Ecosystem processes and the regeneration niche

Abstract

Removal experiments, in which certain organisms are eliminated from established natural communities, have received renewed attention as tools for assessing the effects of functional biodiversity on ecosystem processes. The approach has several advantages (e.g. realism), but also some drawbacks. One of the drawbacks is that, when an established species is removed from a community and an ecosystem effect is detected, it is difficult to distinguish between the effect of the absence of the removed organism and the effect of the act of removal. This is particularly true for terrestrial plant communities in which removing whole plants inevitably causes major soil disturbance. Amy Symstad and David Tilman have just presented a fine example of another aspect to consider in the interpretation of removal experiments 1xDiversity loss, recruitment limitation, and ecosystem functioning: lessons learned from a removal experiment. Symstad, A.J. and Tilman, D. Oikos. 2001; 82: 424–435CrossrefSee all References1: recruitment limitation, that is, the effect of the species that gets into the space – and takes up the resources – left behind by the removed species.They performed a plant-removal experiment in which they manipulated functional group diversity (forbs, cool-season C3 graminoids and warm-season C4 graminoids) and composition of a sand prairie–grassland in Minnesota, USA. They monitored the effects of these manipulations on ecosystem properties, such as plant biomass production, nitrogen dynamics in the soil and community resistance to a simulated drought, and found that the removal of different functional types affected ecosystem function. However, rather than attributing these responses to the absence of the functional groups that had been eliminated, the authors attributed them to differential recruitment abilities of the remaining functional groups. In plots where forbs and C3 graminoids were removed, there was a larger proportion of open ground, and higher nitrogen leaching from the soil, than in plots from which forbs and C4 graminoids were removed. That was because C4 graminoids (remaining in the first case) have stronger recruitment limitations, that is, occupy free space and thus take up available resources more slowly than do C3 graminoids (remaining in the second case), which are able to fill in the gaps much more quickly.Does this mean that removal experiments are basically flawed? I believe not. On the contrary, this study adds a new dimension to the search for causal links between biodiversity and the way ecosystems work. Traditionally, biodiversity–ecosystem function studies have focused on the niche of established plants. The fact that recruitment limitation can influence ecosystem processes calls attention to the regeneration niche (sensu Grubb 1977) 2xThe maintenance of species richness in plant communities: the importance of the regeneration niche. Grubb, P.J. Biol. Rev. 1977; 52: 107–145CrossrefSee all References2. Differences among organisms in their local recruitment ability, hardly taken on board in experimental studies in this area, might be important in determining at least the transient effects of diversity loss on ecosystem function. If the fast-growing research field of biodiversity and ecosystem function is to contribute to the understanding and management of natural ecosystems, it needs a diversity of approaches. The study by Symstad and Tilman adds to that diversity by stressing that not only what is lost, and how many different species are involved, but also what gets in and how fast, are functionally important.