Climate Change, Phenological Shifts, and Species Interactions: Case Studies in Subalpine Plant and Migratory Fish Populations

Abstract

Phenology, the timing of biological events across the year, is shifting in response to climate change. Not all species within a community are responding to the same environmental cues by shifting their phenology to the same degree. As a consequence, the strength and direction of species’ interactions are also undergoing rapid changes. In this dissertation, I used observations, experiments, and demographic modeling to explore the relationship between climate, phenology, and species interactions in one terrestrial and one aquatic field system. With these two systems, I attempted to answer the central question, “What are the environmental drivers and ecological consequences of phenological shifts?”In chapter two, I examined how co-flowering subalpine spring ephemerals (Claytonia lanceolata, Mertensia brevistyla, and Mertensia fusiformis) in the Colorado Rocky Mountains, USA compete with and facilitate one another for biotic and abiotic resources. These flowering species were the first to emerge and flower shortly after snowmelt. As a result of phenological shifts, these species showed greater temporal overlap under early snowmelt conditions. Using field experiments, I found that these species did not facilitate one another or compete for pollinator resources, but they did affect each other’s vital rates in two years.In chapter three, I simulated how the presence of neighbors, including all heterospecific neighbors, affected population growth of Claytonia and Mertensia under future predictions of spring snowmelt date. I used data from field experiments to parameterize integral projection models and forecast population growth into the future. I found that neighbors significantly influenced population growth rate under average snowmelt conditions, but not under early snowmelt conditions. Under future predictions of early snowmelt, populations declined rapidly regardless of neighbor presence.In chapter four, I worked with the Northeast Climate Adaptation Science Center to determine the environmental drivers of alewife (Alosa pseudoharengus) migration in Massachusetts. First, I used field-collected daily fish counts to assess how the timing of migration has changed. I found that some of the streams showed significant advances in run timing, while others did not. Second, I combined migration timing metrics with publicly available climate data. I found that shorter, wetter winters and mild spring temperatures were correlated with earlier run initiation dates.Collectively, my research from subalpine meadows and coastal streams revealed three significant conclusions. First, the temporal variability in the strength and direction of species interactions may be important for predicting future coexistence. Second, unequal phenological shifts between members of a community may not only impede coexistence, but could facilitate coexistence in the future through positive effects on population growth. Finally, experiments mechanistically linking phenology and species interactions are necessary for understanding implications of phenology on coexistence.