Stream distance and vegetation structure are among the major factors affecting various groups of arthropods in non-riparian chestnut forests

Abstract

Streams are likely to be increasingly important for the conservation of non-riparian forest diversity with increased aridity due to climate change in many parts of the world. However, the importance of streams as promoters of non-riparian forest diversity has not been explored in detail. Given the likely stronger effect of surrounding habitats on open forests, we examined chestnut woodlands in northern Spain (n = 32) to explore the contribution of streams to variation in arthropod measures. We captured 5,490 arthropods (21 orders and 60 families) from which we calculated total abundance and richness and the abundance of trophic guilds. Vegetation structure at the tree and understory levels often made the largest independent contributions to explained variation in arthropod numbers and effects suggested alterations in arthropods’ food-web. Negative effects of stream-distance on taxon richness and the abundance of predators, omnivores and phytophages seemed to be largely mediated by vegetation structure. The exception was a strong univariate inverse association between the abundance of aerial predators and stream distance, possibly because wasps might be exploiting riverine habitats. Overall, effects of stream-distance were weaker for ground than for aerial arthropod groups. Moreover, arthropod numbers were greater at sites with low tree species richness and tall and species-rich understories, raising concerns for the ecological consequences of the abandonment of traditional forest practices in these woodlands. There are likely to be advantages for arthropod diversity and ecosystem services (e.g. pest control, nutrient cycling, pollination) if non-riparian forests are managed in a way that depends on stream-distance. Inferences may improve with a more complete knowledge of the biology of poorly studied groups, such as flies.