Lecture 5: Competition and predation

 Distinguish between intraspecific competition and interspecific competition. Provide an example of each

 Intraspecific = same species; interspecific = different species.

What are the 3 main conclusions we can draw from Gause’s experiments with Paramecium?

Gause showed that:

1. direct competition for the same resource leads to exclusion

2. coexistence requires different niches, and

3. competition lowers growth for both species.

How can different species avoid competition and thus competitive exclusion?

Species avoid competition by dividing resources through niche differentiation—such as using different resources, using them at different times, or occupying different microhabitats—thereby preventing competitive exclusion.

Compare and contrast resource partitioning vs. character displacement. What is the key difference between these phenomena?

Resource partitioning is how species currently divide resources to coexist (ecological, often behavioral or spatial).
Character displacement is the evolutionary change in traits that reduces competition over time.

Think of it this way: resource partitioning is what species do, and character displacement is how species evolve to make it easier to do it.

What are the 4 primary types of exploitative interactions? How do these interactions differ? Why are they all still considered “exploitative” interactions? 

1. Predation - one organism (predator) eats another (prey)

2. Herbivory - an organisms eats part of a plant or algae

3. Parasitism - one organism (parasite) lives on or inside another (the host) deriving nutrients over time

4. Parasitoidism - A specialized form of parasitism in which the parasitoid eventually kills its host.

In lizard-spider predator-prey study, why do you think lizards had a larger effect on spider species richness when lizards were introduced to islands versus when they were present naturally on islands?

The larger effect of introduced lizards is due to prey naivety and lack of evolved defenses, whereas naturally coexisting prey have already adapted or been filtered by predation pressure.

 How can “keystone predation” lead to an increase in species richness? Explain how keystone predation can be considered an “indirect interaction” in your answer.

Keystone predation can increase species richness because it removes dominant competitors, indirectly allowing other species to coexist. The effect is considered indirect because the predator doesn’t act on the weaker species directly—it alters the competitive dynamics in the community.

Suppose that an invasive fungus killed most of the mussels, the preferred prey of sea stars. In the absence of mussels, how would species richness be affected if sea stars were removed?

 Species richness would likely remain unchanged or increase slightly, because sea stars are no longer controlling competition among species—the “keystone” effect is gone without their preferred prey.

Which trophic level (predator, prey, resource) do you expect to increase due to a trophic cascade? Which trophic level will be reduced in the trophic cascade model?

Increase: Resource (bottom trophic level)

Decrease: Prey/herbivore (middle trophic level)

Predator: May remain stable, but initiates the cascade

If sea otters were extirpated from a coastal region, how would you expect the kelp forest community structure to change? 

Top predator removed → prey increases → primary producer decreases → community diversity declines
This is a classic example of a trophic cascade.

Consider a grassland with 5 trophic levels: grasses, mice, snakes, racoons and bobcats. Assuming this system was controlled from the top-down, how would grass biomass change if you release additional bobcats into the grassland? 

Adding bobcats paradoxically reduces grass biomass in this system because of a trophic cascade that increases mouse populations (the primary herbivores).
This is called a “top-down trophic cascade,” but the effect depends on which intermediate predators are most affected.

What is meant by the term “top-down control”? What are the 3 examples of top-down control we discussed in lecture?

Top-down control shows that predators can indirectly shape entire communities, not just by consuming prey but by altering competition, habitat, and resource availability.

What type of population cycles do mathematical models of predator-prey interactions predict?

Mathematical models of predator-prey interactions predict cyclical population cycles:

• Prey numbers rise → predator numbers rise → prey numbers fall → predator numbers fall → cycle repeats.
  
  

• Predator populations lag behind prey populations.
  
  

• Cycle size and timing depend on prey growth, predation rate, and predator efficiency.

These cycles can be regular or, in more complex situations, irregular or damped.