Experimental island biogeography demonstrates the importance of island size and dispersal for the adaptation to novel habitats

Abstract

Aim: Island biogeography theory describes how island size and isolation determine population colonization success. Large islands sustain larger populations than small ones and experience less demographic stochasticity, thus a lower extinction risk. Nearby islands are more likely to be colonized than distant ones, because they receive more immigrants from the mainland. However, local conditions on islands are often different from those on the mainland; therefore, populations on recently colonized islands also need to adapt. Island size and isolation are known to impact the build-up of genetic variation necessary for adaptation; hence, we integrated island biogeography with evolution experimentally to gain a better understanding of the roles of island size and isolation in biodiversity patterns. Location: Laboratory, Ghent University, Belgium. Time period: October 2013 to June 2014. Major taxa studied: Two-spotted spider mite (Tetranychus urticae). Methods: Using experimental evolution, we studied the effects of island size and isolation on colonization, extinction and adaptation of the two-spotted spider mite to new islands. The mainland population consisted of bean plants and the islands of tomato plants (a known challenging condition). Islands differed in their size (number of plants) and in the number of immigrants (females, the dispersive stage) they received from the mainland. Results: Island size and dispersal decreased extinction risk and increased colonization success and adaptation. Populations on small islands, which are most affected by extinction, were rescued demographically by an increase in dispersal. However, they were never able to adapt. Main conclusions: Evolutionary rescue via dispersal is possible only when populations are sufficiently large; small populations cannot adapt, because they lack the genetic variation necessary for local adaptation. Hence, in addition to the effects of island size and dispersal on the ecological processes of colonization and extinction, our results show that island size and dispersal can jointly affect the evolutionary process of adaptation to novel habitats.