Islands are a bit of an evolutionary oddity in nature, after all the majority of species that manage to successfully colonise islands tend to go extinct rather fast. The insularity and isolation of colonizing species tends to lead to high levels of specialism that increase the vulnerability of colonizers to changes in their environments. Islands, therefore, tend to have a one way (unidirectional) movement of species from continents to islands with very little exchange occurring in the other direction.
In the attempt to explain why this occurs several hypotheses have arisen, most notably that of the Taxon Cycle coined by E. O. Wilson in 1961 when examining Pacific ant species. Many other authors have modified Wilson’s original theory to fit other species and other islands leading to several different versions of the Taxon Cycle. Whilst each is slightly different the main premise of each is as follows:
- Evolution takes place on the Island in question
- The colonizer undergoes niche shifts
- Niche shifts are partly driven by competition from newly arrived species
Melanesian Ants and the Three Stage Taxon Cycle
Whilst Wilson was observing Melanesian ant species that inhabit geographically isolated, high elevation niches he came to realise that the ants of Melanesia may have descended from more geographically widespread, low elevation and disturbance tolerant ancestors. In order to prove this the case Wilson had to come up with a way of explaining how such a shift in the niche of the ancestor population could have occurred on the Melanesian Islands. He came up with the Three Stage Taxon Cycle
Stage 1: In stage one the species to be found are those that dominate the marginal habitats and have good dispersal abilities. These species tend to be commonly found throughout the marginal habitats of Melanesia. These species tend to be the same species or species closely related to those found in Asia.
Stage 2: The widespread coloniser moves into new habitats on the islands (mainly rainforests) becoming specialized to the new niche becoming a subspecies of the original coloniser. At this point newly arrived colonisers may arrive and compete with these subspecies for resources forcing either extinction or further niche change.
Stage 3: The colonising species has evolved to the point where it no longer resembles its mainland counterparts becoming a new species in its own right. By this point the ant species has lost much of its dispersal ability due to ever increasing specialisation and therefore little return dispersal occurs.
Whilst the Taxon Cycle provides a convincing framework for the unidirectional movement of species from continents to islands it is not without problems. Most importantly, the Taxon Cycle tends to focus on biotic forcing factors rather than abiotic factors from a changing environment. Environments are assumed to be stable without experiencing changes that greatly effect the species present and therefore their distributions. Evidence suggests that the environment of some Islands was dramatically different during the last period of glaciation. New Providence Island, for example, contained far more Xeric (arid, with little moisture) habitats during the last glaciation which meant that species preferring Xeric habitats may have had far greater distributions in the past. The loss of avifauna (Flying Animals) on New Providence Island is thought to be as high as 50% after the end of the glacial period and the return to wetter climates. Whilst competition is important in understanding the evolutionary history of islands it is equally important to focus on environmental changes that often have just as dramatic effects on species mortality.