Ecology & Evolution of Species Range Limits
All organisms are limited in geographic distribution, but how & why range limits occur are still poorly understood. This prevents realistic forecasting of whether species will shift ranges or go extinct during climate change and weakens effective conservation of Canadian species at risk most of which are at their northern range limits in southern Canada. The goal of our research is to better understand the ecological and evolutionary processes that impose range limits over the short-term and longer-term constraints that prevent range expansion through evolutionary adaptation over the long term. To this end, we combine large-scale geographic surveys of trait variation and population demography with reciprocal transplant experiments and genomic analysis. We use two main study systems one involving geographic ranges and the other involving elevational ranges because the relative importance of the ecological and evolutionary factors at play may differ in constraining these two major dimensions of species distributions.
Cross & Eckert 2020. Amer. J. Bot doi:10.1002/ajb2.1400
Lopez-Villalobos & Eckert 2019 Molecular Ecology
Eckert, Samis & Lougheed 2008 Molecular Ecology
Elevational range limits in the Canadian Rocky Mountains
As you hike up a mountain it’s impossible not to notice the striking changes in vegetation, even over short spatial scales. So, mountains offer excellent opportunities to test the constraints on elevational range limits because these limits are replicated on virtually every mountain slope and often over short spatial scales. We’ve been working with short-lived flowering plants along elevational gradients in the Kananaskis Valley just west of Calgary Alberta. The most widely accepted hypothesis is that range limits are a spatial expression of a species realized niche. In other words, a species cannot exist beyond its range limit because survival and reproduction is too low there to allow populations persist. Our transplant experiments support this hypothesis, and further suggest that populations right at the limit are demographic sinks that exist only because of an influx of immigrants from within the range.
Hargreaves & Eckert 2018 Ecology Letters
Ensing & Eckert 2019 New Phytologist
Geographic range limits of Pacific. coastal dune plants
Plants endemic to the Pacific coastal dunes of western North America offer exceptional opportunities to better understand species range limits because they have near 1-dimensional distributions that can be thoroughly quantified with focussed field surveys. Emerging evidence from beyond-range transplant experiments suggest that species can persist beyond their ranges, suggesting that the range might be limited by constraints on dispersal rather than low fitness (aka niche limits). The dune plants we have studied are clear examples of this. Our experimental populations have persisted up to 200km beyond the northern range limits and, at places, for more than 10 generations. So we are now investigating whether dispersal constraints may account for the range limit.
Samis, López-Villalobos & Eckert 2016 Evolution
Samis & Eckert 2009 Ecology
Adaptation
A major goal of evolutionary biology is to understand how plants and animals become so well suited to their environments. This adaptive fit likely starts with the local populations of the same species that inhabit different conditions diverging through natural selection, each becoming best suited to its particular environment. However, whether this is commonly found in nature remains unclear. Reciprocal transplant experiments reveal that “often” genotypes from the local population outperform those from foreign populations (“the home-site advantage”), but often they don’t. There are several explanations for this apparent maladaptation (e.g. small population size and gene flow) but these are rarely tested. We are addressing this knowledge gap by performing reciprocal transplanting of populations across ecological gradients with genomic analysis to clarify the long-term demography of populations and the extent of gene flow between them.
Ensing & Eckert 2019 New Phytologist
Samis, López-Villalobos & Eckert 2016 Evolution
Brady et al. 2017. American Naturalist
Natural selection
Natural selection should favour different traits in populations that experience different ecological conditions. One of the most predictable patterns of ecological variation involves variation in growing season length with variation in elevation and latitude. And, growing season length will be strongly altered by climate warming. We expect that growing season length exerts strong selection on the timing of growth and reproduction across the season. Surprizingly, relatively few studies have shown that the direction and strength of natural selection change predictably with growing season length. We are measuring natural selection in natural and experimental populations of montane plants distributed across elevational gradients of growing season length to test this fundamental expectation. However, measuring selection on phenological traits, like the timing of first flower for example, is tricky because these traits along with survival and reproductive success (i.e. fitness) can be influenced by individual condition. Individuals in the best condition, by virtue of their genes or growing environment, may flower earliest and gave the highest fitness, which can induce a correlation between flowering time in fitness regardless of whether flowering time actually influences fitness directly. Our work addresses this and other potential pitfalls of measuring natural selection in the wild by using both natural and genetically -structure experimental populations where we can control for the effects of individual condition.
Ensing & Eckert 2020 Evolution (submitted)
Mating systems
The mating system can be broadly defined as who mates with who, when, where and how, and the pattern of mating dictates the movement of genes in time and space and thereby influences the evolutionary potential of natural populations. In this way, traits that influence the mating system can affect their own evolution. This is especially true in plants because most species are hermaphroditic and can, therefore, self-fertilize as well as outcross with unrelated individuals. As a result, the transition from outcrossing to self-fertilization is among the most commonly trod evolutionary pathways in plants, with myriad consequences for the genetics and ecology of populations. Biologists long-standing interest in the evolution of the mating system has produce a mountain of mathematical theory. Our goal is to test the assumptions and predictions of theory with experiments where we manipulate the mating system of populations (by experimentally modifying flowers, individuals or the stricture of populations) and by measuring the resulting consequences for mating and fitness. This has involved digging deep into diverse plant study systems from across North America and beyond.
Floral biology
Because individual plants are rooted in space, they require vectors, usually animals, to move pollen from the anthers of one flower to the stigma of another. This process is mediated by uniquely colourful, complex, smelly and altogether awesome structures that we call flowers. Because flowers directly affect reproductive success, we expect them to be under very strong natural selection. Hence a change in the reproductive system is usually initiated and further refined by a change in a species’ flowers. We have been particularly interested in how patterns of mating within natural populations are influences by variation in key floral traits. We’re also investigating how a shift in the reproductive system feeds back on the evolution of flowers. For example, does natural selection favour a reduction in investment towards floral attractive traits (petals, nectar, colour, fragrance) in selfing or asexual populations that no longer need to attract insect pollinators?
Doubleday, Raguso & Eckert 2014 American J of Botany
Loss of sex in clonal plants
Most perennial plants combine sexual reproduction via seed with some form of asexual vegetative propagation, and the relative investment in the two reproductive modes varies widely among species. The balance between sexual vs. asexual reproduction also varies across the geographic range within species, with populations at the range edge transitioning to asexuality with a consequent reduction in genotypic diversity and gene flow. Theory and experimental evolution suggest that the loss of sex may impede adaptation to edge conditions and constrain species ranges. Alternatively, asexuality may facilitate range expansion by protecting multi-locus genotypes from recombination, so that rare advantageous allele combinations accumulate when linked to mutations causing sexual sterility at range edges. Loss of sex may also permit the spread of alleles increasing survival and vegetative performance at the expense of sexual performance.
Thomsen, Bartkowska & Eckert 2018 International J Plant Sciences
Eckert, Dorken & Barrett 2016 Aquatic Botany
Ecological & evolutionary consequences of biological invasion
The movement of species from one part of the world to another is being facilitated by human activities at an alarming rate. During introduction and subsequent range expansion, an invasive species will encounter novel environments and selective pressures, which amounts to a massive, though largely unplanned experiment in evolution. We combine multi-continent surveys of trait and demographic variation with common-garden experiments and population genetic analysis to understand how evolutionary processes act and interact during invasion and to determine the extent to which adaptive evolution during invasion contributes to successful range expansion. Our results join emerging evidence from many labs that evolutionary change can occur over contemporary time scales and feed back on the ecology of invasive populations.
Eckert, Dorken & Barrett 2016 Aquatic Botany
Conservation Biology
There is a strong synergy between our research on the ecology & evolution of species range limits and conservation in Canada. More than 75% of plants designated “at-risk” in Canada are at their northern range limits in southern Canada but much more widely distributed south of the US border. So, species conservation in Canada is all about conserving range edge populations. And whether these peripheral populations are worth conserving has been debated for decades in the absence of definitive evidence. Are they small, demographically unstable, low in genetic variation and difficult to manage? Or, are they genetically uniques and poised to initiate range shifts during rapid environmental change caused by humans? Addressing these questions requires studying at-risk edge populations in the context of their whole geographic ranges to better understand their ecological, and genetic properties and the ecological and evolutionary pressures impinging on them. We have leveraged our expertise with species ranges to determine the conservation value of peripheral populations at risk.
Caissy et al. 2020 Biological Conservation
Yakimowski & Eckert 2008 New Phytologist
Yakimowski & Eckert 2007 Conservation Biology