A recent study looks at how invasive plants take polinators away from native plants. In turn, the native plants evolve to deal with this new challenge.
Invasive species are a growing problem in our world. Invasive species are plants and animals that move from their native area into new territory. In the new territory, they compete with the native species. This competition can impact the survival of native species, sometimes even driving native species into extinction. Invasive species can also help drive natural selection and impact the evolution of a species.
A new study by Carolyn Beans and Deborah Roach, from the University of Virginia, looks at what happens to pollinator and plant interactions when an invasive species moves into an area where a closely related plant, called a congener, is native. Pollinators, such as bees, hummingbirds, and flies, are extremely important to plants. As pollinators move between plants in search of nectar, they carry pollen between the plants helping with fertilization and seed production. The nectar that plants provide is an important food source for their pollinators. This makes the relationship between plants and pollinators a mutualistic relationship, where both species benefit.
In order to attract pollinators, flowers have specific coloration, patterns, and scents. These conditions all together are called a flower’s “floral traits”. Certain floral traits will attract different types of pollinators. For example, butterflies have a poor sense of smell and they are able to see the color red, so most butterfly-pollinated flowers are orange and red, but rarely have a scent. On the other hand, bees see yellow, blue, and ultraviolet colors. They also have a developed sense of smell. Bees are attracted to flowers with yellow or blue petals and a sweet smell. What about those flowers that aren’t very showy and smell bad? They attract flies with their rotting meat scent.
This study focused on the competition for pollinators between Impatiens glandulifera, an invasive jewelweed, and Impatiens capensis, a native jewelweed, as well as how these interactions might influence the evolution of the native jewelweed. Both plants produce similarly shaped pink flowers, though the invasive flower is slightly larger. Both species are pollinated by bumblebees.
The scientists began their study by looking at pollinator preference and pollinator constancy. Pollinator preference occurs when the bumblebee visited one species instead of visiting the other. Pollinator constancy occurs when the bumblebee pollinator moves between flowers of the same species instead of moving back and forth between flowers of both species. In this study, pollinators visited I. glandulifera four times more often then they visited the native I. capensis. The pollinators also showed pollinator constancy. They moved between flowers of the same species more often than they moved between the two species.
When two congeners live together in the same environment and share the same pollinators, it is possible that the pollen from the two species will cross and end up on the other species flowers. Sometimes when this happens seeds are produced normally. In other cases, the pollen from one species is too different from the other and no viable seeds are produced. When I. capensis received a mixture of pollen from I. glandulifera and itself, 42% less seeds were produced. This means that when the invasive species is near by the native species will produce fewer seeds and therefore fewer offspring.
When invasive species were present, the scientists found that there was selection for shorter flowers (corolla) in the native species. Over time, this selection would lead to I. capensis having different sized flowers from the invasive jewelweed. Having different flowers could be beneficial for the native species by attracting pollinators that preferred it to the invasive species. This is especially important when the two congeners share the same pollinators. When invasive species attract more pollinators than their native counterparts, the native species begins to produce fewer seeds. With fewer viable offspring, the population size will begin to decrease. If the native species can adapt to increased competition and adjust its floral traits to increase pollinator visits, there is a possibility of both species successfully existing in the ecosystem together.
Studies like this one are important to our understanding of how changing ecosystems are driving the evolution of plants and animals. As invasive species continue to spread beyond their natural ranges, the competition between invasive and native species will rise. With increased selection pressure, some native species will adapt and others will perish.