The Distribution of Organisms Over Landscapes

An organism may only persist where the physical conditions (temperature, humidity, etc.) are tolerable and the food resources are adequate. In other words, the environment should match the niche space of the species (see Ecological Niche). Consequently, all organisms are associated with a specific type of habitat, and hence view the area around them in widely different manners. In this context, it is useful to think of the landscape as a mosaic, consisting of patches of favorable habitat surrounded by uninhabitable areas. Within such patches the species may form more or less permanent local populations (also called subpopulations), while the species is only rarely found in the intervening areas.

The dynamics of a subpopulation are driven by several processes, as shown in Figure 1. Individuals enter the population through birth and immigration, and leave it as a result of death and emigration. Of these processes, birth and death rates may be controlled by the number of individuals already present, their density, and the resource availability. Immigration and emigration may also be affected by these factors, as well as by the favor-ability of the surrounding landscape and the size of the adjacent subpopulations.

If we increase our vantage point to include several such subpopulations located over the landscape, which are separated by uninhabitable areas but with migration occurring between them, then a very complex picture appears (Figure 2). All these interacting subpopulations can be viewed as one large spatially structured population, known as a metapopulation (see Metapopulation Models). The complex spatial dynamics of the metapopulation result from the patterns of between-patch movement and local birth and death rates, as well as local extinction and colonization of subpopulations which occur over larger timescales. These processes can be modeled by a set of connected differential equations, which allows quantitative predictions to be made about the dynamics of the metapopulation.

Births

Births

Figure 1 The abundance of a subpopulation within a patch of suitable habitat is affected by the processes of birth, death, immigration, and emigration.
Figure 2 In a landscape consisting of several suitable habitat patches, the population dynamics become dependent on dispersal and colonization between interconnected subpopulations. Such a system is described by metapopulation dynamics.

Other factors, which are not accounted for by the basic model described above, may be included in more sophisticated representations. These factors include the size and favorability of patches, their relative isolation, and the nature of the intervening habitat relative to the dispersal ability of the organism. There are also dynamic processes affecting the occupancy of each patch over time. These processes rely on the observation that over time, the subpopulation in many cases causes a reduction in the quality of the patch which it inhabits (e.g., reduces the amount of resources and/or attracts predators, parasites, and pathogens). As the environment deteriorates, mortality and emigration is likely to increase while the rate of reproduction correspondingly decreases. In addition, Allee effects (see later) can hasten the demise of the subpopulation. A further complication to this pattern is that these effects are not restricted to the species in question, but are equally likely to affect its predators and the vital bioresources, such as food resources on which it depends.

An interesting consequence of the spatial movement of individuals between subpopulations is that some patches may be occupied, even though they cannot in themselves sustain viable populations of the species. The subpopulation of such a patch is actually kept alive by the immigration of individuals from more productive patches in the vicinity. The movement of individuals resembles water flowing from its source to a sink, and correspondingly this type of system is usually termed source-sink dynamics (see Dispersal-Migration). In some areas, especially in transition zones between habitats or biomes and in areas severely affected by human disturbance, such source-sink dynamics may play a key role in structuring the occurrence and distribution of individuals.

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