Effects of habitat fragmentation on population structure and fitness components of the wetland specialist Swertia perennis L. (Gentianaceae)

We assessed the effects of habitat fragmentation on the demography of the locally abundant, long-lived fen plant Swertia perennis L., by examining population structure, components of fitness and herbivory of 17 populations in Switzerland. We distinguished between large MAIN, small, barely isolated NEAR and small, geographically isolated DISTANT habitat islands. Large isolation distance of small sites significantly affected the transition from sub-adult to adult age states: the mean population density was reduced by 30%, the density of vegetative adults by 60% and the density of reproductive adults by 78% on DISTANT compared with NEAR islands. Seedling survivorship decreased significantly from MAIN to NEAR to DISTANT islands. Moreover, several vegetative components of fitness were significantly reduced on DISTANT islands, i.e. the length and number of leaves and the stern height. Finally, leaf herbivory was highest on small NEAR islands. Our results indicate that population viability of S. perennis is reduced on small, isolated fens, possibly because of arrested individual adult growth due to inbreeding. In contrast, environmental influences such as higher productivity through increased nutrient influx on small sites may be responsible for changed plant-herbivore interactions.