Genetic and environmental contributions to dispersal distance in the two-spotted spider mite, Tetranychus urticae

Dispersal, the movement of individuals leading to gene flow, is a life-history trait found in virtually all organisms. Understanding the mechanisms behind why some individuals move when others remain is crucial to the study of population dynamics and conservation biology. Dispersal is comprised of three phases: emigration, transfer, and settlement. While emigration is well studied, the causes of differing dispersal distances resulting from decisions made at each of these phases are less well investigated. In this thesis, we attempted to distinguish genetic mechanisms of dispersal distance and to quantify plastic responses to different environmental and individual condition. To achieve this goal, we first attempted two different methods of artificial selection: one experiment on emigration and another on dispersal distance. While unable to produce differently dispersing lines in either of these experiments, the results revealed the importance of different environmental factors on dispersal decisions, i.e. the importance of phenotypic plasticity in the expression of this trait. We then performed experiments investigating the effects of population density, genetic relatedness, and maternal density on dispersal distance and the shape of the dispersal kernel. We found that increases in population density and relatedness increased dispersal distances, and that relatedness increased the distance traveled by the furthest moving 10% of individuals. We also found that in general, maternal and even grand-maternal density can influence the distance at which offspring will disperse, almost regardless of the offspring’s own environment. Our results contribute to the field of dispersal ecology by demonstrating that dispersal distance, in addition to emigration, can be affected by the interaction between external environmental factors (context-dependent dispersal) and the phenotypic condition of the individual (condition-dependent dispersal). We propose additional experiments to further clarify and augment the results of this thesis. Finally, we suggest that these results can be used in predictive modeling to know how a population might disperse when faced with differing environments. Thus, our results are useful in terms of predicting dispersal behavior by species undergoing range expansion, and for conservation biologists attempting to rescue populations that are affected by habitat fragmentation.