Vicariance versus Dispersal: Chatham Islands

The flora and fauna composition of the Chatham Islands is distinctive from the New Zealand mainland and includes many endemic species. The uniqueness of these endemic species has arisen due to the 800 kilometres of open ocean separating the Islands from the mainland. There are multiple theories regarding the origin of the ancestors of Chatham Islands biota. The vicariance theory explains the cause of speciation as the separation since the areas were geographically connected in the past (now separated by plate tectonics). The dispersal theory states that founding taxa historically crossed a significant barrier by dispersal and the difference has arisen due to the subsequent separation. In the past, testing of these theories has been based on morphological differences. With the combination of the relatively new techniques of DNA sequencing and molecular clocks, the time period when species divergence occurred can be estimated, allowing better testing of these theories.

A study by Steven Trewick in the paper ‘Molecular evidence for dispersal rather than vicariance as the origin of flightless insect species on the Chatham Islands, New Zealand’ investigated the origin of Chatham Island biota using mitochondrial gene sequences to test the vicariance versus dispersal argument. The study had three main hypotheses for the origin of Chatham biota; Gondwanan vicariance, vicariance created by the loss of a land bridge, and recent oversea dispersal.

There are multiple theories regarding the most recent connection to the mainland. The Chathams may have adjoined the mainland in Gondwanan times over 70 million years ago. For the current fauna to have Gondwanan origins, the theory has to assume that the Chathams area did not submerge since disconnection. A more likely theory is that a post-Gondwanan land bridge may have existed linking the Chathams and southern New Zealand, that has since submerged. This theory is supported by the fact that high endemicity is primarily at the species level, implying relatively recent biological isolation of the Chathams.

For the study, genera found both on the Chathams and the mainland that contain large flightless species were selected . Taxa with these characteristics are expected to have a low dispersal ability over sea, making them susceptible to vicariance. Therefore, this was expected to exclude dispersal as a cause of origin, better testing the vicariance hypotheses. Two beetle genera (Mecodema and Geodorcus), a cave weta genus (Talitropsis) and one cockroach genus (Celatoblatta) were selected for the study. Within the selected genera, samples of all available species from the Chathams and species present at sampling locations on the mainland were collected for analysis. The mitochondrial gene, cytochrome oxidase 1 (CO1) was used as the source of the DNA sequences for the comparison.



Geodorcus spp. stag beetle exemplifying the large size and flightlessness of this genus. (Photo: Ian Phillipps)


It was found that the Chatham Islands taxa are closely related to New Zealand taxa and in the case of Mecodema, are not genetically separable as distinct species. Chatham diversity appears to be a subset of New Zealand diversity with all genera colonising the Islands over one time period. This episode of colonisation was estimated using the observed divergence within the genera and the standard molecular clock calibrations of 2–2.3% sequence divergence per million years. It seems that the colonization occurred within the Pliocene period from 2-6 million years ago, discounting the Gondwanan vicariance hypothesis.

With the important addition of a colonisation time frame, the land bridging hypothesis can also be discounted, as geology has so far failed to reveal evidence of any such structure during the Pliocene. Therefore, the ancestors of current taxa seem to have arrived by oversea dispersal. The study targeted taxa with little potential for active migration over water. However, it may be that the genera have a susceptibility to passive dispersal. Mecodema and Geodorcus beetles have their larval stage within decaying logs, and Talitropsis and Celatoblatta often have their daytime roosts in logs. Therefore, it is possible that these insects travelled to the Chathams in floating logs.

The lack of deeply diverging lineages among the studied genera may be caused by pre-Pliocene submergence of the island group. This could have removed the species that occupied the Chatham Islands before the current biota colonised. Therefore, despite the new revelations allowed by molecular techniques, it can only be concluded that the current observed biota of the Chathams arrived by dispersal. The origin of biota that possibly preceded the suggested submergence event still remains unsolved.

The full article for the study can be found in the Journal of Biogeography, volume 27, issue 5, pages 1189-1200. A link to the article can be found here.