Only a few parasites thrive on islands
A study of avian malaria parasites in the Canary and Madeira archipelagos shows that the typical characteristics of island communities of parasites can evolve very quickly.
6 JUL 2014 - 20:56 CET
The role of islands as natural laboratories for studying evolutionary processes (simplified and independent replicates of the processes occurring at larger scales in continental areas), makes the origin and evolution of island communities a favorite research topic in evolutionary ecology. In this context, to study insular parasite communities is useful to understand what features allow pathogens to colonize new areas, or rather fail in the attempt. Moreover, there are many cases where the accidental establishment of a parasite in remote islands brought disastrous consequences for the endemic fauna. Therefore, the study of parasites on islands is also very relevant from the perspective of conservation biology.
Most community studies of island parasites have focused either on recently introduced parasites (which could fail to establish in the long term) or on pathogens of insular endemics (which often have diverged from their continental relatives). In other words, we know fairly well what happens at the beginning and end of the colonization process... but we know very little about what happens in between. A study published in Global Ecology and Biogeography by researchers from our group, in collaboration with Dr. David S. Richardson (University of East Anglia), fills this gap by studying blood parasites of blackcaps Sylvia atricapilla in the Canary Islands and Madeira. Although the blackcaps living on these islands are sedentary, they originated from continental migratory populations not very long ago (in evolutionary time: blackcaps colonized these islands around the end of the Last Glacial Maximum, some 10,000 years ago). In addition, many continental blackcaps continue visiting each year both archipelagos during migration and wintering periods. The blackcap (which in the continent harbours an extraordinary diversity of parasites) is therefore an ideal model in which to study a community of parasites halfway in the isolation process.
The study found that despite the incomplete isolation of blackcaps on the islands, their community of parasites differs markedly from the one observed in the continent: only 10% of the continental parasites appear on the islands (with a decreasing richness with increasing distance to the mainland), and these also reach much lower prevalence on islands. In addition, all island parasites are generalists (they are able to infect several bird species), in contrast to the continental parasites, most of which are in exclusive to blackcaps. These results show that what is a usual host-parasite relationship in the continent can be compromised on the islands, even if isolation is recent and incomplete. Therefore, the typical characteristics of insular communities of parasites (low species richness, reduced specialization and frequent host switching, together termed island syndromes) can evolve well before isolation is complete. This kind of studies help us understand the mechanisms involved in the evolution of island biotas, and enable us to identify some characteristics of pathogens that may facilitate their expansion into remote regions of the planet.
Pérez-Rodríguez, A., Ramírez, Á., Richardson, D. S. & Pérez-Tris, J. 2013. Evolution of parasite island syndromes without long-term host population isolation: parasite dynamics in Macaronesian blackcaps Sylvia atricapilla. Global Ecology and Biogeography. DOI: 10.1111/geb.12084.
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