Mixed biodiversity benefits of agri-environment schemes in five European countries.

Agri-environment schemes are an increasingly important tool for the maintenance and restoration of farmland biodiversity in Europe but their ecological effects are poorly known. Scheme design is partly based on non-ecological considerations and poses important restrictions on evaluation studies. We describe a robust approach to evaluate agri-environment schemes and use it to evaluate the biodiversity effects of agri-environment schemes in five European countries. We compared species density of vascular plants, birds, bees, grasshoppers and crickets, and spiders on 202 paired fields, one with an agri-environment scheme, the other conventionally managed. In all countries, agri-environment schemes had marginal to moderately positive effects on biodiversity. However, uncommon species benefited in only two of five countries and species listed in Red Data Books rarely benefited from agri-environment schemes. Scheme objectives may need to differentiate between biodiversity of common species that can be enhanced with relatively simple modifications in farming practices and diversity or abundance of endangered species which require more elaborate conservation measures.

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

Effects of habitat fragmentation on species diversity and herbivore-parasitoid interactions were analyzed using the insect community of seed feeders and 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 species diversity and population abundance of endophagous insects. Effects of isolation were further analyzed experimentally using 16 small plots with potted vetch plants isolated by 100-500 m from vetch populations on large old meadows. The results showed that colonization success greatly decreased with increasing isolation. In both cases, insect species were not equally affected. Parasitoids suffered more from habitat loss and isolation than their phytophagous hosts. Minimum area requirements, calculated from logistic regressions, were higher for parasitoids than for herbivores. In addition, 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 absence on meadows and isolated plant plots were not only characterized by their high trophic level, but also by low abundance and high spatial population variability. Thus conservation of large and less isolated habitat remnants enhances species diversity and parasitism of potential pest insects, i.e., the stability of ecosystem functions.

Habitat fragmentation, species loss, and biological control.

Fragmentation of habitats in the agricultural landscape is a major threat to biological diversity, which is greatly determined by insects. Isolation of habitat fragments resulted in decreased numbers of species as well as reduced effects of natural enemies. Manually established islands of red clover were colonized by most available herbivore species but few parasitoid species. Thus, herbivores were greatly released from parasitism, experiencing only 19 to 60 percent of the parasitism of nonisolated populations. Species failing to successfully colonize isolated islands were characterized by small and highly variable populations. Accordingly, lack of habitat connectivity released insects from predator control.