I

Ei E2 E

Total population size

Winter mortality (d)

Net breeding output (b)

Ei E2 E

Total population size capita rate of decrease in reproduction with rising population size (or decreasing area) (slope b). If slope b > slope d, loss of breeding habitat would have most impact on overall population size, and if d > b, loss of wintering habitat would have most impact. If the two density-dependent relationships were known, the effect of loss of habitat (or food supply) on equilibrium population size could in theory be calculated as b/(b + d) for the breeding area, or as d/(b + d) for the wintering area.

If, in an extreme case, all the density-dependence occurred in winter habitat, say, with no density-dependence in breeding habitat (which at prevailing population levels was present in excess, as in Figure 26.1A), then loss of winter habitat would cause a matching reduction in population size. In this situation, loss of breeding habitat would have no effect up to the point where density-dependent decline in breeding success set in. Although as yet there can be few species for which enough information is available to test the model in Figure 26.2, or to judge the form of density-dependent relationships over a wide range of densities at both seasons, attempts have been made for the Oystercatcher Haematopus ostralegus in Britain (Goss-Custard et al. 1995, Sutherland 1996).

The above considerations (model in Figure 26.2) lead to a number of conclusions about the effects of habitat (or food) loss on the equilibrium population sizes of migrants: (1) knowledge of the density-dependent response within just the wintering or breeding area cannot be used to predict precisely the effects of habitat or food loss in either, for it is the ratio of density-dependence in the two areas that is important; (2) unless there is no density-dependence acting during one of the seasons, a loss of habitat or food supply in either summer or winter areas could result in population decline; and (3) the consequence of habitat or food loss is greatest for the season in which density-dependence is strongest (winter in Figure 26.2). In practice, all migrants are likely to be affected more by changes in one area than the other, although whether breeding or wintering areas are most limiting may change through time. They could also change from year to year in species subject to large annual fluctuations in habitat, food supplies or other conditions.

The above generalisations on the role summer and winter conditions hold most clearly for populations limited by resources - by the available habitats and food supplies. They could also hold for populations limited below the levels that resources would permit by factors such as parasitism, predation and human persecution. In some circumstances, the latter factors can kill an unsustainably large number of individuals each year, sending populations into decline, and leaving a surplus of unused habitat and food. For example, if for some reason the predation pressure on eggs and chicks in the breeding areas increased so much that loss of annual production could not be offset by improved annual survival, the population would decline below the levels that both breeding and wintering habitats would support. Similarly, if shooting pressure on full-grown birds increased in winter quarters, so that the loss could not be offset by improved reproduction or natural survival, the population could again decline below the carrying capacities of both breeding and wintering habitats. In both these examples, decline would continue while ever that situation held (eventually to extinction), the trend being driven primarily in whichever area the effects of adverse factors on individual reproduction or survival were greatest.

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