Community interactions are classified by whether they help, harm, or have no effect on the species involved Ecologists call relationships between species in a community interspecific interactions Example: Interspecific competition Interspecific interactions can affect the survival and reproduction of each species, and the effects can be summarized as positive (+), negative (–), or no effect (0)

Competition Interspecific competition (–/– interaction) occurs when species compete for a resource in short supply

Competitive Exclusion Strong competition can lead to competitive exclusion, local elimination of a competing species The competitive exclusion principle states that two species competing for the same limiting resources cannot coexist in the same place

Ecological Niche A plant's or animal's ecological niche is a way of life that is unique to that species. Niche and habitat are not the same. While many species may share a habitat, this is not true of a niche. Each plant and animal species is a member of a community. The niche describes the species' role or function within this community.

For example, the red fox's habitat, which might include forest edges, meadows and the bank of a river, is shared with many animals . The niche of the red fox is that of a predator which feeds on the small mammals, amphibians, insects, and fruit found in this habitat. Red foxes are active at night. They provide blood for blackflies and mosquitoes, and are host to numerous diseases. The scraps, or carrion, left behind after a fox's meal provide food for many small scavengers and decomposers. This, then, is the ecological niche of the red fox. Only the red fox occupies this niche in the meadow-forest edge communities. In other plant communities different species of animal may occupy a similar niche to that of the red fox.

If you moved this stone the whole arch would fall down Changing one factor in an ecosystem can affect many other factors. A keystone species is a species that has an unusually large effect on its ecosystem. Keystone If you moved this stone the whole arch would fall down

Reintroducing Wolves to Yellowstone National Park http://vimeo Beaver http://vimeo.com/28055044

Keystone species form and maintain a complex web of life. creation of wetland ecosystem increased waterfowl Population increased fish population nesting sites for birds keystone species

Diversity and trophic structure characterize biological communities In general, a few species in a community exert strong control on that community’s structure Two fundamental features of community structure are species diversity and feeding relationships

Species Diversity Species diversity of a community is the variety of organisms that make up the community It has two components: species richness and relative abundance Species richness is the number of different species in the community Relative abundance is the proportion each species represents of all individuals in the community

A B C D Community 1 Community 2 A: 25% B: 25% C: 25% D: 25% A: 80% Figure 54.10 A B C D Community 1 Community 2 Figure 54.10 Which forest is more diverse? A: 25% B: 25% C: 25% D: 25% A: 80% B: 5% C: 5% D: 10%

Communities with higher diversity are More productive and more stable in their productivity Better able to withstand and recover from environmental stresses More resistant to invasive species, organisms that become established outside their native range