Each is a major determinant of the others population

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Each is a major determinant of the other’s population size.When predators are removed from an area,most prey populations increase,which implies that predator pressure keeps prey populations down. When prey numbers are high, predators may respond with in- creased reproduction,decreased mortality,or increased immigration.Often,predator–prey populations fluctuate in a predictable fashion. Initially, prey populations increase, fol- lowed by increases in predator populations, followed by decreases in prey populations (Figure 15-19). Yellow-rumped Warbler Bay-breasted Warbler (b) Cape May Warbler (c) Black-throated Green Warbler (d) Blackburnian Warbler (e) (a) Figure 15-18 Several species of warblers forage in evergreen trees in the northeastern United States and southeastern Canada. Each species forages in a particular area of the trees (denoted by the red color), thus minimizing competition between species for foraging sites.
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15-4 How Do Populations Interact? 511 snowshoe hare lynx Thousands of hares Thousands of lynx Year 1850 0 40 0 3 6 9 80 120 160 1875 1900 1925 Figure 15-19 (a) In some parts of their ranges, numbers of snowshoe hares (b) and lynx (c) are interrelated. Large numbers of hares support large numbers of lynx. In the face of intense predator pressure, hare numbers fall.With food lacking, lynx numbers decline.With lynx numbers low, hare populations increase, and a new cycle is initiated. (Graph adapted from Machulich, 1937.) (a) (b) (c) Remember that predation is a density-dependent limiting factor. For this reason, predation, by itself and under natural conditions, usually will not result in the extinction of a prey species. Here’s one way density dependence might prevent extinction: Red foxes prey on rabbits only after the number of rabbits becomes relatively high.As rab- bit numbers decrease due to predator pressure, the number of squirrels, an alternative prey of the foxes, may increase. Eventually, the squirrels outnumber the rabbits, and the foxes switch to preying on squirrels. (This scenario assumes that rabbits and squirrels are equally available to the foxes.)
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512 C HAPTER 15 Population Ecology: How Do Organisms Interact to Form Populations? Figure 15-20 The accidental introduction of the brown boa has had drastic effects on Guam’s birds. Sometimes,predation can result in extinction.In Guam,an island in the South Pacific, the brown boa, a snake native to Australia and New Guinea, arrived accidentally some- time during World War II (Figure 15-20). Their preferred foods are the eggs and young of ground- and tree-nesting birds, which were once common on the island.As a result of relentless predation by the snake,most of Guam’s birds have become extinct,and still the snake populations thrive.Why haven’t their populations fallen? Rats,another introduced species on Guam, are a secondary food source for the snakes. First, the snakes seek out young birds and eggs. Finding none, they subsist on rats. Since the rats and snakes are r -selected and the birds are K -selected species, the birds on Guam were doomed.
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