The Ecology of Exotic Plant Invasions
Shade Tolerant Invaders and Exotic Plant Invasions of Forest Ecosystems
Invasion ecology has traditionally focused on exotic plant species with early successional life-history traits, adapted to colonize areas following disturbance. However, the ecological importance of these traits may be overstated, in part because most invasive plants originate from intentional introductions. Furthermore, this focus neglects the types of plants most likely to invade established communities, particularly forests – namely shade-tolerant, late-successional species. In invasion ecology, it is generally assumed that undisturbed forests are highly resistant to plant invasions. Our review shows that this assumption is not justified: in temperate and tropical regions around the world, at least 139 exotic plant species are known to have invaded deeply shaded forest understories that have not undergone substantial disturbance. This list of shade tolerant invaders is available here. These exotics present a particular management challenge, as they often increase in abundance during succession. While forest invasions may develop comparatively slowly under natural disturbance regimes, anthropogenic processes, including the spread of exotic pests and pathogens, can be expected to accelerate the rate of invasion.
In a nutshell:
- The vast majority of invasive exotic plant species originate from intentional introductions, most of which are fast-growing, shade-intolerant species
- The number of shade-tolerant species that have been deliberately introduced is much lower, but a very high percentage of these shade-tolerant exotics invade deeply shaded forests
- Forests are not immune to invasion; while the rate of invasion by these shade-tolerant species may be comparatively slow, many shade-tolerant invasives have detrimental and long-term impacts on forest ecosystems worldwide
Biotic Resistance to Invasion
Theories of ecosystem resistance to plant invasions remain intensely debated. Invasion ecology is hampered by the prevalence of descriptive studies of extant invasions. Moreover, most invasives research has focused on disturbed ecosystems and the exotic plants which invade these open environments; research of long-lived, slower-growing organisms has been neglected. Hence, there is a gap in our knowledge regarding the long-term ability of exotic trees to invade and dominate the intact forests that characterize much of the matrix vegetation and nature preserves throughout the U.S. To test invasion theories, I conducted an experiment of invasion ability of Norway maple (Acer platanoides ) into closed-canopy forests. This study is unique with 5 years of growth and survivorship of Norway maple seedlings under a variety of in situ conditions. This study demonstrates the absence of any emergent ecosystem property which prevents Norway maple invasions even in healthy, undisturbed forests. Rather, the life-history traits of the species are paramount: the high shade tolerance of Norway maple enables a very high percentage of its seedlings to establish and survive for 5 years even in deep shade. This research is published in the Journal of Ecology (PDF).
Causes and Consequences of Colonization by an Exotic Tree Species
Invasions by exotic plants are hard to predict and harder to control once underway. Some exotic species become serious invasives while others remain benign. The exotic tree Norway maple (Acer platanoides) has been shown to be a problematic invader in some contexts, capable of dominating and altering forest stands. When I began research on Norway maple, however, almost no research had been conducted on this potentially serious invader. This dearth of research and the economic value of this species contributed to the lack of efforts to control the spread of this species. This research became one of the first to document Norway maple\’s strongly negative impacts on native flora abundance and diversity. Today, greater efforts are devoted to the control of Norway maple. This research is published in Biological Invasions (PDF).