Separating the chance effect from other diversity effects in the functioning of plant communities

The effect of plant species diversity on productivity and competitive abili ty was studied in an experiment carried out simultaneously in five European countries: Czech Republic (CZ), the Netherlands (NL), Sweden (SE), Spain ( SP), and United Kingdom (UK). The aim was to separate the 'chance' or 'samp ling effect' (increasing the number of sown species increases the probabili ty that a species able 'to do a job' will be included) from the complementa rity effect (species-rich communities are better able to exploit resources and to take care of ecosystem functions than species-poor communities). In the experiment, low diversity (LD) and high diversity (HD) mixtures of gras sland species were sown into fields taken out of arable cultivation. The HD mixture consisted of five grass species, five legumes and five other forbs . The LD mixtures consisted of two grasses, one legume and one other forb, with different plant species combinations in each replicate block. The desi gn of the experiment was constructed in such a way that the total number of seeds of each species over all the replications was exactly the same in HD and LD treatments, and the total number of grass seeds, leguminous seeds a nd other forb seeds were the same in both LD and HD. The responses measured were the total above-ground biomass las a measure of productivity) and the average number of naturally establishing species in a plot las a measure o f the competitive ability of the mixture), both measured in the third year of the experiment. The results show that, on average, the HD plots performe d better (i.e., attained higher biomass, had better weed suppression), but that the best LD mixture was as good as the best HD mixture. On the contrar y, the worst LD mixture was always less successful than the worst HD replic ate. The performance of particular species in the HD mixtures was a good pr edictor of the success of a certain species combination in a LD mixture (ex plaining 61% of variability between particular LD mixtures). In all sites, the LD mixture composed of species which were the most abundant in HD mixtu res was as efficient in suppressing weeds as the HD mixture itself. It is a rgued that the performance of a species assemblage is influenced mostly by the identity of species and the diversity effect is mainly due to the 'chan ce' or 'sampling' effect with increasing number of species the probability that an important species will be included in the mixture increases. Cautio n is urged in interpreting experiments with manipulated diversity and the p ossible limitations of such experiments are discussed.