Many have commonly believed that more diverse habitats are less likely to be invaded by exotic plants since ecological niches are occupied by species already present. Recent evidence shows that invasibility is a much more complex issue and may be determined by multiple factors. Observations show that species-rich communities—those with many different plant species—are more likely invaded by exotic species if they are in early stages of succession (ecological development over time) and the biomass of the community is low. Work by university with USDA scientists and partners indicates a community’s ability to stave off species invasions may be dependent upon a threshold level of both species richness and abundance, below which the importance of species interactions is only a weak force (Guo et al. 2015).
Japanese stiltgrass, an exotic carpet in a mature Eastern Forest of the USA. Photo: Q. Guo, USFS, with permission.
…” knowledge about invasibility is more pertinent to the prevention of future invasions, whereas knowledge of DI [the degree of invasion] may help a manager prioritize the management of existing exotics species” p. 261
Invasibility represents susceptibility to invasion and reflects the number of open niches in the community. It is influenced by community species composition, diversity, and biomass. Biomass is included in the considerations because it is related directly to competition from surrounding species. Invasibility is intrinsic to a community because it is controlled by availability of resources. Invasibility will probably change over time as the populations of dominant species fluctuate.
The degree of invasion, or the extent already invaded and abundance of exotics, is largely influenced by previous interactions of, for example, propagule/colonization pressure, time since invasion and past disturbance. It is based more on the proportion of exotic richness and biomass in the community to the total richness and species biomass – since the starting point has to account for what was there and what has moved in. Exotic species may continue to invade and could persist unless checked by management actions.
The metric for quantifying invasibility, is based on a habitat’s resource availability as inferred from relative resident species richness and biomass (observed and maximum), whereas for DI, the metric is based on the ratios of exotic to total species richness plus exotic to total biomass. The study acknowledges the possibility of skewed data, especially if the sample sizes or number of plots are small, and suggests methods for calculating maximum species richness and biomass to deal with potentially bias estimates from an outlying maximum value. See the links in the literature source cited below to find the metric calculations in the published paper.
These indices for invasibility and DI can be useful in determining patterns of invasions across systems and actions for restoration of ecological habitats during land management.
The richness of species and of amount of species biomass may switch in relative importance across habitats and as succession stages change. Communities in early successional stages are likely more invasible than a community in a later successional stage, and in almost all cases, DI will increase and stabilize over time unless stopped or reversed by management activities. So over time, both short and long term, “increasing DI would reduce invasibility due to increased community saturation, at least until the next major disturbance occurs” says Guo.
Current research efforts are specifically targeted at U.S. forest ecosystems, but future comparisons among the major community types within and among geographic regions can provide new insights into invasion biology to assist scientists, resource managers, policymakers, and the general public in managing and controlling invasive species.
For better control and management of invasive plant species, research must uncover the factors that contribute to habitat invasibility, degree of invasion, and species invasiveness as well as how these factors can be measured. Scientists are collecting and comparing extensive data on habitat characteristics and invasibility from diverse ecosystems in U.S. forests and other ecosystems around the world to identify these factors and to provide information for land management and policy making into the future.
Partners engaged in this collaborative work include: Purdue University; Auburn University; Brown University; North Carolina State University; University of Washington; University of Tennessee-Knoxville; University of Nevada-Reno, University of Georgia; University of Missouri; U.S. Geological Survey; South Florida Water Management District; Chinese Academy of Sciences; University of California-Berkeley; USGS-EROS Data Center; German Centre for Integrative Biodiversity Research (iDiv); University of Hong Kong; Taiwan National University; Biota of North America Program with US Forest Service scientists at the Southern Research Station, Eastern Forest Environmental Threat Assessment Center, Forest Inventory and Analysis Program; Pacific Northwest Research Station; and Northern Research Station (Northern Institute of Applied Climate Science).
Adapted from: Guo, Q. , S. Fei, J.S. Dukes, C.M. Oswalt, B.V. Iannone III, and K.M. Potter. 2015. A unified approach for quantifying invasibility and degree of invasion. Ecology 96:2613–2621. http://dx.doi.org/10.1890/14-2172.1
Adapted by: Sarah Workman, University of Georgia - Warnell School and US Forest Service Southern Research Station, Eastern Forest Environmental Threat Assessment Center.