Ch. 17 - Species Interactions & Community Structure

Food Web

Summary of feeding interactions within a community.

What are the most biologically significant trophic interaction?

Strong feeding relationships

Strong interactions

Feeding interactions of a few spp. that have a dominant influence on community structure.

How are strong interactions defined?

Degree of influence on community structure.

Wetland Reed Food Web Interactions in order:

1. Plant

2. Herbivores

3. Parasites

4. Blue tit (keystone)

Pros/Cons of emphasizing strong interactions in food webs:

• Pros

  • Simple

  • Directly quantify relationships

• Cons

  • Easy to overlook important interactions

  • Doesn’t provide complete picture (oversimplifying)

Direct Interactions

Positive or negative interactions between two species w/o involvement of intermediary spp.

• competition, predation, herbivory, mutualism

Indirect Interactions

Positive or negative interactions between two species that is mediated by a third species.

• apparent competition, indirect commensalism,

Indirect Commensalism

Activity of one spp. benefits another spp. through an intermediary spp.

Examples of Indirect Commensalism

• ex. Beaver mowing down trees chemically changes and helps with new growth of cottonwood trees which benefits leaf beetle

Apparent Competition

Negative effects of two spp. sharing predator or herbivore

or one spp. facilitating predators or herbivores of another spp.

Example of apparent competition

B. nigra shelters small mammals which feed on surrounding vegetation

Keystone Species

Spp. who have high impact on community structure despite low biomass.

examples of keystone species

• Orcas

• sea otters

• gray wolves

Foundation (dominant) Species

Have substantial influence on community structure due to high biomass.

Examples of foundation species

• corals in coral reefs

• kelp in kelp forests

• mangroves in mangrove forests

• redwoods in forests

How are keystone species important to the environment?

• keep prey pop. below carrying capacity

• reduce likelihood of competitive exclusion

Mutualistic Keystone species

have an impact on community structure through mutualistic relationships unlike other keystone species who predate on other organisms to maintain the community structure. 

Examples of mutualistic keystone species

• ex. Cleaner Wrasse Fish acts as mutualistic keystone spp. in coral reefs by removing thousands of parasites from other fish

• ex. Native African ants disperse up to 30% of the seeds in the scrublands of the fynbos