Actually, there is a way to do both—to elude predators and to access more food—and it explains the greatest spectacle on the Serengeti. MIGRATION: HOW TO EAT MORE WITHOUT GETTING EATEN"
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Predation on those resident populations accounts for almost 87 percent of their deaths, whereas predation on the migrants accounts for only about one-fourth of migrant deaths. Moreover, only about 1 percent of the migrants are taken in a given year, while up to 10 percent of residents may be killed. The migrants, therefore, experience much less predation per capita. Studies of lion and hyena behavior explain why they cannot take advantage of all that meat on the go: the predators cannot follow the herds, because they are confined to territories to raise and protect their own young. The combined effect of evading predators and accessing more food allows the migratory wildebeest to achieve a much greater density (about sixty-four animals per square kilometer) than that of the resident populations (about fifteen animals per square kilometer).
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Migration, then, is another ecological rule, or more aptly a rule-breaker, a way of exceeding the limits imposed by density-dependent regulation: SERENGETI RULE 6 Migration increases animal numbers Migration increases animal numbers by increasing access to food (reducing bottom-up regulation) and decreasing susceptibility to predation"
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"SERENGETI RULE 5 Density: The regulation of some species depends on their density
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Density-dependent regulation has the “virtue” of buffering change in both directions, slowing expansion as populations get large, and slowing declines as populations drop. It has been likened to a thermostat that triggers"
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"What the curves revealed was that the rate of increase of each species was higher when their numbers were fewer, decreased as the populations grew, and then turned negative (the populations decreased). In other words, the rate of change in the population depended on its density."
//////////////////////////////////"There was a fairly sharp threshold around 150 kilograms of body weight, below which smaller animals were generally regulated by predation, and above which larger animals were not. For example, most smaller antelopes, such as oribi (eighteen kilograms), impala (fifty kilograms), and topi (120 kilograms), die from predation"
////////////////////////////////////"But larger mammals, such as buffalo, experience much less predation (by lions only) and adult giraffe, rhino, hippos and elephants essentially none at all (see Figure 7.5, bottom right). The latter herbivores, so-called megaherbivores, appear to have escaped regulation by predators by evolving large bodies (and defenses) that make them just too difficult or dangerous to bring down, even by lions. Since elephants and the other large mammals above the size limit are not regulated from the top down by predators, it follows then that they must be regulated from the bottom up, by the availability of food.
////////////////////////////////
"All five small-bodied prey they monitored (oribi, Thomson’s gazelle, warthog, topi, and impala) increased in numbers during the period of predator removal, but the giraffe population did not. And all five small-bodied populations decreased again once the predators returned, demonstrating that these species, but not the giraffe, are negatively regulated from the top down by predators."
......///////////////////
Predation on those resident populations accounts for almost 87 percent of their deaths, whereas predation on the migrants accounts for only about one-fourth of migrant deaths. Moreover, only about 1 percent of the migrants are taken in a given year, while up to 10 percent of residents may be killed. The migrants, therefore, experience much less predation per capita. Studies of lion and hyena behavior explain why they cannot take advantage of all that meat on the go: the predators cannot follow the herds, because they are confined to territories to raise and protect their own young. The combined effect of evading predators and accessing more food allows the migratory wildebeest to achieve a much greater density (about sixty-four animals per square kilometer) than that of the resident populations (about fifteen animals per square kilometer).
////////////////////////
Migration, then, is another ecological rule, or more aptly a rule-breaker, a way of exceeding the limits imposed by density-dependent regulation: SERENGETI RULE 6 Migration increases animal numbers Migration increases animal numbers by increasing access to food (reducing bottom-up regulation) and decreasing susceptibility to predation"
/////////////////////////
"SERENGETI RULE 5 Density: The regulation of some species depends on their density
/////////////////////////
Density-dependent regulation has the “virtue” of buffering change in both directions, slowing expansion as populations get large, and slowing declines as populations drop. It has been likened to a thermostat that triggers"
////////////////////////////
"What the curves revealed was that the rate of increase of each species was higher when their numbers were fewer, decreased as the populations grew, and then turned negative (the populations decreased). In other words, the rate of change in the population depended on its density."
//////////////////////////////////"There was a fairly sharp threshold around 150 kilograms of body weight, below which smaller animals were generally regulated by predation, and above which larger animals were not. For example, most smaller antelopes, such as oribi (eighteen kilograms), impala (fifty kilograms), and topi (120 kilograms), die from predation"
////////////////////////////////////"But larger mammals, such as buffalo, experience much less predation (by lions only) and adult giraffe, rhino, hippos and elephants essentially none at all (see Figure 7.5, bottom right). The latter herbivores, so-called megaherbivores, appear to have escaped regulation by predators by evolving large bodies (and defenses) that make them just too difficult or dangerous to bring down, even by lions. Since elephants and the other large mammals above the size limit are not regulated from the top down by predators, it follows then that they must be regulated from the bottom up, by the availability of food.
////////////////////////////////
"All five small-bodied prey they monitored (oribi, Thomson’s gazelle, warthog, topi, and impala) increased in numbers during the period of predator removal, but the giraffe population did not. And all five small-bodied populations decreased again once the predators returned, demonstrating that these species, but not the giraffe, are negatively regulated from the top down by predators."
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