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Abstract

Plants rely on seed dispersal to track favorable habitats and escape unsuitable conditions in variable environments. One likely driver of phenotypic dispersal trait plasticity is the prevalence and intensity of parasites and pathogens in the plant’s environment. If the effects of parasites and pathogens follow broad scale biogeographic patterns, and plant dispersal traits vary with parasite and pathogen prevalence and intensity, then corresponding biogeographic patterns in dispersal trait variation will likely be observed. To test this, I explored the literature to identify potential biogeographic trends in parasites and pathogens and evaluate support for the hypothesis that biotic interactions increase with decreasing latitude. In parallel, I reviewed and synthesized empirical studies that have tested for intraspecific variation in dispersal traits and evaluated the extent of evidence for a latitudinal gradient in dispersal propensity. These literature surveys revealed that there is mixed evidence at best for the hypothesis that parasite and pathogen pressure increase with decreasing latitude, and—consistent with these mixed results—there was little support for a consistent latitudinal gradient in plant dispersal propensity. However, very few studies have quantified biogeographic variation in dispersal, and I found that none of these studies simultaneously tested for covariation between dispersal traits and disease pressure. Nonetheless, it is clear that both disease pressure and plant dispersal traits each show high spatial variation. I propose an experimental design that tests if host plants exhibit phenotypic plasticity in their dispersal traits in response to the presence of parasites or pathogens, as well as the genetic and environmental sources of dispersal trait variation across geographic gradients. Understanding the relationship between dispersal and the presence of parasites and pathogens will be key to predicting the responses of these relationships and the species evolved to a rapidly changing world.