Variation of annual apparent survival and detection rates with age, year and individual identity in male Weddell seals ( Leptonychotes weddellii ) from long‐term mark‐recapture data

Insights from the first global population estimate of Weddell seals in Antarctica

The Weddell seal is one of the best-studied marine mammals in the world, owing to a multidecadal demographic effort in the southernmost part of its range. Despite their occurrence around the Antarctic coastline, we know little about larger scale patterns in distribution, population size, or structure. We combined high-resolution satellite imagery from 2011, crowd-sourcing, and habitat modeling to report the first global population estimate for the species and environmental factors that influence its distribution. We estimated ~202,000 (95% confidence interval: 85,345 to 523,140) sub-adult and adult female seals, with proximate ocean depth and fast-ice variables as factors explaining spatial prevalence. Distances to penguin colonies were associated with seal presence, but only emperor penguin population size had a strong negative relationship. The small, estimated population size relative to previous estimates and the seals’ nexus with trophic competitors indicates that a community ecology approach is required in efforts to monitor the Southern Ocean ecosystem.

Demystifying individual heterogeneity

Among-individual variation in vital rates, such as mortality and birth rates, exists in nearly all populations. Recent studies suggest that this individual heterogeneity produces substantial life-history and fitness differences among individuals, which in turn scale up to influence population dynamics. However, our ability to understand the consequences of individual heterogeneity is limited by inconsistencies across conceptual frameworks in the field. Studies of individual heterogeneity remain filled with contradicting and ambiguous terminology that introduces risks of misunderstandings, conflicting models and unreliable conclusions. Here, we synthesise the existing literature into a single and comparatively straightforward framework with explicit terminology and definitions. This work introduces a distinction between potential vital rates and realised vital rates to develop a coherent framework that maps directly onto mathematical models of individual heterogeneity. We suggest the terms "fixed condition" and "dynamic condition" be used to distinguish potential vital rates that are permanent from those that can change throughout an individual's life. To illustrate, we connect the framework to quantitative genetics models and to common classes of statistical models used to infer individual heterogeneity. We also develop a population projection matrix model that provides an example of how our definitions are translated into precise quantitative terms.

Challenges and opportunities for comparative studies of survival rates: An example with male pinnipeds

Survival rates are a central component of life-history strategies of large vertebrate species. However, comparative studies seldom investigate interspecific variation in survival rates with respect to other life-history traits, especially for males. The lack of such studies could be due to the challenges associated with obtaining reliable datasets, incorporating information on the 0-1 probability scale, or dealing with several types of measurement error in life-history traits, which can be a computationally intensive process that is often absent in comparative studies. We present a quantitative approach using a Bayesian phylogenetically controlled regression with the flexibility to incorporate uncertainty in estimated survival rates and quantitative life-history traits while considering genetic similarity among species and uncertainty in relatedness. As with any comparative analysis, our approach makes several assumptions regarding the generalizability and comparability of empirical data from separate studies. Our model is versatile in that it can be applied to any species group of interest and include any life-history traits as covariates. We used an unbiased simulation framework to provide "proof of concept" for our model and applied a slightly richer model to a real data example for pinnipeds. Pinnipeds are an excellent taxonomic group for comparative analysis, but survival rate data are scarce. Our work elucidates the challenges associated with addressing important questions related to broader ecological life-history patterns and how survival-reproduction trade-offs might shape evolutionary histories of extant taxa. Specifically, we underscore the importance of having high-quality estimates of age-specific survival rates and information on other life-history traits that reasonably characterize a species for accurately comparing across species.