Two simultaneous processes are altering biodiversity in the Earth’s ecosystems, i.e., species losses through extinction, and species gain though biological invasion and range expansion. Many studies have explored, through experiments in which species richness is varied by random draws of species from a species pool, how the resultant changes in biodiversity may impact on ecosystem functioning. However, species are neither equal in their ecosystem impacts nor gained and lost at random by the community. As such, those species that are gained or lost by the community could have very different effects on ecosystem functioning to those comprising the remainder of the community. I will address how biodiversity change in natural ecosystems, as explored through manipulative and natural ‘removal experiments’, may impact on ecosystem functioning, and in doing so will explore how biodiversity change may be affected by species that have disproportionate effects. This will be done by discussing two natural study systems, i.e., biological invasion of New Zealand forests, and species change on lake islands in northern Sweden.
Results/Conclusions
In the New Zealand system I show how invasions by individual species or species groups profoundly transform the ecosystem when compared to the resident species. This includes invasive deer that remove high quality plant species from forest understories, invasive possums that remove native nitrogen-fixing shrubs and redirect succession, invasive rats that thwart nutrient inputs by native burrowing seabirds, and non-native riverbed plants that modify ecosystem processes excess of what is expected from their relative biomass contribution. In the Swedish system I show how the biomass-dominant plant group (trees) and two other plant groups that comprise a small fraction of the resident biomass (dwarf shrubs and feather mosses) each contribute in disproportionate ways to different ecological processes. I then describe the results of long term removal experiments which show that the relative importance of these groups (and of species within these groups) to ecosystem functioning is determined by ecological context. From these examples I make the case that species that are gained or lost frequently have transformative effects on natural ecosystems that greatly exceed their contribution to species richness, and that are therefore greater than predicted by experiments in which diversity is varied through random draws of species.