Grassland connectivity explains entomophilous plant species assemblages in an agricultural landscape of the Pampa Region, Argentina

摘要

The Pampa grassland of Argentina is one of the most highly threatened biomes in the world. A hig hproportion of the original grassland cover has been transformed into land for agriculture or degraded. In the southern part of the region, fragmented semi-natural grasslands over exposed rock still persist and connectivity between them is assumed to be crucial for maintaining viable populations. We quantified overall connectivity of grassland patches in a sector of the Southern Pampa region, and investigated the degree to which landscape connectivity explains entomophilous plant species assemblages in a subset of patches. We characterized each of the301 patches in the landscape by their degree of intra-patch and inter-patch connectivity based on graph theory,and considering threshold dispersal distances from 100 to 1000 m. We surveyed entomophilous plant species in39 grassland patches and classified the species in three categories (annual herbs, perennial herbs and shrubs)considering their different growth form and longevity. The influence of connectivity variables on entomophilous plant species assemblages variation was explored using Canonical Correspondence Analysis. Although grassland patches were poorly connected at all threshold distances, some of them were found to be critical for global connectivity. Connectivity significantly explained total, annual-biennial and shrub assemblages for all threshold dispersal distances (6–13% of total variation). Variation in annual species assemblages was associated with intra-patch and inter-patch connectivity at short distance (100 m), while variation in shrub species assemblages was explained by intra-patch and inter-patch connectivity for distances between 100 m and 1000 m. This study evidenced the low connectivity of the study system, allowed the identification of critical areas for conservation, and provided valuable information to develop management strategies in increasingly human-dominated landscapes