The most ubiquitous spatial gradient in diversity is the latitudinal decline of species richness from the Tropics to the poles. This richness gradient is assumed to be accompanied by a gradient in evenness, where single species gain more dominance in temperate or boreal areas compared to tropical ones. However, systematic evidence is rare for this pattern or similar changes along altitudinal gradients. Many macroecological studies on range-abundance relationships have shown that widespread species have higher local densities, but the mechanisms for these patterns are still widely discussed and comprise purely statistical reasons, population dynamics, or resource use.
In addition to geographical gradients, dominance also varies along environmental conditions. Dominance tends to be high under harsh environmental conditions, such that entire organism guilds are represented by single species. An example are saltmarshes, where the most stressful marine side of the marsh is often inhabited by few species (e.g., Salicornia, Spartina), whereas, more landward (and less affected by salt), the number of species increases and the dominance of a single species decreases. Also in freshwater-terrestrial contact zones, the aquatic side of the gradient is often dominated by single terrestrial species adapted to the stressful conditions such as reed. The reason for this pattern is the strong selection for certain adaptations in the harsh environments, which favors only few specialized species.
Also gradients in productivity are often related to dominance patterns. Dominance tends to be high under high productivity, whereas habitats of low to intermediate productivity seem to be less probable to be dominated strongly by a single species. Fertilization studies often enhance the dominance ratio (cf. Figure 2a). Many studies concur on a decrease in evenness with increasing community biomass, which then translates into an increase of dominance with increasing biomass production.
Dominance varies not only over spatial gradients but also in time. This can be strongly seen not only in chron-osequences of succession, but also in many paleoecological investigations. In early primary succession, the habitat tends to be harsh and few species dominate early seral stages. Later on, dominance ratios might decline when more species contribute more evenly to community biomass. When the chronosequence remains undisturbed, late successional stages may exhibit increasing dominance ratios again, as highly competitive species displace other organisms. Also in secondary succession, dominance is also most pronounced in early developing species guilds (colonizers) and in late stages (competitors), whereas mid-stages show lower dominance levels.
Paleoecological analyses of pollen or of diatom community composition nicely show the shifts in dominance ratios over time. Sequences of strong single species dominance are often interrupted by phases of more even abundance distribution. These shifts are often related to alterations of the abiotic environment on local (changes in nutrient input or water chemistry) or regional (climate) scales.
For all these temporal and spatial gradients, it should be noted that not only the degree of dominance changes but also the identity of the dominant species. Species dominating early and late successional stages differ as do species between the extremes of environmental or spatial gradients. Both the dominance ratio and the identity of species have strong consequences for the functioning of ecosystems.
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