The degree and identity of dominance strongly differ between ecosystems, but does this variation matter for the functioning of ecosystems.? Obviously the processes in ecosystems (such as productivity, respiration, energy transfer, resource retention) are mainly driven by the dominant species. The traits of dominant species thus have a higher importance for ecosystem functioning than the traits of rare species. The consequences of dominance are separate from the consequences of keystone species (sensu Payne) and foundation species (sensu Dayton). Whereas keystone species are species affecting ecosystem processes by having very high per capita impacts (e.g., top predators) and foundation species comprise species which have strong impact on ecosystem structure by enabling community structure (e.g., trees), the importance of dominance is based on the relative fraction of matter and energy flow canalized through one species. However, most foundation species are locally dominant, whereas most keystone species are not.
The question becomes highly important in the face of strong alterations of global diversity and of dominance patterns by human actions such as habitat destruction, species homogenization, and fertilization. Recent theory and experiments suggest that local species richness can affect local ecosystem processes. In many of these experiments, the identity of the species had a major effect on the outcome. The identity of the dominant species consistently affects primary production, decomposition rates, consumption rates, and overall community respiration. Species identity effects have in many studies been more important than the effects of species richness per se.
Few studies have manipulated not only the number of traits present but also their distribution. Studies looking for effects of evenness on ecosystem processes such as productivity are few to date, but one emerging pattern is that higher evenness in plants tends to reduce primary productivity. This can easily be explained by the fact that high evenness prevents the community from being dominated by the most productive species. Thus, important ecosystem processes tend to be best performed in communities characterized by strong dominance.
This primacy of dominant species in ecosystem functioning does not necessarily mean that rare species are unimportant. Many ecologists have proposed that the importance of subdominant species is an insurance against spatial or temporal fluctuations in abiotic conditions. Thus, rare species represent an insurance as they may be able to perform major ecosystems processes under different environmental constraints. More evenly structured communities may perform such shifts more easily than communities characterized by a strongly dominating species. However, this evenness insurance effect has not been analyzed theoretically or experimentally yet.
See also: Average Taxonomic Diversity and Distinctness; Competition and Competition Models; Dispersal-Migration; Metacommunities; Seed Dispersal; Succession.
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