When areas of habitat are lost or added through human action, bird numbers often change accordingly. To take some recent examples from Britain, through much of the twentieth century Siskins Carduelis spinus increased in numbers and expanded in range as new conifer plantations matured and provided additional breeding habitat (Gibbons et al. 1993). In contrast, Redshanks Tringa totanus declined and contracted as summer nesting habitat was destroyed by land drainage (Norris et al. 2004), and Twite Carduelis flavirostris declined as areas of winter saltmarsh were lost to reclamation projects (Atkinson 1998). Many other examples have been described in the literature from Europe and North America, including the massive declines in numbers of waterbirds which followed the drainage of marshes. While many such examples may represent causal relationships, some may be due to coincidence between population decline (caused by some other factor) and habitat loss. Clearly, not all bird population changes occur in response to habitat changes, and some species seem to have far more potential habitat than they currently occupy. In the case of migrants, numbers may be limited in wintering areas at a level lower than habitat in breeding areas would support (or vice versa).
In recent years, much thought has been given to predicting the effect of habitat loss (equivalent to food loss) in both resident and migratory bird species. For resident birds, in which breeding and wintering areas are the same, population declines should be roughly in proportion to habitat loss, if habitat were of uniform quality and fully occupied throughout. In other words, if half the habitat (or food supplies) were lost, we would in general expect the population to be roughly halved (but see later). The situation is more complicated for migrant birds because habitat loss may occur in breeding or wintering areas or both (Figure 26.2). The actual population change following loss of breeding or wintering habitat would be expected to depend on where the tightest bottleneck occurred; that is, on the relative strengths of density-dependent constraints in the two areas (Sherry & Holmes 1995, Sutherland 1996). Such constraints include those various pressures, such as competition for space and food that can affect an increasing proportion of individuals as their density rises, resulting in an increased per capita mortality or decreased per capita reproduction. In the wintering area, the strength of density-dependence is measured by the per capita rate of increase in mortality that occurs as a result of rising population size (or decreasing area) (slope d). In the breeding area, the strength of density-dependence is measured by the per
Figure 26.2 Model of relationship between per capita mortality and reproduction (continuous lines), and equilibrium population size (E). Per capita winter mortality increases, and per capita reproduction decreases, with increasing population size. The equilibrium population size is where the two lines intersect. The model shows how loss of habitat (or food supply) results in population decline. If wintering habitat (or food supply) is reduced by 50%, this results in a displacement of the relationship between mortality and total population size in direct proportion to the degree of habitat loss (dashed line). The equilibrium population size is reduced accordingly (E1). A 50% loss of breeding habitat (or food supply) similarly results in a displacement of the relationship between net breeding output and total population size (dashed line) and a reduced equilibrium population (E2). In this example, 50% loss of breeding habitat results in a smaller reduction in equilibrium population size than does 50% loss of wintering habitat, because density-dependence is stronger in winter than in summer. Modified from Sutherland (1996).
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