Aim: Different hypothesis have been proposed to explain differences in species richness among islands. However, few studies have attempted to compare the explanatory power of multiple hypotheses using a large data set. Here we analyse how different types of predictors (energetic/climatic, environmental heterogeneity, island biogeography and anthropogenic) affect variation in dung beetle species richness on Mediterranean and Macaronesian islands.

Location: Mediterranean and Macaronesian islands.

Taxon: Dung beetles.

Methods: Using a large dataset of islands (n = 147), we extracted the species richness of dung beetles on each island using 362 bibliographic reference sources. We performed GLMs to analyse the relationship between the species richness of dung beetles and eleven explanatory variables (temperature, evapotranspiration, aridity, area, maximum elevation, connection to continent during LGM, geological origin, distance from continent, nearest continent, years since first human colonization and human density) representing four types of causal hypotheses. We also included as a covariate the number of published papers studying dung beetles as a surrogate of the survey/study effort carried out in each island.

Results: GLMs suggest that the years since first human colonization, the number of published papers, and island area were the predictors with a higher explanatory capacity. The volcanic character of the islands and the distance from the mainland had some relevance in the case of Scarabaeinae and Geotrupinae, and maximum elevation appeared relevant in the species richness of Scarabaeinae and Aphodiidae. The anthropogenic and island biogeography hypotheses on the variation in species richness were the ones that have the strongest explanatory capacity, regardless of the inclusion of the surrogate of survey effort as a covariate in the models.

Main conclusions: The long history of human movements and agricultural activities has facilitated the colonization of dung beetles and provided trophic resources for their persistence, leading to increased species richness. Thus, the importance of anthropogenic factors in shaping the biodiversity patterns of island biogeography cannot be ignored. These human-induced influences may play a fundamental role in altering the biogeographic patterns of islands, even overriding the importance of other variables. Consequently, our findings underline the profound impact of historical human actions on islands biodiversity.

The study focused on the islands of the Mediterranean and Macaronesian regions, totalling 147 islands. Islands were included in this study when they had faunistic studies or published records of dung beetle species. The great variability in the environmental features of all these islands maximize the capacity of finding geographical species richness patterns able to be explained by the used explanatory variables and factors.

The dung beetle species considered were those belonging to the superfamily Scarabaeoidea, which mainly feed on mammal faeces at any stage of their development. In other words, dung beetles strictly belong to the subfamilies Geotrupinae, Scarabaeinae, and Aphodiinae. However, in our analysis we used data of dung beetles sensu lato because we added data of all the family Aphodiidae, which also includes the subfamilies Psammodiinae, Aegialiinae, and Eremazinae, which have a saprophagous feeding behaviour. Thus, we studied here the geographical variation in the species richness of dung beetle species belonging to Geotrupinae, Scarabaeinae, and Aphodiidae.

All available published manuscripts concerning dung beetles in the Mediterranean and Macaronesian regions were compiled, such as atlases, books, peer-reviewed papers, and grey literature. Most of these references come from the authors’ exhaustive library. A total of 362 bibliographic reference sources published from 1838 to 2019 were used to extract dung beetle occurrence data in islands (see Supplementary Material S1 for more details). All these data were checked, and the nomenclature was standardized following an up-to-date taxonomical criterion avoiding subspecies, recent species splitting, and doubtful citations. All dung beetles species finally listed (n = 220) are considered native to the biogeographic region in which the islands studied are located.

The primary data used are provided in excel format.