Lecture 27
Topics
- The Prudent Predator and The Efficient Prey
- Functional and Numerical Response
Study Sources
- Chapter 23 in Ricklefs; especially pp. 466-468
- Study Guide: Predation
The Main Points
- When the predator isocline intersects the prey isocline at the "peak" of the prey isocline, three things happen:
- The prey density is at about K/2. At this density the prey has its maximum rate of growth or so called "recruitment" (recruitment = additions to the population by births). The prey isocline could just as well be called the prey recruitment curve.
- The prey at this density can support the largest number of predators without changing the size of the prey population, i.e. the predators live just off the recruitment. Above and below this prey density, recruitment is less and fewer predators can be supported without eating into the population.
- The geometry of the isocline intersection is most symmetrical here and one would expect a rather constant oscillation of the populations in the vicinity of the intersection. This relationship is most like the stable limit cycle of the Lotka-Volterra model.
- Slobodkin used the metaphore of the "prudent predator and efficient prey" to describe this arrangment. The metaphore is that in order to receive the maximum energy flow from the prey, the predator should manage its exploitation so that the prey numbers remain at about K/2. At that point, the recuitment of the prey is maximum and energy flow through the prey to the predator is also maximum.
- He proposed this metaphore in the process of asking the question, "would predator/prey systems evolve in such a way as to maximize energy flow." His experiments with Daphnia to find the predation rate that would produce the highest recruitment rate indicated that removal of mostly the larger individuals resulted in the largest recruitment. For the Daphnia this makes sense because the energy that is used for maintenance of the large bodies could be turned into young produced by the smaller animals. The message is that we need to look around in nature to examine closely the predation patterns and specific behaviors of the predators that could lead to the same effect as "prudence." The problem with prudence is that greed will produce more young, and in evolution, reproduction is very important.
- Numerical and Functional Response - This is a body of theory that was first introduced by Solomon in 1949 and studied extensively by Holling in 1959 (Section 23.5, pp. 458-462). It is important because it suggests:
- There are two aspects to predation: The numerical response is what we have been considering as reproductive response of the predator to increasing numbers of prey. The Functional Response is a behavioral response of the predator during a time when the prey is increasing and its own population is stable. This could occur, for instance, if the yearly reproductive cycle of the predator is long compared to that of the prey.
- That predation may not be in direct proportion to the sizes of the prey and predator populations as has been assumed for the Lotka-Volterra model.
- Study Figure 21-11 to see Solomon's functional response models. I presented these in Lecture, but I drew the Type I curve differently than Ricklefs. He draws a straight line which is really indeterminant in length. In fact, like the other two functional-types, this "filter feeder" type will also become satiated and level off when the density of food is so great that the predators simply cannot eat any faster.
- Ricklefs cites Holling's work (pp. 458-460) in connection with the functional responses and with good reason. His work is interesting, field oriented, elegant and deals in detail with the issues we have been discussing. The reference to his work with with small mammal predation on European sawfly is listed in the Bibliography (Holling, 1959). I recommend it.