Ecological mechanisms and evolutionary patterns of introduced species

Abstract

It has long been assumed that introduced species have higher seed dispersal and survival than do native species. These assumptions are central to our understanding of introduced species’ fast spread rates, competitive ability, and dominance in their new environments. However, studies show mixed results when comparing proxy traits for dispersal and survival. In this thesis, using data collected from the global literature, I compared seed dispersal and recruitment success (survival through germination, one week survival after germination, and survival from germination to first reproduction) of introduced and native species. Against all expectations, I found no significant difference between introduced and native species’ seed dispersal distance or recruitment success. I then explored the relative importance of nature (species’ level of invasiveness) and nurture (introduced vs. native ranges) on invasive species’ life history characteristics. I found that invasive species have higher seed production due to both their nurture and nature, but no differences in survival or dispersal. Lead by evolutionary theory, we assume that introduced species’ fast phenotypic changes could lead to diversification events. Nevertheless, we know little about the trajectories of changes in these species. I used herbarium specimens to track changes in leaf area, leaf shape and plant height for three annual introduced species. I found that introduced species keep changing even 200 years after introduction and that across species there is a lag-phase in the rates of phenotypic change. Although introduced species sometimes show incredible dominance over native species, there is no fundamental reason to expect native species to be at a general disadvantage in dispersal or survival. It is possible that introduced species’ advantages such as higher seed production, enemy release or increase competitive ability might be countered, or at least matched, by native species superior adaptation to local conditions. However, that our three introduced species demonstrate rapid changes in phenotype after a lag phase suggests that some invaders may yet demonstrate their full potential. Overall, this thesis reshapes our knowledge about introduced species’ ecological strategies and shows that trait-based studies are deceiving when used as a proxy for processes, such as dispersal, recruitment and survival.