In systems where disturbances act on larger spatial and temporal scales, competition-colonization tradeoffs can maintain regional species diversity. Large-scale disturbances, such as fire, logging, or sedimentation-erosion dynamics, open up tracts of unoccupied habitat. These newly opened areas are colonized by a predictable sequence of species in a process called ecological succession (see Succession). Newly disturbed areas are first colonized by ruderal pioneer species (see Pioneer Species). These species may be outcompeted or overgrown by larger or longer-lived organisms. Over time, competitively superior species will dominate. When different areas in a landscape are disturbed at different times, the landscape becomes a patchwork of communities in different stages of succession, called a 'successional mosaic'. When disturbance is moderate, this successional mosaic is thought to result in high diversity at the regional scale because communities in all successional stages are represented. This is called the 'intermediate disturbance hypothesis' and is discussed in detail in another article in this volume (see Intermediate Disturbance Hypothesis).
The intermediate disturbance hypothesis has been proposed as an explanation for patterns of macroinvertebrate diversity in lotic streams in the Taieri River catchment in New Zealand. In this system, the intensity and frequency of disturbance are determined by the flow rate and frequency, respectively, of periodic high-discharge events. Disturbance varies from reach to reach in the catchment due to stream slope, distance from headwaters, and local topography, among other factors. Macroinvertebrate diversity is a unimodal function of disturbance intensity. At high disturbance intensity, some stream macroinvertebrates cannot survive a disturbance or colonize fast enough after a disturbance to persist in high-disturbance reaches. At low disturbance intensity, macroinvertebrates are distributed evenly, indicating competition among the species that remain long after a disturbance. For more on this examples, see section entitled 'Further reading'.
A critical piece of this 'successional mosaic' is the availability of colonists from other populations on the landscape. In streams, these colonists can come from several sources: upstream habitat, flying adults from another reach, or refuge habitat that organisms use to escape the effects of disturbance. If disturbance is too frequent (or to rare) across the entire landscape, then there will be no areas in the later (or earlier) stages ofsuccession to provide colonists to areas in other stages of succession. At this scale, colonization-extinction balance can influence the diversity of local communities (e.g., stream reaches) with different disturbance regimes and whole regions (e.g., river catchment) through the successional mosaic.
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