A new invasion, or what happens next

From our previous study we knew that body size evolution of mammals on islands is strongly influenced by ecological interactions, or said more simple, when competitors and / or predators are around, body size of the focal mammal will not or hardly change. When there is not a single competitor or predator, body size change may be spectacular, as we saw for Elephas falconeri, the Pleistocene dwarf elephant of Sicily, that reduced so much in size that in the end it was just 2 percent of the body mass of its ancestor. The big question now is, what happens when a new colonization of the island takes place? What if a new dormouse manages to reach an island where there is already an endemic dormouse? The endemic dormouse may have become gigantic after thousands of years of evolution in isolation. The prediction is that the new, tiny dormouse stands no chance against this big brother. The opposite is, however, true. Generally, the new invasion is successful. This is one of the reasons that insular biodiversity is so vulnerable. We studied the pattern through time for 19 endemic small mammals across four large islands. We found that initially, these small mammals all became large as predicted. Then, after a new colonization (or invasion), something interesting happened. A reverse took place, the endemic mammals became smaller again. At second thought, this was to be expected. The new invasion meant the introduction of a competitor, and in some cases a predator as well. Under such ecological conditions, body size increase is only moderate. The endemic mammal was in fact too large for the new ecological setting and evolution went backwards. In most cases, however, not for long, as often the old endemic eventually lost the competition and went extinct. This study made it clear that evolution is driven by interaction and is not a sole business, each species on its own. From the article: Body size evolution of palaeo-insular mammals: temporal variations and interspecific interactions, by A.A.E. van der Geer, G.A. Lyras, M.V. Lomolino, M.R. Palombo and D.F. Sax, in Journal of Biogeography(2013) 40, 1440–1450.