Azorean arthropods do it fast in dark caves

Azorean arthropods have diversified according to the age, area and relative isolation of each island within the archipelago. However, each group experiences these factors differently.

This is the main result of a study conducted by Joaquín Hortal from the NERC Centre of Population Biology of the Imperial College, and Paulo Borges from the Azorean Biodiversity Group (CITA-A) of the University of the Azores, which is going to be published in the Journal of Biogeography.

The authors show that although the shape of the relationship between diversification and time is in general the same, different groups show different rhythms of evolution. They do so in the first independent evaluation of the General Dynamic Model of Oceanic Island Biogeography, recently proposed by Robert J. Whittaker and colleagues to merge the geological evolution of islands with the biological evolution happening on them. Paulo Borges and Joaquín Hortal used the framework provided by this new model to study the relationship between the number of species that are single island endemics (i.e., exclusive of each island) at the Azores, its age, and different measures of area and isolation for each island. The Azores is placed in the middle of the North Atlantic Ocean, and is composed by islands of volcanic origin but at different stages of geological development, that emerged from the sea in very different moments of time, from almost ten million to a few hundred thousand years. Thus, “the characteristics of the archipelago provide an excellent experiment for the study of evolutionary pattern and process”, says Joaquín Hortal.

Cave species have evolved quite fast, producing a number of species in the initial stages of development of the islands, when cave systems formed by lava tubes and volcanic pits are abundant and pristine due to the high volcanic activity. When the island settles, erosion takes part, so most lava tubes and pits start to collapse, diminishing dramatically the area available for the species living there. According to Paulo Borges “cave species eventually end up facing their extinction, for they have nowhere to run, except sometimes surviving in the little crevices of the soil under the forest, where the conditions are obviously not the ones they are used to”.

This rapid pace of diversification and early decline is exclusive of cave arthropods within the groups studied. When all arthropods or all beetles are considered, older islands show more exclusive species than younger ones. Only the older island, Santa Maria, shows some decrease in the pace of diversification. This evidences that in most islands some lineages are still evolving into new species. Such differences between groups are caused by the opposing roles of the two components of diversification. The rate at which diversification occurs reflects a trade-off between speciation and extinction processes. When speciation is predominant, diversification is positive and the number of endemic species on an island increases. This pace slows down as extinction takes the lead, and diversification gets slower and eventually negative when the islands age and erode and they start to loose species numbers. “While for most arthropods the Azores is a land full of opportunities, those inhabiting caves already feel the pressure of living in aging islands”, said Paulo Borges.

Other factors also affect the pace of diversification at the Azores. For example, how easy is for each particular species to disperse either from the continent or from other islands of the archipelago. Also, the diversification of groups with high dispersal ability shows no strong relationship with either the age or the isolation of the islands. All these differences point out that the diversity of evolutionary responses of organisms with different characteristics is so wide that no general model like the one proposed by Whittaker and colleagues is able to predict the pattern and process of diversification of all living groups. According to Joaquín Hortal, “what Whittaker’s model does, however, is to allow the integration of all the deviations from the general pattern into a general theoretical framework; by relating these deviations with the characteristics of each group, we might be able to ascertain how and why evolutionary processes happen on the isolated archipelagos that constitute some of the few long-term experiments provided by nature”.

Source Article: Borges, P. A. V., and J. Hortal. (2009). Factors driving arthropod speciation at the Azorean Archipelago. Journal of Biogeography, doi:10.1111/j.1365-2699.2008.01980.x.
Available online at http://www3.interscience.wiley.com/journal/121403376/abstract