We consider how Antarctic seals may respond to changes in climate, realizing that
anthropogenic alteration of food webs will influence these responses. The
species considered include the ice-obligate ...- crabeater (Lobodon carcinophaga), Weddell (Leptonychotes weddellii), Ross (Ommataphoca rossii) and leopard (Hydrurga leptonyx) seal - and the ice-tolerant Antarctic fur
seal (Arctocephalus gazella) and southern
elephant seal (Mirounga leonina). The data
analysed are from long-term censuses of Weddell seals in McMurdo Sound
(1997–2006), and of Weddell, fur and elephant seals at Arthur
Harbour, Antarctic Peninsula (1974–2005). After considering their
responses to recent changes in environmental features, as well as projected and
current changes to their habitat our conclusions are that the distribution and
abundance of 1) crabeater and Weddell seals will be negatively affected by
changes in the extent, persistence and type of annual sea ice, 2) Ross and
leopard seal will be the least negatively influenced by changes in pack ice
characteristics, although, as may be the case for crabeater and Weddell,
population size and distribution may be altered through changes in food web
dynamics, and 3) southern elephant and fur seals will respond in ways opposite
to the pack ice species, but could also be influenced most immediately by
changes in their food resources due to factors other than climate.
Wolf restoration has become a widely accepted conservation and management practice throughout North America and Europe, though the ecosystem effects of returning top carnivores remain both scientific ...and societal controversies. Mathematical models predicting and describing wolf-ungulate interactions are typically limited to the wolves' primary prey, with the potential for prey switching in wolf-multiple-ungulate systems only suggested or assumed by a number of investigators. We used insights gained from experiments on small taxa and field data from ongoing wolf-ungulate studies to construct a model of predator diet composition for a wolf-elk-bison system in Yellowstone National Park, Wyoming, USA. The model explicitly incorporates differential vulnerability of the ungulate prey types to predation, predator preference, differences in prey biomass, and the possibility of prey switching. Our model demonstrates wolf diet shifts with changes in relative abundance of the two prey, with the dynamics of this shift dependent on the combined influences of preference, differential vulnerability, relative abundances of prey, and whether or not switching occurs. Differences in vulnerability between elk and bison, and strong wolf preference for elk, result in an abrupt dietary shift occurring only when elk are very rare relative to bison, whereas incorporating switching initiates the dietary shift more gradually and at higher bison-elk ratios. We demonstrate how researchers can apply these equations in newly restored wolf-two-prey systems to empirically evaluate whether prey switching is occurring. Each coefficient in the model has a biological interpretation, and most can be directly estimated from empirical data collected from field studies. Given the potential for switching to dramatically influence predator-prey dynamics and the wide range of expected prey types and abundances in some systems where wolves are present and/or being restored, we suggest that this is an important and productive line of research that should be pursued by ecologists working in wolf-ungulate systems.
Declines in calf recruitment in some Rocky Mountain elk populations have raised concerns that recovering carnivore populations may be limiting elk population growth. To help resolve continuing ...questions about the influence of wolves (Cants lupus) reintroduced during 1995-1997 on elk calf production and survival, we conducted retrospective modeling of factors such as weather, elk density, and recovering carnivore populations potentially affecting northern Yellowstone elk pregnancy and calf survival using a longterm data set collected during 1985-2008. We found elk density negatively affected yearling pregnancy rate but had little effect on adult pregnancy rate. Adult pregnancy rate declined as individuals aged and increased following summers with higher precipitation but did not vary across a wide range of carnivore densities. Fall lactation rates, an indicator of early-season calf survival, increased with elk age and decreased as the ratio of grizzly bear (Ursus arctos horribilis) per elk increased. Over the 23 years of this study, the index of calf recruitment decreased as the wolf/elk ratio increased, decreased following severe winters, and increased as elk density decreased. Estimates of the wolf/elk ratio effect on calf recruitment were negative and confidence intervals did not overlap 0, whereas estimates of the wolf/elk ratio effect on elk pregnancy and early calf survival were positive and confidence intervals did overlap 0. Although a myriad of factors affect each stage leading to calf recruitment, and some of these factors such as weather are out of management control, management actions aimed at reducing wolf densities may result in increasing calf survival and elk population growth rates. However, the desire to manage wolf numbers to achieve a balance with ungulate population objectives must be balanced with other social considerations such as the desire to maintain wolf populations for tourism and wildlife-watching opportunities.
1. Age-related changes in maternal reproductive allocation for long-lived species are a key prediction from life-history theory. 2. Theoretical and empirical work suggests that allocation may ...increase with age due to constraint (increases with experience) or restraint (increases with age in the face of declining residual reproductive value), and may decrease among the oldest aged animals due to senescence in reproductive function. 3. Here, we use a hierarchical modelling approach to investigate the age-related patterns of change in maternal reproductive effort in the Weddell seal, a long-lived marine mammal with a protracted period of maternal care during which mothers allocate a large proportion of body mass while feeding little. 4. We find that maternal allocation increases with age for young mothers during both the pre-natal and post-natal periods. In contrast, older mothers demonstrate a senescent decline in pre-natal allocation but allocate more of their declining resources to their offspring during the post-natal period. We also find strong evidence for the importance of individual effects in reproductive allocation among mothers: some mothers consistently produce heavier (or lighter) pups than expected. 5. Our results indicate that maternal allocation changes over a mother's reproductive life span and that age-specific differences differ in notable ways in pre-natal and post-natal periods.
In colonial-breeding species, prebreeders often emigrate temporarily from natal reproductive colonies then subsequently return for one or more years before producing young. Variation in ...attendance-nonattendance patterns can have implications for subsequent recruitment. We used open robust-design multistate models and 28 years of encounter data for prebreeding female Weddell seals (
Leptonychotes weddellii
Lesson) to evaluate hypotheses about (1) the relationships of temporary emigration (TE) probabilities to environmental and population size covariates and (2) motivations for attendance and consequences of nonattendance for subsequent probability of recruitment to the breeding population. TE probabilities were density dependent (β̂
BPOP
= 0.66,
= 0.17; estimated effects β and standard errors of population size in the previous year) and increased when the fast-ice edge was distant from the breeding colonies (β̂
DIST
= 0.75,
= 0.04; estimated effects and standard errors of distance to the sea-ice edge in the current year on TE probability in the current year) and were strongly age and state dependent. These results suggest that trade-offs between potential benefits and costs of colony attendance vary annually and might influence motivation to attend colonies. Recruitment probabilities were greatest for seals that consistently attended colonies in two or more years (e.g.,
= 0.56, SD = 0.17) and lowest for seals that never or inconsistently attended prior to recruitment (e.g.,
= 0.32, SD = 0.15), where
denotes the mean recruitment probability (over all years) for 10-year-old seals for the specified prebreeder state. In colonial-breeding seabirds, repeated colony attendance increases subsequent probability of recruitment to the adult breeding population; our results suggest similar implications for a marine mammal and are consistent with the hypothesis that prebreeders were motivated to attend reproductive colonies to gain reproductive skills or perhaps to optimally synchronize estrus through close association with mature breeding females.
Individual variation in reproductive success is a key feature of evolution, but also has important implications for predicting population responses to variable environments. Although such individual ...variation in reproductive outcomes has been reported in numerous studies, most analyses to date have not considered whether these realized differences were due to latent individual heterogeneity in reproduction or merely random chance causing different outcomes among like individuals. Furthermore, latent heterogeneity in fitness components might be expressed differently in contrasted environmental conditions, an issue that has only rarely been investigated. Here, we assessed (i) the potential existence of latent individual heterogeneity and (ii) the nature of its expression (fixed vs. variable) in a population of female Weddell seals (Leptonychotes weddellii), using a hierarchical modeling approach on a 30‐year mark–recapture data set consisting of 954 individual encounter histories. We found strong support for the existence of latent individual heterogeneity in the population, with “robust” individuals expected to produce twice as many pups as “frail” individuals. Moreover, the expression of individual heterogeneity appeared consistent, with only mild evidence that it might be amplified when environmental conditions are severe. Finally, the explicit modeling of individual heterogeneity allowed us to detect a substantial cost of reproduction that was not evidenced when the heterogeneity was ignored.
Using a hierarchical modeling approach, we investigated specific hypotheses concerned with the prevalence and temporal expression of individual variation in Weddell seal reproductive rates. Our results provide strong evidence for the existence of biologically significant levels of individual variation in the study population and also suggest that differences among individuals remain relatively consistent across contrasted environmental conditions. We also found, for the first time, strong evidence of an important reproductive cost in this species.
Variation in vital rates of an unharvested elk (Cervus elaphus) population was studied using telemetry for 7 consecutive years, 19911998. We found pronounced senescence in survival rates, but no ...evidence for reproductive senescence. Prime-age females (<10 years old) experienced very high annual survival rates (mean = 0.97, SE = 0.02), with lower survival rates for senescent animals (
10 years old; mean = 0.79, SE = 0.06). There was evidence that the severity of snowpack conditions had little effect on survival of prime-age animals except during the most extreme winter, while survival of senescent animals was progressively depressed as the severity of snowpack conditions increased. Reproductive rates remained essentially constant, near their biological maxima (mean = 0.91, SE = 0.02). Annual re cruitment was highly variable. Snowpack had a pronounced effect on recruitment (r
2
= 0.91), the most severe snowpack conditions resulting in the virtual elimination of a juvenile cohort. Population estimates and recruitment rates obtained during this investigation and historic data collected from 1965 to 1980 support the premise that the population has been maintained in a dynamic equilibrium for at least three decades despite the stochastic effects of climate variation on vital rates. We conclude that the population is resource-limited, with variation about the equilibrium caused primarily by variable recruitment driven by stochastic annual snowpack.
S. Creel et al. reported a negative correlation between fecal progesterone concentrations and elk : wolf ratios in greater Yellowstone elk (
Cervus elaphus
) herds and interpreted this correlation ...as evidence that pregnancy rates of elk decreased substantially in the presence of wolves (
Canis lupus
). Apparently, the hypothesized mechanism is that decreased forage intake reduces body condition and either results in elk failing to conceive during the autumn rut or elk losing the fetus during winter. We tested this hypothesis by comparing age-specific body condition (percentage ingesta-free body fat) and pregnancy rates for northern Yellowstone elk, one of the herds sampled by Creel et al., before (1962-–1968) and after (2000-–2006) wolf restoration using indices developed and calibrated for Rocky Mountain elk. Mean age-adjusted percentage body fat of female elk was similarly high in both periods (9.0%% ±± 0.9%% pre-wolf; 8.9%% ±± 0.8%% post-wolf). Estimated pregnancy rates (proportion of females that were pregnant) were 0.91 pre-wolf and 0.87 post-wolf for 4-–9 year-old elk (95%% CI on difference == −−0.15 to 0.03,
P
== 0.46) and 0.64 pre-wolf and 0.78 post-wolf for elk >9 years old (95%% CI on difference == −−0.01 to 0.27,
P
== 0.06). Thus, there was little evidence in these data to support strong effects of wolf presence on elk pregnancy. We caution that multiple lines of evidence and/or strong validation should be brought to bear before relying on indirect measures of how predators affect pregnancy rates.
Exploring age‐ and sex‐specific survival rates provides insight regarding population behavior and life‐history trait evolution. However, our understanding of how age‐specific patterns of survival, ...including actuarial senescence, compare between the sexes remains inadequate. Using 36 years of mark‐recapture data for 7,516 male Weddell seals (Leptonychotes weddellii) born in Erebus Bay, Antarctica, we estimated age‐specific annual survival rates using a hierarchical model for mark‐recapture data in a Bayesian framework. Our male survival estimates were moderate for pups and yearlings, highest for 2‐year‐olds, and gradually declined with age thereafter such that the oldest animals observed had the lowest rates of any age. Reports of senescence in other wildlife populations of species with similar longevity occurred at older ages than those presented here. When compared to recently published estimates for reproductive Weddell seal females, we found that peak survival rates were similar (males: 0.94, 95% CI = 0.92–0.96; females: 0.92, 95% CI = 0.93–0.95), but survival rates at older ages were lower in males. Age‐specific male Weddell seal survival rates varied across years and individuals, with greater variation occurring across years. Similar studies on a broad range of species are needed to contextualize these results for a better understanding of the variation in senescence patterns between the sexes of the same species, but our study adds information for a marine mammal species to a research topic dominated by avian and ungulate species.
Male Weddell seals exhibit variation in age‐specific survival rates and commence actuarial senescence immediately following the mean age at sexual maturity. Survival patterns of male Weddell seals differ from those of female Weddell seals in the same population.
1. Organisms balance current reproduction against future survival and reproduction, which results in life-history trade-offs. These trade-offs are also known as reproductive costs and may represent ...significant factors shaping life-history strategy for many species. 2. Using multistate mark-resight models and 26 years of mark-resight data (1979-2004), we estimated the costs of reproduction to survival and reproductive probabilities for Weddell seals in Erebus Bay, Antarctica and evaluated whether this species either conformed to the 'prudent parent' reproductive strategy predicted by life-history theory for long-lived mammals or alternatively, incurred costs to survival in order to reproduce in a variable environment (flexible-strategy hypothesis). 3. Results strongly supported the presence of reproductive costs to survival (mean annual survival probability was 0·91 for breeders vs. 0·94 for nonbreeders), a notable difference for a long-lived mammal, demonstrating that investment in reproduction does result in a cost to survival for Weddell seals, contrary to the prudent parent hypothesis. 4. Reproductive costs to subsequent reproductive probabilities were also present for first-time breeders (mean probability of breeding the next year was 31·3% lower for first-time breeders than for experienced breeders), thus supporting our prediction of the influence of breeding experience. 5. We detected substantial annual variation in survival and breeding probabilities. Breeding probabilities were negatively influenced by summer sea-ice extent, whereas weak evidence suggested that survival probabilities were affected more by winter sea-ice extent, and the direction of this effect was negative. However, a model with annual variation unrelated to any of our climate or sea-ice covariates performed best, indicating that further study will be needed to determine the appropriate mechanism or combination of mechanisms underlying this annual variation.
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