1. 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)

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

3. 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

4. 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.

5. 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.

6. 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

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

8. 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

9. 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

10. 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

11. 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 Which forest is more diverse?
A: 25%
B: 25%
C: 25%
D: 25%
A: 80%
B: 5%
C: 5%
D: 10%

12. 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