Ecology is increasingly becoming a study of variation. Most of my current research is aimed at teasing apart the effects of environmental variation on population and metapopulation dynamics: how much variation do we see at individual- and population-level scales, and how important is this variation for explaining the patterns we see at larger scales? I am particularly interested in what happens when population responses to environmental variation are nonlinear, a common but often ignored pattern in ecological systems.
Climate change and range shifts
Due to climate change, species are expected to shift their ranges upwards in elevation and latitude to track suitable climatic conditions. There is evidence that some species ranges have already shifted, and species distribution models predict future shifts for many species. However, species responses to climate can be complex in ways that are not accounted for by models that make simple assumptions, so we require a better understanding of the factors that determine range limits. I have two projects using demographic modeling to test for the effects of climate and competition on pine species and predict future range shifts in response to climate change. The first looks at elevation shifts in whitebark pine at the local scale, while the second looks at regional range dynamics of common pinyon pine, across its entire range.
WHITEBARK PINE:
Whitebark pine is a high-elevation pine tree found in western North America. As such, it's populations are in danger of contracting at low elevations if the climate becomes too hot and dry for the species. However, if the low range limit is determined by other factors, such as competition from other tree species, whitebark pine might tolerate warmer and drier conditions that it currently experiences, buffering it against the direct effects of climate. I am building demographic models to predict the climate conditions suitable for whitebark pine population growth to predict the extent of potential range contractions over the next century.
To study whitebark pine populations, I travel to the Elkhorn Mountains in northeastern Oregon, where I backpack into the mountains to collect demographic data. I get data on growth and survival of adult trees by collecting tree cores, and I climb the trees to collect cones and get data on reproduction. I also have a seedling planting experiment to test for the effects of elevation and density on the growth and survival of seedlings.
PINYON PINE: Common pinyon pine is a low- to mid-elevation pine found in the Four Corners states. This species is found at the lower limit of the forest biome, so a contraction at the low elevation limit of pinyon pine would mark a contraction of the forest biome. I am building demographic models, parameterized with Forest Inventory and Analysis data (www.fia.fs.fed.us/), to predict the population growth rate of pinyon pine across the entire region, based on environmental predictors. To compare the importance of different factors that might determine range limits, I am comparing models that include only climate predictors to models that include both climate and competition predictors. By comparing the output of the different models to the true distribution, I can determine where in environmental space different factors are important for determining range limits and where we might still be missing important processes.
WHITEBARK PINE:
Whitebark pine is a high-elevation pine tree found in western North America. As such, it's populations are in danger of contracting at low elevations if the climate becomes too hot and dry for the species. However, if the low range limit is determined by other factors, such as competition from other tree species, whitebark pine might tolerate warmer and drier conditions that it currently experiences, buffering it against the direct effects of climate. I am building demographic models to predict the climate conditions suitable for whitebark pine population growth to predict the extent of potential range contractions over the next century.
To study whitebark pine populations, I travel to the Elkhorn Mountains in northeastern Oregon, where I backpack into the mountains to collect demographic data. I get data on growth and survival of adult trees by collecting tree cores, and I climb the trees to collect cones and get data on reproduction. I also have a seedling planting experiment to test for the effects of elevation and density on the growth and survival of seedlings.
PINYON PINE: Common pinyon pine is a low- to mid-elevation pine found in the Four Corners states. This species is found at the lower limit of the forest biome, so a contraction at the low elevation limit of pinyon pine would mark a contraction of the forest biome. I am building demographic models, parameterized with Forest Inventory and Analysis data (www.fia.fs.fed.us/), to predict the population growth rate of pinyon pine across the entire region, based on environmental predictors. To compare the importance of different factors that might determine range limits, I am comparing models that include only climate predictors to models that include both climate and competition predictors. By comparing the output of the different models to the true distribution, I can determine where in environmental space different factors are important for determining range limits and where we might still be missing important processes.
Environmental heterogeneity and metapopulation dynamics
With habitat fragmentation resulting from human activities, species are increasingly becoming organized into metapopulations - geographically isolated populations that are connected to each other via dispersal between populations. A contemporary challenge in the study of spatially-structured populations is to understand the importance of realistic sources of spatial variance, which are ignored in classic models of metapopulation dynamics.
I use whitebark pine metapopulations as a model system to study the effects of nonlinear responses to spatial heterogeneity within population patches and whether these effects matter for metapopulation dynamics. Specifically, I am testing whether small-scale variation in elevation and density of whitebark pine trees affects the extinction and colonization dynamics that happen at a larger scale. Whitebark pine has a patchy distribution because it is found on mountain tops and replaced by other species at lower elevations. However, there is dispersal between patches because whitebark pine seeds are dispersed by Clark's Nutcracker, which will sometimes carry seeds more than 30 km to cache them for winter. Whitebark pine is also threatened by mountain pine beetle outbreaks and white pine blister rust, an invasive fungus. Along with fires, which are a natural part of this system, these threats put individual patches in danger of extinction. However, extinct patches can then be recolonized by dispersal by Clark's Nutcracker.
I use whitebark pine metapopulations as a model system to study the effects of nonlinear responses to spatial heterogeneity within population patches and whether these effects matter for metapopulation dynamics. Specifically, I am testing whether small-scale variation in elevation and density of whitebark pine trees affects the extinction and colonization dynamics that happen at a larger scale. Whitebark pine has a patchy distribution because it is found on mountain tops and replaced by other species at lower elevations. However, there is dispersal between patches because whitebark pine seeds are dispersed by Clark's Nutcracker, which will sometimes carry seeds more than 30 km to cache them for winter. Whitebark pine is also threatened by mountain pine beetle outbreaks and white pine blister rust, an invasive fungus. Along with fires, which are a natural part of this system, these threats put individual patches in danger of extinction. However, extinct patches can then be recolonized by dispersal by Clark's Nutcracker.