Researchers have commonly believed that more diverse habitats are less likely to be invaded due to niche occupation by species present, but recent evidence shows that invasibility is a much more complex issue and may be determined by multiple factors. To identify these factors, scientists at the U.S. Forest Service Eastern Forest Environmental Threat Assessment Center scientists are collecting and comparing data on various habitat characteristics and invasibility from diverse ecosystems in U.S. forests and other ecosystems around the world. Researchers' recent observations show that species-rich communities, those with many different species, are invasible if they are in early stages of succession (development over time) and biomass is low. A plant community's ability to resist 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. Current research efforts are 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.