Grantee Research Project Results
Final Report: An Experimental Study of Biological Invasions in Forests of the Eastern United States.
EPA Grant Number: R828900Title: An Experimental Study of Biological Invasions in Forests of the Eastern United States.
Investigators: Gurevitch, Jessica , Lerdau, Manuel
Institution: The State University of New York at Stony Brook
EPA Project Officer: Packard, Benjamin H
Project Period: September 20, 2001 through February 19, 2004 (Extended to September 19, 2005)
Project Amount: $451,553
RFA: Exploratory Research to Anticipate Future Environmental Issues (2000) RFA Text | Recipients Lists
Research Category: Ecological Indicators/Assessment/Restoration , Water , Aquatic Ecosystems
Objective:
The spread of invasive, exotic species increasingly is threatening native ecosystems, with serious consequences both for humans and for the preservation of native species and natural communities. Our understanding of the factors governing community invasion, and of the consequences of invasions for communities and ecosystems, however, is at present too vague and superficial to be predictive. The objectives of the present study were twofold. First, we experimentally tested several hypotheses as to why some forest communities are more resistant to invasion, whereas others are more susceptible. No previous experiments had been conducted in forests specifically to test hypotheses regarding the degree to which the community resists or is vulnerable to invasion, and no experimental study in any system has tested the effects of more than a single factor in inhibiting or promoting community invasion. Second, we investigated the the impacts of invasion on key ecosystem properties by examining the dynamics of mass and element transfer from invasive species to ecosystems. To our knowledge, no previous research on biological invasions in eastern forests had examined this primary step in ecosystem processes.
This work contributes to a better understanding of why invasion by exotic species is more problematic in some ecosystems than in others. The results of this study improve our ability to predict which ecological communities are most likely to face serious threats from biotic invasion in the future, as anthropogenic environmental changes increase. Furthermore, our research in these eastern forests lends insight into the impacts that invasions have on ecosystem processes for a major system in which these impacts not yet been well studied. Ultimately, we hope to be able to use the information obtained to develop management recommendations for protection of natural communities and control of exotic species, leading to better protection of these economically important and recreationally invaluable resources.
Forests of the eastern United States particularly are at risk from invasions both because of the large number of exotic species growing nearby in heavily perturbed systems and because of the large anthropogenic influences, such as nitrogen deposition and disturbance, that are affecting these systems. Eastern forests today exist within an urban/suburban matrix that provides a large potential donor pool of exotic species poised to become invasive. In contrast to more extensive or remote ecosystems where dispersal of exotics to the ecosystem may often limit invasions, invasions of forests in the eastern United States often are more influenced by factors other than dispersal.
Although documenting patterns of invasion is a critical step in making progress in understanding the underlying processes governing invasions, alternative hypotheses may explain the same (survey) data equally well, and it usually is impossible to separate the effects of co-occurring factors. These kinds of studies, therefore, offer limited potential for testing which factors are of greatest importance in promoting or inhibiting the invasion of natural communities. We believe that manipulative field experiments provide the most robust tests of the factors determining community invasibility. The failure or success of an invasion is determined not simply by the growth and reproductive characteristics of the invader but also by the interaction between the invasive species and the target community.
Summary/Accomplishments (Outputs/Outcomes):
Our experiments focused on two major issues regarding ecosystems and invasions in eastern forests. First, we explored the factors that may be causally related to susceptibility to invasion, using multifactorial experiments. We looked at the effects of three environmental factors: calcium, nitrogen, and light. Second, in an attempt to gain a preliminary understanding of the impacts that invasive taxa might have on ecosystem processes, we examined the dynamics of carbon and nutrient loss from litter to the ecosystem. Few data exist (especially in comparison to the amount of theory) on this question, and a solid investigation of the first stage on ecosystem impacts is necessary before more complexities can be investigated experimentally. A key element, common to all of our experiments, is that they were conducted in each of three ecosystems: hardwood invaded, hardwood uninvaded, and pine/oak uninvaded (no pine/oak-dominated sites were ever found to be invaded). By examining species responses and effects in all three ecosystems, we were able to gain important insights into the mechanisms underlying community susceptibility to invasion, mechanisms that transcend particular species and communities. In addition, the ecosystem impact experiments allowed us to distinguish species- or system-specific phenomena from more general ones that could play major causal roles in determining these impacts. Unlike previous single-factor experimental studies on invasibility, we conducted factorial experiments to identify the role of interactions among hypothesized causal factors in promoting or inhibiting invasion. The experiments also indirectly tested for the importance of the availability of propagules in determining invasion success. In each of the experiments, transplants of the exotics were introduced to sites where these species are already present (“control” sites) and to sites which have not been previously colonized. If invasion is largely dependent on introduction to a site rather than on site characteristics, the species would have performed well wherever introduced.
Results from all experiments can be summarized as follows. Responses to gaps were strongest for invasives in hardwood forest sites; responses of both invasives and natives to gaps in the pine barrens were smaller. Gaps consistently had far greater effects on plant performance than did nutrient manipulations. Invasive species did not grow faster than natives on average, but invasive plants had a greater positive response to gaps. Invasives allocated more biomass to shoots than did natives; natives shifted allocation in response to site differences and nitrogen addition, but invasives did not.
There was an interaction between nitrogen availability and loss of leaves to natural enemies that favored invasive species in hardwood forests. Native species always received more damage by herbivores, and all plants received more damage under nitrogen addition treatments, but in the hardwood forests, nitrogen addition increased herbivory on native species to a much greater degree than on exotic species, to the extent that they were no longer able to tolerate herbivory and had reduced growth rates.
Decomposition rates of both native and exotic taxa were accelerated in invaded hardwood forest communities compared to uninvaded hardwood forests, and all species decomposed more slowly in the pine barrens than in hardwood forest sites. Litter decomposition rates and nutrient release rates of exotic taxa were faster than those of closely related native taxa, despite similar macrodecomposer communities. There were differences in decomposition between community types. Average mass loss was more rapid in hardwood forest than pine barrens sites, mass and nitrogen loss were more rapid in invaded than uninvaded sites, and decomposition was faster for exotic than native species. In addition, leaf nitrogen was higher in invaded than uninvaded hardwood forest sites, suggesting either that invasive species were altering nutrient dynamics in these sites, or that there were underlying differences between sites that were invaded and those that were not. We also found that when litter from different species is placed in mixtures, patterns of mass-loss, nutrient transfer, and decomposer activity and abundance frequently are nonadditive (that is, these patterns cannot be predicted by the dynamics of each species decaying singly).
Our results suggest that gaps created in the pine barrens are unlikely to facilitate invasions, but gaps in these hardwood forests are likely to strongly increase invasion. Nitrogen addition may increase enemy pressure on native species, which could confer a competitive advantage to invasive species. In addition, our litter decomposition experiments suggest that invasive plant taxa may have large effects on ecosystem processes that may facilitate further invasion. This research has confirmed some of our hypotheses and offered some surprises that offer numerous insights into the process of invasion of exotic species in forests. It has suggested fruitful avenues for future research that promise to extend our understanding of the interaction between invasive species and the ecological communities and ecosystems they invade.
This project addresses the effects of exotic species on forest communities and ecosystems, aids in developing our predictive abilities regarding the effects of anthropogenic activities in promoting invasions, and touches upon the role of abiotic factors and the links between protected areas and their surroundings. We evaluated critical factors and interactions between factors affecting resistance and susceptibility of different forest ecosystems to biological invasions by exotic plants. Our results on mass and nutrient loss from native and invasive species increases our understanding of the ecosystem impacts of these invasive species. These results will be very useful when extending our results to other ecosystems to determine the impacts of invasions.
Uses of the Results by Scientists, Managers, and Policymakers
The information provided by this study will have broad application to other forest systems in the United States that are experiencing invasions. Our results will be directly applicable to questions of ecosystem management and conservation. The results will help conservation managers design efficient rational strategies to minimize the risk of biological invasions, control already ongoing invasions, and identify sites that are particularly susceptible to invasions. This research will provide information on where to focus necessarily limited control efforts in forest preserves and other forest systems potentially threatened by invasion.
Journal Articles on this Report : 2 Displayed | Download in RIS Format
Other project views: | All 35 publications | 2 publications in selected types | All 2 journal articles |
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Type | Citation | ||
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Ashton IW, Hyatt LA, Howe KM, Gurevitch J, Lerdau MT. Invasive species accelerate decomposition and litter nitrogen loss in a mixed deciduous forest. Ecological Applications 2005;15(4):1263-1272. |
R828900 (2004) R828900 (Final) |
Exit Exit |
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Gurevitch J, Padilla DK. Are invasive species a major cause of extinctions? Trends in Ecology & Evolution 2004;19(9):470-474. |
R828900 (2004) R828900 (Final) |
Exit Exit |
Supplemental Keywords:
forest invasion, invasibility, exotic plants, decomposition, pine barrens, Long Island, northeastern US, experimental manipulation, field experiments, invasion ecology, water, land, soil, ecological effects, population, susceptibility, nitrogen oxides, ecosystem, terrestrial, habitat, public policy, conservation, biology, ecology, northeast, midatlantic, New York, forestry,, RFA, Scientific Discipline, Ecosystem Protection/Environmental Exposure & Risk, Ecosystem/Assessment/Indicators, Ecosystem Protection, Forestry, Ecological Effects - Environmental Exposure & Risk, Monitoring/Modeling, Ecological Risk Assessment, Biology, Exp. Research/future, Futures, emerging environmental problems, extinction risk, ecological exposure, biodiversity, endangered species, forest, biopollution, runoff, shrubland, Eastern U.S., exploratory research, invasive species, forests, ecological dynamics, rainfallProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.