Ecology Practice Questions

Correct answer: Ecosystem

An ecosystem includes both the biotic components (living communities of plants, animals, and microbes) and the abiotic factors (air, water, soil, and climate) interacting as a system. A community includes only the living organisms across different species. A population is restricted to individuals of a single species in a specific area.

Correct answer: 10%

According to Lindeman's Ten Percent Law, only about 10% of the organic energy stored as biomass at one trophic level is successfully converted into biomass at the next level. The remaining 90% is lost primarily as metabolic heat during respiration, or through undigested waste material. This dramatic energy loss limits the total number of trophic levels a food chain can support.

Correct answer: Commensalism

Commensalism is a long-term biological interaction where one species benefits significantly while the other species derives neither benefit nor harm. In this case, the orchid gains structural support and solar access, while the host tree experiences no physiological impact. Mutualism involves both species benefiting, while parasitism involves harm to one party.

Correct answer: Primary decomposers

Decomposers, or saprotrophs, chemically break down complex organic compounds from dead organisms and metabolic wastes into simpler inorganic substances. This recycling process is vital for sustaining primary productivity because it restocks the soil with nitrogen, phosphorus, and other minerals. Without decomposers, nutrients would remain locked permanently within dead biomass.

Correct answer: Niche

An ecological niche encompasses everything an organism does to survive, including its diet, reproductive habits, physical habitat, and direct interactions with biotic and abiotic factors. While a habitat is simply an organism's 'address', the niche is its 'profession' within the ecosystem. Two species cannot occupy the exact same niche indefinitely due to competitive exclusion.

Correct answer: Primary succession

Primary succession begins in an environment that is totally devoid of life and organic soil, requiring pioneer species like lichens and mosses to slowly weather rock down into substrate. Secondary succession, conversely, happens in areas where an existing community has been disrupted by a disturbance like a forest fire or human farming, but the fertile soil layer remains intact.

Correct answer: Biomagnification

Biomagnification is the process where toxin concentrations increase significantly at each subsequent link in the food chain because apex predators consume vast quantities of lower-level biomass over time. Bioaccumulation is a related term, but it refers specifically to the buildup of a chemical within a single individual's lifetime. Eutrophication involves nutrient over-enrichment in aquatic bodies.

Correct answer: Tundra

The tundra biome is defined by its extreme cold, short growing seasons, and a thick layer of underlying permafrost that prevents deep root growth and water drainage. These harsh conditions limit the landscape to low-lying vegetation like mosses and grasses, entirely excluding large trees. The taiga lies just south of the tundra and features dense coniferous forests instead.

Correct answer: Eutrophication

Eutrophication is typically triggered by agricultural fertilizer runoff or sewage entering aquatic systems, which over-fertilizes the water and creates dense algal mats. When these short-lived algae die, aerobic decomposers multiply rapidly to consume the dead organic mass, draining the dissolved oxygen from the water column. This drop in oxygen levels creates hypoxic 'dead zones' where fish and other aquatic animals suffocate.

Correct answer: Carrying capacity

Carrying capacity, denoted as 'K' in population ecology equations, represents the balance point where birth rates equal death rates due to limited resources like food, water, and nesting space. If a population overshoots its carrying capacity, resource depletion will trigger an increase in mortality until the population size drops back down. Biotic potential is the maximum possible growth rate under ideal conditions.

Correct answer: Wildfires

Density-independent factors are typically abiotic environmental events, such as natural disasters, extreme weather, or volcanic eruptions, which kill a similar percentage of a population regardless of its density. Density-dependent factors, like disease transmission, food scarcity, and predation, intensify in their effects as the population grows more crowded.

Correct answer: Nitrogen-fixing bacteria

Atmospheric nitrogen has a strong triple covalent bond that plants and animals cannot break directly. Nitrogen-fixing bacteria, such as Rhizobium living in the root nodules of legumes, possess the nitrogenase enzyme needed to convert this gas into ammonia, which plants can readily absorb. Denitrifying bacteria perform the reverse process, turning soil nitrates back into nitrogen gas.

Correct answer: Keystone species

A keystone species holds an ecosystem together; its removal causes a dramatic collapse in local biodiversity and community structure, as seen with sea otters controlling sea urchin populations to preserve kelp forests. Indicator species are organisms whose health status reflects the overall condition of the environment, while endemic species are native to a single distinct geographic area.

Correct answer: Ecotone

An ecotone is an area of marshy or variable terrain acting as a buffer between two biomes, such as an estuary between a river and the open sea, or a grassland bordering a forest. These zones frequently feature the 'edge effect', containing species from both overlapping ecosystems alongside unique species specialized for the boundary itself.

Correct answer: The deep oceans

The deep ocean acts as a massive carbon sink, containing far more dissolved inorganic carbon in the form of carbonate and bicarbonate ions than the atmosphere, soil, and living terrestrial plants combined. Carbon shifts between the surface ocean and the deep sea over long ocean currents. Fossil fuels hold more carbon overall, but it remains locked out of active biological cycling unless extracted and burned.

Correct answer: Photosynthesis

Photosynthesis is the primary biological pathway that captures atmospheric carbon dioxide, using solar energy to bond it with water into glucose. Respiration, decomposition, and combustion all do the opposite, breaking down carbon compounds and releasing carbon dioxide back into the atmosphere.

Correct answer: Invasive species

Invasive species lack natural predators, parasites, or competitors in their new environments, allowing them to reproduce rapidly and dominate local resources at the expense of native species. Examples include kudzu vines and cane toads. Simple exotic species are non-native but do not necessarily trigger widespread environmental damage.

Correct answer: One species will eventually outcompete and displace the other.

Gause's Principle of Competitive Exclusion states that two species competing for the exact same limiting resources cannot coexist stably if their ecological niches are identical. The more efficient species will gradually drive the other to local extinction or force it to adapt toward an alternative niche via resource partitioning.

Correct answer: Age structure

An age structure diagram (or population pyramid) illustrates the balance of pre-reproductive, reproductive, and post-reproductive individuals within a population. This data helps ecologists project whether a population is primed for rapid expansion, stable maintenance, or future decline. Survivorship curves look instead at the statistical probability of individuals surviving at various points throughout their lifespans.

Correct answer: Earth's axial tilt and uneven solar radiation across latitudes.

Because Earth is a sphere tilted on its axis, solar radiation strikes the equator directly but hits polar regions at a sharp angle, spreading the energy across a wider surface area. This differential heating drives global atmospheric wind belts and ocean currents, which shape the temperature and moisture regimes that dictate where specific biomes can develop.