Environmental filtering of agroforestry systems reduces the risk of biological invasion

Abstract

Prevention is the most effective way of mitigating the negative impacts of exotic species invasions on biodiversity. Preventative measures include inhibiting the further dispersal and recruitment of established invasive species. We expect coffee production in agroforestry systems to reduce the recruitment of exotic species relative to monoculture stands because intercropped trees function as environmental filters that select for individuals with traits such as shade tolerance. If ecologically similar species are also closely related phylogenetically, such environmental filters should reduce phylogenetic diversity, supporting the coexistence of closely related species. Here, we test whether the taxonomic and phylogenetic diversity of native and exotic species in agroforestry systems is lower than in monocultures. We sampled the understory in 180 plots distributed in both habitats in three study sites within the Araponga municipality, Brazil, and measured understory light availability. Phylogenetic diversity was calculated as the net relatedness index (NRI) and the nearest taxon index (NTI). Understory light availability was lower in agroforestry systems. Of the 48 species found, 25 occurred in agroforestry systems, three of which were exotics. Forty-two species were found in monoculture stands (including 13 exotics). As expected, NRI and NTI indicated lower phylogenetic diversity in agroforestry systems than in monoculture stands. Agroforestry systems provide less favorable habitats for exotic species, filtering them from the understory. The mechanisms that select against exotic species remain unknown; however, shading might exclude predominantly heliophytic exotic species from the agricultural matrix. Therefore, agroforestry systems may function as buffer zones preventing invasion into sensitive ecosystems.