All aboard the emergent ark: biogeography of the dune insect fauna of New Zealand and Chatham Island

Abstract

Oceanic islands, and the processes by which they are colonised from mainland source populations of flora and fauna, have been a major component of biogeographical research for over half a century. This study addresses the biogeographical relationship between the coastal dune insect fauna of mainland New Zealand (NZ) and those of Chatham Island (CI), an outlying oceanic island some 850 km distant across open ocean. I conducted a comprehensive entomological survey of Coleoptera, Diptera and Hymenoptera from the coastal dune habitat of the NZ east coast and CI. Over 5000 insect samples were collected, consisting of over 650 species. Comparison of the NZ and CI insect communities using the survey data showed that CI has a lower number of insect species per site than NZ. The degree to which the CI diversity was lower approximately equated with the relative dune area available in NZ and CI. Re-sampling techniques were used to create a null hypothesis assuming random immigration from NZ to CI. The observed number of taxa found on CI was compared to the null hypothesis. CI was found to have significantly fewer Hymenoptera and significantly more Diptera than NZ. The likelihood of a specific taxon immigrating successfully varied considerably between taxa. The higher proportions of Diptera and lower proportions of Hymenoptera on CI appeared to be driven by a few very successful or unsuccessful taxonomic groups. There was no clear effect due to taxonomic order on immigration likelihood. Factors which were deemed to have a potential effect on immigration success were modelled using GLM models. Abiotic and biotic effects were modelled against immigration likelihood. The factor most influencing immigration success were the relative difference in mean temperature, taxa with an NZ range which included sites of a similar mean temperature to the CI had a higher the probability of successful immigration to CI. Immigration success was also found to be significantly influenced by the number of sites a taxon had been collected from. The more widely distributed taxa were the most successful immigrants. Genetic analysis of mitochondrial cytochrome oxidase I sequences from six taxon groups showed that genetic divergence has occurred between NZ and CI populations. All but one of the taxon groups have a distinct monophyletic CI population. Two of the CI populations appear to derive from a NZ SI population. Randomisation was also used to detect whether the barcoding gap between NZ and CI populations varied significantly from that of randomised samples. For all the taxa tested the genetic divergence between NZ and CI populations was significantly different from a randomly determined sample pool. The results of this project provide valuable information for the ecology and conservation of NZ dune systems and extend the entomological knowledge of these rapidly declining areas of habitat and have given insight into the biogeographical relationship between NZ and CI dune insect populations and the factors which influence immigration between NZ and CI.