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Communities are complex networks of direct and indirect species interactions that vary in strength and direction. Strongly interacting species such as keystone species, foundation species, and ecosystem engineering species often regulate community structure through their interactions with other species. In communities where species interactions are more equivalent, chance plays a greater role in determining community structure.
learning outcomes
You should be able to:
Describe, analyze, and give examples of the importance of indirect interactions in forming and maintaining community structure.
Explain the term “trophic cascade” as it relates to indirect interactions in communities.
Define and compare keystone and foundation species, giving examples of each.
Define a lottery, or neutral, model in community ecology, and describe its usefulness in explaining the chance occurrence of species in a community.
Suppose wolves were once again excluded from Yellowstone National Park. What direct and indirect interactions would change, and how would this affect aspen forests?
Elk would be released from the direct interaction of predation by wolves and would increase in abundance. The increase in elk would result in more grazing pressure on aspens, causing them to decline. Thus, fewer wolves would have indirect effects for aspen. With fewer aspens, there would be other indirect effects of the lack of wolf predation: fewer snowshoe hare, beaver, and other rodents that depend on aspens for food. Fewer small herbivores could affect other carnivores in the system such as coyote, raven, and short-
Make an argument for why beavers could be considered ecosystem engineering species and keystone species but not foundation species.
Beavers most closely fit the ecosystem engineering species definition because they are able to create, modify, and/or maintain physical habitat by cutting down (and killing) trees and using them to dam streams and create ponds and wetlands that provide habitat for themselves and other species. Beavers might also be considered a keystone species because their effect is large relative to their size or abundance, but keystone species are thought to mostly act through food webs by creating trophic cascades. Beavers would not be considered foundation species because foundation species have large effects on the communities as a consequence of their large size or great abundance.
Using the lottery model as your guide, explain how species that depend on the same set of limiting resources can coexist.
The lottery model poses that in communities where species use similar limiting resources and have similar effects on one another, the element of equal chance for all individuals to obtain those resources determines coexistence. The model assumes that when resources are made available, they are used at random by individuals of different species that happen to be in the “right place at the right time.” As long as all individuals have similar chances of obtaining resources (or “winning the lottery”) and no clear advantage in population growth, then their presence in the community should be maintained by chance events that free up resources for individuals competing for those resources.
When Mount St. Helens erupted in 1980, it was clear that the surrounding ecological communities would change forever. Change is a regular feature of all communities, but not all changes are as dramatic as those involving a volcanic eruption. In the next section we will consider how both subtle and catastrophic factors change communities over time.