Community interactions

Populations rarely, if ever, live in isolation from populations of other species. In most cases, numerous species share a habitat. The interactions between these populations play a major role in regulating population growth and abundance. All populations occupying the same habitat form a community: populations inhabiting a specific area at the same time. The number of species occupying the same habitat and their relative abundance is known as species diversity. Areas with low diversity, such as the glaciers of Antarctica, still contain a wide variety of living things, whereas the diversity of tropical rainforests is so great that it cannot be counted. Ecology is studied at the community level to understand how species interact with each other and compete for the same resources. Any interactions between two or more species is referred to globally as interspecific interactions , and there are specific terms for some of unique interactions that will be discussed throughout this section. These unique interactions can have a positive benefit (+), a negative effect (-) or no benefit or negative effect (0) for the individual of the species.

Niche

In a community, no two species can have exactly the same ecological requirements. The way an organism interacts with other species and with the physical environment is unique. These ideas make up the concept of niche. So, the unique role and organism plays in its natural community is its niche. For example, the Black-tailed Prairie Dog ( Cynomys ludovicianus ) [link] has such a unique niche that the animal is considered a keystone species of the short grass prairie.

Black-tailed prairie dog

Predation and herbivory

Perhaps the classical example of species interaction is predation: the hunting of prey by its predator where the predator has a positive effect (+) and the prey has a negative effect (-). Nature shows on television highlight the drama of one living organism killing another. Populations of predators and prey in a community are not constant over time: in most cases, they vary in cycles that appear to be related. The most often cited example of predator-prey dynamics is seen in the cycling of the lynx (predator) and the snowshoe hare (prey), using nearly 200 year-old trapping data from North American forests ( [link] ). This cycle of predator and prey lasts approximately 10 years, with the predator population lagging 1–2 years behind that of the prey population. As the hare numbers increase, there is more food available for the lynx, allowing the lynx population to increase as well. When the lynx population grows to a threshold level, however, they kill so many hares that hare population begins to decline, followed by a decline in the lynx population because of scarcity of food. When the lynx population is low, the hare population size begins to increase due, at least in part, to low predation pressure, starting the cycle anew.