RELATING SPECIES-DIVERSITY TO ECOSYSTEM FUNCTIONING - MECHANISTIC BACKGROUNDS AND EXPERIMENTAL APPROACH WITH A DECOMPOSER FOOD-WEB

Hypotheses have recently been formulated to elucidate the relationship between species diversity and ecosystem functioning. Using previously published mechanisms as a starting point we suggest that common mecha nisms can be provided for this relationship by using the concepts of n iche and trophic-level dynamics. The reasoning is the following: if re maining species within a trophic level can modify their niches as othe r species disappear, production within the level remains unchanged, wh ereas in the absence of niche modification production decreases. Decre ased production within a trophic level affects biomass and production at other levels as predicted by trophic-dynamic models. Changes in bio mass and production finally bring about changes in ecosystem functioni ng. In the redundant species hypothesis remaining species can modify t heir niches, and so functioning remains unchanged. In the predictable change hypothesis (our counterpart for the rivet hypothesis), and in t he idiosyncratic response hypothesis, remaining species cannot modify their niches, leading to predictable and unpredictable changes in func tioning, respectively. Unpredictable changes are due to differences in the characteristics of species and indirect interactions between popu lations. We tested the hypotheses and the suggested mechanisms using a soil food web with three trophic levels: microbes, microbivorous nema todes and a predatory nematode. We established one diverse (3 bacteriv ores and 3 fungivores) and three simple (1 bacterivore and 1 fungivore ) food webs and found that differences in trophic-level biomasses betw een the diverse and simple food webs were idiosyncratic. Unpredictabil ity resulted from differences in microbivore characteristics - their e fficiency in resource utilisation and vulnerability to predation and c ompetition. Changes in microbial respiration and total mineralisation of C and N, i.e., system functioning, were also idiosyncratic rather t han redundant or predictable when diversity was reduced, although idio syncracy was not as clear as in the case of trophic-level biomasses. W e conclude that predicting the influence of declining species diversit y on trophic-level dynamics and ecosystem processes is difficult, at l east in food webs with a small initial number of species, unless the c haracteristics of species and the nature of their interactions are kno wn.