A key area of interest to ecologists is why a species occur in a particular range of environments, often called the species niche. Researcher have typically investigated how abiotic limitation of an organism (e.g. thermal tolerance) as well as negative species interactions (e.g. competition and predation) impact where organisms can successfully persist. While this work has proved invaluable, the effect of equally important and widespread positive species interactions on species niches has been virtually ignored. The PIs' research investigates the question: How participation in mutualism (a positive interaction in which all partners benefit) affects the niche of an organism? They examine this question in the association between endophytic fungi (fungi that lives inside plants) and a native grass, Chinook brome (Bromus laevipes). Models based on their endophyte occurrence data from ~100 populations of the grass and climate data strongly suggest that endophytes expand the niche of the grass (i.e. the grass can persist in habitats that would be untenable in the absence of the mutualistic partner). With NSF funding, the PIs will test this result by planting experimental gardens throughout California. Within each garden, they will plant Chinook brome with and without endophyte from 12 populations and measure plant performance (e.g. stem/leaf/seed production), phenology (e.g. flowering/dormancy timing), and herbivore damage.
This research will not only elucidate conceptual questions in ecology (e.g. species distributions and diversity, community assembly, and speciation) by incorporating positive interactions into niche theory, but it will also inform applied fields (e.g. agricultural and conservation activities). Although fungal endophytes live inside all plant species examined, we know little about their ecology or evolution in most cases. Given the large impact of endophytes on plant success in different environments, the information about endophytes garnered from this research can help make conservation, restoration, agriculture, or development decisions.
A key area of interest to ecologists is why a species occur in a particular range of environments, often called the species niche. Researcher have typically investigated how abiotic limitation of an organism (e.g. thermal tolerance) as well as negative species interactions (e.g. competition and predation) impact where organisms can successfully persist. While this work has proved invaluable, the effect of equally important and widespread POSITIVE species interactions on species niches has been virtually ignored. The PIs’ research investigated the question: How participation in mutualism (a positive interaction in which all partners benefit) affects the niche of an organism? They examined this question in the association between endophytic fungi (fungi that lives inside plants, Epichloë spp.) and a native grass, Chinook brome (Bromus laevipes) across the host species range. Species distribution models using data from their surveys of natural populations of the grass as well as climate data suggested that fungal endophytes have expanded the range of Chinook brome by ~20% into novel, drier habitats (i.e. mutualists allow plants to uniquely occupy dry areas). To determine if the endophytes were in fact causing this expansion of the grass’s range, rather than simply being correlated with habitat moisture, the PIs conducted a series of field and greenhouse experiments, which collectively demonstrated that endophytes caused range and niche expansion by enhancing drought tolerance of their hosts. For example, the PI planted seeds with and without endophytes at 10 sites throughout northern and central California (ca. 1/3 of the grass range), finding that survival of plants with the mutualist was higher in drier sites and survival of mutualist-free plants was higher in wetter sites. Further, the PIs conducted experiments looking at other possible causes of endophyte-mediated range and niche expansion. They set up five experimental gardens containing plants with and without endophyte from 11 populations and measured plant performance (e.g. mortality and stem/leaf/seed production), phenology (e.g. flowering/dormancy timing), and pest damage for the three years of the grant, finding that endophytes deter pest damage and increased plant fitness across the grass’s range. These results indicated that microscopic mutualistic partners can have substantial impacts on where species occur at very large spatial scales and, for the first time, clearly demonstrated that positive interactions can expand the niche and range. Intellectual Merit: By incorporating positive interactions into niche theory, this research informed important topics in ecology, such as species distributions and diversity, community assembly, and speciation. This work also demonstrated the importance of accounting for biotic interactions in determining species range limits and their response to climate change. In particular, the research emphasized the large impact and importance of considering positive interactions, including those with microscopic partners, in determining where organisms occur. This work was the first clearly demonstrated example of positive interactions causing range and niche expansion at a large spatial scale. Broader Impacts: The research also contributed to applied fields, such as agricultural and conservation. Although fungal endophytes live inside nearly all plant species examined, we know little about their ecology or evolution in most cases. Given the large impact of endophytes on plant success in different environments, the information about endophytes garnered from this research will help make conservation, restoration, agriculture, or development decisions. For example, because grasses are key components of many ecosystems (estimated to cover more than one-fifth of land on Earth) and are often the "workhorses" of restoration, understanding the role of these fungal symbionts in the persistence of grass populations may be vital for restoration of natural habitats. Further, this work has provided the opportunity for the PIs to educate/mentor students and the public. Since the beginning of the work on fungal endophytes, the PIs have mentored ~30 University of California undergraduates, encouraging female and under-represented minority participation in science (~15 female students and ~5 under-represent minority students). These experiences have allowed more than 1/3 of these students to go on to graduate school and/or to hold science research jobs in academic, government, or corporate environments. The PI’s research has also provided a platform for mentorship of high school students, education of elementary school children, and community outreach on native plant gardening.
