Investigating the global drivers of earthworm species diversity on islands

Abstract

Large gaps exist in knowledge of the global patterns and drivers of biodiversity, particularly in belowground systems and island environments. Earthworms are a key group of soil invertebrates in many systems due to their ability to act as ecosystem engineers, yet our understanding of their distributions globally is limited. To investigate the drivers of earthworm species diversity on islands, I conducted literature searches to obtain lists of earthworm species on 379 islands. Earthworm species records were found for 99 of the searched islands, with many of the islands for which no species records were found comprising entire archipelagos. The relationship between earthworm species richness and each island’s underlying geology, latitude, distance from the mainland, temperature, precipitation, area, and plant species richness were analyzed using a zero-truncated negative binomial regression model. As predicted, earthworm species richness was positively correlated with island area, plant species richness, and mean annual precipitation and negatively correlated with distance from the mainland. Less expected results were a strong correlation between earthworm species richness and oceanic island geology, and temperature and latitude having no effect on earthworm species richness. Species composition on islands was also explored, and the most widespread species across islands was found to be Pontoscolex corethrurus, a dominant invader in the tropics. Overall, it is clear that earthworm species data is severely lacking for a great number of islands, and future research should focus on further sampling of islands chosen by model predictions. In addition, future work should investigate whether drivers differ for the diversity of native versus non-native species of earthworms on islands.