Competition-driven Selection on a Native Plant Following a Species Invasion

Invasive plants often evolve rapidly in response to novel environments. We are only now beginning to explore whether native plants may also evolve in response to these novel competitors. Research suggests that at least some invaded native plant populations are capable of adaptive evolutionary responses. The underlying mechanisms driving this evolutionary response, however, remain largely unexplored. For example, our knowledge of how invasive plants may alter the strength and direction of natural selection on specific traits of native plants is limited. Here I examine the ecological and evolutionary influence of an invasive jewelweed, Impatiens glandulifera, on a native congener, I. capensis. I begin with a review of our current understanding of competition-driven plant evolution in the context of the character displacement literature. Then, using a series of greenhouse, manipulated field, and natural plant community studies, I explore how pollinator-mediated competition and vegetative competition with the invasive plant influence the reproductive success and selection regime experienced by the native congener.
The results show that pollinator sharing between the invasive and native jewelweeds was common and may affect selection on floral traits. When the invasive jewelweed was present, selection favored native jewelweed plants with shorter corolla heights. A follow-up study shows, however, that this result may not be consistent across years. While negative directional selection on corolla height was maintained in a complex community that included two additional competitors, it was no longer significant under pair-wise competition with the invasive jewelweed. In addition to potentially affecting phenotypic selection on floral traits, the invasive jewelweed also altered selection on vegetative traits in the native congener. When the invasive plant was present in both the greenhouse and the field, selection favored native jewelweed individuals investing less in rapid upward growth and more in branching and fruiting potential. Other factors, however, may limit the ability of native jewelweed populations to evolve in response to this altered selection pressure. Overall, this dissertation demonstrates that both pollinator-mediated and vegetative competition with invasive plants can significantly alter phenotypic selection in native plant communities.