Invasive plants often evolve in response to novel habitats. Is it also possible that native plants may evolve in response to invasive competitors? The aim of this project is to investigate the potential for a native jewelweed, Impatiens capensis, to evolve in response to competition from an invasive jewelweed, Impatiens glandulifera, in New England plant communities. In greenhouse and experimental field settings, previous research showed that the invasive jewelweed alters natural selection on vegetative and floral traits of the native plant. This study will test whether this same altered selection occurs in natural plant communities. Using removal experiments and a reciprocal transplant experiment, the study will measure natural selection on the native plant both with and without the invasive jewelweed present, and will test whether seedlings from invaded communities have evolved to better compete with the invasive.
Invasive species challenge plant and animal communities throughout the world. This project will demonstrate how the long-term evolutionary impact of invasive species can be estimated in natural populations and should be considered when formulating conservation and restoration decisions. Results from this study will be shared with conservation organizations including The Nature Conservancy and The Massachusetts Invasive Plant Advisory Group. The project will also provide an undergraduate researcher with the opportunity to learn how to study natural selection in the field.
Invasive plants reduce the biodiversity of native plant communities through strong vegetative competition. While the negative ecological impact of these invaders is clear, we wondered whether invasive plant competitors may also influence the evolutionary trajectory of native plant populations. A deeper understanding of whether and how native plants evolve in response to invasive competitors can both aid conservation efforts and further our basic understanding of competition-driven plant evolution. From a conservation perspective, understanding the underlying evolutionary mechanisms at play in invaded communities can aid in predicting which plant populations will survive an invasion and which require remediation efforts. From a basic science perspective, understanding whether native plants evolve in response to invasive competitors can offer valuable insight into the role of competition-driven evolution in shaping plant communities. We studied the interaction between an invasive plant from India, Impatiens glandulifera, and a closely related native species, Impatiens capensis, in Coastal Maine. These species, both referred to as jewelweeds, are annual plants that grow completely intermixed throughout much of New England. The invasive jewelweed grows to nearly three meters tall and can create a dense shade over the native jewelweed, which is often half its size. We knew from a previous study that in the greenhouse the invasive jewelweed drastically alters natural selection on the native jewelweed. In the presence of this invasive species, natural selection favors native jewelweed plants that invest more energy in branching out at the cost of growing taller. This result makes biological sense. The number of seeds a jewelweed plant produces is largely dependent on whether that plant is able to reach sunlight by growing taller than its neighbors. Growing taller costs a lot of energy, but this expense is worth it if the payoff is more sunlight to fuel seed production. Because the native jewelweed almost never grows taller than the invasive jewelweed, investment in height rarely results in additional sunlight. Instead, these plants are most successful when they invest whatever energy they have in branching and seed production, rather than height. Because results in the greenhouse do not always predict what is happening outside, we repeated this study with an invasive species removal experiment in a natural plant community in Camden, Maine. In early May as both jewelweed species were just starting to emerge, we marked plots where the two jewelweeds grew densely intermixed along a small stream. We then divided each of these plots in half and removed the invasive species from one half of each plot. We tracked the survival, growth rate, branching patterns, and fruit production of the plants. The results of this study matched those of our previous greenhouse study very closely. In the plots where the invasive species was still present, natural selection favored native jewelweed individuals that invested less in growing fast and more in branching out. These results suggest it is possible that the invasive plant may alter the evolutionary trajectory of native jewelweed populations to make them grow more slowly and have more branching potential. We looked for an evolutionary response to this altered selection pressure in three additional experiments. In the first experiment, called a reciprocal transplant experiment, we moved native jewelweed plants between invaded and uninvaded sites to see if the plants from invaded sites had evolved to better compete with the invasive species. Unfortunately, this study failed due to bad weather conditions that prevented our transplanted seedlings from growing quickly enough to thrive alongside the surrounding vegetation. We also conducted a greenhouse experiment with native jewelweed seed collected from invaded and uninvaded plant communities. Again, the goal was to test whether native jewelweed plants from invaded communities had evolved to survive alongside the invasive competitor better than those from uninvaded communities. This experiment showed no evidence of an evolved response. Finally, we also conducted a survey of natural populations of the native jewelweed growing with and without the invasive competitor. We visited 16 populations and measured over 700 native jewelweed plants to see if there was any evidence that suggested that the plants in invaded communities had evolved in response to the altered selection pressures we detected in our removal and greenhouse experiments. Again, we did not find any evidence of an evolutionary response. Although we have not yet detected evolution in response to the invasive jewelweed, the results of our research are important because they show how an invasive species may alter natural selection on specific vegetative traits in a native species. Additionally, this research provided a deeper understanding of the ecological and potential evolutionary impact of this invasive species. These results will be shared with the Nature Conservancy and the Massachusetts Invasive Plant Advisory Board to help in making decisions about where to focus conservation efforts. Finally, this project provided hands-on training in experimental design and implementation for two undergraduate research assistants.
