Dominance in Metacommunities

Ecological communities are not isolated, but connected via dispersal, which links local communities into meta-communities. Metacommunities provide a framework to understand the scale dependence of dominance and to highlight important controlling factors of dominance such as ecological interactions, dispersal, and species sorting. Regional and local dominance can strongly differ as do the processes leading to regional versus local dominance. Most locally dominant species are competitively superior species. However, depending on the frequency of disturbance, the overall probability of dispersal and the presence of source sink dynamics, regionally dominant species may be either superior competitors or superior dispersers. The degree of regional (and local) dominance further depends on the spatial heterogeneity in environmental conditions across the local patches.

The effect of dispersal on dominance is clearly rate dependent (Figure 3). At very low dispersal rates, neither competitive nor consumer-prey interactions are affected and the impact of dispersal is low. The identity of the competitive dominant species may strongly differ between the local communities, as each community will have a different species composition. At highest dispersal, all species are able to reach each of the local communities, and competitive dominance still plays the most important role, but now one species, the superior competitor in the regional species pool, will dominate all local communities. At intermediate dispersal frequencies, the dominance of the best competitor will be disturbed by frequent invasion of good colonizers as long as colonization-competition tradeoffs persist. The dominance can also be reduced in source-sink metacommunities, where local populations in harsh local (sink) patches are maintained by substantial dispersal from benign (source) patches.

The frequency of disturbances (or other sources of mortality) in the local patches determines how important different dispersal regimes can be. With no opening of space, dispersal cannot rescue colonizers from being excluded by competitors. However, with disturbance space is opened for colonizers to persist and the importance of colonization traits compared to competitive traits increases with disturbance frequency and intensity.

The identity of the dominant species can be highly dissimilar between local patches in a metacommunity, depending on dispersal rate (see above) and the environmental and geographical distance between the local patches. Higher geographical distance enhances the probability of dispersal limitation and higher environmental distance allows for species sorting. Beta-diversity, which measures the turnover in species composition between local patches, increases with increasing spatial distance and with decreasing environmental similarity.

Including metacommunity dynamics and community assembly into the analysis of dominance also allows addressing the effect of stochasticity and assembly sequence on dominance ratio and the identity of dominance species. As exemplified in Figure 1 , nearby habitats of similar abiotic conditions can have different species setup and can be highly dominated by one species, which is almost missing in the other system. These differences can occur due to founding effects, which stress the importance of colonization sequence. Overall dominance can be enhanced by a species colonizing an enemy-free space and growing into a size refuge (which is the mechanism probably leading to the dominance of green algae in the right rock pool (Figure 1 )) or by a species arriving able to monopolize and defend space (which is congruent with inhibition mechanisms in succession, see temporal patterns above).

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