Update: The way back journey: islands as sources of biodiversity

Abstract

news and update ISSN 1948‐6596 update The way back journey: islands as sources of biodiversity Until very recently, biogeographers and ecologists considered continents as the main source of biodi‐ versity for islands. This notion of islands as the end of the colonization journey was held mainly for two reasons (Bellemain and Ricklefs 2008). First, species colonizing islands from the mainland may lose dispersal ability making the process of reverse colonization less likely. Second, continen‐ tal populations evolve under intense interspecific competition and a high diversity of predators and pathogens, whereas island taxa usually evolve in a low‐competition environment and in the absence of such natural enemies; this makes mainland habitats more difficult to invade by island popula‐ tions. However, Bellemain and Ricklefs (2008) sug‐ gested that the way back journey to the continent should happen more often than previously thought, and this may be crucial for the mainte‐ nance of biodiversity on continents. A recent study by Hutsemekers et al. (2011), based on population genetic analysis of the spore‐producing moss Platyhypnidium ri‐ parioides from North Atlantic archipelagos, pro‐ vided strong evidence that (i) this bryophyte had not lost dispersal ability in island populations; (ii) the migration rates from continent to islands and vice versa were statistically similar; and (iii) conti‐ nental, instead of island populations, experienced a severe bottleneck during the last glacial maxi‐ mum. According to their analyses, the sea is not a major barrier for migration by island mosses. They did find a geographical pattern in genetic diversity in P. riparioides within the archipelagos of the Azores, Canary Islands, and Madeira, but it was not different from the isolation by distance pat‐ tern found for the Iberian Peninsula and Morocco. Moreover, although present air currents have fa‐ vored island colonization, the emergence of sea‐ mounts in the region may have acted as stepping stones between the archipelagos and the conti‐ nent. The major implications of their findings go beyond the biogeography of spore‐producing plants. Although bryophytes can be regarded as among the best candidates to show reverse colo‐ nization—due to their high dispersal ability and widespread distributions—these results alto‐ gether suggest that islands have the potential to act as refugia. Population size on islands can re‐ main constant in periods of intense disturbance on the continent, and such refugial biodiversity can be the source for recolonization of mainland areas. This link was made by Hutsemekers et al. (2011) by showing that, opposed to what hap‐ pened in the archipelagos, there was a decrease of suitable areas in the Iberian Peninsula during the last glacial maximum, and that island‐ continent air currents might have predominated during the arid phases of the last glaciation. The challenge now is to test for similar patterns for other groups—especially those with active disper‐ sal such as birds, bats and insects—and other ar‐ chipelagos with varying distances to the continent to test the generality of these findings. Testing the ‘way back journey’ and other emerging hypothe‐ ses in oceanic island biogeography has the poten‐ tial not only to improve our understanding about the factors that affect diversity across insular com‐ munities, but also to help the development of a comprehensive new model of biogeography (Heaney 2007). Tadeu Siqueira UNESP – Universidade Estadual Paulista, Departamento de Ecologia, Rio Claro, SP, Brazil. tsiqueira@rc.unesp.br References Bellemain, E. & Ricklefs, R. (2008) Are islands the end of the colonization road? Trends in Ecology and Evolution, 23, 461–468. Heaney, L.R. (2007) Is a new paradigm emerging for oceanic island biogeography? Journal of Bio‐ geography, 34, 753–757. Hutsemekers, V., Szovenyi, P., Shaw, A.J., Gonzalez‐ Mancebo, J.‐M., Munoz, J. & Vanderpoorten, A. (2011) Oceanic islands are not sinks of biodiver‐ sity in spore‐producing plants. Proceedings of the National Academy of Sciences USA, 108, Edited by Marcus Cianciaruso frontiers of biogeography 4.2, 2012 — © 2012 the authors; journal compilation © 2012 The International Biogeography Society