Demographic Consequences of Dispersal through Space and Time

Abstract

As habitat fragmentation and changing climatic conditions continues to pose threats to species persistence, it is important to study the traits that may ensure survival of individual plants and the population-level effects of those traits. Previous studies have highlighted the importance of one such trait, dispersal, for population persistence under changing conditions. Dispersal allows plants to track spatial and seasonal changes in the environment. Plants can disperse through space, i.e., pollen and seed dispersal, and through time, i.e., delaying germination within seasons (seed dormancy) or between years (seed banking). Spatial sorting of good dispersers has been highlighted as a mechanism to facilitate spatial habitat tracking, but few studies have evaluated how interactions between spatial sorting and plasticity of dispersal-related traits may interact to impact dispersal dynamics. In addition, theory predicts that both spatial dispersal as a form of bet-hedging, and seasonal seed dormancy as a form of habitat selection, may stabilize population demography across years, thereby reducing population extinction risk. Few studies have experimentally manipulated spatial and temporal dispersal in the field to test these theoretical predictions. My dissertation quantifies spatial dispersal at a small scale, tests for the genetic basis and plasticity of dispersal-related traits, tests the metapopulation consequences of local spatial dispersal versus spatial isolation, and quantifies the population-level consequences of seasonal seed dormancy. I combined field, greenhouse, and quantitative genetic approaches to assess the demographic effects of spatial and temporal dispersal in the model plant species Arabidopsis thaliana. In my first chapter I show that limited dispersal results in predictable variation in post-dispersal density across space. Further, I show that the traits that enhance dispersal ability in the field have a genetic basis, exhibit plasticity to density and season length, and genetic variation in that plasticity. Plasticity and genotypic differences in plasticity can alter the effects of spatial sorting on dispersal ability across a species range. Specifically, plasticity can augment the effects of spatial sorting of genotypes, by enhancing dispersal at low-post-dispersal density. Genetic differences in plasticity of good versus poor dispersers can mask genetic differences in dispersal ability and thereby slow spatial sorting of genotypes at high density but augment genetic differences and spatial sorting at low density. In my second chapter I show that, compared to isolated populations, populations open to dispersal had smaller between-year population size fluctuations, increased survival of individuals within years, and less between-population differentiation in morphological traits. Those demographic effects of dispersal may have increased the effective population size of populations open to dispersal, facilitating a recovery from the effects of harsh environmental conditions. Finally, my third chapter demonstrates that seasonal seed dormancy may allow populations to better take advantage of favorable conditions by increasing population size and stabilizing population demography over time in more permissive environments. However, contrary to expectations, dormancy did not reduce the effects of environmental variation due to its inability to counter the effects of population bottlenecks induced by harsh environmental conditions. The findings of my dissertation highlight the importance of considering the interaction between spatial sorting, phenotypic plasticity, and genetic variation in plasticity when projecting range expansion dynamics. My dissertation also provides some of the first experimental validation of the theory that predicts that spatial and temporal dispersal can stabilize population demography and facilitate population persistence. Therefore, when predicting how species may respond to anthropogenic changes it is important to not only consider the effects of dispersal on environmental tracking, but also the effects of dispersal on population demography. Finally, given the limited dispersal of Arabidopsis thaliana and many annual species like it, it is important to measure population dynamics at a micro-scale, otherwise researchers risk underestimating regional-level extinction risk.