Modeling motives for movement: Theory for why animals migrate

Abstract

Migration, the seasonal movement of organisms among different locations, is a ubiquitous phenomenon in the animal kingdom: there are migratory species found in all major vertebrate groups (birds, fish, mammals, reptiles, amphibians), as well as in many invertebrate groups (insects, crustaceans). Despite this, most discussion of migration tends to be taxonomically restricted, and little work has been done to draw comparisons across taxonomic groups. Furthermore, although scientists have long been fascinated by migratory species, we still have little understanding of why migration is such a common strategy and what specific ecological factors have favored its evolution and maintenance. At a basic level, organisms migrate because they benefit through growth, survival, and/or reproduction, and though it has long been suggested that organisms can migrate for different reasons, the distinct motivations for migration have generally received little attention.

In this dissertation, I examine migration as an adaptive response to variable ecological conditions, and use the motivations that drive migration to gain an understanding of the conditions favoring migration, spanning taxonomic boundaries. To achieve this, I use a variety of approaches: meta-analyses of the migration literature to determine the types of motivation that drive migration and how these combine into different round-trip patterns (Chapter 2), individual-based simulations to determine what types of spatiotemporal resource distributions select for migration (Chapter 3), analytic models based on these motivations for migration (Chapter 4-5), and fieldwork to study a migratory terrestrial crab species in more detail (Chapter 6-7). The main conclusions of this dissertation research are that animal migration is driven by a few distinct reasons, and that these motivations span taxonomic boundaries, shape both the ecological conditions selecting for migration as well as the tradeoffs organisms face when making the decision to migrate, and influence what impact a changing environment will have on both the migratory behavior and survival of a species.