To assess land-use impacts, we need to quantify effects on different components of diversity, at different spatial scales. The species absent from a community but present in the species pool, so-called “dark diversity” are an important diversity component. The dark diversity concept has mostly been applied to plant communities, however, it is unclear how levels of dark diversity and their spatial distribution differ between trophic or functional groups. What are the relationships between dark diversity for different functional groups? What are the characteristics of the missing species? Is dark diversity greater at higher land-use intensity and in fragmented landscapes?
To answer these questions we estimated dark diversity based on species co-occurrence probabilities for different functional groups (plants, pollinators, herbivores, symbionts, predators, decomposers, tertiary consumers) in 300 plots. Data were collected in temperate forests (4120 species, including OTUs) and grasslands (3594 species) in the framework of the Biodiversity Exploratories project in Germany. We analysed correlations between the dark diversity of different functional groups. We then compared dispersal characteristics of species belonging to the dark diversity pool vs species present in the plots. We finally constructed linear models to estimate the effect of land use intensity and landscape fragmentation on dark diversity.
Dark diversity of belowground organisms was larger than dark diversity of aboveground groups. The patterns of pairwise relationships between different trophic groups were different when considering richness or dark diversity. For instance plant and herbivores richness were significantly correlated while there was no correlation between the dark diversity of these two groups. When considering all species together, dark diversity was positively correlated to land-use intensity and landscape fragmentation. However this effect was not consistent among groups of species. Pollinators, herbivores and autotrophs dark diversity increased significantly with land-use intensity while we did not find any significant effect of land-use intensity on decomposers or tertiary consumers. When corrected by richness differences, forests and grasslands showed similar levels of dark diversity but generally, landscape had stronger effects in grasslands than in forests compared to land-use intensity. All together, these results suggest that species pools are filtered differently for different trophic groups. Dispersal limitation plays an important role in both temperate grasslands and forests but land-use intensity also strongly filters the species pool. Analysing dark diversity brings a new and interesting information that complements our knowledge on community assembly in land-use intensity gradients.