Evolutionary integration of invasive species
Exotic species often gain ecological advantages over their competitors in their new range due to a mismatch of evolutionary histories. While the native species are enmeshed in a web of coevolved interactions which can act to limit their population growth, the exotic species has left behind most of its coevolved interactors. For several years now, we have been interested these evolutionary mismatches, and if, and how, they are “resolved” over time due to evolutionary change in both the exotic and native species. Beginning during his time at the Illinois Natural History Survey, Rick has been investigating this process with garlic mustard (Alliaria petiolata) invasions into forest understories. Garlic mustard produces toxic secondary compounds that inhibit the growth of mycorrhizal fungi. Most native plants partner with these fungi to aid in nutrient acquisition, but garlic mustard does not. We have found that over time, garlic mustard populations tend to decrease their genetic investment to these chemicals. Mycorrhizal communities tend to decline in diversity during garlic mustard invasions, but seem to recover some of this lost diversity in sites with the oldest invasions. Finally, at least one native plant appears to be evolving in response to garlic mustard, becoming more tolerant to competition from the invader in part due to a reduced dependency on soil microbes, and a better ability to retain mycorrhizal colonization when competing with the invader.
Chelsea Cunard is continuing with this line of inquiry, studying how population dynamics and plant-soil feedbacks in another aggressive forest understory invader, Microstegium vimineum, change over its invasion history. A central goal of both projects is to understand how these invasions change through time, and how our management and restoration of invaded habitats might be improved by considering these changes.
Plant-soil interactions and climate change
Climates are currently warming at unprecedented rates. Species must move to track the changing climate or evolve to tolerate warmer conditions; those that fail to do so face extinction. Most plant species, especially forest trees, rely on intimate associations with microbial species living in soil in order to capture the resources they need for proper growth. Little is known about how these invisible, but very important, soil microbes are distributed across the continent, and how they will respond to climate change. With funding from the NSF’s new Dimensions of Biodiversity panel, Daniel Keymer and I are investigating the genetic, taxonomic, and functional biodiversity of soil microbial communities from forests across the eastern US to test for parallel latitudinal patterns with respect to climate. Trees and soil microbes will likely not move at equal speeds as climates change. Therefore, we are also using experiments to test the functional consequences for tree growth for situations where microbial species migrate slower or faster than trees. Our hope is that this research will allow for more precise predictions about how forests will change as a function of the changing biodiversity of the fungal symbiont community during climate warming.
Partner choice in plant-mycorrhizal fungal interactions
The vast majority of plant species form symbiotic relationships with soil fungi in the phylum Glomermomycota to form arbuscular mycorrhizae, in which the fungi provide increased access to soil resources (phosphorous, nitrogen, and water) to the plants and the plants provide sugars to the fungi. While this relationship is often presented as a classic example of a mutualism, in which both parties benefit, in reality the outcome of the interaction can range from mutualistic to parasitic for either partner. Arbuscular mycorrhizal fungi (AMF) are considered broad host generalists, since almost any AMF species can form a connection with almost any AM plant. However, we know that 1) the functional consequences for plant and fungal fitness can vary widely depending on the species involved, and 2) in nature, plant species interact with a non-random set of the available AMF species. Rick is collaborating with Chang Hyun Khang to study the transcriptional responses of rice plants to colonization by beneficial and harmful AMF species, along with a true fungal pathogen, to see if rice plants are able to resist colonization by harmful fungi via defensive responses. Jackie Freeman is exploring similar ideas in tomato, testing whether wild vs. domesticated tomato varieties prefer different AMF species, and are better or worse at preferentially interacting with beneficial fungi. Our ultimate goal in both projects it to understand the genetic control over plant-AMF interactions, potentially leading to breeding programs to maximize plant benefits from AMF in agricultural settings.
Mutual feedbacks in the maintenance of genetic and species diversity
During Rick’s PhD thesis at UC Davis, he investigated possible feedbacks in the maintenance of genetic diversity in a chemical trait of Brassica nigra and species diversity among several competing plant species. The selective value of sinigrin, a secondary compound of B. nigra with defensive and allelopathic properties, depended on the complexity of the surrounding community, including specialist and generalist herbivores and the species of plant competitor. Additionally, the fitness of competing plant neighbors depended on the sinigrin concentration of B. nigra individual. Together, these effects resulted in a “rock-paper-scissors” type competitive intransitivity between high and low sinigrin B. nigra genotypes and three other plant species. This intransitivity promoted the maintenance of genetic and species diversity because it prevented any one species or genotype from dominating the community. Models parameterized with field data suggest that the observed genetic variation was sufficient to allow long term coexistence of all three species pairs, even though two pairs were not predicted to coexist in the absence of genetic variation. Further work showed that this pattern arose primarily due to feedbacks with altered soil communities, especially mycorrhizal fungi.
Rick is still very interested in the role of intraspecific competitive trade-offs, genetic variation, and rapid evolution in the coexistence of species and structuring of communities, and recently published a review of the topic (see “Publications”).