Tracy S. Feldman: Current Research Interests

My work focuses on the behavioral and population ecology of multi-species interactions, including mutualisms between plants and pollinators, facilitation between plants that share pollinators, and interactions between plants, plant pathogens and insect vectors. In general, these interactions can be seen as interactions between plants and animal vectors of pollen or pathogens. I use a combination of field and lab experiments, mathematical models, and molecular tools. In general, I am interested in the direct and indirect effects of behavior and species interactions on population-level phenomena. I am also interested in the ways human-induced changes (habitat fragmentation and introduced species) affect the behavior and population dynamics of species involved in multi-species interactions. My work so far addresses two main questions related to the maintenance and persistence of mutualisms, conditionality of mutualisms, and population dynamic consequences of species interactions. (1) What factors might enable plant-pollinator mutualisms to persist when plants are at low densities? (2) What are the behavioral, fitness, and population consequences of competitors on mutualisms and interactions between pathogens, hosts, and vectors?

(1) What factors might enable plant-pollinator mutualisms to persist when plants are at low densities?

In particular, I have focused on pollination facilitation, a process by which one plant species that co-occurs and shares pollinators with a second species actually increase each the reproductive success of the second species by increasing pollinator visits. For plants occurring at low densities, these increases in the quantity or quality of pollinator visits, leading to increased reproductive success, might be critical to population persistence. Thus, through facilitation, plant species may rescue one another from negative effects of growing at low density, by increasing the quantity or quality of pollinator visits, thus enhancing seed production. I explored this question for my dissertation work (with Dr. W. F. Morris at Duke University), using a variety of systems and approaches, including mathematical models and field experiments.

From two separate field experiments with two different focal plants, Brassica rapa and Piriqueta caroliniana, I found strong positive effects of increasing plant density on pollinator visitation (numbers of visits to patches and numbers of visits to plants per foraging bout) and reproductive success (seed production). In addition, evidence from a density-dependent matrix model for population growth in P. caroliniana demonstrates that populations may grow more slowly at low densities due to reduced seed production, but do not decline (Allee effects occur but are weak). P. caroliniana plants also receive more outcross pollen and produce more seeds in the presence of a co-flowering species, Coreopsis leavenworthii, indicating that facilitation occurs in this species, through increased visit quality (increased length of visits to flowers and probably to patches) in the presence of the co-flowering species.

Evidence from mathematical model I developed (with Dr. W. F. Morris and Dr. W. G. Wilson) suggests that one plant species can also facilitate another’s pollination when the number of pollinator visits to patches of plants per unit time (the aggregative response) accelerates at low densities. To my knowledge, these two mechanisms by which facilitation might occur have not been addressed previously. However, I found that in Brassica rapa the pollinator aggregative response did not accelerate at low densities. Thus, when facilitation occurs among plants at low density, it may be more likely to occur through disproportionate increases in visit quality rather than through increases in numbers of visits to patches of plants.

(2) What are the behavioral, fitness, and population consequences of competitors on mutualisms and interactions between pathogens, hosts, and vectors?

In almost all known mutualisms, additional species exploit or compete with one or both partners. However, the consequences of this exploitation are not often well understood. I have been developing a research project to address this question, using a system of multi-species interactions involving Claviceps, a genus of fungal pathogens that infect grasses. In the future, I plan to combine fieldwork, laboratory/greenhouse experiments, and molecular work to study the effects of a common pathogen (Claviceps paspali) on fitness of two invasive grass species (Paspalum notatum and P. dilatatum ) that are hosts to the pathogen. C. paspali can both severely damage grain crops and poison livestock that eat infected forage grasses. However, effects of this pathogen on whole-plant fitness remain largely unknown. During infections, C. paspali translocates sugars from its host grasses, producing sugar-rich exudates (laden with spores) that may provide a major food source to the hundreds of species of insects, including moths, flies, and wasps, that visit infected grass inflorescences and disperse fungal spores. Some of the insects attracted to infected inflorescences are capable of spreading fungal spores, thereby potentially spreading the disease. Thus, the fungal pathogen Claviceps paspali and its insect vectors are likely mutualists.

A second fungal species, Fusarium heterosporum, is a likely an exploiter of the mutualism between C. paspali and its insect vectors. F. heterosporum grows on the exudates produced by C. paspali, thereby potentially affecting fitness of the pathogen while also decreasing the potential reward reaped by insects that visit inflorescences infected by both fungi. F. heterosporum may also affect insect attraction to infected inflorescences, by altering volatile compounds produced by C. paspali, or by producing its own volatiles. Further, these infections may alter exudates produced by C. paspali, potentially affecting fitness of insects that feed on these exudates. Evidence suggests that moths may also disperse spores of F. heterosporum.

Further, several species of fungal endophytes live inside the leaves and stems of Paspalum species that are hosts of C. paspali, with unknown effects on fitness of host grasses, C. paspali, F. heterosporum, or insect vectors. These endophytes may affect pathogen resistance, which may indirectly affect insects that feed on fungal exudates. As a future research project, I hope to use this system as a model for insect-vectored pathogens of plants.