Elevational gradients provide powerful natural systems for testing hypotheses regarding the role of environmental variation in the evolution of life history strategies. Case studies have revealed shifts towards slower life histories in plants and animals living at higher elevations. Our goals were to determine (1) how life history traits change with elevation among matched pairs of populations of bird species around the world, and (2) which biotic and abiotic factors are responsible for observed life history shifts. We compiled population-level comparisons for seven life history traits and four reproductive phenology variables from 105 published studies resulting in 441 variable/taxon comparisons. We compared elevational differences in traits with multiple methods, including calculation of effect sizes with formal meta-analyses.
Results/Conclusions
Our analyses revealed consistent declines in fecundity at higher elevations due to smaller clutch size and fewer reproductive attempts per year. In contrast, traits associated with shifts toward increased survival or parental investment were highly variable, with only nest success increasing at higher elevations. High elevation populations of birds nested consistently later, had shorter breeding seasons, but longer developmental periods implying that temporal constraints likely contribute to reductions in fecundity at high elevations. The magnitude of elevational shifts in life history trait values was poorly explained by geographic (absolute altitude, latitude), environmental (vegetation structure), or intrinsic (body mass, migratory status) covariates. Overall, our results failed to reveal predicted life-history trade-offs between components of reproductive effort and survivorship. Methodological considerations, metapopulation structure, or differences in factors that limit high and low-elevation range limits could explain these results. Climate change is predicted to negatively affect vertebrate populations at high elevations as biogeoclimatic zones shift and reassemble. Our analyses have identified another risk factor for montane and alpine populations: consistently lower fecundity will result in lower reproductive potential to recover from future environmental perturbations.