Marked compositional changes in harvestmen assemblages in Amazonian forest islands induced by a mega dam

Mega hydroelectric dams have become one of the main drivers of habitat loss in tropical forests, converting large tracts of pristine forests into isolated forest islands. Understanding how biodiversity cope with landscape modification in these archipelagic landscapes is of paramount importance to assess the environmental consequences of dam infrastructure and propose mitigation actions for biodiversity conservation. In this context, harvestmen (Opiliones, Arachnida) comprise a good indicator taxon of habitat quality, given their high sensitivity to desiccation and microclimatic change. Here, we investigate the effects of landscape change induced by a mega hydropower dam on forest harvestmen species richness, abundance and composition within the Balbina Hydroelectric Dam, Central Brazilian Amazon. Using both nocturnal search and diurnal beating tray surveys, we sampled 20 islands and five mainland continuous forests, relating our biological response variables to local, patch and landscape scale metrics. We captured 783 harvestmen representing 31 species, 22 genera and 9 families considering all sampling sites, with species composition differing across islands and mainland sites. Although species richness was unaffected by any local, patch and landscape variables, species composition and abundance were differentially affected by a set of predictor variables at different scales. Forest cover and fallen woody stems were significant predictors of species composition patterns, whereas vegetation density, forest cover, island area, abundance of palm trees, and fallen woody stems best explained harvestmen abundance. We recommend retaining large forest habitat patches surrounded by a great amount of forest cover to prevent the rapid decay in tree communities and minimize forest disturbance effects in large hydroelectric dams.