Prioritisation of conservation interventions should be informed by population ecology. Across vital rates (such as: adult survival, fertility), some have greater influence than others. However, certain rates are often overlooked. Population models generally assume consecutive breeding attempts, but, in many species, some established breeders skip breeding attempts in subsequent years. Such ‘intermittent breeding’ is increasingly considered a major demographic knowledge gap in wildlife management.
Our objective was to synthesise existing data, in order to explore the demographic consequences of different breeding assumptions in life cycle structures. We built matrix population models parameterised by demographic meta-analysis, including literature review and a call for data. Mean vital rate values were determined for the common eider (Somateria mollissima), an increasingly threatened species which offers insight into the other less abundant – and consequently less data-rich – seaducks.
Models can incorporate intermittent breeding as proportionally reduced fertility: if breeding propensity is 0.75, only three-quarters of established breeders attempt reproduction each year. We modelled it more realistically as a separate ‘non-breeder’ stage, with breeding propensity built into the transitions – a formulation which also facilitates inclusion of a survival cost of breeding. Our models further partitioned ‘pre-breeder’ into ages 1-4, making use of available age-stratified recruitment data.
Results/Conclusions
To the best of our knowledge, this is the first application to seaducks of this combination of formulations. We found that population growth rate and its determinants depended on whether, and how, intermittent breeding was incorporated. The effects of differing paths through the life cycle are not accounted for if individuals are assumed to breed every year, or even if average breeding propensity is used. Elasticity of the transition ‘established breeder breeding again’ was: 80% when ignoring breeding propensity; 83% with breeding propensity (0.735) in fertility; and 45% with separate non-breeders – in the latter, ‘established breeder skipping’ and ‘non-breeder returning to breeding’ each contributed a further 16.5%.
In the full model, transitions between breeding and non-breeding states (elasticities summing to 32%) had a strong influence, secondary only to continued breeding (41%), and much greater than fertilities (<5%). Across surveyed literature and contributed data, however, the number of studies documenting components of fertility (n=74) greatly exceeded the number of studies quantifying adult survival (n=24) or breeding propensity (n=6).
We highlight a disconnect between data availability and information needs of population ecologists and wildlife managers. Interdisciplinary methods and better stakeholder collaboration are required to facilitate improved modelling and decision-making.