Invasion disharmony in the global biogeography of native and non-native beetle species

Aim
The concept of “island disharmony” has been widely applied to describe the systematic over- and under-representation of taxa on islands compared to mainland regions. Here, we explore an extension of that concept to biological invasions. We compare biogeographical patterns in native and non-native beetle (Coleoptera) assemblages from around the world to test whether beetle invasions represent a random sample of species or whether some families are more prone to invade than others.

Methods
Numbers of non-native beetle species established in ten regions worldwide were compared with the land area of each region. The distribution of species among families was compared with the distribution among families for all species native to the same region and with the distribution among families for the global pool of all known beetle species. Ordination analysis was used to characterize differences among native and non-native assemblages based upon the distribution of species among families.

Results
We report a total of 1,967 non-native beetle species across all ten regions, and a classic log–log relationship between numbers of species per region and land area though relationships are generally stronger for native assemblages. Some families (e.g., Dermestidae and Bostrichidae) are over-represented and others (e.g., Carabidae, Scarabaeidae and Buprestidae) are under-represented in non-native assemblages. The distribution of species among families is generally similar among native assemblages with greatest similarities among nearby regions. In contrast, non-native species assemblages are more similar to each other than to native species assemblages.

Main conclusions
Certain families are over-represented, and others are under-represented in non-native beetle assemblages compared to native assemblages, indicating “invasion disharmony” in the global representation of beetle families. Similarities in composition among non-native assemblages may reflect unobserved associations with invasion pathways and life-history traits that shape invasion success of different insect groups.