The influence of landscape fragmentation, expressed by the 'Effective Mesh Size Index', on regional patterns of vascular plant species richness in Lower Saxony, Germany

Landscape fragmentation has been identified as a major threat to biodiversity worldwide. Several landscape metrics have been developed to quantify the extent of fragmentation, of which the Effective Mesh Size Index (meff) is one of the most widely used. However, its relevance for analysing the effect of fragmentation on biodiversity patterns has been rarely tested. We analysed the explanatory power of meff for richness patterns of different groups of vascular plant species (all species, and species groups by naturalisation and threat status) in Lower Saxony, Germany, by using a grid of 1386 analysis units. Since we assumed species richness to be influenced by abiotic conditions and spatial autocorrelation, we used variation partitioning to separate the effects of these variables from that of fragmentation. We tested five types of meff based on various fragmentation geometries. We found that meff largely influenced richness of neophytes and, to a lesser extent, that of archaeophytes whilst the richness of native species was only slightly affected and threatened species were not affected. All species groups, except threatened species, showed a negative correlation with meff, i.e. richness was highest in highly fragmented and lowest in less fragmented grid cells. We conclude that meff is a meaningful tool to explain richness patterns of non-native plant species, if relevant fragmentation geometries are chosen, but not of native and threatened plant species. Our approach may help future studies to determine correct fragmentation geometries to use with meff and may facilitate the unravelling of fragmentation impacts on the landscape-scale.