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Browsing by Subject "geographic range limit"

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    Biotic interactions and edaphic variation modulate geographic range limits in Clarkia xantiana ssp. xantiana
    (2019-09) Benning, John
    The study of species’ geographic distributions, especially limits to those distributions, lies at a fruitful nexus of ecology and evolutionary biology. At these distributional limits, the ecological interactions that determine population mean fitness across and beyond the range limit collide with the evolutionary limits to adaptation. Species’ geographic distributions comprise the spatial extent of their populations, and vary greatly in size, shape, and the arrangement and abundance of populations contained therein. However variable these distributions are, they all are bounded by an invisible perimetric line on the landscape beyond which populations of that species cannot be found, i.e., the species’ geographic range limit. Why are individuals able to persist on one side of this border but are excluded from regions directly adjacent? This deceptively simple question is a perennial one that underlies many foundational questions about ecological interactions and adaptation. Distributions are structured by myriad factors with large, small, and interactive effects, but the essential determinants of the spatial patterns are relatively simple — populations persist where long term growth rates are equal to or greater than replacement (λ ≥ 1). But given sufficient time and adequate heritable variation for ecologically important traits, species’ ranges should theoretically be able to continually expand outward through sequential adaptation by populations at the range edge. Of course, this is not the observed pattern in nature; most species are restricted to a relatively small fraction of the planet’s available habitat. This dissertation is an attempt to untangle the complex environmental gradient that occurs across and beyond C. x. xantiana’s distribution, and evaluate the relative importance of precipitation, mammal herbivory, pollinator limitation, and biotic and abiotic edaphic factors in setting the subspecies’ geographic range margin. In Chapter 1, I focus on fatal mammal herbivory and evaluate its potential as a range limiting factor, with a conceptual approach based on foundational range limits theory. I show that probability of herbivory increases sharply near and beyond C. x. xantiana’s range margin, that this interaction has large effects on population mean fitness at the transplant site beyond the range edge, and that susceptibility to herbivory is largely mediated by plant phenology. In Chapter 2, I follow up on these results with a large field experiment at multiple sites inside and outside the range, estimating the effects of geography, source population, herbivory, and pollen limitation on lifetime fitness across two years. Protection from herbivory and supplementation of pollen increased plant fitness three to seven-fold outside the range margin, and there was only limited evidence of local adaptation of C. x. xantiana populations. Both of the transplant experiments reported in these chapters captured both a relatively wet and a relatively dry year, and showed that the effect of herbivory on population mean fitness differed across abiotic contexts — in dry years, precipitation limited fitness outside the range edge, but when C. x. xantiana was largely “released” from abiotic stress wet years, herbivory strongly limited population mean fitness. Chapters 3 and 4 focus on belowground - aboveground interactions. In Chapter 3, I use greenhouse and field experiments to ask how spatial variation in soil microbial communities influences plant local adaptation and the potential for range expansion in C. x. xantiana. Microbial communities from one site inside the range positively affected components of fitness in both the greenhouse and field, especially near to and beyond the range margin, and there was no evidence of local adaptation to microbial communities among plant populations. In Chapter 4, I report on an intensive field experiment where I factorially manipulated complete (i.e., biotic and abiotic) edaphic environments (growing plants with soil sourced from either within or beyond their native range) and precipitation to quantify the relative effects of within-range soil and increased precipitation on C. x. xantiana fitness outside its range margin. Across two years, edaphic environment had large effects on plant lifetime fitness that were similar in magnitude to the effects of precipitation. Moreover, mean fitness of plants grown with within-range soil in the low-water treatment approximated that of plants grown with beyond-range soil in the high-water treatment.