Do pinch points in conservation corridors matter? A test case using localized butterflies and grasshoppers

Conservation corridors are important for supporting biodiversity conservation across many transformed landscapes. However, these corridors vary in terms of their structural layout. Constrictions, or pinch-points, are localized narrowing of corridor width and in extreme cases the corridors are blocked off, creating cul-de sacs. Little is known about the effect of pinch-points on biodiversity within conservation corridors. We focus here on pinch-points along remnant natural grassland corridors designed to mitigate the effects of adjacent plantation stands. We use grasshoppers and butterflies to assess this constriction effect, as these taxa are known to be sensitive to subtle changes in habitat. Effects of constriction size and habitat quality were assessed for butterfly and grasshopper species richness, abundance, and assemblage composition. Three pinch-point types were selected (narrow (< 50 m wide), wide (> 50 m wide), and cul-de-sac (one entrance/exit)). These were compared with open grassland reference sites. Cul-de-sacs significantly reduced butterfly and grasshopper abundance due to their inability to connect existing remnant grassland areas along and because they promote high levels of alien bramble, Rubus cuneifolius. Wide pinch-points supported the most species-rich butterfly assemblages, while grasshopper species preferred narrow pinch-points. Butterflies responded more strongly to environmental changes than grasshoppers, with bare ground, rocky areas, and R. cuneifolius cover being the most significant predictors. As the wide and narrow pinch points are present for commercial timber production purposes, yet have minimal adverse effect on the insects, they can be largely left without any concern for biodiversity. However, where possible cul-de-sacs should be opened at the closed end to improve structural and functional connectivity.