Inferring the dynamics of ecosystem switch from the scaling relationships and spatial pattern of habitat patches

Background/Question/Methods

Ecosystem switch from one land cover type to another often occurs over time scales too long to be observed directly, and hence the dynamics must be inferred from indirect evidence. Increasingly, the present-day spatial distribution of colonists is used to test models of underlying processes. Here, I consider cases where ecosystem switch can be conceptualized as a sequence of nucleation and coalescence of discrete patches of ‘pioneers’ within a background matrix of ‘residents’, analogous to surface growth processes in the physical sciences. Nucleation refers to establishment of discrete ‘clumps’ of pioneers, which subsequently expand outward through vegetative growth or enhanced seed recruitment. Over time, adjacent expanding patches may merge or coalesce. Using a range of examples, I discuss how spatial patterns and scaling relationships of pioneer patches can provide insight on ecosystem dynamics.

Results/Conclusions

• The distribution of pioneer ‘nuclei’ can be tested for spatial randomness across the background matrix as a whole, or more specifically in relation to the distribution of safe sites (which themselves may be distributed non-randomly). On lava flows, for example, moss colonization occurs at random across micro-scale (< 1 cm) irregularities on the rock surface. The spatial distribution of vegetation during the early stages of succession is determined largely by the spatial distribution of these micro-scale safe sites.
• The subsequent growth or expansion of pioneer patches can be analysed using scaling relationships, which indicate how patch shape changes over time. On peatlands, for example, patches of the bog-moss Sphagnum fuscum grow vertically as well as horizontally, leading to the development of raised hummocks. There is some evidence that the shape of these hummocks is related to regional climate: increasing potential evapotranspiration favors the development of lower, flatter hummocks, indicating a climate-related shift in the relative rates of vertical accretion and lateral expansion.
• Coalescence of pioneer patches can be inferred from size-structured patch distributions and patch statistics. On some patterned peatlands, for example, many small open-water pools are located around the periphery of a complex dominated by fewer, larger pools at the center. This pattern is consistent with a model whereby pools initially form at a central point and, as they grow and coalesce, hydrological conditions change, allowing new pools to form at the margin of the expanding pool complex.

The analysis of scaling relationships and spatial patterns offers insight on ecosystem dynamics, but care must be taken to consider patterns as the results of spatial processes.