Introduction

The coexistence of species in ecological communities is almost always asymmetrical such that some species are dominant, but most are rare. This inequality of species contribution is reflected by the degree of dominance in species composition and the identity of the dominant species. The analysis of dominance has been a cornerstone in general ecology, tightly linked to questions of community structure and diversity. Understanding the degree of dominance and the identity of the dominant species is implicit in studies on community assembly and organization, on causes and consequences of diversity, and on evolutionary and macroecological constraints of regional species pools.

Dominance in a local community can be highly obvious as one species monopolizes the use of space and visually characterizes the structure of the assemblage (Figure 1). Such striking dominance of single or few species can be seen in many terrestrial and aquatic communities. However, even in communities with seemingly lower dominance, only a few species contribute the majority of the biomass or the count of individuals.

The degree of dominance can be characterized by the maximum proportion of individuals (or biomass)

Figure 1 Two adjacent rock pools at the Swedish west coast. The right rock pool is strongly dominated by ephemeral green algae (Enteromorpha sp.), the left rock pool is characterized by a large snail population (Littorina sp.) and crustose algae. Picture courtesy of Monika Feiling.

contributed by a single species in an assemblage (dominance ratio). The dominance ratio can fluctuate between 0 and 1, but values >0.5 are very common, both for individuals and for biomass (Figure 2). Another way of addressing dominance is represented by rank-abundance curves, where species are ordered in ranks according to

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Figure 2 Dominance patterns exemplified by data on benthic marine microalgae from the western Baltic Sea. (a) Relative biomass proportions of the ten most common microalgal species in control and fertilized treatments. (b) Correlation between evenness (measured as Pielous's index) and the dominance ratio in fertilization experiments from 1996 to 1998. (c) Correlation between dominance ratios calculated from abundances and dominance ratios calculated from biomass proportions. (d) Correlation between dominance ratios calculated from biomass proportions and species richness. All data are from Hillebrand H and Sommer U (2000) Diversity of benthic microalgae in response to colonization time and eutrophication. Aquatic Botany 67: 221-236.

their proportional contribution. All classical rank-abundance models - broken-stick, log series, or neutral models - predict the numerical dominance of just a few species out of all species present. Similar relationships can be derived between rank and biomass instead of abundance. The shape of the rank-abundance curve is a strong visualization of dominance (Figure 2a) and a readily assessable way of analyzing the effects of ecological interactions and abiotic drivers on species dominance.

The complementary term to dominance is evenness, which describes the equality of the distribution of proportions across species. Communities with less well pronounced dominance have a higher evenness (a higher degree of equality in the distribution of abundance or biomass on different species). Consequently, the correlation between dominance (as maximum proportion of one species) and evenness is negative (Figure 2b).

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