Assignment 4

Ecologists have identified a phenomenon called "predator-mediated coexistence" (Caswell 1978), where a predator feeds on a dominant competitor, which allows the weaker competing species to coexist. Working within PopulationEcologyLab and referring to the detailed instructions on Assignment 4, answer the questions below.


1 .		Design experiments where the brown sparrow always outcompetes the blue sparrow in the absence of any hawks, but by adding hawks that feed more heavily on the brown sparrow, all three species coexist. Were you able to achieve these conditions? Explain your results.

2 .		Some predators are "specialists," which survive by focussing on one or a few abundant prey species. Others are "generalists," which need to eat a variety of less common prey to survive. Design an experiment where a predator is unable to survive with only one prey source, but can persist when two prey species are available. Were you able to achieve these conditions? Explain your results.

3 .		Ecological systems are complex. Sometimes one species can indirectly affect another through its effect on a third species. Design an experiment where hawks eat only brown sparrows (i.e., blue sparrows have the maximum flight speed needed to avoid hawks) and hawks coexist with brown sparrows when blue sparrows are absent. However, when blue sparrows are present, their competition with brown sparrows is enough to drive hawks to extinction. What did you discover? What parameters enabled you to create this situation?