Individual heterogeneity in fitness in a long-lived herbivore

Heterogeneity in the intrinsic quality and nutritional condition of individuals affects reproductive success and consequently fitness. Black brant (Branta bernicla nigricans) are long-lived, migratory, specialist herbivores. Long migratory pathways and short summer breeding seasons constrain the time and energy available for reproduction, thus magnifying life-history trade-offs. These constraints, combined with long lifespans and trade-offs between current and future reproductive value, provide a model system to examine the role of individual heterogeneity in driving life-history strategies and individual heterogeneity in fitness. We used hierarchical Bayesian models to examine reproductive trade-offs, modeling the relationships between within-year measures of reproductive energy allocation and among-year demographic rates of individual females breeding on the Yukon-Kuskokwim Delta, Alaska, using capture-recapture and reproductive data from 1988 to 2014. We generally found that annual survival tended to be buffered against variation in reproductive investment, while breeding probability varied considerably over the range of clutch size-laying date combinations. We provide evidence for relationships between breeding probability and clutch size, breeding probability and nest initiation date, and an interaction between clutch size and initiation date. Average lifetime clutch size also had a weak positive relationship with apparent survival probability. Our results support the use of demographic buffering strategies for black brant. These results also indirectly suggest associations among environmental conditions during growth, fitness, and energy allocation, highlighting the effects of early growth conditions on individual heterogeneity, and subsequently, lifetime reproductive investment.

Long-term research and hierarchical models reveal consistent fitness costs of being the last egg in a clutch

Maintenance of phenotypic heterogeneity in the face of strong selection is an important component of evolutionary ecology, as are the consequences of such heterogeneity. Organisms may experience diminishing returns of increased reproductive allocation as clutch or litter size increases, affecting current and residual reproductive success. Given existing uncertainty regarding trade‐offs between the quantity and quality of offspring, we sought to examine the potential for diminishing returns on increased reproductive allocation in a long‐lived species of goose, with a particular emphasis on the effect of position in the laying sequence on offspring quality.

To better understand the effects of maternal allocation on offspring survival and growth, we estimated the effects of egg size, timing of breeding, inter‐ and intra‐annual variation, and position in the laying sequence on gosling survival and growth rates of black brant Branta bernicla nigricans breeding in western Alaska from 1987 to 2007.

We found that gosling growth rates and survival decreased with position in the laying sequence, regardless of clutch size. Mean egg volume of the clutch a gosling originated from had a positive effect on gosling survival (β = 0.095, 95% CRI: 0.024, 0.165) and gosling growth rates (β = 0.626, 95% CRI: 0.469, 0.738). Gosling survival (β = −0.146, 95% CRI: −0.214, −0.079) and growth rates (β = −1.286, 95% CRI: −1.435, −1.132) were negatively related to hatching date.

These findings indicate substantial heterogeneity in offspring quality associated with their position in the laying sequence. They also potentially suggest a trade‐off mechanism for females whose total reproductive investment is governed by pre‐breeding state.

Changes in behavior are unable to disrupt a trophic cascade involving a specialist herbivore and its food plant

Changes in ecological conditions can induce changes in behavior and demography of wild organisms, which in turn may influence population dynamics. Black brant (Branta bernicla nigricans) nesting in colonies on the Yukon-Kuskokwim Delta (YKD) in western Alaska have declined substantially (~50%) since the turn of the century. Black brant are herbivores that rely heavily on Carex subspathacea (Hoppner's sedge) during growth and development. The availability of C. subspathacea affects gosling growth rates, which subsequently affect pre- and postfledging survival, as well as size and breeding probability as an adult. We predicted that long-term declines in C. subspathacea have affected gosling growth rates, despite the potential of behavior to buffer changes in food availability during brood rearing. We used Bayesian hierarchical mixed-effects models to examine long-term (1987-2015) shifts in brant behavior during brood rearing, forage availability, and gosling growth rates at the Tutakoke River colony. We showed that locomotion behaviors have increased (β = 0.05, 95% CRI: 0.032-0.068) while resting behaviors have decreased (β = -0.024, 95% CRI: -0.041 to -0.007), potentially in response to long-term shifts in forage availability and brood density. Concurrently, gosling growth rates have decreased substantially (β = -0.100, 95% CRI: -0.191 to -0.016) despite shifts in behavior, mirroring long-term declines in the abundance of C. subspathacea (β = -0.191, 95% CRI: -0.355 to -0.032). These results have important implications for individual fitness and population viability, where shifts in gosling behavior putatively fail to mitigate long-term declines in forage availability.