Breeding on the leading edge of a northward range expansion: differences in morphology and the stress response in the arctic Gambel's white-crowned sparrow

Individuals at the forefront of a range shift are likely to exhibit phenotypic traits that distinguish them from the population breeding within the historic range. Recent studies have examined morphological, physiological and behavioral phenotypes of individuals at the edge of their range. Several studies have found differences in the hypothalamic-pituitary-adrenal (HPA) axis activity in response to acute restraint stress in individuals at the range limits. HPA axis activation leads to elevations in glucocorticoids that regulate physiology and behavior. Here we compare the hormonal profiles and morphometrics from Gambel's white-crowned sparrows (Zonotrichia leucophrys gambelii) breeding at the northern limit of the population's range to those birds breeding within the historic population range. Birds breeding at the northern limit experienced a harsher environment with colder temperatures; however, we found no differences in arthropod prey biomass between the northern limit and more southern (historic) sites. Males at the northern limit had higher body condition scores (mass corrected for body size) compared to individuals within the historic range, but no differences were found in beak and tarsus lengths, wing chord, muscle profile or fat stores. In males during the pre-parental stage, before breeding commenced, HPA axis activity was elevated in birds at the northern limit of the range, but no differences were found during the parental or molt stages. Females showed no differences in HPA axis activity during the parental stage. This study suggests that "pioneering" individuals at the limits of their breeding range exhibit physiology and morphology that are distinct from individuals within the historic range

Habitat matching and spatial heterogeneity of phenotypes: implications for metapopulation and metacommunity functioning

Spatial heterogeneity in the distribution of phenotypes among populations is of major importance for species evolution and ecosystem functioning. Dispersal has long been assumed to homogenise populations in structured landscapes by generating maladapted gene flows, making spatial heterogeneity of phenotypes traditionally considered resulting from local adaptation or plasticity. However, there is accumulating evidence that individuals, instead of dispersing randomly in the landscapes, adjust their dispersal decisions according to their phenotype and the environmental conditions. Specifically, individuals might move in the landscape to find and settle in the environmental conditions that best match their phenotype, therefore maximizing their fitness, a hypothesis named habitat matching. Although habitat matching and associated non-random gene flows can produce spatial phenotypic heterogeneity, their potential consequences for metapopulation and metacommunity functioning are still poorly understood. Here, we discuss evidence for intra and interspecific drivers of habitat matching, and highlight the potential consequences of this process for metapopulation and metacommunity functioning. We conclude that habitat matching might deeply affect the eco-evolutionary dynamics of meta-systems, pointing out the need for further empirical and theoretical research on its incidence and implications for species and communities evolution under environmental change