Understanding small-scale insect diversity patterns inside two nature reserves: the role of local and landscape factors

Insect decline has become a major topic in scientific research, yet the relative roles of multiple factors on insect communities are still incompletely known. Our aim was to elucidate the significance of variation in local habitat quality and landscape context on moth diversity in two nature reserves in North-eastern Italy. We evaluated 14 local descriptors like plant diversity or forest structure, and 14 landscape-scale components like habitat diversity and the distance to forest edges, condensed through a Principal Component Analysis. PC-axes served as predictors in linear mixed effects models, with moth diversity and catch size (corrected for the influence of temperature and humidity) as response variables. Furthermore, sites were sorted into four groups: ‘high habitat quality’, ‘high landscape diversity’, ‘moderate quality’ and ‘low quality’. This was done to test whether local or landscape factors interact in modulating insect diversity. Our results indicate a strong influence of local factors, especially plant richness and biomass, on small-scale moth diversity. High diversity of nearby natural habitats also had a positive effect, while there was no correlation with landscape-scale attributes. Contrastingly, moth numbers were influenced by landscape diversity in 500–1000 m radius. So, although high local habitat quality supports higher moth alpha diversity, it is also important to maintain the diversity of natural habitats on the landscape-scale to preserve insect biomass and gamma diversity inside isolated reserves. Conservation efforts should be directed to preserve high vegetation and habitat diversity within reserves, while simultaneously keeping more varied landscapes around reserves might help stabilizing local insect assemblages.