514 c hapter 15 population ecology how do organisms

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514 C HAPTER 15 Population Ecology: How Do Organisms Interact to Form Populations? These relationships are highly complex. In some cases, herbivores able to tolerate defensive plant chemicals are attracted only to those plants producing such chemicals. This may be the primary reason why many plants produce defensive chemicals only after being damaged by herbivores. Herbivore–Carnivore Interactions The relationships between herbivores and the car- nivores that feed on them may seem deceptively simple:As we saw in Chapter 14, car- nivores must find, approach, and secure prey; herbivores must sense the presence of predators and avoid them (Figure 15-22).As with plant–herbivore interactions,situations in nature are generally far from simple. Energy is precious to both predator and prey, and we can understand much of the interactions between carnivores and their prey from the perspective of energy expended and energy obtained. Compared with plant material, animal material is generally easier to digest, and the energy and nutrients are more highly concentrated.This simplifies life for the carnivore. Figure 15-22 The puma has seen the deer and must now approach it.The deer has seen the puma and must now elude it. Figure 15-21 Plants are not always passive in their response to herbivory. Proteins of oak leaves, for example, react with tannins and become indigestible to caterpillars.
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15-4 How Do Populations Interact? 515 Among the larger ones, except when they are feeding young, most must obtain prey only once every few days. In between feedings, life is relatively easy.They sleep, rest, or socialize. Small carnivores may need to feed more often, but not as often as herbivores. Herbivores feed nearly continuously. As was pointed out earlier in the chapter, it is to a carnivore’s advantage to secure the most food while expending the least energy. Often, this means concentrating on the prey that is most abundant or the most available. To hunt squirrels in an environment where rabbits are more abundant is a waste of a fox’s energy, assuming that both are equally “catchable.”Where rabbits are more abundant, it is more efficient to hunt rab- bits. Even if a fox happens upon a squirrel, it passes it by because, today, foxes are hunt- ing rabbits. Carnivores often seem to formulate a prey image that is what they then seek to the exclusion of other possible prey. What vastly complicates the task is that some herbivores are highly mobile and po- tentially dangerous when cornered.A healthy moose is quite capable of successfully de- fending itself against a pack of hungry wolves.To push their point is not only a waste of the wolves’ energy, but also invites serious injury.Thus, a predator spends time studying a particular prey animal before attacking it.Carefully,predators assess chances of success, looking especially for signs of weakness—individuals that are sick, injured, old, or very young.Only when faced with starvation will they chance challenging healthy individuals.
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