Niche models do not predict experimental demography but both suggest dispersal limitation across the northern range limit of the scarlet monkeyflower (Erythranthe cardinalis)

Abstract

AbstractAimGeographical ranges largely reflect the projection of species’ environmental niches onto the landscape, but dispersal limitation can cause ranges to fall short of niche limits. Understanding the prevalence of niche and dispersal limitation is a fundamental problem in ecology and biogeography, and it is relevant in predicting climate‐driven range shifts. Dispersal limitation could also cause widely used ecological niche models (ENM), which relate records of occurrence to environmental predictors, to underestimate properties of the ecological niche. Using a combination of experimental transplants and ENM, we tested for (a) dispersal and niche limitation across a species’ range boundary and (b) associations between ENM predictions and population performance.LocationOregon, USA.TaxonScarlet monkeyflower (Erythranthe cardinalis).MethodsWe created experimental populations within and beyond the northern range edge and used integral projection models (IPM) to infer potential population growth trajectories. We also built two classes of ecological niche models (ENM), one using climatic predictors and the other using fine‐scale stream habitat variables, to estimate habitat suitability across the northern range edge. Finally, we tested whether higher ENM suitability scores predict greater demographic performance.ResultsConsistent with dispersal limitation, experimental populations beyond the range were projected to persist or spread in three of four sites (compared to two of four sites within the range) and stream habitat ENM projected abundant suitable stream microhabitat within the species’ thermal envelope beyond the range edge. In contrast, climatic ENM suggested decreasing habitat suitability and availability at the northern range edge. Unexpectedly, higher climatic ENM scores were associated with negative population growth rates, while higher stream habitat ENM scores were unrelated to population growth.Main conclusionsThe northern range edge falls short of the species niche limit and is instead limited by dispersal into suitable habitat. Dispersal limitation caused correlative niche models to underestimate the climatic niche and to poorly predict demographic performance in a short‐term field study. These results highlight key challenges to applying predictions from correlative ENM to understanding range and niche limits.