Species richness and parasitism in a fragmented landscape: experiments andfield studies with insects on Vicia sepium

Effects of habitat fragmentation on species diversity and herbivore-parasit oid interactions were analyzed using the insect community of seed feeders a nd their parasitoids in the pods of the bush vetch (Vicia sepium L.). Field studies were carried out on 18 old meadows differing in area and isolation . The area of these meadows was found to be the major determinant of specie s diversity and population abundance of endophagous insects. Effects of iso lation were further analyzed experimentally mentally using 16 small plots w ith potted vetch plants isolated by 100-500 m from vetch populations on lar ge old meadows. The results showed that colonization success greatly decrea sed with increasing isolation. In both cases, insect species were not equal ly affected. Parasitoids suffered more from habitat loss and isolation than their phytophagous hosts. Minimum area requirements, calculated from logis tic regressions, were higher for parasitoids than for herbivores. In additi on, percent parasitism of the herbivores significantly decreased with area loss and increasing isolation of Vicia sepium plots, supporting the trophic -level hypothesis of island biogeography. Species with high rates of absenc e on meadows and isolated plant plots were not only characterized by their high trophic level, but also by low abundance and high spatial population v ariability. Thus conservation of large and less isolated habitat remnants e nhances species diversity and parasitism of potential pest insects, i.e., t he stability of ecosystem functions.