Life History Consequences of Bioenergetic and Biomechanical Constraints on Migration

In this paper I test the hypothesis that bioenergetic and biomechanicalconstraints to migration play a pivotal role in shaping thelife history characteristics of migrants. Firstly, I examinebioenergetic constraints on the ability to migrate by activetransport and how they shape the life histories of insects andfish, and, secondly, the consequences of biomechanical constraintsto the migration by passive transport in insects and spiders. In both insects and fish the mass-specific energetic costs ofactive transport (flight and swimming, respectively) decreasewith body size, and hence selection should favor large sizein migrant species. Because their habitats are ephemeral, migrantinsects must grow rapidly. In fish, migrant species are ableto exploit resources unavailable to more sedentary species andhence should also show an enhanced rate of growth. These predictionsare supported by comparisons within populations, between populations,and among species in both groups. In contrast to the above, biomechanical factors limit the uppersize at which insects and spiders can migrate by passive transport.Theory predicts that ballooning will be most likely in spidersconsiderably less than 6 mm in length. Therefore, species thatmigrate as adults are predicted to be smaller than those thatdo not. This prediction is supported by a comparison of migratoryand non-migratory spider species from the United Kingdom. Theaverage length of species that migrate as adults, and of migratingyoung of spiders too large to balloon as adults, is about 2mm. Further, within this geographic species assemblage, thesize distribution of adult spiders is markedly peaked in the2 mm region, suggesting that biomechanical constraints on ballooningmay have a major influence on the evolution of body size inspiders.