered by the loss of a top predator" describes a biotic, species-interaction-driven process, which is unrelated to the abiotic water-availability mechanism causing this biome shift. Recognizing that biome boundaries are fundamentally controlled by precipitation and temperature explains why long-term climate shifts can convert one biome type into another.

By dividing the tree's vertical space, the warblers reduce their niche overlap and avoid the direct competition that the competitive exclusion principle predicts would otherwise eliminate one species. The distractor "Eventual extinction of one of the two competing warbler species" describes what would happen without resource partitioning, not the actual coexistence outcome achieved through niche differentiation. This example illustrates how resource partitioning is a common evolutionary solution that allows ecologically similar species to persist together in the same habitat.

Ecological efficiency is limited because organisms use much of their consumed energy for respiration, movement, and heat production rather than storing it as biomass, so only a small fraction remains available to the next trophic level. The distractor "Decomposers capture and return all the energy that would otherwise be lost between levels" is incorrect because decomposers only recycle chemical matter, not the metabolic heat energy that is permanently lost from the ecosystem. This thermodynamic energy loss is the fundamental reason food chains rarely extend beyond four or five trophic levels.

Because the disaster kills individuals across all ages equally rather than concentrating mortality in old age as is typical for Type I species, the curve would temporarily flatten and show elevated mortality throughout the age range instead of only near the maximum lifespan. The distractor "It would remain completely unchanged since Type I curves cannot shift" ignores that survivorship curves reflect actual observed mortality patterns and will change if underlying mortality causes change. This scenario shows that survivorship curve shapes are descriptive of current conditions and can shift when unusual density-independent mortality events occur.

Metapopulation structure allows individuals to migrate between patches, so if one subpopulation goes locally extinct, nearby subpopulations can recolonize that empty patch, increasing the overall species' long-term persistence across the landscape. The distractor "It guarantees that no individual subpopulation will ever go locally extinct" is incorrect because local extinctions are actually expected and normal within metapopulation dynamics, with recolonization being the compensating mechanism. This metapopulation concept is especially important in conservation biology when designing networks of protected habitat patches.

Because herons and otters both rely on the same fish population as a food source, they are in interspecific competition even though they never directly encounter one another, since increased consumption by one reduces food available to the other. The distractor "Direct predation of one species by the other" is incorrect because the food web explicitly states the two species do not interact directly at all. Recognizing indirect competition through a shared resource, sometimes called exploitative competition, is an important skill for interpreting complex food web diagrams.

Since births and deaths cancel each other out, the population's net change is determined entirely by the migration terms, and because immigration exceeds emigration, the population experiences net growth. The distractor "The population remains exactly stable with no change in size" ignores the migration imbalance, which is a real and often significant contributor to population change beyond birth and death rates alone. This example shows that a complete population growth model must always include immigration and emigration alongside births and deaths.

When decomposer activity fails to keep pace with rising atmospheric carbon dioxide, carbon is not returned to sinks like soil or biomass fast enough, causing atmospheric carbon to accumulate and disrupt the cycle's natural balance. The distractor "The carbon cycle immediately re-equilibrates without any noticeable effect" contradicts the premise that decomposer activity is insufficient to offset the rising input, meaning equilibrium cannot be maintained. This scenario illustrates how disruptions to any single process within a biogeochemical cycle, such as decomposition, can have cascading effects on the entire cycle's balance.