16: Species Interactions and Community Structure

What is a food web?

A summary of feeding relationships within a community that describes how energy is transferred from producers upward.

What is the example of a food web in the North Atlantic?

Phytoplankton → Zooplankton → Invertebrates → Fish → Birds & Mammals.

What forms the basis of a food web?

Producers (e.g. plants or phytoplankton).

What are Primary Consumers?

Herbivores that eat producers.

What are Secondary Consumers?

,Carnivores or omnivores that eat herbivores.

What are Tertiary Consumers / Top Predators?

Species that feed on secondary consumers or other predators.

Can species feed at multiple levels of a food web?

Yes (e.g. cod feed on both invertebrates and fish).

Why are food webs considered complex?,

Many species feed on multiple other species creating overlapping links and strong interactions embedded in many weak ones.

What is meant by the "strength" of community interactions?

How much one species affects another within a community.

How are interactions typically distributed in a food web?

They are dominated by a few strong interactions embedded within many weak interactions.

What was the result of Tscharntke’s (1992) marsh study?

The top predator (blue tit) reduced herbivore abundance which indirectly affected parasitoid wasps and plant health.

What distinguishes strong interactions from weak ones?

Strong interactions have large effects on other species and community structure

Weak interactions have smaller or less noticeable effects.

What is a foundation species?

A species at the base of the food web that provides both food and habitat structure for the community.

What is an example of a foundation species?,

Phragmites australis (marsh reed) which supports insects birds and parasitoids.

What is a direct interaction?

When one species affects another without intermediaries

What is an indirect interaction?

When one species affects another via a third species.

What is a trophic cascade?

An indirect interaction where a predator reduces prey which benefits a third species

trophic cascade example

fish reduce zooplankton causing algae to increase

What is indirect commensalism?

An interaction where the activity of one species benefits another without harm

indirect commensalism example

beavers cutting trees providing shoots for beetles

What is apparent competition?

When two species share a predator

an increase in one prey increases the predator population which then harms the second prey species.

What is an example of apparent competition?

Exotic mustard increasing herbivore populations which leads to a decline in native mustard (Orrock et al. 2008).