Are Pest Plants... Born to Be Bad?
by Jim McGlone, Urban Forest Conservationist, Virginia Department of Forestry
(Conservation Currents, Northern Virginia Soil and Water Conservation District, Fall 2007-Winter 2008)
An outstanding question in natural resource management is why some non-native plants become aggressive invaders. How do particular species become pest plants? Are they just born to be bad?
Consider the case of the Brazilian pepper-tree (Schinus terebinthifolius). According to the Smithsonian Research Station at Fort Pierce, the Brazilian pepper-tree was introduced to Florida horticulture by the 1840s. Nicknamed the Florida holly, the glossy-leaved, red-berried ornamental remained limited to gardens and maintained landscapes for over 100 years. A 1942 inventory of Everglades flora fails to even mention the plant. By 1969, Everglades National Park staff recognized the Brazilian pepper as a threat. The plant is now considered a serious pest in Florida’s natural areas and one of the most aggressive non-native invasive plants in the state.
What made a seemingly well-behaved horticultural specimen suddenly explode across the landscape after 100 years? What changed—either in the environment or in the plant or both? Did something change? Why do some non-native plants develop into aggressive invading pests? Many hypotheses exist to explain such lags between introduction and invasion, and given the variety of ways that plants become invasive, they may all be right.
The simplest explanation is one of population dynamics. It can take years for a plant’s population to reach sufficient density either to be noticed as a problem or for the plant to be able to spread rapidly. Kudzu is a pest plant that may fit this category. Although kudzu spreads aggressively once it is established, sexual reproduction is rare according to the Southeast Exotic Pest Plant Council Invasive Plant Manual. Plants like kudzu that produce few seeds per plant or have low germination rates would need a large number of individuals to produce enough seed to begin spreading across large areas. Aggressive promotion of kudzu as animal fodder and to prevent soil erosion aided its spread in the first half of the 20th century.
A change in the environment could also trigger invasive behavior in a plant. Natural changes like a hurricane, fire or flood can disturb a local ecosystem and create an opening into which a non-native plant could move. Human land use and natural resource actions like clearing forests, abandoning agricultural land and building roads can also disturb ecosystems. Many invasive plants thrive in these conditions in their native ranges and establish footholds in new territory on disturbed sites.
Environmental changes may explain the aggressive spread of the Brazilian pepper. In the 1950s and 60s, at the same time that Brazilian pepper exploded across South Florida, the hydrology of the Everglades was significantly altered to drain farmland and provide flood control. These hydrologic changes may have transformed what had been an inhospitable environment for the plant into a hospitable one.
Human actions and natural processes also can combine to create conditions for the expansion of non-native species. In Fairfax County, land use changes and the elimination of predators have created ideal conditions for explosive growth of the deer population. Deer preferentially browse on native vegetation and at some Fairfax County sites, deer have eaten literally every scrap they can reach! However, many non-native invasive plants are unpalatable to the animals. Invasive plants may be spreading in our area to replace native plants removed from our woodlands by deer.
Over time, scientists hypothesize, a plant can also change. Plants, like other living things, have an energy budget. They use energy for growth, reproduction or to defend themselves from disease or pests. Successful individuals strike a balance between these energy demands.
When a plant is moved to a place where pests or disease are absent, additional energy can be spent on growth and reproduction. Under these conditions, natural selection will favor aggressive growth and/or prolific seed production, and more invasive plants may come to dominate a population. This type of genetic drift towards invasiveness is more common in plants that already have the latent trait in their genes, but it also can happen as a result of mutation after a plant has been moved from its native range.
Scientists testing evolution of invasive tendencies in plants have had mixed results. Under controlled conditions, some plants show increased growth or reproduction when released from the pressures of pests and disease, but others do not. Scientists are now trying to more carefully identify the circumstances in which plants evolve an increased ability to out-compete other plants. A difficulty is that is it may take several generations of a plant before invasive characteristics are expressed.
So are invasive plants “born to be bad?” We don’t know. But, in exploring the question we have learned that plants don’t always show their invasive potential immediately. The lag between introduction and invasion varies among species. According to Ingo Kowarik, a German scientist studying invasiveness, this lag is on average 170 years for trees, 131 years for shrubs, and generally less for herbaceous plants.
On the positive side, less than 10 percent of introduced plants evolve into invasive pests. However, the lag between introduction and invasion suggests one more reason to be wary of using non-natives in your landscape, especially new introductions. We still don’t know what causes a plant to go “bad” and how time and a changing climate may affect the alien species among us.
Photo: Amy Ferriter, South Florida Water Management District, Bugwood.org.