Community and Succession for AP Biology

Practice problems for these concepts can be found at: Ecology Review Questions for AP Biology

Community

Most species exist within a community. Because they share a geographic home, they are bound to interact. These interactions range from positive to neutral to negative.

Forms of Species Interaction

1. Symbiosis. A symbiotic relationship is one between two different species that can be classified as one of three main types: commensalism, mutualism, or parasitism.
   1. Commensalism. One organism benefits while the other is unaffected. Commensalistic relationships are rare, and examples are hard to find. Cattle egrets feast on insects that are aroused into flight by cattle grazing in the insects' habitat. The birds benefit because they get food, but the cattle do not appear to benefit at all.
   2. Mutualism. Both organisms reap benefits from the interaction. One popular example of a mutualistic relationship is that between acacia trees and ants. The ants are able to feast on the yummy sugar produced by the trees, while the trees are protected by the ants' attack on any potentially harmful foreign insects. Another example involves a lichen, which is a collection of photosynthetic organisms (fungus and algae) living as one. The fungus component pulls its weight by helping to create an environment suitable for the lichen's survival, while the alga component supplies the food for the fungus. Without each other's contribution, they are doomed.
   3. Parasitism. One organism benefits at the other's expense. A popular example of a parasitic relationship involves tapeworms, which live in the digestive tract of their hosts. They reap the benefits of the meals that their host consumes by stealing the nutrients and depriving the host of nutrition. Another less well-known example of parasitism involves myself and my younger brother's Playstation 2 console.
2. Competition. Both species are harmed by this kind of interaction. The two major forms of competition are intraspecific and interspecific competition. Intraspecific competition is within-species competition. This kind of competition occurs because members of the same species rely on the same valuable resources for survival. When resources become scarce, the most fit of the species will get more of the resource and survive. Interspecific competition is competition between different species.
3. Predation. This is one of the "negative" interactions seen in communities (well, for one half of those involved, it is negative.) J One species, the predator, hunts another species, the prey. Not all prey give in to this without a fight, and the hunted may develop mechanisms to defend against predatory attack. The next section describes the various kinds of defense mechanisms developed by prey in an effort to survive.

Defense Mechanisms

Aposematic coloration is a very impressive-sounding name for this defense mechanism. Stated simply, it is warning coloration adopted by animals that possess a chemical defense mechanism. Predators have grown cautious of animals with bright color patterns due to past encounters in which prey of a certain coloration have sprayed the predator with a chemical defense. It is kind of like the blinking red light seen in cars with elaborate alarm systems. Burglars notice the red light and may think twice about attempting to steal that car because of the potential for encountering an alarm system.

In Batesian mimicry, an animal that is harmless copies the appearance of an animal that is dangerous to trick predators. An example of this is a beetle whose colors closely resemble those of bees. Predators may fear that the beetle is a bee and avoid confrontation.

In cryptic coloration, those being hunted adopt a coloring scheme that allows them to blend in to the colors of the environment. It is like camouflage worn by army soldiers moving through the jungle. The more you look like the terrain, the harder you are to see.

Some animals have patterns called deceptive markings, which can cause a predator to think twice before attacking. For example, some insects may have colored designs on their wings that resemble large eyes, causing individuals to look more imposing than they truly are.

In Mullerian mimicry, two species that are aposematically colored as an indicator of their chemical defense mechanisms mimic each other's color scheme in an effort to increase the speed with which their predators learn to avoid them. The more often predators see dangerous prey with this coloration, the faster the negative association is made.

Looking at Figure 18.5, we can see how the predator–prey dance plays out. When the prey population starts to decrease because of predation, there is a reactionary reduction in the predator population. Why does this happen? Because the predators run low on a valuable resource necessary to their survival—their prey. Notice in the figure that as the predator population declines, an increase in the population of the prey begins to appear because more of those prey animals are able to survive and reproduce. As the prey population density rises, the predators again have enough food available to sustain a higher population, and their population density returns to a higher level again. Unless disturbed by a dramatic environmental change, this cyclical pattern continues.

Coevolution is mutual evolution between two species and is often seen in predator-prey relationships. For example, imagine that the hunted prey adapts a new character trait that allows it to better elude the predator. In order to survive, the predator must evolve so that it can catch its victim and eat.