Microclimate and vegetation structure shape harvestman (Opiliones) assemblages in an isolated forest fragment and adjacent potential biological corridors

The conservation value of forest fragments is highly dependent on the internal habitat quality and functional connectivity. Biological corridors are widely used to mitigate fragmentation, but their efficacy for many arthropod groups, including harvestmen, remains understudied in fine-scale agricultural landscapes. We assessed how microclimatic and vegetation structural conditions influenced ground-living harvestman (Opiliones) assemblages in the protected floodplain forest fragment Jarovská bažantnica (Slovakia) and in adjacent linear habitats of varying width that could potentially serve as biological corridors. A total of 3,067 individuals representing 11 species were captured. Multivariate ordination revealed that surface air temperature was the sole significant predictor structuring overall assemblage composition, segregating xero-tolerant species in warmer microsites from moisture-dependent species in cooler microsites. Coverage-standardised species richness (Hill q = 0) increased significantly with higher tree-layer cover and decreased with increasing minimum surface temperature. Effective Shannon diversity (Hill q = 1) was positively associated with litter depth and negatively with increasing mean surface temperature. Activity-density, expressed as captures per trap-day, decreased with increasing soil moisture. Amongst the three linear habitats examined, the widest corridor (15 m) supported the highest coverage-standardised richness and effective diversity, whereas 5 m and 10 m elements exhibited lower values. The 15 m element also most closely resembled forest-interior conditions in terms of vegetation structure and microclimate and observed effective diversity. Overall, our results indicate that harvestman assemblages in this fragmented landscape are primarily associated with fine-scale microclimatic and vegetation-structural conditions. Wider and structurally complex linear habitats may provide more suitable conditions for forest associated harvestmen. However, because our study is limited to a single forest fragment and only one linear habitat represented each width, these findings should be interpreted as site-specific patterns rather than general evidence of corridor effectiveness or a minimum functional width.