Measure Based Patterns

The dominance ratios calculated for biomass often differ strongly from dominance ratios calculated for abundance, which is mainly due to the fact that the identity of the species dominating abundance differs from the species dominating biomass. As a consequence, the rank of any species within a community according to the proportion of abundance and the proportion of biomass tend to be negatively correlated. Therefore, the maximum dominance found for abundance is often uncorrelated to dominance assessed by biomass (Figure 2c).

As a consequence of these different outcomes, ecologists have to define whether dominance in abundance or dominance in biomass has to be the main target of their study. The answer to this question is mainly driven by the ecological processes focused on. Number-driven processes such as dispersal and intra- or interspecific encounters are best reflected by abundance dominance, whereas other processes such as competition for resources or energy flux are mainly affected by dominance in biomass.

The degree of dominance is largely independent of the number of species present in a local assemblage, as becomes evident by the lack of correlation between taxo-nomic richness and dominance ratios (Figure 2d). This pattern is corroborated by the repeatedly found lack of correlation between evenness and taxonomic richness in many community types. This uncoupling is mainly due to the fact that the overall species number is a consequence of the number of rare species, which have negligible influence on the share of dominant species in a community. In other words, a species can be highly dominant independent of the length of the tail of rare species in the assemblage.

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