Biocoenotic Interactions between Different Ecotopes

In comparison with a relatively uniform landscape organisms in a complex one have to cope with a distinctly higher number of problems. On the one hand, the irregular habitat pattern may be inducive to higher biodiversity, on the other isolation of habitats may result in a decrease in species numbers, if extinction is not compensated by immigration. In fractal landscape models, the number of species coexisting in microsites increased up to the fractal dimension of 2.75, but decreased with higher dimensions (Palmer 1992). Studies on ground beetles in the Netherlands showed a significantly higher proportion of species with high migration potentials in small habitats than in large ones (Vries et al. 1996). In stressed habitats an opposite effect might develop. Thus, Fahrig and Jonson (1998) found an increasing species richness in alfalfa fields with increasing isolation. In diverse landscapes the quality of habitat fringes is, therefore, important for the interactions of habitats through species and the species richness of the habitats. Species interacting between different habitats can also influence ecosystem functions, e.g. seed dispersal or fructification of plants. For the pollinator-plant interaction Steffan-Dewenter and Tscharnke (1999) found that the proportion of large bees and bumblebees was higher in small habitats than in large habitats, which was attributed to the better flying ability of the larger species. Borders between ecosystems (ecotones) may either serve as guidelines for migrating animals or as barriers that inhibit the dispersion of species. Corridors between ecosystems are generally regarded as structural prerequisites to 'optimize the migration into isolated habitats. However, corridors with suboptimal living conditions are only used by a few species (Collinge 2000) and only over short distances (Haas 1995). Furthermore, the interactions between habitats depend on their sink or source qualities (Cornelsen et al. 1993; Irmler et al. 2000). In the present inquiries different borders between habitats were investigated: the transitional zone between terrestrial ecosystems and Lake Belau, the ecotones between the beech forest and the arable land, between the beech forests and the mixed forest on the slope, and between the littoral and pelagic of Lake Belau (cf. 1.5). Research focused on the quantification of movements across the habitat boundaries for different groups of animals and the function of boundaries for the orientation of animals. The interaction processes between habitats and at habitat margins were additionally simulated by models to analyse ecotone effects on the animal populations. Furthermore, model simulations should help to predict the influence of a future landscape development on biodiversity and ecosystem functions.JRC.G.4-Maritime affair