Abstract
Phenological events of plants and animals are sensitive to climatic processes. Migration is a life-history event exhibited by most large herbivores living in seasonal environments, and is thought to occur in response to dynamics of forage and weather. Decisions regarding when to migrate, however, may be affected by differences in life-history characteristics of individuals. Long-term and intensive study of a population of mule deer (Odocoileus hemionus) in the Sierra Nevada, California, USA, allowed us to document patterns of migration during 11 years that encompassed a wide array of environmental conditions. We used two new techniques to properly account for interval-censored data and disentangle effects of broad-scale climate, local weather patterns, and plant phenology on seasonal patterns of migration, while incorporating effects of individual life-history characteristics. Timing of autumn migration varied substantially among individual deer, but was associated with the severity of winter weather, and in particular, snow depth and cold temperatures. Migratory responses to winter weather, however, were affected by age, nutritional condition, and summer residency of individual females. Old females and those in good nutritional condition risked encountering severe weather by delaying autumn migration, and were thus risk-prone with respect to the potential loss of foraging opportunities in deep snow compared with young females and those in poor nutritional condition. Females that summered on the west side of the crest of the Sierra Nevada delayed autumn migration relative to east-side females, which supports the influence of the local environment on timing of migration. In contrast, timing of spring migration was unrelated to individual life-history characteristics, was nearly twice as synchronous as autumn migration, differed among years, was related to the southern oscillation index, and was influenced by absolute snow depth and advancing phenology of plants. Plasticity in timing of migration in response to climatic conditions and plant phenology may be an adaptive behavioral strategy, which should reduce the detrimental effects of trophic mismatches between resources and other life-history events of large herbivores. Failure to consider effects of nutrition and other life-history traits may cloud interpretation of phenological patterns of mammals and conceal relationships associated with climate change.
Highlights
Climate change is expected to alter ecosystem structure and function, including community composition and distributions of species (Walther et al 2002)
We modeled all possible combinations of the extrinsic variables that were significant in the first stage of the modeling approach, and individual life-history characteristics that we hypothesized would affect the timing of seasonal migration including: age; summer residency; nutritional condition; fetal rate; and recruitment
Phenological patterns of seasonal migration are likely to be affected by climate change (Walther et al 2002, Stenseth et al 2003, Bolger et al 2008)
Summary
Climate change is expected to alter ecosystem structure and function, including community composition and distributions of species (Walther et al 2002). Overwhelming evidence from long-term research supports the influence of climate change on phenology (i.e., timing of seasonal activities) of plants and animals (Stenseth et al 2002, Badeck et al 2004, Gordo and Sanz 2005). Spring activities of numerous taxa have occurred progressively earlier and in the direction expected from climate change since the 1960s, including breeding by birds, arrival of migrant birds, appearance of butterflies, chorusing and spawning of amphibians, and flowering in plants (Walther et al 2002, Parmesan and Yohe 2003 for reviews). Addressing questions related to climate change, requires long-term studies to disentangle influences of large-scale climate and individual lifehistory patterns on phenological events
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