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Conclusion Population

Control

One hare population cycle (11 years)

Control

One hare population cycle (11 years)

Conclusion: Population cycles of the snowshoe hare are influenced by their food supply as well as by interactions with their predators.

54.12 Prey Population Cycles May Have Multiple Causes

Experiments showed that both food supply and predation (but not food quality) affect the population densities of snowshoe hares.

ing stage. At least three butterfly subpopulations became extinct during a severe drought in 1975-1977. The largest patch of suitable butterfly habitat, Morgan Hill, typically supports thousands of butterflies (Figure 54.13). It probably served as a source of individuals that dispersed to and recolonized small patches where the butterflies had become extinct.

In another study, ecologists manipulated the habitat of tiny arthropods (springtails—tiny insects without wings— and mites) to investigate the subpopulation dynamics of these animals. In one experiment, they created isolated patches of the animals' habitat—mosses growing on rocks— by clearing moss from parts of the rock surface (Figure 54.14, Experiment 1). The number of species present in these patches declined about 40 percent within a year, with more rare species than common species disappearing from the patches. The experiment illustrated that small, isolated populations are more likely to become extinct than large populations are.

In a second experiment, the investigators created similar patches, but these patches were connected by narrow corridors of moss that were either intact or disrupted by a barrier only 10 mm wide (Figure 54.14, Experiment 2). Moss patches connected by unbroken corridors contained more species of arthropods a year later than patches whose corridors were discontinuous. Thus, a gap of only 10 mm was sufficient to reduce the rescue effect.

Rescue Effect

Serpentine outcrops

54.13 Subpopulation Dynamics The bay checkerspot butterfly population is divided into a number of subpopulations confined to patches of habitat (serpentine rock) that contain the food plants of its larvae. Extinction of these subpopulations is common.

Serpentine outcrops

54.13 Subpopulation Dynamics The bay checkerspot butterfly population is divided into a number of subpopulations confined to patches of habitat (serpentine rock) that contain the food plants of its larvae. Extinction of these subpopulations is common.

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