As climate change shifts species’ climatic envelopes across the landscape, equilibrium between geographic ranges and niches is likely diminishing due to time lags in demography and dispersal. If a species’ range and niche are in disequilibrium, then population performance should decrease from cool, “leading” edges, where populations are expanding into recently ameliorated habitats, to warm, “trailing” edges, where populations are contracting from newly unsuitable areas. Population contraction signals that compensatory changes in vital rates are insufficient to buffer population growth from deteriorating environments. Life history theory predicts tradeoffs between fast development, high reproduction, and short longevity at low latitudes and slow development, less frequent but multiple bouts of reproduction, and long lifespan at high latitudes. If demographic compensation is driven by life history evolution, there may be compensatory negative correlations in vital rates that are associated with this fast-slow continuum. An outstanding question is whether range and niche limits reflect inadequate compensatory life history evolution along environmental gradients, causing population growth rates to fall below replacement at range edges. We surveyed demography of 32 populations of the scarlet monkeyflower (Erythranthe cardinalis, Phrymaceae) spanning 11˚ latitude in western North America and used integral projection models to infer population dynamics and assess demographic compensation.
Population growth rates increased from low to high latitudes, consistent with leading-trailing dynamics. Differences in growth, survival, and recruitment drove spatial variation in population growth rates, with growth and recruitment probabilities increasing with latitude and survival probabilities decreasing from mid latitudes towards the northern and southern edges. The presence of statistically significant demographic compensation indicates that vital rates respond individualistically to the same environmental gradient and that single vital rates fail to predict population performance. Although there was demographic compensation, the small, positive effects of a higher probability of flowering and greater fruit number did not buffer southern, low-latitude populations from the large, negative effects of low survival, growth, and recruitment. Thus, demographic compensation may not be sufficient to rescue populations at the trailing edge from extinction.