The effects of predation are often density

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The effects of predation are often density dependent.As long as numbers of rabbits are low, foxes tend to ignore them, seeking more abundant prey—quail, perhaps.With foxes hunting other prey, rabbit numbers increase. Only when rabbits are relatively abundant do they draw the attention of foxes.As rabbit numbers increase further,so does the intensity of predation, which may eventually limit the numbers of rabbits. Disease is also often density dependent. Periodic outbreaks of rabies among rac- coons in the northeastern United States, for example, are most intense when raccoon numbers are relatively high. When population numbers are low, so is contact between individuals.With little contact, many diseases, rabies included, spread slowly. Only with frequent contact do such diseases spread quickly and reach epidemic proportions. So far, the examples we have discussed are extrinsic factors. Density-dependent factors may also be intrinsic. Superficially, periodic outbreaks of rabies in arctic foxes (a) (b) Figure 15-12 Food and space are density-dependent limiting factors. (a) In this dense stand of trees, all available space is filled with individuals.Although new individuals may replace those that die, further increases in overall numbers are not possible. (b) When populations of deer become so large that all available food is eaten, they may not only fail to increase, but the population may decrease.
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15-3 How Do Populations Grow? 503 resemble those of raccoons; that is, soon after population densities reach high levels, rabies breaks out, causing declines in the number of foxes. Microscopic analysis shows Negri bodies (small dark-colored dots) always to be present in the brain tissue of diseased foxes. But examination of extensive numbers of arctic foxes shows that Negri bodies are also present in the brains of apparently healthy foxes. As long as the population densities are low, rabies is latent in the arctic fox.What triggers the rabies is not under- stood, but when densities surpass a certain level, something apparently changes within individual foxes and the disease is expressed. High population densities are known to be stressful to many mammals, quite often affecting their ability to reproduce. Some environmental constraints that limit population size are apparently density- independent factors .Freezing temperatures,for example,kill many flowering plants.If an unusual frost in,say,southern Florida killed all individuals of a species,regardless of pop- ulation density, we might say that sensitivity to cold is a density-independent factor. Ecologists have argued long,hard,and sometimes bitterly on the importance and,indeed, the existence of density-independent factors. For example, frost in Florida might not kill all individuals of the species.Those individuals fortunate enough to live in protected sites might survive. Indeed, freezing temperature may be lethal only when population densi- ties are so high that some individuals live in unprotected sites. If this is true, then the effects of freezing temperatures may, in fact, be density dependent. As we learn more
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