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Unformatted text preview: OPTIMAL FORAGING THEORY Lab 7 7-1 Reminder! Bring a Calculator I. FUNCTIONAL RESPONSE In lecture, you studied the Lotka-Volterra predator-prey model: cNP rN t N = , dP acNP t P = where N is the prey population, P is the predator population, r is the intrinsic rate of prey increase, c is the encounter rate or efficiency of capture, a is the rate at which the predator converts victim biomass into predator offspring, and d is the predator death rate. As you probably remember, there were four major assumptions underlying the LV predation model. In this lab, one of the points we will be examining is the assumption that individual predators consume a constant fraction of the prey at any given time. The change in the predators feeding rate with respect to its preys density is termed the predators functional response . In general, when prey are more abundant we expect a predator to consume more of the prey. In the Lotka-Volterra model, the functional response of the predator is given as c N, where c is a constant (efficiency of capture) and N is the prey population size or density. This relationship leads to a simple linear functional response for the predator (Type I, Figure 1, slope = c). Unlike our simple model, real predators are often expected to exhibit different feeding behaviors in response to varying prey densities. Consequently, there are more complex functional response curves (Type II, and Type III) that are frequently used by ecologists to model predator-prey interactions (Figure 1). A Type I functional response assumes that the predator consumes a constant fraction of the prey. This translates into a linear increase in the number of prey consumed as the prey density increases. An example would be a mussel that filters plankton from a fixed volume of water each day. As prey density in the water increases, more prey are consumed. The linear nature of the Type I response makes it the simplest of the three response curves to model, accounting for why the Lotka-Volterra predation model assumes this type of functional response. A Type II functional response curve describes an initial increase in feeding rate with increasing prey density. However, as prey density increases further, the predators response to the increase becomes less. Finally, a maximal feeding rate is approached wherein additional increases in prey density do not lead to higher predation rates. A Type II functional response is probably the most common of the three types among real predators. The declining rate of increase at higher prey densities can be explained by predator satiation-- the predators are eating at their maximum rate per day. Under these circumstances, they have plenty of food and they wont consume prey at higher rates even if prey density increases more. Another way to think about the leveling of predation at high prey densities is that at very high prey densities, the predator is spending all of its time handling its prey (killing and eating), so the...
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