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Graduate Student Supervision
Doctoral Student Supervision (Jan 2008 - May 2021)
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.
Master's Student Supervision (2010 - 2020)
Limber pine (Pinus flexilis) is one of the North American white pines under duress. Pressured by several forces including an introduced fungal pathogen (Cronartium ribicola) and changing climate, this is a species of conservation concern in many regions. The ability to adapt to shifting conditions depends on the amount and distribution of standing genetic variation. To evaluate the patterns and extent of variation in limber pine, I examined needle traits with regards to pathogen infection, and the distribution of quantitative traits in the context of climate. Specifically, I measured how leaf traits differ after surviving an inoculation with C. ribicola and found that survivors of infection had significant differences in needle size, stomatal density and specific leaf area compared to uninoculated controls. In addition, the variance of each trait shifted modestly, pointing to signs of both phenotypic selection and plasticity. Next, I examined the structure of quantitative genetic variation across 16° of latitude by phenotyping traits in a common garden experiment. This trial revealed that population differences explained between 1-24%, and family between 1-20% of the total phenotypic variance, depending on the trait under inspection. This corresponded to a mean Qst estimate of 0.158 (range 0.02-0.19), with growth traits exhibiting the greatest population differentiation. Precipitation-related climate variables were the strongest predictors of differences among populations. These results suggest that limber pine has relatively low levels of quantitative genetic variation among populations, but an almost equivalent amount within populations. Whether or not it will be sufficient to cope with the many stresses this species contends with remains unclear.
Under the pressure of anthropogenic climate change, species that are negatively impacted must rapidly respond or risk extirpation. The most immediate option for many species will be to track changing distributions of suitable habitat. Comparisons of contemporary data to historical baseline data indicate that climate change has already altered ranges and abundances of numerous species. Though general patterns are slowly emerging, there is still considerable variation in responses among species. Further, a number of species do not appear to be undergoing any change in their distributions or abundances, despite possible fitness costs of stasis. Given this variation, mechanisms underlying whether species shift or do not shift must be elucidated to allow for the creation of a predictive framework that can be extended to other systems. One way to achieve this end is to associate species functional traits with their magnitude of response. To detect elevational range shifts and changes in abundance of plant species, I and a team of surveyors resurveyed historical vegetation plots in North Cascades National Park. Since the original 1983 survey, the area has warmed by approximately 0.8 ⁰ C. I then tested whether variation in range shifts among species could be associated with functional traits. Overall, most species exhibited range stasis. Of the species that initially appeared to exhibit a range shift, more than half were eliminated after accounting for fires and differences in survey effort between years. Species tended to decrease in abundance within their range, though this trend was often not significant. Predictions from trait models were inconsistent, depending on the modeling framework, the metric used for range shifts, and the inclusion of an outlying species. Range stasis was likely driven by dispersal limitation, but may have also resulted from acclimation, slow demographic processes, microclimate buffering of atmospheric temperatures by landscape features, or some combination of these and other factors. The variation in the degree of range shifts could not be explained satisfactorily by functional traits, casting doubt on their use in a general framework to predict future responses.
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.
- Geographic and climatic drivers of reproductive assurance in Clarkia pulchella (2018)
- Demographic compensation does not rescue populations at a trailing range edge (2018)
Proceedings of the National Academy of Sciences, 115 (10), 2413--2418
- Floral trait variation and links to climate in the mixed-mating annual Clarkia pulchella (2018)
Botany, 96 (7), 425--435
- Gene flow improves fitness at a range edge under climate change (2018)
- Genetic differentiation is determined by geographic distance inClarkia pulchella (2018)
- Moving Character Displacement beyond Characters Using Contemporary Coexistence Theory (2018)
Trends in Ecology & Evolution, 33 (2), 74--84
- Species Ranges (2018)
Reference Module in Life Sciences,
- The effect of range overlap on ecological niche divergence depends on spatial scale in monkeyflowers (2018)
Evolution, 72 (10), 2100--2113
- Local Adaptation Interacts with Expansion Load during Range Expansion: Maladaptation Reduces Expansion Load (2017)
The American Naturalist, 189 (4), 368--380
- When are species invasions useful for addressing fundamental questions in plant biology? (2017)
American Journal of Botany, 104 (6), 797--799
- A synthesis of transplant experiments and ecological niche models suggests that range limits are often niche limits (2016)
Ecology letters, 19 (6), 710--722
- Artificial selection reveals high genetic variation in phenology at the trailing edge of a species range (2016)
The American Naturalist, 187 (2), 182--193
- Effects of range-wide variation in climate and isolation on floral traits and reproductive output of Clarkia pulchella (2016)
American journal of botany, 103 (1), 10--21
- Grow with the flow: a latitudinal cline in physiology is associated with more variable precipitation inErythranthe cardinalis (2016)
- The scale of local adaptation in Mimulus guttatus: comparing life history races, ecotypes, and populations (2016)
New Phytologist, 211 (1), 345--356
- Assessing the potential for maladaptation during active management of limber pine populations: a common garden study detects genetic differentiation in response to soil moisture in the Southern Rocky Mountains (2015)
Canadian Journal of Forest Research, 45 (4), 496--505
- Where and when do species interactions set range limits? (2015)
Trends in ecology & evolution, 30 (12), 780--792
- Disentangling the drivers of context-dependent plant--animal interactions (2014)
Journal of Ecology, 102 (6), 1485--1496
- High water-use efficiency and growth contribute to success of non-native Erodium cicutarium in a Sonoran Desert winter annual community (2014)
Conservation Physiology, 2 (1), cou006
- Identifying the paths leading to variation in geographical range size in western North American monkeyflowers (2014)
Journal of Biogeography, 41 (12), 2344--2356
- Phenotypic constraints and community structure: Linking trade-offs within and among species (2014)
Evolution, 68 (11), 3149--3165
- The evolution of environmental tolerance and range size: a comparison of geographically restricted and widespread Mimulus (2014)
Evolution, 68 (10), 2917--2931
- Using among-year variation to assess maternal effects in Pinus aristata and Pinus flexilis (2014)
Botany, 92 (11), 805--814
- Climate change and species interactions: ways forward (2013)
Annals of the New York Academy of Sciences, 1297 (1), 1--7
- Phenotypic selection favors missing trait combinations in coexisting annual plants (2013)
The American Naturalist, 182 (2), 191--207
- Photosynthetic temperature responses of co-occurring desert winter annuals with contrasting resource-use efficiencies and different temporal patterns of resource utilization may allow for species coexistence (2013)
Journal of arid environments, 91, 95--103
- Understanding past, contemporary, and future dynamics of plants, populations, and communities using Sonoran Desert winter annuals (2013)
American journal of botany, 100 (7), 1369--1380
- Water-use efficiency and relative growth rate mediate competitive interactions in Sonoran Desert winter annual plants (2013)
American journal of botany, 100 (10), 2009--2015
- Fitness and physiology in a variable environment (2012)
Oecologia, 169 (2), 319--329
- Range-wide patterns of genetic population structure and potential geographical range shifts of Pinus contorta (ssp. latifolia, murrayana, contorta, and bolanderi) (2012)
97th ESA Annual Meeting,
- Variation in photosynthetic response to temperature in a guild of winter annual plants (2012)
Ecology, 93 (12), 2693--2704
- Differences in the timing of germination and reproduction relate to growth physiology and population dynamics of Sonoran Desert winter annuals (2011)
American Journal of Botany, 98 (11), 1773--1781
- Do species’ traits predict recent shifts at expanding range edges? (2011)
Ecology Letters, 14 (7), 677--689
- Incorporating population variation in thermal niche properties into geographic range shift predictions (2011)
Integrative and Comparative Biology, 51, E3--E3
- Incorporating population-level variation in thermal performance into predictions of geographic range shifts (2011)
Integrative and comparative biology, , icr048
- Quantifying the impact of gene flow on phenotype-environment mismatch: a demonstration with the scarlet monkeyflower Mimulus cardinalis (2011)
The American Naturalist, 178 (S1), S62--S79
- The effect of geographic range position on demographic variability in annual plants (2011)
Journal of Ecology, 99 (2), 591--599
- Contemporary climate change in the Sonoran Desert favors cold-adapted species (2010)
Global Change Biology, 16 (5), 1555--1565
- Phenotypic plasticity and precipitation response in Sonoran Desert winter annuals (2010)
American Journal of Botany, 97 (3), 405--411
- SYMP 7-4: Can species' traits predict recent range shifts? (2010)
The 95th ESA Annual Meeting,
- Time scales of biogeochemical and organismal responses to individual precipitation events (2010)
AGU Fall Meeting Abstracts, 1, 02
- COS 53-1: Climate induced changes in plant community composition in the Sonoran Desert (2009)
The 94th ESA Annual Meeting,
- Evolution and ecology of species range limits (2009)
- Functional tradeoffs determine species coexistence via the storage effect (2009)
Proceedings of the National Academy of Sciences, 106 (28), 11641--11645
- OOS 28-2: Population dynamics across species' ranges: Comparative demography of central and marginal populations of Mimulus cardinalis and M. lewisii (2009)
The 94th ESA Annual Meeting,
- Phenology, stochasticity, and the demography of plant populations (2009)
94th ESA Annual Meeting,
- The niche, limits to species' distributions, and spatiotemporal variation in demography across the elevation ranges of two monkeyflowers (2009)
Proceedings of the National Academy of Sciences, 106 (Suppl), 19693--19698
- COS 108-4: Functional tradeoffs promote species coexistence via the storage effect (2008)
The 93rd ESA Annual Meeting,
- COS 29-6: Phenological differences promote coexistence in Sonoran Desert winter annuals (2008)
The 93rd ESA Annual Meeting,
- Phenological Differences Promote Coexistence in Sonoran Desert Winter Annuals (2008)
AGU Fall Meeting Abstracts, 1, 0384
- Photosynthetic resource-use efficiency and demographic variability in desert winter annual plants (2008)
Ecology, 89 (6), 1554--1563
- Using experimental evolution to investigate geographic range limits in monkeyflowers (2008)
Evolution, 62 (10), 2660--2675
- Linking growth strategies to long-term population dynamics in a guild of desert annuals (2007)
Journal of Ecology, 95 (2), 321--331
- Long-term ecological research on Colorado shortgrass steppe (2007)
- Demography of central and marginal populations of monkeyflowers (Mimulus cardinalis and M. lewisii) (2006)
Ecology, 87 (8), 2014--2025
- Growth and leaf physiology of monkeyflowers with different altitude ranges (2006)
Oecologia, 148 (2), 183--194
- The Ecology and Evolution of Elevation Range Limits in Monkeyflowers (mimulus Cardinalis and M. Lewisii) (2005)
- The evolution of species' distributions: reciprocal transplants across the elevation ranges of Mimulus cardinalis and M. lewisii (2005)
Evolution, 59 (8), 1671--1684
- Trade-offs and the evolution of altitude range limits in monkeyflowers (2004)
INTEGRATIVE AND COMPARATIVE BIOLOGY, 44 (6), 516--516
- Microhabitat use and thermal biology of the collared lizard (Crotaphytus collaris collaris) and the fence lizard (Sceloporus undulatus hyacinthinus) in Missouri glades (2002)
Journal of Herpetology, 36 (1), 23--29