The interplay between predation and competition

Answers to the reading theory exercise

Make sure you have read your handout (or download the handout here) for the summary of this paper.

Question 1: Notice the assumptions of logistic growth by both resources and predators. This means that the resources will increase rapidly if their abundance is close to zero, but cannot increase higher than a set carrying capacity. Is this a good characterization of a biotic or abiotic resource?

Biotic resources. At small population sizes, logistic growth is approximately exponential growth – whereby each individual gives birth to the same number of individuals and populations rapidly rise, this is characteristic of things that can reproduce, biotic resources.

Question 2: Two cases are mentioned in the last paragraph where niche overlap can be reduced. In theory, there is also a somewhat “trivial” case where overall niche overlap is even lower than those mentioned. What would that be? How does that promote coexistence?

Niche overlap would be the lowest if the two focal species have distinct predators as well as distinct resources. In this case, their “niches” would be so distinct that the species do not influence one another at all, and thus they could coexist even when their differences in average competitive ability are very large.

Question 3: Do you “buy” the insight that resource competition and predation play similar roles in focal species coexistence? Are there missing differences between predation and competition in the model that you think could affect this insight?

Conceptually, it is a much older idea that predators can affect the competitive outcomes of focal species similar to resource competitions, a phenomenon termed “apparent competition”. However, whether their contributions are quantitatively comparable depends on how the focal species interact with the predators and resources. Some potential differences between predation and competition missing in this model include: the resources could be abiotic rather than biotic, predation could be further complicated by intra-guild predation, different functional responses for the predation and resource consumption, the different timescales at which resource and predator species may be able to respond to changes in the focal species abundance, etc. Determining the specific effect of these differences on model results would likely need further theoretical assessment.