Coupled dynamics of body mass and population growth in response to environmental change

Abstract

Climate change affects the timing of regular events of plant and animal life, such as budding, migration and hibernation, as well as population dynamics and morphology. It is difficult to monitor all these interacting factors at once, but an extended life-history study of a hibernating mammal — a yellow-bellied marmot (Marmota flaviventris) population in a subalpine habitat in the Upper East River Valley, Colorado — provides data suited to the task. Climate change over the period 1976–2008 has caused earlier emergence from hibernation, lengthening the animals' growing season so that they are now heavier when they start to hibernate. At the same time, the fitness of large individuals has increased, leading to a rapid increase in population size. As Marcel Visser explains in the accompanying News & Views, the major challenge in climate-change ecology is to predict the impact of future climate change on populations. This work on marmots provides the type of data needed to achieve that aim. Climate change can affect the phenology, population dynamics and morphology of species, but it is difficult to study all these factors and their interactions at once. Using long-term data for individual yellow-bellied marmots, these authors show that climate change has increased the length of the marmot growing season, leading to a gradual increase in individual size. It has simultaneously increased the fitness of large individuals, leading to a rapid increase in population size. Environmental change has altered the phenology, morphological traits and population dynamics of many species1,2. However, the links underlying these joint responses remain largely unknown owing to a paucity of long-term data and the lack of an appropriate analytical framework3. Here we investigate the link between phenotypic and demographic responses to environmental change using a new methodology and a long-term (1976–2008) data set from a hibernating mammal (the yellow-bellied marmot) inhabiting a dynamic subalpine habitat. We demonstrate how earlier emergence from hibernation and earlier weaning of young has led to a longer growing season and larger body masses before hibernation. The resulting shift in both the phenotype and the relationship between phenotype and fitness components led to a decline in adult mortality, which in turn triggered an abrupt increase in population size in recent years. Direct and trait-mediated effects of environmental change made comparable contributions to the observed marked increase in population growth. Our results help explain how a shift in phenology can cause simultaneous phenotypic and demographic changes, and highlight the need for a theory integrating ecological and evolutionary dynamics in stochastic environments4,5.