Invasive Species: They’ve Got “Good Genes”

Contact: Matt Salo, Naturalist, Town of Cheverly | Kerrie L. Kyde, Maryland DNR

Callery Pear Field

Callery Pear (Pyrus calleryana) invasion of an open field in Maryland. Photo: K. Kyde

ANNAPOLIS, MD (December 01, 2016) – Invasive plants cost the U.S. economy billions of dollars each year due to losses they incur in agriculture, forestry, fisheries and the recreation industry, plus the damage they do to native ecosystems. Does genetic make-up affect a plant’s ability to become invasive? Knowledge of genetic factors involved in the introduction and diffusion of species that become invasive in new environments can help us understand the dynamics involved and consequently aid us in devising better methods to control them. The Maryland Invasive Species Council has chosen invasion genetics as the topic for the December Invader of the Month.

Introduced species arrive in novel environments with genetics that are evolutionarily adapted to their areas of nativity, where their spread is controlled by limiting factors such co-evolved competitors, predators or disease. Their new environments offer different degrees of hospitality and opportunities for colonization. Most of the newcomers will not make it, but a few, perhaps already generalists in their original habitats, will adapt, thrive, and sometimes even overcome the natives to establish themselves as what biologists call “invasive species.”

The ability of an introduced plant to thrive in a new environment initially depends on genes that endow it with structural, physiological or behavioral attributes that allow it to fit within given parameters of growing space, temperature, moisture and sunlight. Among the most cited plant characteristics favoring establishment are prolific seed production, good seed dispersal methods, ability to take root in disturbed and often marginal soils, high germination rates, rapid growth, early reproduction, and drought resistance. A new arrival may also gain an advantage over native species by “ecological release” or the absence of predators, parasites, disease and the competitors that had kept its numbers in check back home.

Callery Pear along highway

Callery Pear (Pyrus calleryana) invasion along Maryland highway. Photo: Sara Tangren

The more numerous the introduced individuals with high invasive potential are, the greater the risk of their becoming established and spreading. But to do more than merely persist, a species must have traits that allow it to excel in activities that contribute to survival and reproduction. The latter is of primary importance; it requires genetic variability and a large enough population to allow natural selection to occur. When a species is given further boosts — by often unwitting cultivators trying to improve their stock by adding varieties and cross breeding — the outcomes became more complicated and possibly more threatening.

The history of Callery pear in the U.S. provides a prime example. The original Callery pear introductions, primarily from China, were assumed to be self-incompatible. This was true for trees with the same genotypes, but plants with varying genotypes were able to cross-pollinate and produce numerous, fertile seeds. Massive importation of seeds from various locations occurred over decades, along with intensive campaigns to produce better cultivars, primarily for their ornamental value, and with scant attention paid to their potentially invasive qualities. Callery pears of diverse genotypes crossbred to produce hybrids from a wide range of parental stock, which then spread into natural areas, where they continue to expand and diversify. Both the numbers and the genetic diversity of the original imports were a hotbed for creating an army of invaders which now exhibit a high degree of genetic admixture. Today they are among the worst plant invaders in Maryland.

Other species, both plant and animal, exhibit similar trajectories. Studies of genetic variance and clonal diversity of Asiatic kudzu show genetic diversity born of multiple introductions. Kudzu, infamous as the “vine that ate the South,” has a patchy presence in Maryland, where it can be just as damaging, by growing over native vegetation.

Japanese Knotweed is described as a complex species of hybrids, and polyploids, individuals with more than the usual two sets of chromosomes. Knotweed has expanded aggressively throughout North America as well as in other continents. It is a pest of riparian areas that can spread both vegetatively and by seed, which makes it doubly difficult to eradicate. Studies have shown that allopolyploids, which retain chromosomes from both parents, are implicated in facilitating invasiveness, and that these genetic oddities accumulate in a population over time, thus magnifying the threat they pose.

Likewise, the high levels of genetic diversity among nutria introduced to the U.S. as compared to native South American populations indicate that multiple sources provided the invasive U.S. populations. Nutria, which are beaver-like semi-aquatic rodents, were introduced to several states, including Maryland, to bolster the fur trade. In large numbers, they impact wetlands by destroying salt marsh plants and structure and also damaging farm crops in adjacent areas.

Multiple introductions facilitate initial survival of a species, but do not guarantee its invasiveness. Genetic variance due to hybridization and the additive effects of multiple forms of single genes may be even more important. One such condition is polyploidy. Another is epistasis, the impact of one gene’s action on another gene’s functioning. Both variances have been considered important by some researchers. The number of possible changes in genetic heritage is large and researchers are still finding new ones and arguing about which ones are more important.

It appears clear that hybridization of genotypes from multiple sources creates new genetic combinations, and that individuals with these combinations have greater adaptive potential through competitive advantage than their progenitor species. Whatever the genetic architecture, variance provides ample material for natural selection that promotes rapid evolution for invasive behavior. Researchers will eventually clarify which genetic mechanisms are involved and under which selective conditions.

For more information about other Invasive Species of Concern in Maryland, visit the Maryland Invasive Species Council or call the Maryland Department of Agriculture at 410-841-5920.

For more information on Callery Pear:

Invader of the Month: April 2007

Are Callery Pear (Pyrus calleryana) Cultivars Invasive? – University of Cincinnati

See also:

How weeds emerge: a taxonomic and trait-based examination using United States data.