The ecological niche is an essential concept for studies in ecology, evolution and biogeography. Geographic distributions are largely determined by species’ ecological niches. In turn, niches evolve via selection stemming from where species occur, which has implications for coexistence and the breadth of environmental tolerance. With modern comparative methods, we can improve our understanding of interactions among niche, range and diversification across spatial scales. To select variables for quantifying niche properties, I first applied generalized linear models with occurrence data of 71 western North American monkeyflowers (Mimulus sensu lato). Then I evaluated the relative importance of four bioclimatic variables by ranking them based on the magnitudes of model-averaged regression coefficients. Thus three out of four bioclimatic variables were identified as important predictors in determining geographic distributions of Mimulus species, while one variables was negligible due to its small effect. To determine how geographic overlap affects niche divergence, I quantified niche divergence for 16 closely related Mimulus species pairs. I found that macrohabitat niche divergence decreased with increasing range overlap, consistent with environmental filtering operating in sympatry and divergent selection operating in allopatry. For species pairs with partially overlapping ranges, greater microhabitat niche divergence was found in sympatry, consistent with competition driving divergence where species interact. Phylogenetic distance was positively related to niche divergence for two macrohabitat axes but negatively related for one microhabitat axis. This suggests increasing coarse-scale niche similarity with increasing sympatry following allopatric speciation, while greater local-scale niche divergence accumulates through time. Given differences in evolutionarily lability of niche axes across spatial scales, I next examined evolutionary trends in niche breadth. For 82 Mimulus species, I converted niche breadths into binary states, generalist or specialist. Then I tested whether niche breadth affected diversification rate and explored evolutionary transitions. My results showed higher diversification rates for generalists and weak generalist-to-specialist trends for three bioclimatic variables, but higher diversification rates for specialists and weak specialist-to-generalist trends for two microhabitat variables. Together, these results suggest that ecology plays an essential role in diversification processes, but underlying mechanisms might differ across spatial scales.
Every species experiences limits to its geographic distribution on the landscape. Sometimes the barriers that limit geographic ranges are obvious. For example, oceans and topographic features may prevent a species from colonizing the areas beyond them. However, species' distributions frequently end at places on the landscape where no obvious barrier or abrupt shift in the environment occurs, and this raises the question of what limits the range at these edges, both proximately and in evolutionary time. This thesis investigates the contributions of pollination, climate, and gene flow to limiting range edge populations of an annual wildflower, Clarkia pulchella. Pollinators may be important at range edges because many of the proposed characteristics of edge populations (small, isolated, or low density) are also features that might make pollination less reliable and in some cases favour the evolution of self-pollination. I found that climate influences floral morphology and that the capacity of plants to set seed in the absence of pollinators was slightly higher in northern range edge populations. All populations benefit from the service of pollinators. Another factor that may limit populations at geographic range edges is the influence of asymmetric gene flow from central populations, which could prevent local adaptation in range edge populations. Alternatively, edge populations might have low genetic variance and therefore might benefit from gene flow. I tested these competing predictions by simulating gene flow between populations from across the species' range in the greenhouse and planting the progeny into common gardens at the northern range edge. This experiment took place during an extremely warm year. As a result, gene flow from warmer provenances improved performance. I also found a small benefit of gene flow independent of climate. Finally, I found no evidence that environmental differences contribute to genetic differentiation of populations, though geographic distance is a strong predictor of genetic differentiation. Contrary to expectations, genetic variation was higher near the northern range edge. Together, these chapters shed light on important drivers of reproductive success and local adaptation in this species and allow for insights into what processes are likely (or unlikely) to generate range limits.
Correlative ecological niche models, built with species’ occurrence records, have become the most widespread methods to forecast range shifts with climate change, but these models assume species’ range limits are driven by their niche limits. If a species range limit is instead the result of dispersal limitation, then these correlative based models will be poorly calibrated and largely inaccurate. I used experimental field transplants within and beyond the northern range limit of the scarlet monkeyflower (Mimulus cardinalis) to test for dispersal limitation and to see if climatic-based ecological niche models were able to accurately predict site-level suitability. I also compared predictions from the niche models to a previous study that transplanted the species beyond its upper elevational range limit, which is known to be fitness limited rather than dispersal limited. Predictions from the niche model closely matched observed fitness from the field transplant experiment across the species’ elevational range limit, but not across the species’ northern latitudinal range limit. Consistently high fitness was maintained within and beyond the northern range limit and even in sites of low predicted suitability, suggesting the northern range limit is dispersal limited. I then constructed an alternative ecological niche model for M. cardinalis with stream habitat variables, rather than climatic variables and controlled for the influence of climatic mechanistically, with a simple thermal envelope. This alternative model demonstrated a large amount of suitable habitat beyond the northern range limit, further supporting that this range limit is largely dispersal limited rather than fitness limited. Dispersal limitation presents a serious systemic challenge for the correlative niche modeling framework and its associated applications. By combining niche models with field experiments, I was able to show both the strengths and weaknesses of these methods and use existing theory of dispersal limitation as a framework to assess the accuracy of these models.