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Decreasing species diversity

Decreasing species diversity

Decreasing species diversity

Decreasing species diversity

Decreasing species diversity

| Fig. 3.2.6. A Changes in above-ground biomass production in experimental fields with differing diversities, with and without leguminous plants. The fields were planted in 1996 and mown twice a year. In 1998, productivity of plots containing legumes was significantly higher than those without. In contrast, the productivity of plots without legumes decreased after five mowings (Scherer-Lorenzen 1999). B Plots of the biodiversity experiment in Bayreuth. (Photo E.-D. Schulze)

cies results, even in mixed stands, in massive losses of yield. There are also minor species which appear to be redundant (Fig. 3.2.5 B) with little influence on the immediate ecosystem processes (e.g. orchids in a meadow). The absence of these plants hardly affects the biomass production of the stand. If the loss of biodiversity is associated with a change of life form (see Chap. 2.4.5), very different yield levels may be reached by the same numbers of species according to the presence of a specific life form at the site (Fig. 3.2.5 C). The system may, with a loss of a species, jump from one level to another with consequences for all processes in the ecosystem.

There are deviations from this general observation. It may happen that one of the introduced species has a much greater effect on the ecosystem than would have been assumed based on its fractional share of resources. Such species are called keystone species (Bond 1994). An example of such keystone species are nitrogen fixers in a meadow (Fig. 3.2.6 A). In an experiment in which meadow communities of differing diversity were sown (Fig. 3.2.6 B), an increase in above-ground biomass occurred with increasing number of species. This effect was, however, only observed if Leguminoseae (particularly Tri-folium rubrum) were included in the stand. In the first year after sowing, there were still sufficient nutrients available in the soil and the effects were not seen. With increasing depletion of the soil because of the loss of nutrients through harvesting, the effect of Leguminoseae became stronger in the following years (Scherer-Lorenzen 1999). Keystone species are often animals, which affect vegetation composition although this may not be obvious (see, e.g., Ernest and Brown 2001).

them to survive. In the unfertilised meadow about 75% of the biomass is in roots. Comparing this with a well-fertilised meadow, then the number of species decreases to 10 or 20 species, with new species invading. The remaining species have higher stems and may form a closed canopy at considerable height above the soil surface (Fig. 3.2.7 D). At first, grasses dominate. With further eutrophication and availability of nutrients, dicotyledonous species which grow even taller become dominant. The large leaf area index (LAI) near the ground in Fig. 3.2.7 A originates from the lower leaves of these tall, growing species during early development of the vegetation, but these lower leaves will die off with further growth of the stand. Because of the dense and high canopy little light penetrates to the ground and thus low growing species are excluded with time. Tall growth is stimulated by only investing 30% of the biomass into roots. The example shows that the reduction of root competition changes the competition for light, regulated predominately by the height of the vegetation.

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