Short-term succession of aquatic plant species richness along ecosystem productivity and dispersal gradients in shallow lakes
The highest species richness is usually expected at an intermediate stage of development since the last major disturbance event, but some studies have shown that ecosystem productivity and dispersal may modify this pattern, suggesting the need for further studies on the effects of productivity and dispersal on the dynamics of species richness through succession. In this study, we analysed aquatic plant species richness in relation to (1) succession stage, measured as numbers of years since the last disturbance that affected the ecosystems; (2) lake productivity, measured as the chlorophyll a concentration; and (3) connectivity to similar nearby ecosystems, a proxy for the potential input of diaspores.
Shallow lakes of the Dombes region, France.
Every 5–7 yr these shallow lakes are emptied and left to dry out for 1 yr. These drought disturbances lead to complete destruction of the submerged aquatic plant communities. Sixty lakes arranged along a gradient of productivity were selected. The probability of diaspore input was considered to increase from upstream to downstream, as lakes are organized in hydrologically connected networks via ditches, through which the downstream lakes receive water from the upstream lakes. For each lake, the aquatic plant species richness (from systematic summer vegetation sampling), time since the last disturbance (last summer drying), productivity (estimated as chlorophyll a concentration) and probability of diaspore input (assessed from position in the network) were recorded.
The aquatic plant species richness decreased with the time since the last disturbance for all of the lakes, but there was a significant interaction with the chlorophyll a concentration and position of the lake in the network. At the lowest ecosystem productivities, the relationship between successional stage and species richness was hump-shaped, whereas the species richness decreased with increasing time since the last disturbance when productivity increased. The lake's position in the network did not influence species richness during the first 2 yr after disturbance, but from year 3 and thereafter, lakes connected to high numbers of upstream lakes consistently exhibited decreased richness, contradicting the expected trend of increasing species richness with increasing diaspore inputs.
This study indicates that both ecosystem productivity and connectivity strongly affected the relationship between aquatic plant species richness and succession, and that these factors should be taken into account in further developments of the intermediate disturbance hypothesis.