Biology Practice Test: Ecology and Ecosystems

Primary consumers get 10% of 20,000 = 2,000 kcal, and secondary consumers get 10% of that, 200 kcal. The answer 2,000 kcal stops one level too early. The rest of the energy is mostly lost as heat from respiration.

Grass is the producer, the mouse eats grass (primary consumer), and the snake eats the mouse, making it a secondary consumer. The hawk, which eats the snake, is a tertiary consumer.

With fewer predators, more deer survive and the herd grows. More deer eat more young trees, so tree seedlings decline. This is a trophic cascade: a change at the top of the food web affects levels below it.

In commensalism one species benefits and the other is not affected. The barnacles get a ride to food-rich water, and the whale is unaffected. In mutualism both partners would benefit, and in parasitism the host would be harmed.

The population grows quickly at first and then levels off near 500, staying close to that number from year 5 to year 7. That level is the carrying capacity, the largest population the resources can support over time.

As the meadow filled with rabbits, each rabbit had less food and space, so births fell and deaths rose until they balanced. More grass or a larger meadow would raise the limit, not slow growth.

A density-dependent factor has a stronger effect when the population is more crowded; disease spreads more easily at high density. Storms, frosts and fires affect a population about the same way whatever its size, so they are density-independent.

In primary succession there is no soil yet, so pioneer species such as lichens and mosses grow on the rock and slowly help break it down into soil. Grasses, shrubs and later trees can only grow after soil has formed.

Producers take in CO₂ during photosynthesis and build it into sugars. Respiration, decomposition and burning fossil fuels all break down carbon compounds and release CO₂ back into the air.

Nitrogen-fixing bacteria, some living in the root nodules of legumes, convert N₂ into ammonia, which other bacteria can turn into nitrates that plants absorb. Denitrification does the reverse, returning N₂ to the air.

Each consumer eats many organisms from the level below and stores the pesticide in its tissues, so the concentration rises at every step of the food chain. This biomagnification makes top predators carry the highest levels.

An invasive species with no predators can grow quickly and outcompete native species for food, which can reduce biodiversity. The lack of predators is exactly what lets it spread, so the ecosystem does not stay unchanged.

Two hours are 120 minutes, which is 6 doubling periods: 100 × 2⁶ = 6,400. The answer 1,200 adds 100 × 2 each time instead of doubling, and 3,200 counts only 5 doublings.

Abiotic factors are nonliving parts of an ecosystem, such as temperature, sunlight, water pH and dissolved oxygen. Algae, bacteria and frogs are living things, so they are biotic factors.

Nutrients from fertilizer cause an algal bloom. When the algae die, decomposers break them down and use large amounts of dissolved oxygen in the process, so the fish suffocate. This chain of events is called eutrophication.