Herbivores can have strong impacts on plant populations and communities. However, human activities have profoundly altered herbivore populations, particularly large vertebrates, via habitat modification, hunting, and removal of top predators. Utilizing field experimentation, the Community Ecology lab is examining the impacts of these altered plant-herbivore dynamics on plant communities and particularly exotic plant invasions, with the long-term goal of understanding how anthropogenic modifications of biotic interactions ultimately influence the ecological goods and services provided by natural communities.
For example, in 2009, about 200 Citizen Scientists from Earthwatch helped to install a factorial experiment manipulating the presence/absence of white-tailed deer and introduced plants. This experiment has been running for five years in both intact and logged forests, resulting in several novel observations. First, logging increases soil compaction, decreases insect diversity, and disproportionately enhances the cover and diversity of alien over native plants. These patterns mimic results from separate landscape scale analyses in which historical patterns of land use history (e.g. logged vs. unlogged, or use by Native Americans) were linked to contemporary patterns of plant community composition (Biological Invasions 2010, Landscape Ecology 2014). Moreover, deer have stronger negative impacts on native plants in the presence of invasive plants, and invasive plants have negative impacts on tree regeneration. These findings support the notion that invasive plants are major disturbances with significant impacts on native communities, an area of surprising controversy in invasion biology.
A major goal in ecology and evolution, and in the Community Ecology laboratory, is to unravel how biological invasions of non-native species alter community structure and function. A fundamental assumption in invasion biology is that invasive species perform better in their introduced range than in their home ranges. This idea has given rise to numerous hypotheses explaining ‘invasion success’, but the fundamental assumption has been little tested. In 2013, we finished a meta-analysis comparing performance data from both the native and the introduced range for a total of 53 species from over 100 countries and territories.
We found that invasive species generally perform better in their introduced ranges but that there was considerable variation across species with roughly half of the species showing no evidence of increased performance (Ecology 2013). We also have another paper (Neobiota 2014) that utilizes these findings to derive better quantitative metrics of invader performance. Both of these papers were identified as key manuscripts by the Faculty of 1000 Prime.
Another area of considerable interest is in the Enemy Release Hypothesis, which posits that invasive plants thrive because they are unpalatable to native herbivores. In contrast, exotic prey may be particularly vulnerable in their new ranges because they are defensively naïve to novel enemies. Indeed, native generalist herbivores in both aquatic and terrestrial systems often preferentially consume exotic over native plants (Ecology Letters 2005; Science 2006; Oecologia 2007), and invasive plants can often be no more chemically noxious than native plants (PlosOne 2010). Thus, in some cases native generalists can provide Biotic Resistance to invasions, a pattern we are continuing to investigate in a variety of contexts.