Smithsonian Environmental Research Center

I am interested in experimental and empirical tests of theory in community ecology, specifically theories that address the mechanisms generating community composition, coexistence, and the maintenance of diversity.  For almost a decade, I have conducted laboratory, field, and greenhouse projects to evaluate the diversity and abundance of microbial endophytes to build up to assess how plant-microbial interactions maintain ecosystem processes at larger scales.  Moreover, I have experimentally tested the degree to which invasive species respond to environmental change compared to equally dominant native species, which have a variety of implications in the context of global climate change, agricultural and horticultural practices, and sustainability. Below is a description of the projects that I have been and am currently involved in.

webpage: https://ericgriffin742.wixsite.com/mysite

Though the impacts of plant diversity on ecosystem function are well documented, the degree to which these interactions are related to plant-associated microbial communities has rarely been considered.  Fungal and bacterial endophytes, or those that reside inside plant tissues, are critical mediators of plant functional traits and trophic interactions.  Whether foliar endophyte communities mediate plant-herbivore interactions, however, is almost entirely unexplored for trees, which store the bulk of carbon worldwide.  I am simultaneously evaluating the degree to which plant diversity and herbivore damage are related to foliar endophyte community structure and composition using experimental forest experiments.  I am using high-throughput sequencing to quantify how fungal and bacterial endophyte community composition and structure vary with host tree diversity and identity, plant traits, mycorrhizal status, and herbivore damage.  My goal is to critically evaluate what structures endophyte communities and provide evidence for a novel dimension that links plant diversity to ecosystem processes.

The bacteria that inhabit the phyllosphere are abundant and diverse.  Moreover, bacteria can either be pathogenic or conversely produce anti-fungal compounds that protect their host plants.  Nonetheless, our understanding of phyllosphere–bacterial interactions in situ is negligible.  I began a research initiative to evaluate whether foliar bacteria are beneficial or harmful for woody species in a mature tropical forest in Panama.  To address this, I isolated strains of bacteria from leaves in the field for inoculation experiments in growth chambers and the greenhouse to assess the degree to which these strains are pathogenic and specific to plant hosts.  Moreover, I am interested in determining whether limiting soil resources mediate these responses using a 15-yr, factorial resource supply (N, P, K) experiment.  Ultimately, variation in the impacts of foliar bacteria across host species along resource gradients represents an unrecognized dimension of niche differentiation.  Additionally, foliar bacteria may be a cryptic but powerful driver of negative density dependence in plant communities, with important implications for species coexistence.  To date, my results suggest that phyllosphere bacteria-plant interactions are primarily pathogenic and can constrain the realized niches of host plants, with potentially large ramifications for species coexistence and maintenance.

Beta diversity of bacterial endophytes

Foliar bacterial endophytes are abundant and diverse among trees, but the scale of their diversity is unclear; unfortunately, studies are generally not comparable due to differences in endophyte isolation methods and bacterial species concepts.  Ultimately, beta diversity of endophytes remains unknown, and the degree to which ecological factors shape endophyte communities has not been examined.  I use high-throughput sequencing to quantify how ecological factors impact bacterial endophyte community composition and structure among forest trees.  These methods provide us with a critical tool for hypothesis testing with regard to the importance of forest diversity, host plant frequency, and abiotic factors in shaping host-endophyte associations, and for explicit examination of beta diversity at larger scales.

The recent advent of novel NMR metabolomic and liquid chromatography (LC)-in tandem mass spectrometry (mass spec) techniques allows us to identify molecular networks that capture the structural similarity of known and unknown compounds in plant samples.  With collaborators from the Smithsonian Tropical Research Institute and the University of Nevada at Reno, we are empirically testing how host species, soil nutrient availability, and foliar microbes structure leaf chemistry.  Ultimately, I am interested in using field and greenhouse experiments to see how feedbacks occur between soil properties, soil-associated microbes, herbivores, and leaf chemistry and determine which of these is most important in mediating plant performance and survivorship.

Ecological theory predicts that introduced species will respond positively to release from their natural enemies and that nutrient enrichment may enhance this response.  Moreover, it remains unknown how large-scale discrete disturbances disrupt the invasion process, and to what degree invaders recover from such disturbances.  I test this hypothesis using two species that commonly dominate North American wetlands: the invasive purple loosestrife (Lythrum salicaria) and the native broad-leaved cattail (Typha latifolia).  To date, my data suggest that invasive species benefit from natural enemy release and this effect is exacerbated with elevated nutrient levels.  Moreover, loosestrife recovers much more quickly than cattail after disturbances, suggesting that this invader may be more resilient to environmental fluctuations in a changing world.  These projects have led to many new questions and inspired plans for future work with invasive species and their impacts on diversity, sustainability and conservation, and human welfare.