Spatial Patterns and Processes

Spatial processes lead to the formation of distribution patterns. Seed dispersal, for instance, often produces concentrations of seedlings around parent plants and leads to the formation of clumped distributions. When local dispersal is combined with patchy disturbance, such as fire, the result is a distribution composed of patches. When combined with environmental gradients, such as soil moisture, local dispersal can produce zone patterns, with different species dominating different areas (Figure 4).

Fragmentation is one of the most important consequences of landscape connectivity. When the density of (randomly located) objects in a landscape falls below a critical density, they are mostly isolated individuals. When the density exceeds the critical threshold, they become connected. The density at which the critical threshold occurs depends on the size of the neighborhood of the objects. There are many cases where landscape connectivity plays an important role. Epidemic processes require a critical density of resources to spread. Instances include disease outbreaks (susceptible individuals), fire spread (fuel), and invasions of exotic plants (suitable sites). Populations become fragmented if individuals cannot interact with one another. For instance, in wet years the water bodies of central Australia are essentially connected for water birds, which can fly from one body to another almost anywhere in the continent. In dry years, however, many water bodies shrink or dry up and become too widely separated for birds to migrate between them (Figure 5).

Figure 4 Emergence of spatial patterns from dispersal. This CA model shows the hypothetical distributions of two plant populations that result in three different scenarios. (a) Global dispersal, in which seeds can spread anywhere, results in random distributions of plants. (b) Dispersal from local seed sources leads to clumped distributions. (c) The combination of local dispersal and environmental gradients (from top to bottom) creates vegetation zones.

Figure 4 Emergence of spatial patterns from dispersal. This CA model shows the hypothetical distributions of two plant populations that result in three different scenarios. (a) Global dispersal, in which seeds can spread anywhere, results in random distributions of plants. (b) Dispersal from local seed sources leads to clumped distributions. (c) The combination of local dispersal and environmental gradients (from top to bottom) creates vegetation zones.

Subcritical Critical Supercritical

Figure 5 Critical phase changes in connectivity within a fragmented landscape. In this CA model, grid cells represent sites in a landscape. Gray and black cells represent vegetation and white cells have no cover. The black cells show examples of patches of vegetation sites that are connected, for example, by spread of afire ignited in the center of the grid. Notice that only a small increase in the density of covered sites makes the difference between subcritical and supercritical.

Subcritical Critical Supercritical

Figure 5 Critical phase changes in connectivity within a fragmented landscape. In this CA model, grid cells represent sites in a landscape. Gray and black cells represent vegetation and white cells have no cover. The black cells show examples of patches of vegetation sites that are connected, for example, by spread of afire ignited in the center of the grid. Notice that only a small increase in the density of covered sites makes the difference between subcritical and supercritical.

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