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About the Paper

Understanding the mechanisms that limit species’ distributions is one of the most fundamental questions in ecology. Explanations for the existence of distribution limits often appeal to either a pattern of declining environmental suitability and demographic performance towards the edge of the distribution (niche limitation) or an inability to disperse to suitable habitat beyond the distribution’s boundary (dispersal limitation). These two explainations are often viewed as mutually-exclusive, and the vast majority of studies examine only one of these mechanisms when attempting to decipher the determinants of distributional boundaries. In contrast, ‘metapopulation’ models posit that niche and dispersal limitation act in tandem to determine the boundary of species’ distributions, but simultaneous evaluations of performance across the distribution and dispersal limitation beyond its edge are rare.

In this study, we used a demographic monitoring approach to determine how stochastic population growth rate of populations changes from the center to the northern edge of the distribution of the perennial forb Astragalus utahensis, and whether this pattern is also reflected in abundance. We also used a transplant experiment conducted at and beyond the northern range edge to simultaneously evaluate dispersal limitation of this edge. Stochastic population growth rate and abundance declined from populations at the center of the distribution to those at the northern edge, indicating niche limitation of this portion of the distribution. However, transplants beyond the northern range edge performed similarly to those at the edge, indicating simultaneous dispersal limitation. Taken together, these results reveal that the northern range boundary of A. utahensis results from both niche and dispersal limitation in accordance with the predictions of ‘metapopulation’ models for range limits. 

This study is among the first to simultaneously examine an integrative measure of demographic performance across a significant portion of a species’ latitudinal distribution and multiple aspects of its performance in transplanted populations beyond its boundary. This is, to our knowledge, one of the first empirical demonstrations of a distribution boundary imposed by both niche and dispersal limitation. Our findings suggest that the determinants of species’ geographical distributions may be more complex than previously recognized, and suggest that further research is needed to evaluate the extent to which this pattern applies across taxa along with and the environmental mechanisms underpinning variation (or lack thereof) in demographic performance across and beyond species’ distributions.

About the Research

This paper represented a portion of my Ph.D. dissertation, which focused on developing a comprehensive understanding of how environmental conditions and dispersal limitation limit the distribution of A. utahensis. Studies of this kind are logistically challenging, and this was no exception. Tracking the demography of plants in populations spanning a distance of over 400 km over the course of several years while performing weekly experimental manipulations in two-thirds of the populations was quite difficult and made for long days. The results of these experiments are also published in Journal of Ecology: Pre‐dispersal seed predation and pollen limitation constrain population growth across the geographic distribution of Astragalus utahensis (Baer & Maron, 2018). We developed creative methods for carrying trays of transplants up steep, rocky slopes, and occasionally conducted demographic monitoring by the light of headlamps. Perhaps the most exciting part of the research was seeing transplants nearly 200 km north of the nearest population produce mature seed pods. That was the point at which we knew we there might be a really interesting story to tell that could contribute to shifting the conversation about the processes limiting geographic distributions.

About the Author

I am from Alaska, and a childhood spent largely outside led to a curiosity about all things natural and wild. I fell in love with ecology’s big-picture, integrative focus in college. I earned my bachelor’s degrees in Biological Sciences and Spanish at the University of Alaska Fairbanks in 2007 and my Ph.D. in Ecology and Organismal Biology from the University of Montana in 2017 under Dr. John Maron. I worked as a postdoctoral fellow with the US Department of Agriculture Forest Service in Anchorage, Alaska from 2017-2019 and was recently hired as a research ecologist with the US Forest Service’s Pacific Northwest Research Station. I am currently working on a broad range of projects that mainly focus on how rapidly changing climatic conditions in subarctic Alaska may affect the distribution of important subsistence plant resources, disturbance regimes and resultant plant community composition and successional trajectories, and pollinator community composition in Alaskan forests. I am also part of a Citizen Science Initiative in interior Alaska focused upon collaboratively answering questions of importance to local community members.

Working as an ecologist is both rewarding and challenging. It took me a while to realize that there is no handbook for the ‘right way’ to be an ecologist, and that the only rule is to try anything you can think of and pay attention to everything along the way. I am incredibly fortunate that my job is to ask questions that I think are interesting and find a creative way to answer those questions and share the answers.

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