Life history variation between high and low elevation subspecies of horned larks Eremophila spp.

Environmental variation along elevational gradients can strongly influence life history strategies in vertebrates. We investigated variation in life history patterns between a horned lark subspecies nesting in high elevation alpine habitat Eremophila alpestris articola and a second subspecies in lower elevation grassland and sandy shoreline habitats E. a. strigata. Given the shorter breeding season and colder climate at the northern alpine site we expected E. a. articola to be larger, have lower fecundity and higher apparent survival than E. a. strigata. As predicted, E. a. articola was larger and the trend was toward higher apparent adult survival for E. a. articola than E. a. strigata (0.69 vs 0.51). Contrary to our predictions, however, there was a trend toward higher fecundity for E. a. articola (1.75 female fledglings/female/year vs 0.91). The larger clutch sizes, higher nest survival, and shorter renesting intervals have apparently allowed E. a. articola to compensate for the short breeding season. Estimates of population growth rate (lambda) predicted a stable population for E. a. articola (lambda=1.00) and a rapidly declining population for E. a. strigata (lambda=0.62) this may provide an explanation for the deviations from our expectation of higher reproduction in E. a. strigata. We suggest that anthropogenic influences (e.g. habitat loss and degradation and increased nest predator abundance) to E. a. strigata nesting sites may be responsible for reducing annual fecundity to the point where it is almost half that of E. a. articola. This result suggests that human influenced habitat changes that, in turn, change demographic rates may result in vital rates that do not accurately reflect historically divergent life histories between the two groups. Our results underscore the importance of including estimates of multiple traits in life history studies to provide insight into compensatory interactions among components of demographic rates and to identify recent changes to demographic rates that might result in a mismatch between observed and predicted life history strategies.