Surrounding vegetation matters for arthropods of small, natural patches of indigenous forest

Natural forests are important for terrestrial biodiversity conservation. In South Africa, many forest patches are naturally small and surrounded by either natural grassland or exotic plantations. Here we compared arthropod diversity and composition among natural forest patches, pine plantations and grassland. We also assess how arthropod assemblages respond to the environmental changes at the boundaries between natural forest patches and the two different surrounding vegetation types. Twenty transects were selected, ten natural forests surrounded by pines, and ten natural forests surrounded by natural grassland. Transects ran from natural forest interiors, across the boundary into the interior of the surrounding vegetation. Arthropods were sampled using pitfall traps and active searches. Natural forests surrounded by grassland had higher species richness than those surrounded by pines. Greater arthropod abundance was in grassland edges than other stations. Pairwise comparisons revealed significant differences in assemblages between all vegetation types. This implies that forest biodiversity in these patches is altered by the main vegetation type in the matrix or in close proximity. Qualitative differences in assemblages between natural forest‐grassland boundaries were greater than differences between natural forest‐pine boundaries, indicating greater sharing of species between pines and natural forests. Our findings emphasise the value of natural forest‐grassland edges for conserving arthropod diversity as they add to the local species pool. Overall, we show that when maintaining small isolated natural forest patches, the role of the surrounding vegetation must be considered as it has a major influence on the local patch fauna.