10.7.1 Metapopulations and diversity
There is a need for a conceptual framework to facilitate the description of ecological processes occurring on a landscape scale (see Section 10.7.3) above the level of the local population. This is provided by the concept of the metapopulation, an assemblage of partially isolated populations of the same species existing in a balance between extinction and colonization. Animal and plant species are seldom distributed continuously in space; they are usually organized into local populations (each of which may be increasing or decreasing) connected to a greater or lesser extent through dispersal. This spatial structure implies that the demography and genetics of populations will be the product of both local environmental conditions and processes operating on a regional scale. The metapopulation concept is still vigorously discussed, here we will define some of the most important variables involved and discuss a relevant woodland example.
Whether regional populations form a linked metapopulation or a series of discrete populations depends on the proportion of suitable habitat potentially available to the species concerned. Where suitable (S) habitats cover a much larger area than unsuitable (U) habitats, i.e. S » U, plants and animals can be described as developing spatially extended populations. Where the reverse applies, i.e. U » S, species form regional ensembles in which migration between patches of suitable habitat is effectively prevented by isolation, and re-colonization does not occur after local extinctions. If, however, suitable habitats cover an intermediate area relative to those which are unsuitable, it is considered by Freckleton and Watkinson (2002, 2003) that metapopulations are particularly likely to exist. In such cases, the regional dynamics (movement between populations) are determined by the relationship between local birth and death processes (B and D) and regional immigration and emigration rates (I and E).
Whilst it is generally accepted that the relative size of I and E versus B and D describes the dynamics within the local population units, it is also essential to realize that variation in any or all of I, E, B or D can be crucial for regional dynamics. In practice very few plant metapopulations have been described, and there are good reasons why plants may not develop such populations in the same way as short-lived mobile animals (Ehrlen and Eriksson, 2003). The existence of long-lived seeds or vegetative ramets enables local populations to exist for a long time even though the habitat patch has become unsuitable. Additionally, successful dispersal and recruitment may be very sporadic so re-colonization may be unlikely after local extinction, while dispersal over long distances can be governed by chance events.
Consideration of metapopulations can be a useful tool for identifying constraints on populations. In the central hardwoods region of Indiana in central USA, the North American red squirrel (Tamiasciurus hudsonicus) has gradually colonized all but the most heavily forested areas since the early 1990s. This range expansion by the red squirrel has coincided with a reduction in numbers of the grey squirrel (Sciurus carolinensis). It may appear at first sight that the red squirrel is detrimentally interacting with or outcompeting the grey in the same way that the same grey squirrel introduced into the UK is detrimentally affecting the native red squirrel (Sciurus vulgaris). The reason for the changes in Indiana, however, is more to do with metapopulations. Goheen et al. (2003) investigated the problem in an area of Indiana where forest patches (covering 10% of the land) are embedded within an agricultural matrix and joined by wooded fencerows (hedgerows). The grey squirrel is more sensitive to forest fragmentation than the red and the former are found only in the largest most continuous forested sites. As importantly, they found that the red squirrel is more able to move between local populations along the fencerows. Overall survival of grey squirrels moved into fencerows was 71% compared with 95% in red squirrels, presumably due to higher predation risks for the grey. The grey squirrel is thus found in fewer, more isolated populations in a regional ensemble rather than a metapopulation, and has a low ability to move between them. Thus the grey squirrel is constrained both by individual area requirements (further fragmentation will reduce their suitable habitat) and poor dispersal ability (local extinction is hard to replace by movement). By contrast the red squirrel can live in smaller fragments but can also move between local populations much more easily to maintain viable numbers if, for example, local disturbance reduces population size in one area. Such movement makes the future survival of the red squirrel metapopulation much more probable.
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