Question

B. Prey-prey Relations. In a classic paper, Terborgh et al. (2001, Science 294:1923-6.) revealed that predators are important in tropical forest ecosystems for stabilizing the system and preventing the overharvesting of the understory. Their study sites were a set of “islands” in Venezuela created by hydroelectric impoundments that excluded large predators. On the “islands,” the authors state, “Predators of vertebrates are absent, and the densities of rodents, howler monkey, iguanas, and leaf-cutter ants are 10 to 100 times greater than on the nearby mainland” (p. 1923).

b1. Assume that the diet of the rodents studied in this paper is primarily seeds from fast growing understory plants. Draw a feed-back loop that describes the relationship between the three components: (1) rodent population and (2) abundance of seeds and (3) density of understory plants. Is this a positive or negative feedback loop?

b2. Is the impoundment caused by the hydroelectric plant, which excludes predators from the “islands” a forcing or perturbation on the system? Explain your answer.

b3. What are the likely long-term consequence of the impoundment to the system? Describe (and/or illustrate with a diagram) the effects to both the rodents and the understory plants initially (after 1-2 years) and in the long-term (10+ years).

Answer 1

b1 initially the population of rodents will increase with abundance of seeds but the density of the understory plants will decrease because these will be explored by excess of rodents damage(plants) and finish(seeds) it. So initially this is a positive feedback which will turn to negative later when the population of rodents will start decreasing due to starving /death/competition for food.