Tracy S. Feldman: Current Research Interests

I study direct and indirect effects of behavioral and species interactions on population-level phenomena: When do interactions between species have fitness consequences for individuals, and when do these fitness effects influence population dynamics? I am also interested in the ways human-induced changes, such as habitat fragmentation and introduced species, affect the behavior and population dynamics of species involved in multi-species interactions (including mutualisms). In my research, I address these issues using a combination of field and lab experiments, statistical and mathematical models, and molecular tools. In addition, I focus primarily on interactions involving fungi, and more recently viruses, which touch on the rapidly growing but still relatively poorly developed fields of microbial and virus ecology. I have studied plant-associated fungi and fungal viruses; indirect interactions involving plants that share pollinators, or between plants, plant-associated fungi and insect vectors; multi-trophic interactions between plants; and mutualisms between plants and pollinators. Thus, I study animal vectors of pollen or fungal pathogens, and their individual and population-level effects on fungi and plants. My work addresses three main questions. (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 involving plant pathogens and their vectors? (3) What role do plant-associated fungal viruses play in ecological communities? To illustrate my approaches to science, I will describe my work in each of these 3 areas below.

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

Mutualisms may be particularly susceptible to extinction in rapidly changing or highly disturbed environments. One mechanism that may allow pollination mutualisms to persist is pollination facilitation, a process by which reproductive success of a rare plant species increases in the presence of a second species due to increased visits from shared pollinators. For plants occurring at low densities, this mechanism 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, and therefore 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 (measured as visits to plants per hour and visits to plants per foraging bout) and reproductive success (seed production). In addition, evidence from a density-dependent projection 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).

To determine when pollination facilitation might rescue plants from negative effects of low density, I conducted experiments using Coreopsis leavenworthii, a co-flowering species that shares pollinators with P. caroliniana. I found that visitation, pollen receipt, and reproductive success of P. caroliniana remain unaffected in the presence of the co-flowering species. Thus, pollination facilitation may not occur when plants simply share pollinators and co-occur at low densities—other criteria are important. Evidence from a 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, this mechanism by which facilitation might occur has not been addressed previously. However, in a second experiment with Brassica rapa, the pollinator aggregative response did not accelerate at low densities. Although pollination facilitation has been found in several systems, and occurs by several different mechanisms (e.g. through disproportionate increases in visits to individual plants or through maintenance of larger pollinator populations in more diverse plant communities), facilitation may be unlikely to occur through increases in visits to patches of plants.

Combining field experiments with mathematical models enabled me to use field data to predict population dynamics under different scenarios. In general, this interdisciplinary approach is necessary for understanding complex processes that are difficult to measure directly. In the future, I hope to continue to address population-level consequences of plant-pollinator interactions, including other factors that may cause population decline, such as self-pollination in outcrossing species.

(2) What are the behavioral, fitness, and population consequences of competitors on mutualisms involving plant pathogens and their vectors?

In almost all known mutualisms, additional species exploit or compete with one or both partners, raising the possibility that exploiters could erode the mutualism over ecological or evolutionary time. However, the consequences of exploitation for most mutualisms are not well understood. I have been developing a research project to address this question in the context of 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 the fitness of two invasive grass species (Paspalum notatum and P. dilatatum) that are hosts to the pathogen. C. paspali can severely damage grain crops and poison livestock that eat infected forage grasses. However, effects of this pathogen on whole-plant fitness remain largely unknown. Claviceps species are likely involved in mutualistic interactions with insect vectors. 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 diverse insects in several orders 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.

Other fungal species, including Fusarium heterosporum, may exploit 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 change the chemistry of the system, affecting insect attraction to infected inflorescences or fitness of insects that feed on the exudates. Further, several species of plant-associated fungi 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. I have submitted a paper coauthored by Dr. H. E. O’Brien and Dr. A. E. Arnold demonstrating that moths carry spores of F. heterosporum and fungal endophytes of Paspalum species. As a future research project, I hope to use this system as a model for insect-vectored pathogens of plants. Also, I believe that many questions in this system are conducive to involving undergraduates in research.

(3) What role do plant-associated fungal viruses play in ecological communities?

Virtually all plant (and perhaps animal) species harbor pathogenic or mutualistic fungi in their tissues. Viruses of these fungi have the potential to affect parasitism by changing virulence of fungal pathogens or mutualisms by altering host tolerance to environmental stress. However, almost nothing is known about fungal viruses in an ecological context. In addition, although the “shotgun” sequencing approach has illuminated extremely diverse and previously unknown fungi, bacteria and viruses, it is often difficult to determine the associations between individual species from these presence/absence data. As part of my postdoctoral research (part of a large NSF-EPSCoR funded project on plant virus biodiversity, with Dr. M. J. Roossinck), I am conducting a study of fungal virus prevalence and diversity in a tall grass prairie ecosystem in Oklahoma. In this system, fungi in diverse genera are hosts to viruses in more than 8 families. All of the 18 viruses I encountered so far (from a sample of 125 plant-associated fungi) are novel, and some are unrelated to known viruses. I am currently working to develop a system to study effects of some of these fungal viruses on their fungal hosts (Alternaria alternata and Stemphylium solani), and on plants associated with these fungi: the parasitic plant Cuscuta cuspidata and one of its host plants, Ambrosia psilostachya. I use several molecular tools, including DNA extraction and polymerase chain reaction (PCR) to identify the Cuscuta species (Cuscuta species are not morphologically distinguishable unless they are flowering) and fungal species, sterile techniques to culture the fungi, and double-stranded RNA extractions and reverse-transcription PCR to obtain partial sequences of fungal viruses. I also use phylogenetic tools to determine species identities of fungi, and hope to use such tools in the future to look at relationships among viruses. I hope to continue work to address effects of fungal viruses on their host fungi and on interactions between fungi and their associated plant hosts.