Cracking the AP Biology Exam


Animal Behavior and Ecology


The study of the interactions between living things and their environments is known as ecology. We’ve spent most of our time discussing individual organisms. However, in the “real world,” organisms are in constant interaction with other organisms and the environment. The best way for us to understand the various levels of ecology is to progress from the big picture, the biosphere, down to the smallest ecological unit, the population.

Just as in taxonomy, there is a hierarchy within the “ecology world.” Each of the following terms represents a different level of ecological interaction.

  • Biosphere—The entire part of the earth where living things exist. This includes soil, water, light, and air. In comparison to the overall mass of the earth, the biosphere is relatively small. If you think of the earth as a basketball, the biosphere is equivalent to a coat of paint over its surface.
  • Ecosystem—The interaction of living and nonliving things.
  • Community—A group of populations interacting in the same area.
  • Population—A group of individuals that belong to the same species and that are interbreeding.


The biosphere can be divided into large regions called biomes. Biomes are massive areas that are classified mostly on the basis of their climates and plant life. Because of the different climates and terrains on the earth, the distribution of living organisms varies. For the AP Biology Exam, you’re expected to know both the names of the different biomes and their characteristic flora (plant life) and fauna (animal life).

Here’s a summary of the major biomes:

Major Biomes


Regions—northernmost regions

Plant life—few, if any, trees; primarily grasses and wildflowers

Characteristics—contains permafrost (a layer of permanently frozen soil); has a short growing season

Animal life—includes lemmings, arctic foxes, snowy owls, caribou, and reindeer


Region—northern forests

Plant life—wind-blown conifers (evergreens), stunted in growth, possess modified spikes for leaves

Characteristics—very cold, long winters

Animal life—includes caribou, wolves, moose, bear, rabbits, and lynx

Temperate Deciduous Forest

Regions—northeast and middle eastern United States, western Europe

Plant life—deciduous trees that drop their leaves in winter

Characteristics—moderate precipitation; warm summers, cold winters

Animal life—includes deer, wolves, bear, small mammals, birds


Regions—American Midwest, Eurasia, Africa, South America

Plant life—grasses

Characteristics—hot summers, cold winters; unpredictable rainfall

Animal life—includes prairie dogs, bison, foxes, ferrets, grouse, snakes, and lizards


Regions—western United States

Plant life—sparse, includes cacti, drought-resistant plants

Characteristics—arid, low rainfall; extreme diurnal temperature shifts

Animal life—includes jackrabbits (in North America), owls, kangaroo rats, lizards, snakes, tortoises

Tropical Rain Forests

Regions—South America

Plant life—high biomass; diverse types

Characteristics—high rainfall and temperatures; impoverished soil

Animal life—includes sloths, snakes, monkeys, birds, leopards, and insects

Remember that the biomes tend to be arranged along particular latitudes. For instance, if you hiked from Alaska to Kansas, you would pass through the following biomes: tundra, taiga, temperate deciduous forests, and grasslands.


Ecosystems are self-contained regions that include both living and nonliving factors. For example, a lake, its surrounding forest, the atmosphere above it, and all the organisms that live in or feed off the lake would be considered an ecosystem. As you probably know, there is an exchange of materials between the components of an ecosystem. Take a look at the flow of carbon through a typical ecosystem:

You’ll notice how carbon is recycled throughout the ecosystem—this is called the carbon cycle. In other words, energy flows through ecosystems. Other substances have similar cycles.


The next smaller level is the community. A community refers to a group of interacting plants and animals that show some degree of interdependence. For instance, you, your dog, and the fleas on your dog are all members of the same community. All organisms within a community fill one of the following roles: producers (or autotrophs), consumers (or heterotrophs), or decomposers.


Producers, or autotrophs, have all of the raw building blocks to make their own food. From water and the gases that abound in the atmosphere, and with the aid of the sun’s energy, autotrophs convert light energy to chemical energy. They accomplish this through photosynthesis.


Consumers, or heterotrophs, are forced to find their energy sources in the outside world. Basically, heterotrophs digest the carbohydrates of their prey into carbon, hydrogen, and oxygen, and use these molecules to make organic substances.

The bottom line is: Heterotrophs, or consumers, get their energy from the things they consume.


All organisms at some point must finally yield to decomposers. Decomposers are the organisms that break down organic matter into simple products. Generally, fungi and bacteria are the decomposers. They serve as the “garbage collectors” in our environment.


Each organism has its own niche—its position or function in a community. Because every species occupies a niche, it’s going to have an effect on all the other organisms. These connections are shown in the food chain. A food chain describes the way different organisms depend on one another for food. There are basically four levels to the food chain: producers, primary consumers, secondary consumers, and tertiary consumers.

Autotrophs produce all of the available food. They make up the first trophic (feeding) level. They possess the highest biomass (the total weight of all the organisms in an area) and the greatest numbers. Did you know that plants make up about 99 percent of the earth’s total biomass?

Primary consumers are organisms that directly feed on producers. A good example is a cow. These organisms are also known as herbivores. They make up the second trophic level.

The next level consists of organisms that feed on primary consumers. They are the secondary consumers, and they make up the third trophic level. Above these are tertiary consumers.

So now you have it. We’ve got our four complete levels of the food chain.

  • Producers make their own food.
  • Primary consumers (herbivores) eat producers.
  • Secondary consumers (carnivores and omnivores) eat producers and primary consumers.
  • Tertiary consumers eat all of the above.

The 10% Rule

In a food chain, only about 10 percent of the energy is transferred from one level to the next—this is called the 10% rule. The other 90 percent is used for things like respiration, digestion, running away from predators—in other words, it’s used to power the organism doing the eating! The producers have the most energy in an ecosystem; the primary consumers have less energy than producers; the secondary consumers have less energy than the primary consumers; and the tertiary consumers will have the least energy of all.

The energy flow, biomass, and numbers of members within an ecosystem can be represented in an ecological pyramid. Organisms that are “higher up” on the pyramid have less biomass and energy, and fewer numbers.

So, for example, primary consumers have less biomass and are fewer in number than producers. But what about decomposers? Where do they fit in on the food chain? They don’t. They are not really considered part of the food chain. Decomposers are usually placed just below the food chain to show that they can decompose any organism.

In addition to the distinctions drawn above, remember:

Toxins in an ecosystem are more concentrated and thus more dangerous for animals further up the pyramid.

This simply means that if a toxin is introduced into an ecosystem, the animals most likely to be affected are those at the top of the pyramid. This occurs because of the increasing concentration of such toxins. The classic example of this phenomenon is DDT, an insecticide initially used to kill mosquitoes. Although the large-scale spraying of DDT resulted in a decrease in the mosquito population, it also wound up killing off ospreys.

Ospreys are aquatic birds whose diet consists primarily of fish. The fish that ospreys consumed had, in turn, been feeding on contaminated insects (bioaccumulation). Because fish eat thousands of insects, and ospreys hundreds of fish, the toxins grew increasingly concentrated (biomagnification). Though the insecticide seemed harmless enough, it resulted in the near-extinction of certain osprey populations. What no one knew was that in sufficient concentrations, DDT weakened the eggshells of ospreys. Consequently, eggs broke before they could hatch, killing the unborn ospreys.

Such environmental tragedies still occur. All ecosystems, small or great, are intricately woven, and any change in one level invariably results in major changes at all the other levels.


Many organisms that coexist exhibit some type of symbiotic relationship. These include remoras, or “sucker fish,” which attach themselves to the backs of sharks, and lichen, the fuzzy, moldlike stuff that grows on rocks. Lichen appears to be one organism, when in fact it is two organisms—a fungus and an alga or photosynthetic bacterium—living in a complex symbiotic relationship.

Overall, there are three basic types of symbiotic relationships:

  • Mutualism—in which both organisms win (for example, the lichen).
  • Commensalism—in which one organism lives off another with no harm to the “host” organism (for example, the remora).
  • Parasitism—in which the organism actually harms its host.