When I first saw kudzu I was not in the South, where entire states were swallowed whole by the vine, but in Pennsylvania. Philadelphians were calling in reports of the invasive plant and the Pennsylvania Department of Agriculture hired me to find out whether the reports were true, or whether the callers were confusing kudzu with some other vine gone rogue.
The callers were correct. I witnessed roadsides, railroad tracks, and bridges devoured by kudzu. An inner city summer camp was forced to abandon a playground because it didn’t have the resources to fight back the aggressive vine. A reverend enlisted congregants to slay kudzu from a church parking lot, only to find it sprawled over more parking spaces the following year.
I saw kudzu grow up and over trees and shrubs, blocking sunlight and sucking nutrients from the soil. Little other plant life survived in its wake. Occasionally, however, I noticed a single Virginia creeper or raspberry vine poking up through the thick mat of kudzu. I wondered whether these individual plants survived by chance or whether they had genes that equipped them to compete with kudzu.
Kudzu is one of our most infamous invasive plants, but there are many others. In total, there are over 5,000 nonnative plant species in the United States that grow and spread independently of human care. Not all of these species present a major threat to train tracks, playgrounds, and all other plant life. But many, such as Japanese honeysuckle, purple loosestrife, Norway maple, and Phragmites, are believed to severely reduce the biodiversity of the environments they invade.
We are still trying to understand why certain introduced plant species become invasive—capable of spreading and displacing other organisms. We do know that many invasive species evolved in response to their new environments, making them better suited to dominate. In dominating these environments, they also change them dramatically. They may alter water or light availability, pollinator abundance, or the density of the litter layer. So in a way, invasive species are presenting native plants with new environments.
Must all native plants fail to thrive in these new plant communities, or, like the invasive species, might they too evolve? This question, sparked by a lone raspberry vine in a sea of kudzu, is now the focus of my biology dissertation. To answer this question, I study two competing species of jewelweed.
Spotted jewelweed (Impatiens capensis) is an annual plant native to much of the United States. It grows along damp roadsides, stream banks, and forest edges. Its cone-shaped flowers are orange with red spots on the lower petals. The cone narrows into a curved tube called a spur where nectar is produced.
Showy jewelweed (Impatiens glandulifera) is originally from India, but has been spreading in the United States for roughly one hundred years. This is one impressive plant. Also an annual, it grows from a seed to nearly eight feet tall in less than four months, whereas the native jewelweed rarely grows taller than four feet. From this high perch, showy jewelweed fruits launch about 800 seeds per plant. Its flowers range in color from white to deep magenta and produce intensely rich nectar.
These two jewelweeds now grow completely intermixed in many plant communities in the northeastern United States. They compete for the same soil nutrients, sunlight, water, and pollinators. Although the invasive jewelweed is a formidable competitor, the native jewelweed persists. The goal of my research is to understand whether this persistence is, at least in part, due to an evolved ability to better compete.
How can native plants evolve to survive alongside invasive species?
Let’s say that before the introduction of an invasive plant, there is wide variation in the root depth of a native plant. Some individual plants have deeper roots, some have shallower roots, but root depth does not have any major influence on the number of seeds a plant produces. Then, when the invasive species joins the plant community, it monopolizes the resources near the surface of the soil. Now only the native plant individuals with deeper roots are able to get enough water and nutrients to produce a large number of seeds. The plants with shallower roots can’t compete with the invasive, and so produce far fewer seeds. If root depth is heritable, then the next generation of this native plant will include a larger proportion of plants with deep roots than those with shallow roots because the deeper-rooted parent plants produced the most seeds. This process is evolution by natural selection.
Invasive species have the potential to influence natural selection on native plants in many ways. In the presence of an invasive species, natural selection on a native plant might favor earlier germination, wider leaves, or brighter flowers—anything that might help the native plant reduce competition with the invasive species. If these traits are heritable, then native plants can evolve in response to this change in natural selection.
We are just beginning to explore this field of research, but a few studies already suggest that some native plants can evolve in response to invasive competitors. Biologist Richard Lankau showed that clearweed (Pilea pumila) evolved to better tolerate the harmful chemicals produced by garlic mustard (Alliaria petiolata). Similarly, biologists Courtney Lowe and Elizabeth Leger discovered that a native grass, big squirrel tail (Elymus multisetus), evolved to survive competition with cheatgrass (Bromus tectorum).
What experiments am I conducting this summer?
I learned from a greenhouse experiment that 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.
This finding suggests that our native jewelweed does have the potential to evolve in response to the invasive jewelweed. But two important questions remain. First, I need to find out whether this same pattern of altered natural selection occurs in natural plant communities. Second, I need to identify whether this altered natural selection results in evolutionary change.
To answer whether the invasive jewelweed alters natural selection on the native jewelweed, I set up an invasive species removal experiment in Camden, Maine. In early May as both jewelweed species were just starting to emerge, I marked plots where the two jewelweeds grow densely intermixed. I then divided each of these plots in half and removed the invasive species from one half of each plot. I am now following the native jewelweeds to track their survival, growth rate, branching patterns, and ultimately their seed production and biomass. I expect that, just as in my greenhouse experiment, native plants in the removal treatment will produce the most seeds when they invest in growing taller, while those growing with the invasive species will produce the most seeds when they invest in branching out.
To uncover whether the native jewelweed is evolving in response to the invasive species, I am using what ecologists call a reciprocal transplant experiment. I identified three pairs of adjacent plant communities in coastal Maine where the invasive jewelweed is present in one and absent in the other. In mid May I collected sixty seedlings of the native jewelweed in each plant community and then distributed these seedlings evenly across all six communities. I am now following their survival, growth, and seed production. If the native jewelweed is evolving in response to the invasive species, then seedlings that came from invaded plant communities should perform better when planted into the invaded communities than seedlings from communities unexposed to the invasion.
If native plants can evolve, does this mean we don’t need to worry about invasive species?
It would be comforting to know that at least some native plants can evolve to survive species invasions. If I find that our native jewelweed is evolving, however, this does not mean that invasive species are harmless. In order for native plants to evolve in response to invasive species, at least some plant individuals in these populations must have genes that equip them to survive the invasion. Or, the plant population must persist alongside the invasive species long enough for new mutations conferring resistance to arise. Although my dissertation was largely inspired by my observations of kudzu, I didn’t want to go anywhere near this plant for my graduate research. Kudzu outcompetes other species so quickly that I doubted there was time for an evolutionary response.
If some native plants evolve in response to invasive species, this does mean that we can work with evolution to restore native plant communities. First, we can identify which native plant communities are persisting because of an evolved response, and then focus our limited time and money on removing invasive species from the plant communities that are unable to evolve. Second, if some populations of a species lack the genetic variation required for adaptive evolution, while other populations of the same species have genes favoring persistence, then we can supplement populations that don’t contain the beneficial genes with seeds from populations that do.
So if the native jewelweed is evolving in response to the invasive species, then we can be hopeful, not only that the native jewelweed will survive, but that we may have a useful new tool for addressing species invasions. I am rooting for this little jewelweed. I’ll keep you posted on the results.
Lankau, R.A. (2012) Coevolution between invasive and native plants driven by chemical competition and soil biota. PNAS 109: 11240-11245.
Pimentel, D., Zuniga, R., and Morrison, D. (2005) Update on the environmental and economic costs associated with alien-invasive species in the United States. Ecological Economics. 52: 273-288.
Rowe, C.L.J. and Leger, E.A. (2011) Competitive seedlings and inherited traits: a test of rapid evolution of Elymus multisetus (big squirreltail) in response to cheatgrass invasion. Evolutionary Applications 4: 485-498.