recap

55.2 recap

Predation is a fundamental species interaction because all animals must feed. Predator–prey interactions result in the evolution of a range of capture mechanisms (extremes of swift pursuer to inconspicuous ambusher) and avoidance mechanisms (including avoiding detection, chemical defenses, warning signals, mimicry, and behavior). Most parasites specialize on hosts, and most hosts have many species of parasites. Predator populations can cycle with their prey and can have dramatic effects on communities.

learning outcomes

You should be able to:

  • Classify prey defenses against predators in terms of type (e.g., behavioral, chemical, morphological).

  • Give examples of plant defenses against herbivores and of reciprocal adaptations of herbivores to these defenses.

  • Describe variations in parasite–host relationships and infer the causes of these variations.

  • Predict future population sizes, given information on current population numbers, in cycling predator–prey populations.

Question 1

Given what you know about lionfish feeding behavior, describe some avoidance mechanisms that might evolve in small reef fish in the Atlantic, to avoid being eaten by lionfishes.

Perhaps the simplest behavior a reef fish might adopt—or that might evolve in reef fish—would be to avoid headfirst capture by the lionfish. This might involve adopting a way to detect a lionfish’s presence and fleeing the encounter before the predator could use jets to create a headfirst capture. Or after being hit by a jet of water, the prey could adopt unpredictable swimming movements to avoid headfirst capture.

Question 2

What characteristics of herbivory and parasitism are likely important in promoting specialization?

Both herbivory and parasitism typically involve a symbiosis in which the herbivore or parasite is smaller than, and may live on, the plant or host. This close relationship naturally leads to species evolving specialized mechanisms that counter their effects on one another. In addition, the prey of herbivores are plants, which are immobile, and thus may also evolve special mechanisms in response to the potential for intense herbivory.

Question 3

Suppose the number of grass species in an alpine meadow is maintained by herbivory by hares. By feeding on the dominant space-occupier, the hares allow less dominant grass species to thrive (similar to the sea star–mussel example in Figure 55.10). Now suppose lynx predators are introduced into the system. When, in the population cycle of lynx and hare (see Figure 55.9), would you predict the number of grass species to be highest? When would it be lowest?

The number of grass species would be highest when the hare population is highest and lowest when the hare population is lowest.

Predator–prey interactions are intrinsically compelling; the idea of a lionfish “vacuuming up” tiny reef fishes naturally captures our attention. But at the same time, lionfishes are not the only predators of reef fishes; sharks, barracudas, and even turtles hunt small reef fishes, potentially reducing the food available for lionfishes. Whenever any shared resource is limiting, organisms may compete to varying degrees to get that resource. In the next section we will consider how competition influences the ecology and evolution of species that overlap in their use of a limiting resource.