Evolutionary Ecology of Range Limits: Conceptual Syntheses and Empirical Tests
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Understanding the environmental, demographic, and evolutionary processes underlying species’ range limits (RLs) is a primary goal of ecology, and increasingly important for conservation in a world of changing species distributions. This thesis addresses three of the most pressing questions: whether RLs typically reflect niche or dispersal constraints; whether generalizations regarding the limiting niche constraints are possible; and how the processes underlying current RLs might affect future RLs. First, I conducted a meta-analysis of transplant experiments beyond species’ ranges, which can directly test the first questions. Performance declined beyond the range for 75% of 111 tests, confirming the broad importance of underlying niche constraints. Results supported the long-proposed but seldom-tested hypothesis that biotic interactions are especially important at low-elevation/equatorial RLs. However, dispersal caused frequent (ca. 50%) mismatches between range and niche limits, often constraining geographic distributions, while extending elevational RLs. Second, I construct a theoretical framework exploring how selection on dispersal varies across species ranges, and how this affects current and future RLs. Whereas dispersal should always increase along invasion fronts, its evolution is likely constrained during expansion of native species, and may play a decisive role in maintaining genetic diversity at RLs contracting due to climate change. Local adaptation to a shifting environmental gradient could select for increased dispersal throughout the range, including contracting limits. Third, I test the importance of niche vs. dispersal constraints and the demographic, environmental and evolutionary patterns underlying the elevational RLs of the annual herb Rhinanthus minor L. Multi-generation demographic surveys of natural populations spanning four range-wide transects reveal that population productivity (net reproductive rate NRR and seed production/m2) declined toward both limits, reflecting low reproductive success at high elevations, but early life-stage failure at low elevations. Surveys suggested an increasing role of competition, but not herbivory, toward the lower RL. A 3-yr reciprocal transplant experiment confirmed that strong niche constraints underlie R. minor’s RLs, as all sources failed beyond both limits (NRR<1). Seed quality declined at upper RLs, but local adaptation in phenology enabled early-flowering high-elevation sources to reproduce in short growing seasons, making them the best poised to expand the upper RL.