Population ecology of breeding Pacific common eiders on the Yukon-Kuskokwim Delta, Alaska

Abstract

Populations of Pacific common eiders (Somateria mollissima v-nigrum) on the Yukon-Kuskokwim Delta (YKD) in western Alaska declined by 50-90% from 1957 to 1992 and then stabilized at reduced numbers from the early 1990s to the present. We investigated the underlying processes affecting their population dynamics by collection and analysis of demographic data from Pacific common eiders at 3 sites on the YKD (1991-2004) for 29 site-years. We examined variation in components of reproduction, tested hypotheses about the influence of specific ecological factors on life-history variables, and investigated their relative contributions to local population dynamics. Reproductive output was low and variable, both within and among individuals, whereas apparent survival of adult females was high and relatively invariant (0.89 ± 0.005). All reproductive parameters varied across study sites and years. Clutch initiation dates ranged from 4 May to 28 June, with peak (modal) initiation occurring on 26 May. Females at an island study site consistently initiated clutches 3-5 days earlier in each year than those on 2 mainland sites. Population variance in nest initiation date was negatively related to the peak, suggesting increased synchrony in years of delayed initiation. On average, total clutch size (laid) ranged from 4.8 to 6.6 eggs, and declined with date of nest initiation. After accounting for partial predation and non-viability of eggs, average clutch size at hatch ranged from 2.0 to 5.8 eggs. Within seasons, daily survival probability (DSP) of nests was lowest during egg-laying and late-initiation dates. Estimated nest survival varied considerably across sites and years (mean = 0.55, range: 0.06-0.92), but process variance in nest survival was relatively low (0.02, CI: 0.01-0.05), indicating that most variance was likely attributed to sampling error. We found evidence that observer effects may have reduced overall nest survival by 0.0-0.36 across site-years. Study sites with lower sample sizes and more frequent visitations appeared to experience greater observer effects. In general, Pacific common eiders exhibited high spatio-temporal variance in reproductive components. Larger clutch sizes and high nest survival at early initiation dates suggested directional selection favoring early nesting. However, stochastic environmental effects may have precluded response to this apparent selection pressure. Our results suggest that females breeding early in the season have the greatest reproductive value, as these birds lay the largest clutches and have the highest probability of successfully hatching. We developed stochastic, stage-based, matrix population models that incorporated observed spatio-temporal (process) variance and co-variation in vital rates, and projected the stable stage distribution (w) and population growth rate (λ). We used perturbation analyses to examine the relative influence of changes in vital rates on λ and variance decomposition to assess the proportion of variation in λ explained by process variation in each vital rate. In addition to matrix-based λ, we estimated λ using capture-recapture approaches, and log-linear regression. We found the stable age distribution for Pacific common eiders was weighted heavily towards experienced adult females (≤4 yr of age), and all calculations of λ indicated that the YKD population was stable to slightly increasing (λ matrix = 1.02, CI: 1.00-1.04); λ reverse-capture-recapture = 1.05, CI: 0.99-1.11; λ log-linear = 1.04, CI: 0.98-1.10). Perturbation analyses suggested the population would respond most dramatically to changes in adult female survival (relative influence of adult survival was 1.5 times that of fecundity), whereas retrospective variation in λ was primarily explained by fecundity parameters (60%), particularly duckling survival (42%). Among components of fecundity, sensitivities were highest for duckling survival, suggesting this vital rate may be a current restriction to further population growth. Increasing adult survival would have the largest positive effects on population growth, but practical ways to influence this vital rate are currently limited. Given that eider laying dates and nest fates may be closely tied to those of sympatrically nesting species, we hypothesize that management actions facilitating early nesting and targeting other species (e.g., increasing overall potential for predator-swamping) may have positive, concurrent effects on nesting common eiders. If enhancing population growth is the objective and assuming the population is not currently at carrying capacity, we recommend an adaptive management approach. This approach would begin with focused efforts towards increasing reproductive output, with particular emphasis on strategies to increase both fecundity and nest survival, such as predator control on breeding grounds, followed by evaluation of population response. However, we conducted our modeling efforts under assumptions of density independence, whereas density-dependent mechanisms may have played an important role in the historic decline and subsequent stabilization of our population at a new, lower equilibrium. In an adaptive-management context, our results can be used to test additional (density-related) models; our predicted response would be compared with actual population response to prescribed management manipulations. Failure of the population to positively respond to increases in vital rates and negative correlations among vital rates would serve as evidence of density-dependent regulation. Rejection of the density-independent model would support management actions focused on habitat improvement. © 2012 The Wildlife Society.