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Rank Dominance and Keystone Species

In the so-called rank dominance distribution one parameter, characterising the presence of a species (number of individual plants, biomass, ground cover per species), is plotted with decreasing proportion of that parameter as shown by the different species of that community (Sala et al. 1996). In many vegetation types a few species, often only one species, comprise the largest fraction of individuals, biomass and canopy cover. Other species obtain a lesser fraction.

Increase or decrease of plant species, or the presence of certain functional types, may have very different, usually non-linear effects on ecosystem processes. Generally, productivity increases with biodiversity because of the better temporal and spatial use of the site (Hector et al. 1999). With the loss of plant species different developments are possible (Sala et al. 1996). There are species which achieve the highest biomass in monocultures (Fig. 3.2.5 A). Removing such spe-

Number of species 3

Number of species 3

Number of species 3

Redundant species

Keystone species

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