Global warming is often hypothesized to facilitate biological invasions. We tested this hypothesis by planting phylogenetically paired invasive, non-invasive exotic, and native species into an experimental field heating array at KBS. Preliminary results suggest that although species vary in warming response, native and invasive species on average respond similarly. Interestingly, invasive species differ dramatically from non-invasive exotic and native species in phenological response to warming. Invasive species substantially advance flowering under warmer conditions, but native and non-invasive exotic species do not. Additional analyses measure herbivory to test how global warming influences the strength of enemy release, building on previous work testing the enemy release hypothesis by former student Liz Schultheis.
The legume-rhizobium symbiosis is a classic example of mutualism. Leguminous host plants trade carbohydrates for the nitrogen fixed by their rhizobium symbionts. In collaboration with Katy Heath (U. of Illinois), we are investigating how long-term nitrogen addition treatments at the KBS LTER have influenced the ecology and evolution of rhizobium populations and their Trifolium host plants. Nitrogen-rich environments have been hypothesized to shift the legume-rhizobium mutualism towards parasitism because plants can obtain nitrogen more efficiently from the abiotic environment than from their rhizobium symbionts. We find that long-term nitrogen addition experiments have caused the evolution of less mutualistic rhizobia (Weese et al. 2015), that horizontal gene transfer has contributed to the evolution of less cooperative rhizobia (Gordon et al. 2016), and have identified the genes underlying these evolutionary responses (Klinger et al. 2016). Our current work investigates the ecological consequences of these evolutionary changes and how the evolution of reduced cooperation influences pollinators, herbivores, and soil nitrogen availability.
Intraspecific genetic variation can have large effects on ecological processes; however, effects may be underestimated because most studies to date are on small spatial-scales and short time-scales. In collaboration with Lars Brudvig (MSU) and Emily Grman (Eastern Michigan University), we are manipulating both plant genetic diversity and plant species richness at the field scale. Our experiment opens up new research possibilities, such as studies investigating the effects of genetic diversity on population demography and plant evolution. We have established permanent sampling transects within each site and are working with students and citizen scientists to monitor these treatments over the coming decades. Shorter-term manipulations explored the effects of climate warming, herbivory by insects and mollusks, seed consumption, soil microbes, and invasive species on native species establishment. Results from these studies document that warming changes which seed sources establish best in prairie restorations; that arthropods are voracious seed predators and potentially more important to prairie establishment than previously thought, and that patterns of prairie plant species establishment are impacted by vertebrate (mammals and birds) consumers and proximity to the restoration site’s edge.
The Enemy Release Hypothesis (sensu Elton) is one of the key hypotheses explaining the success of invasive species. If invasives commonly escape their enemies and if this enemy release is responsible for their dominance over natives, then excluding enemies may help level the playing field and increase native plant abundance. We established long-term enemy exclosures in swamps, old fields, forests, and restored grasslands to test how enemies influence the relative abundance of native vs. exotic plant species.Many of these exclosures were constructed in public areas and Dave Williams (Lawton High School) is currently developing signage and activities for citizen scientists of all ages to engage in their own data collection on a subset of our plots.