Populations of animals interact with each other and their environment in a variety of ways. One of the primary interactions a population has with its environment and other populations is due to feeding behavior.
The consumption of plants as a food source is referred to as herbivory and the animals that do this consuming are called herbivores. There are different types of herbivores. Those that feed on grasses are referred to as grazers. Animals that eat leaves and other portions of woody plants are called browsers, while those that consume fruits, seeds, sap, and pollen are called frugivores.
Populations of animals that feed on other organisms are called predators. The populations on which predators feed are called prey. Often, predator and prey populations cycle in a complex interaction. When prey resources are abundant, predator numbers increase until the prey resources wane. When prey numbers drop, predator numbers dwindle as well. If the environment provides adequate refuge and resources for prey, their numbers may again increase and the cycle begins again.
The concept of competitive exclusion suggests that two species that require identical resources cannot coexist in the same location. The reasoning behind this concempt is that one of those two species will be better adapted to that environment and be more successful, to the point of excluding the lesser species from the enviornment. Yet we find that many species with similar requirements do coexist. Because the environment is varried, competing species can use resources in different ways when competition is intense, thus allowing space for one another.
When two interacting species, for example predator and prey, evolve together, they can influence the evolution of the other. This is referred to as coevolution. Sometimes coevolution results in two species that influence (both positively or negatively) from each other, in a relationship referred to as symbiosis. The various types of symbosis include:
- parasitism - one species (parasite) benefits more than the other species (host)
- commensalism - one species benefits while a second species is neither helped nor injured
- mutualism - both species benefit from the interaction
