Scholarly Work - Ecology
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Item Connectivity between white shark populations off Central California, USA and Guadalupe Island, Mexico(Frontiers Media SA, 2023-07) Kanive, Paul E.; Rotella, Jay J.; Chapple, Taylor K.; Anderson, Scot D.; Hoyos-Padilla, Mauricio; Klimley, Abbott Peter; Galván-Magaña, Felipe; Andrzejaczek, Samantha; Block, Barbara A.; Jorgensen, Salvador J.Marine animals often move beyond national borders and exclusive economic zones resulting in a need for trans-boundary management spanning multiple national jurisdictions. Highly migratory fish vulnerable to over-exploitation require protections at international level, as exploitation practices can be disparate between adjacent countries and marine jurisdictions. In this study we collaboratively conducted an analysis of white shark connectivity between two main aggregation regions with independent population assessment and legal protection programs; one off central California, USA and one off Guadalupe Island, Mexico. We acoustically tagged 326 sub-adult and adult white sharks in central California (n=210) and in Guadalupe Island (n=116) with acoustic transmitters between 2008-2019. Of the 326 tagged white sharks, 30 (9.20%) individuals were detected at both regions during the study period. We used a Bayesian implementation of logistic regression with a binomial distribution to estimate the effect of sex, maturity, and tag location to the response variable of probability of moving from one region to the other. While nearly one in ten individuals in our sample were detected in both regions over the study period, the annual rate of trans-regional movement was low (probability of movement = 0.015 yr-1, 95% credible interval = 0.002, 0.061). Sub-adults were more likely than adults to move between regions and sharks were more likely to move from Guadalupe Island to central California, however, sex, and year were not important factors influencing movement. This first estimation of demographic-specific trans-regional movement connecting US and Mexico aggregations with high seasonal site fidelity represents an important step to future international management and assessment of the northeastern Pacific white shark population as a whole.Item Evaluating the importance of individual heterogeneity in reproduction to Weddell seal population dynamics using integral projection models(Wiley, 2023-07) Macdonald, Kaitlin R.; Rotella, Jay J.; Paterson, J. TerrillIdentifying and accounting for unobserved individual heterogeneity in vital rates in demographic models is important for estimating population-level vital rates and identifying diverse life-history strategies, but much less is known about how this individual heterogeneity influences population dynamics. We aimed to understand how the distribution of individual heterogeneity in reproductive and survival rates influenced population dynamics using vital rates from a Weddell seal population by altering the distribution of individual heterogeneity in reproduction, which also altered the distribution of individual survival rates through the incorporation of our estimate of the correlation between the two rates and assessing resulting changes in population growth. We constructed an integral projection model (IPM) structured by age and reproductive state using estimates of vital rates for a long-lived mammal that has recently been shown to exhibit large individual heterogeneity in reproduction. Using output from the IPM, we evaluated how population dynamics changed with different underlying distributions of unobserved individual heterogeneity in reproduction. Results indicate that the changes to the underlying distribution of individual heterogeneity in reproduction cause very small changes in the population growth rate and other population metrics. The largest difference in the estimated population growth rate resulting from changes to the underlying distribution of individual heterogeneity was less than 1%. Our work highlights the differing importance of individual heterogeneity at the population level compared to the individual level. Although individual heterogeneity in reproduction may result in large differences in the lifetime fitness of individuals, changing the proportion of above- or below-average breeders in the population results in much smaller differences in annual population growth rate. For a long-lived mammal with stable and high adult-survival that gives birth to a single offspring, individual heterogeneity in reproduction has a limited effect on population dynamics. We posit that the limited effect of individual heterogeneity on population dynamics may be due to canalization of life-history traits.Item Density-related reproductive costs and natal conditions predict male life history in a highly polygynous mammal(Elsevier BV, 2023-06) Lloyd, Kyle J.; Oosthuizen, W. Chris; Rotella, Jay J.; Bester, Marthán N.; de Bruyn, P.J. NicoPolygynous males allocate substantial resources to sexual traits and behaviours to improve their chances of winning competitions for mates often at the cost of body maintenance. However, the degree to which males experience these trade-offs can be influenced by external conditions. Studies are needed that assess whether polygynous male resource allocation decisions about life history traits are density dependent. We tested for an influence of density on age-specific life history traits in male southern elephant seals, Mirounga leonina, using a 34-year data set collected at Marion Island. Specifically, we determine whether life history varied with density-related factors for survival, improved breeding success (measured as social status) and recruitment age. This was done by selecting linear models that tested biological hypotheses about density-dependent covariates related to competition intensity during breeding and natal conditions, while accounting for known intrinsic effects such as age and social status. Baseline mortalities were higher for males that had accumulated above average reproductive costs for their age than males with below average costs. This reproductive cost was determined by the number of females per harem relative to the population average. Thus, males likely allocated more resources to reproduction and less to body maintenance at all ages when defending and servicing relatively large harems, and this cost was compounding for males that dominated large harems at a young age. Males born in years with few pups were more likely to be dominant breeders and recruit at an earlier age than males born in years with many pups. Thus, breeding success and recruitment age likely depended on the population density experienced during early life, suggesting lasting effects of natal conditions in a long-lived animal. We show how density-dependent factors interact with intrinsic determinants of resource allocation to determine variation in polygynous male life history with long-term consequences.Item Evidence of an Absence of Inbreeding Depression in a Wild Population of Weddell Seals (Leptonychotes weddellii)(MDPI AG, 2023-02) Powell, John H.; Kalinowski, Steven T.; Taper, Mark L.; Rotella, Jay J.; Davis, Corey S.; Garrott, Robert A.Inbreeding depression can reduce the viability of wild populations. Detecting inbreeding depression in the wild is difficult; developing accurate estimates of inbreeding can be time and labor intensive. In this study, we used a two-step modeling procedure to incorporate uncertainty inherent in estimating individual inbreeding coefficients from multilocus genotypes into estimates of inbreeding depression in a population of Weddell seals (Leptonychotes weddellii). The two-step modeling procedure presented in this paper provides a method for estimating the magnitude of a known source of error, which is assumed absent in classic regression models, and incorporating this error into inferences about inbreeding depression. The method is essentially an errors-in-variables regression with non-normal errors in both the dependent and independent variables. These models, therefore, allow for a better evaluation of the uncertainty surrounding the biological importance of inbreeding depression in non-pedigreed wild populations. For this study we genotyped 154 adult female seals from the population in Erebus Bay, Antarctica, at 29 microsatellite loci, 12 of which are novel. We used a statistical evidence approach to inference rather than hypothesis testing because the discovery of both low and high levels of inbreeding are of scientific interest. We found evidence for an absence of inbreeding depression in lifetime reproductive success, adult survival, age at maturity, and the reproductive interval of female seals in this population.Item Patterns, sources, and consequences of variation in age-specific vital rates: Insights from a long-term study of Weddell seals(Wiley, 2022-12) Rotella, Jay J.Variations in the reproductive and survival abilities of individuals within a population are ubiquitous in nature, key to individual fitness, and affect population dynamics, which leads to strong interest in understanding causes and consequences of vital-rate variation. For long-lived species, long-term studies of large samples of known-age individuals are ideal for evaluating vital-rate variation. A population of Weddell seals in Erebus Bay, Antarctica, has been studied each Austral spring since the 1960s. Since 1982, all newborns have been tagged each year and multiple capture-mark-recapture (CMR) surveys have been conducted annually. Over the past 20 years, a series of analyses have built on results of earlier research by taking advantage of steady improvements in the project's long-term CMR data and available analytical methods. Here, I summarize progress made on four major topics related to variation in age-specific vital rates for females: early-life survival and age at first reproduction, costs of reproduction, demographic buffering, and demographic senescence. Multistate modelling found that age at first reproduction varies widely (4–14 years of age) and identified contrasting influences of maternal age on survival and recruitment rates of offspring. Subsequent analyses of data for females after recruitment revealed costs of reproduction to both survival and future reproduction and provided strong evidence of demographic buffering. Recent results indicated that important levels of among-individual variation exist in vital rates and revealed contrasting patterns for senescence in reproduction and survival. Sources of variation in vital rates include age, reproductive state, year, and individual. The combination of luck and individual quality results in strong variation in individual fitness outcomes: ~80% of females born in the population produce no offspring, and the remaining 20% vary strongly in lifetime reproductive output (range: 1–23 pups). Further research is needed to identify the specific environmental conditions that lead to annual variation in vital rates and to better understand the origins of individual heterogeneity. Work is also needed to better quantify the relative roles of luck, maternal effects, and environmental conditions on variation in vital rates and to learn the importance of such variation to demographic performance of offspring and on overall population dynamics.Item Investigating diverse sources of variation in the amount of time Weddell seal ( Leptonychotes weddellii ) pups spend in the water during the lactation period(Wiley, 2022-07) Petch, Shane M.; Rotella, Jay J.; Link, William A.; Paterson, J. Terrill; Garrott, Robert A.Field observations suggest that time spent in the water by Weddell seal pups during lactation varies among individuals, which could yield important developmental tradeoffs. We analyzed data from 713 pups born to 419 different mothers over 9 years to evaluate total time in the water, age at first entry, and potential sources of variation using temperature loggers attached to the rear flipper of pups. Pups first entered the water at 11–29 days of age (M = 14.9) and spent 4–204 hr (M = 69.3) in the water by 30 days of age. Age at first entry was earlier for pups with higher birth mass and mothers of above average reproductive experience. Total time in the water was related to maternal identity and greater for female pups and for pups that had higher birth mass, mothers of intermediate age, mothers that skipped reproduction in the previous year, and for pups that first entered the water at younger ages. Phenotypic traits explain observed variation in the development of a key life history behavior in the Weddell seal. Strong individual variation in time spent in metabolically costly swimming and diving might lead to variation in growth, energy stores, and survival and fitness outcomes.Item Quantifying fixed individual heterogeneity in demographic parameters: Performance of correlated random effects for Bernoulli variables(Wiley, 2021-10) Fay, Rémi; Authier, Matthieu; Hamel, Sandra; Jenouvrier, Stéphanie; Pol, Martijn; Cam, Emmanuelle; Gaillard, Jean‐Michel; Yoccoz, Nigel G.; Acker, Paul; Allen, Andrew; Aubry, Lise M.; Bonenfant, Christophe; Caswell, Hal; Coste, Christophe F. D. Coste; Larue, Benjamin; Coeur, Christie Le; Gamelon, Marlène; Macdonald, Kaitlin R.; Moiron, Maria; Nicol‐Harpe, Alex; Pelletier, Fanie; Rotella, Jay J.; Teplitsky, Celine; Touzot, Laura; Wells, Caitlin P.; Sæther, Bernt‐ErikAn increasing number of empirical studies aim to quantify individual variation in demographic parameters because these patterns are key for evolutionary and ecological processes. Advanced approaches to estimate individual heterogeneity are now using a multivariate normal distribution with correlated individual random effects to account for the latent correlations among different demographic parameters occurring within individuals. Despite the frequent use of multivariate mixed models, we lack an assessment of their reliability when applied to Bernoulli variables. Using simulations, we estimated the reliability of multivariate mixed effect models for estimating correlated fixed individual heterogeneity in demographic parameters modeled with a Bernoulli distribution. We evaluated both bias and precision of the estimates across a range of scenarios that investigate the effects of life-history strategy, levels of individual heterogeneity and presence of temporal variation and state dependence. We also compared estimates across different sampling designs to assess the importance of study duration, number of individuals monitored and detection probability. In many simulated scenarios, the estimates for the correlated random effects were biased and imprecise, which highlight the challenge in estimating correlated random effects for Bernoulli variables. The amount of fixed among-individual heterogeneity was frequently overestimated, and the absolute value of the correlation between random effects was almost always underestimated. Simulations also showed contrasting performances of mixed models depending on the scenario considered. Generally, estimation bias decreases and precision increases with slower pace of life, large fixed individual heterogeneity and large sample size. We provide guidelines for the empirical investigation of individual heterogeneity using correlated random effects according to the life-history strategy of the species, as well as, the volume and structure of the data available to the researcher. Caution is warranted when interpreting results regarding correlated individual random effects in demographic parameters modeled with a Bernoulli distribution. Because bias varies with sampling design and life history, comparisons of individual heterogeneity among species is challenging. The issue addressed here is not specific to demography, making this warning relevant for all research areas, including behavioral and evolutionary studies.Item Further evidence that Antarctic toothfish are important to Weddell seals(Cambridge University Press, 2020-09) Ainley, David G.; Cziko, Paul A.; Nur, Nadav; Rotella, Jay J.; Eastman, Joseph T.; Larue, Michelle; Stirling, Ian; Abrams, Peter A.Antarctic toothfish Dissostichus mawsoni and Weddell seals Leptonychotes weddellii are important mesopredators in the waters of the Antarctic continental shelf. They compete with each other for prey, yet the seals also prey upon toothfish. Such intraguild predation means that prevalence and respective demographic rates may be negatively correlated, but quantification is lacking. Following a review of their natural histories, we initiate an approach to address this deficiency by analysing scientific fishing catch per unit effort (CPUE; 1975–2011 plus sporadic effort to 2018) in conjunction with an annual index of seal abundance in McMurdo Sound, Ross Sea. We correlated annual variation in scientific CPUE to seal numbers over a 43 year period (1975–2018), complementing an earlier study in the same locality showing CPUE to be negatively correlated with spatial proximity to abundant seals. The observed relationship (more seals with lower CPUE, while controlling for annual trends in each) indicates the importance of toothfish as a dietary item to Weddell seals and highlights the probable importance of intra- and inter-specific competition as well as intraguild predation in seal-toothfish dynamics. Ultimately, it may be necessary to supplement fishery management with targeted ecosystem monitoring to prevent the fishery from having adverse effects on dependent species.Item Estimates of regional annual abundance and population growth rates of white sharks off central California(Elsevier BV, 2021-05) Kanive, Paul E.; Rotella, Jay J.; Chapple, Taylor K.; Anderson, Scot D.; White, Timothy D.; Block, Barbara A.; Jorgensen, Salvador J.Determining population trends is critical for evaluating management actions and prioritizing species protections. In this study, we used empirical data to produce an estimate of the population trend for sub-adult and adult white sharks in central California. We used the unique dorsal fin morphology to build a mark-recapture data set in a modified Jolly-Seber model (POPAN formulation) to estimate annual abundance and then investigate population growth rates using parametric bootstrapping methods for sub-adult and adult sharks (males and females). For all demographic groups combined, we found equivocal evidence for a positive regional population growth (λ = 1.07 (95% CI = 0.91 to 1.23)). However, sex- and size-specific population growth rate estimates provided some evidence of population increases for reproductively mature males (λ = 1.06 (95% CI = 0.99 to 1.13)) and females (λ = 1.06 (95% CI = 0.95 to 1.17)). For sub-adult male and female white sharks, point estimates of λ were positive but uncertainty prevents strong inference (λ = 1.07 (95% CI = 0.85 to 1.29)) and (λ = 1.08 (95% CI = 0.88 to 1.28)), respectively. Our findings of a potential increase in reproductive-aged white sharks in central California may be a result of regional fluxes in density or attributed in part to current protection efforts and subsequent increase in abundance of pinnipeds as well as reduced gill-net fisheries mortality of juveniles. A trend estimate for the entire northeastern Pacific will require obtaining similar data across known aggregation areas along the west coast of North America.Item Size-specific apparent survival rate estimates of white sharks using mark–recapture models(Canadian Science Publishing, 2019-02) Kanive, Paul E.; Rotella, Jay J.; Jorgensen, Salvador J.; Chapple, Taylor K.; HInes, James E.; Anderson, Scot D.; Block, Barbara A.Abstract: For species that exist at low abundance or are otherwise difficult to study, it is challenging to estimate vital rates such as survival and fecundity and common to assume that survival rates are constant across ages and sexes. Population assessments based on overly simplistic vital rates can lead to erroneous conclusions. We estimated sex- and length-based annual apparent survival rates for white sharks (Carcharodon carcharias). We found evidence that annual apparent survival differed over ontogeny in a system with competitive foraging aggregations, from 0.63 (standard error (SE) = 0.08) for newly recruiting subadults to 0.95 (SE = 0.02) for the largest sharks. Our results reveal a potential challenge to ontogenetic recruitment in a long-lived, highly mobile top marine predator, as survival rates for subadult white sharks may be lower than previously assumed. Alternatively, younger and competitively inferior individuals may be forced to permanently emigrate from primary foraging sites. This study provides new methodology for estimating apparent survival as a function of diverse covariates by capture–recapture study, including when sex assignment is uncertain. Résumé : Pour les espèces qui existent en faible abondance ou dont l’étude présente par ailleurs un défi, il est difficile d’estimer des indices vitaux comme la survie et la fécondité et il est couramment présumé que les taux de survie ne varient pas selon le sexe et le groupe d’âge. Des évaluations de populations reposant sur des indices vitaux trop simplistes peuvent mener à des conclusions erronées. Nous avons estimé les taux de survie annuels apparents en fonction du sexe et de la longueur pour de grands requins blancs (Carcharodon carcharias). Nous avons relevé des preuves de variation du taux de survie annuel apparent au fil de l’ontogénie dans un système caractérisé par des regroupements concurrents d’individus en quête de nourriture, ce taux allant de 0,63 (l’écart-type (ÉT) = 0,08) pour les individus subadultes récemment recrutés à 0,95 (ÉT = 0,02) pour les requins les plus grands. Nos résultats révèlent une difficulté potentielle en ce qui concerne le recrutement ontogénique chez un prédateur marin de niveau trophique supérieur très mobile et longévif, puisque les taux de survie de grands requins blancs subadultes pourraient être plus faibles que présumés auparavant. Une autre explication est que les individus plus jeunes ou moins concurrentiels pourraient être forcés d’émigrer des meilleurs sites d’approvisionnement de manière permanente. L’étude présente une nou-velle méthodologie pour estimer la survie apparente en fonction de différentes variables reliées, par une approche de capture–recapture, y compris pour les cas où l’affectation du sexe est incertaine. [Traduit par la Rédaction]