Species richness effects on ecosystem functioning increase with time in an ephemeral resource system

Extrapolating the results from experiments on effects of species loss to natural systems is difficult since most studies, with the exception of microbial microcosm studies, have been performed on relatively short time-scales. One problem with short-term, experimental studies is that complex interactions between species in natural systems may alter the importance of underlying mechanisms over time thus making the effects observed in the experiments representative of a transient phase. Although some evidence indicates that initial effects of species richness observed in short-term experiments may be persistent, other results suggest that effects may change over time. Using detritivorous insect larvae in freshwater mesocosms, where both facilitation and niche differentiation mechanisms were operating and where resources were gradually decreasing, I examined the effects of species richness over time. I found that the initial effect of species richness on ecosystem function was enhanced over time when systems of high and low diversity were compared. There was an overall effect of species composition on larval growth, and when analysing the growth of each species separately it was found that one species was significantly affected by species richness, one showed a marginally insignificant effect of species richness, and the third species was significantly, differently affected by species richness over time. Thus, the results show that species loss initially leading to small, negative, or even undetectable, effects on ecosystem function may increase in importance over the long-term. (c) 2005 Elsevier SAS. All rights reserved.