In many species, temporary emigration (TE) is a phenomenon, often indicative of life-history characteristics such as dormancy, skipped reproduction, or partial migration, whereby certain individuals ...in a population are temporarily unobservable at a particular site. TE may be a flexible condition-dependent strategy that allows individuals to mitigate effects of adverse conditions. Consequently, TE rates ought to be highly variable, but sources of variations are poorly understood for most species. We used data from known-aged female Weddell seals (Leptonychotes weddellii) tagged in Erebus Bay, Antarctica, to investigate sources of variation in TE rates prior to reproduction and to evaluate possible implications for age-specific probability of first reproduction. TE rates were near 1 the year after birth, decreased to an average of 0.15 ( $\hat{\mathrm{S}\mathrm{E}}=0.01$ ) by age 8, and were similar thereafter. TE rates varied substantially from year-to-year and were lower for seals that attended reproductive colonies the previous year than for seals that did not attend (e.g., $\overline{{\hat{\mathrm{\psi }}}_{\mathrm{i},\mathrm{a}\mathrm{g}\mathrm{e} \ 8}^{\mathrm{U}\mathrm{U}}-{\hat{\mathrm{\psi }}}_{\mathrm{i},\mathrm{a}\mathrm{g}\mathrm{e} \ 8}^{\mathrm{P}\mathrm{U}}}=0.22$ ). Recruitment rates were marginally greater for seals that did attend than for seals that did not attend colonies the previous year. For Weddell seals specifically, our results suggest that (1) motivation to attend colonies varied temporally, (2) as seals grew older they had increased motivation to attend even before reproductive maturity, and (3) seals appear to follow various attendance strategies. More broadly, our results support the idea of TE as a variable, condition-dependent strategy, and highlight the utility of TE models for providing population and life-history insights for diverse taxa.
Spring migration may facilitate survival and recruitment in mammals by reducing predation risk and increasing access to higher-quality forage. The Yellowstone pronghorn
Antilocapra americana ...population (<250 animals) retains one of only two pronghorn migrations remaining in the greater Yellowstone region of the western United States. We used 5743 telemetry locations of 44 adult, female pronghorn during June 1999–August 2005 to determine migration patterns, seasonal distributions, and individual fidelity to migratory strategies. Yellowstone pronghorn were partially migratory, with >70% of the pronghorn population migrating 15–50
km to 4 contiguous summering areas and <30% remaining year-round on the winter range. Most radio-collared pronghorn showed fidelity across years to their migration strategy and summer use area, but approximately 20% migrated in some years, but not others. This behavioral flexibility is consistent with the hypothesis that migration in Yellowstone pronghorn is a conditional strategy and likely contributed to dynamic and rapid changes in the proportion of migrants from 80% to 20% and back to 70% during 1967–2005. All migrant pronghorn traveled 10
km over a topographic bottleneck (Mt. Everts) separating the winter and summer ranges, primarily using grassland–sagebrush pathways through conifer forest. We recommend continued protection of this corridor because increased mortality and a decreasing proportion of migrants may be as important a threat to the persistence of partially migratory populations as habitat fragmentation, especially when local resources for non-migrants are inadequate to sustain the entire population.
The Weddell seal population in Erebus Bay, Antarctica, represents one of the best-studied marine mammal populations in the world, providing an ideal test for the efficacy of satellite imagery to ...inform about seal abundance and population trends. Using high-resolution (0.6 m) satellite imagery, we compared counts from imagery to ground counts of adult Weddell seals and determined temporal trends in Erebus Bay during November 2004–2006 and 2009, and December 2007. Seals were counted from QuickBird-2 and WorldView-1 images, and these counts were compared with ground counts at overlapping locations within Erebus Bay during the same time. Counts were compared across years and within individual haul-out locations. We counted a total of 1,000 adult Weddell seals from five images across all years (for a total of 21 satellite-to-ground count comparisons), approximately 72% of the total counted on the ground at overlapping locations. We accurately detected an increase in abundance during 2004–2009. There was a strong, positive correlation (
r
= 0.98,
df
= 3,
P
< 0.003) between ground counts and counts derived from the imagery. The correlation between counts at individual haul-out locations was also strong (
r
= 0.80,
df
= 19,
P
< 0.001). Detection rates ranged from 30 to 88%. Overall, our results showed the utility of high-resolution imagery to provide an accurate way to detect the presence and variation in abundance of Weddell seals. Our methods may be applied to other species in polar regions, such as walruses or polar bears, particularly in areas where little is known about population status.
In long-lived species, juvenile survival typically is lower and more variable than adult survival, and modeling such variation is important for understanding population dynamics. Variability in ...juvenile survival can be related to birth- or current-year influences, and the birth-year influences can be transient, persistent, or intermediate in duration. We used multi-state models and data collected from 5,459 known-aged prebreeder female Weddell seals (
Leptonychotes weddellii
Lesson) tagged in Erebus Bay, Antarctica from 1980–2007 to evaluate the duration of potential birth-year influences on survival rates and the importance of birth- and current-year influences on survival and recruitment rates. Survival rates differed for each birth cohort and were positively related to current-year winter sea-ice conditions. The estimated duration of birth-cohort effects on survival was intermediate (6 years) rather than transient (2 years) or permanent. Estimated survivorship from birth to 6 years of age varied among cohorts from 0.13 (SE = 0.04) to 0.42 (SE = 0.06), and averaged 0.25 (SE = 0.02). Recruitment rates (probability of transitioning from prebreeder to breeder state) varied annually but apparently were not related to birth-year conditions. Our results provide evidence that birth- and current-year conditions act in combination to influence survival. Although for many long-lived species the influences of either birth- or current-year conditions on survival are well-studied, we suggest that modeling survival rates as a function of birth- and current-year influences simultaneously could lead to better understanding of survival and improved stochastic models to project population dynamics.
Understanding mechanisms influencing the movement paths of animals is essential for comprehending behavior and accurately predicting use of travel corridors. In Yellowstone National Park (USA), the ...effects of roads and winter road grooming on bison (Bison bison) travel routes and spatial dynamics have been debated for more than a decade. However, no rigorous studies have been conducted on bison spatial movement patterns. We collected 121 380 locations from 14 female bison with GPS collars in central Yellowstone to examine how topography, habitat type, roads, and elevation affected the probability of bison travel year-round. We also conducted daily winter bison road use surveys (2003-2005) to quantify how topography and habitat type influenced spatial variability in the amount of bison road travel. Using model comparison techniques, we found the probability of bison travel and spatial distribution of travel locations were affected by multiple topographic and habitat type attributes including slope, landscape roughness, habitat type, elevation, and distances to streams, foraging areas, forested habitats, and roads. Streams were the most influential natural landscape feature affecting bison travel, and results suggest the bison travel network throughout central Yellowstone is spatially defined largely by the presence of streams that connect foraging areas. Also, the probability of bison travel was higher in regions of variable topography that constrain movements, such as in canyons. Pronounced travel corridors existed both in close association with roads and distant from any roads, and results indicate that roads may facilitate bison travel in certain areas. However, our findings suggest that many road segments used as travel corridors are overlaid upon natural travel pathways because road segments receiving high amounts of bison travel had similar landscape features as natural travel corridors. We suggest that most spatial patterns in bison road travel are a manifestation of general spatial travel trends. Our research offers novel insights into bison spatial dynamics and provides conceptual and analytical frameworks for examining movement patterns of other species.
We present the first quantitative representation of the intensity of Yellowstone National Park's surficial geothermal activity mapped continuously in space. A radiative thermal anomaly was remotely ...sensed throughout a 19,682-km
2 landscape covering Yellowstone National Park in the northern Rocky Mountains, USA. The anomaly is the residual terrestrial emittance measured using the Landsat Enhanced Thematic Mapper after accounting for elevation and solar effects, and was hypothesized to be an estimator of a lower bound for geothermal heat flux (GHF). Continuous variations in the anomaly were measured ranging from 0 W m
−
2
up to a maximum heat flux of at least 94 W m
−
2
(at the 28.5 m pixel scale). An independent method was developed for measuring GHF at smaller scales, based on inversion of a snowpack simulation model, combined with field mapping of snow-free perimeters around selected geothermal features. These perimeters were assumed to be approximately isothermal, with a mean GHF estimated as the minimum heat flux required to ablate the simulated snowpack at that location on the day of field survey. The remotely sensed thermal anomaly correlated well (
r
=
0.82) with the snowpack-inversion measurements, and supported the hypothesis that the anomaly estimates a lower bound for GHF. These methods enable natural resource managers to identify, quantify and predict changes in heat flux over time in geothermally active areas. They also provide a quantitative basis for understanding the degree to which Yellowstone's famous wildlife herds are actually dependent on geothermal activity.
Irruptive population dynamics appear to be widespread in large herbivore populations, but there are few empirical examples from long time series with small measurement error and minimal harvests. We ...analyzed an 89-year time series of counts and known removals for pronghorn (Antilocapra americana) in Yellowstone National Park of the western United States during 1918-2006 using a suite of density-dependent, density-independent, and irruptive models to determine if the population exhibited irruptive dynamics. Information-theoretic model comparison techniques strongly supported irruptive population dynamics (Leopold model) and density dependence during 1918-1946, with the growth rate slowing after counts exceeded 600 animals. Concerns about sagebrush (Artemisia spp.) degradation led to removals of >1100 pronghorn during 1947-1966, and counts decreased from approximately 700 to 150. The best models for this period (Gompertz, Ricker) suggested that culls replaced intrinsic density-dependent mechanisms. Contrary to expectations, the population did not exhibit enhanced demographic vigor soon after the termination of the harvest program, with counts remaining between 100 and 190 animals during 1967-1981. However, the population irrupted (Caughley model with a one-year lag) to a peak abundance of approximately 600 pronghorn during 1982-1991, with a slowing in growth rate as counts exceeded 500. Numbers crashed to 235 pronghorn during 1992-1995, perhaps because important food resources (e.g., sagebrush) on the winter range were severely diminished by high densities of browsing elk, mule deer, and pronghorn. Pronghorn numbers remained relatively constant during 1996-2006, at a level (196-235) lower than peak abundance, but higher than numbers following the release from culling. The dynamics of this population supported the paradigm that irruption is a fundamental pattern of growth in many populations of large herbivores with high fecundity and delayed density-dependent effects on recruitment when forage and weather conditions become favorable after range expansion or release from harvesting. Incorporating known removals into population models that can describe a wide range of dynamics can greatly improve our interpretation of observed dynamics in intensively managed populations.
Understanding the potential influence of environmental variation experienced by animals during early stages of development on their subsequent demographic performance can contribute to our ...understanding of population processes and aid in predicting impacts of global climate change on ecosystem functioning. Using data from 4178 tagged female Weddell seal pups born into 20 different cohorts, and 30 years of observations of the tagged seals, we evaluated the hypothesis that environmental conditions experienced by young seals, either indirectly through maternal effects and/or directly during the initial period of juvenile nutritional independence, have long-term effects on individual demographic performance. We documented an approximately three-fold difference in the proportion of each cohort that returned to the pupping colonies and produced a pup within the first 10 years after birth. We found only weak evidence for a correlation between annual environmental conditions during the juvenile-independence period and cohort recruitment probability. Instead, the data strongly supported an association between cohort recruitment probability and the regional extent of sea ice experienced by the mother during the winter the pup was in utero. We suggest that inter-annual variation in winter sea-ice extent influences the foraging success of pregnant seals by moderating the regional abundance of competing predators that cannot occupy areas of consolidated sea ice, and by directly influencing the abundance of mid-trophic prey species that are sea-ice obligates. We hypothesize that this environmentally-induced variation in maternal nutrition dictates the extent of maternal energetic investment in offspring, resulting in cohort variation in mean size of pups at weaning which, in turn, contributes to an individual's phenotype and its ultimate fitness. These linkages between sea ice and trophic dynamics, combined with demonstrated and predicted changes in the duration and extent of sea ice associated with climate change, suggest significant alterations in Antarctic marine ecosystems in the future.
Identifying factors affecting juvenile survival is important to understanding the dynamics of populations and may also provide insights into life history theory and the selective forces shaping ...evolution. Quantifying the relative influence of the various potential selective forces for the post-birth, maternal dependency, and independent periods is difficult and often limits investigators to estimating a single juvenile survival rate for the first year of life, or from birth to recruitment. We examined survival of individually marked Weddell seal Leptonychotes weddellii pups during the maternal dependency period in Erebus Bay, Antarctica from 2005-2007. We used mark-recapture models to evaluate competing a priori hypotheses regarding variation in daily pre-weaning survival rates (φ) during the first 42 days of age. The a priori model receiving the most support from the data supported several of our predictions: graphic removed increased with pup age and was higher for pups born later in the season and to older mothers. Increases in graphic removed with increasing pup age may have been due to improved resilience to the environment, phenotypic selection against the frailest pups, or both. Maternal age was more important to graphic removed than was maternal experience or age of primiparity, potentially indicating that age-related increases in body mass allow greater offspring provisioning, or age-related improvements in competitive abilities benefit offspring during the period of maternal care. Depending on the timing of birth and the age of the mother, graphic removed ⁴² days ranged from 0.79 (SE = 0.05) to 0.98 (SE = 0.01). These exceptionally high pre-weaning survival rates contrast with estimates from other large terrestrial and marine mammal species where neonate survival is considerably lower and suggest that in species with similar life histories, pre-weaning survival probability may be buffered from both predators and environmental fluctuations during the period of maternal nutritional dependency. Climatic changes affecting stability of ice used for pupping substrate or extent of fast-ice buffering pupping colonies from predators have the potential to reduce pre-weaning survival and may have important implications for population growth rates.
Synthesis Garrott, Robert A.
Human-wildlife interactions,
04/2018, Letnik:
12, Številka:
1
Journal Article
Recenzirano
Management of wild horse (Equus ferus caballus) populations on western U.S. rangelands has been a challenge since horses were given legal protection through the passage of the Wild Free-Roaming ...Horses and Burros Act (WFRHBA) in 1971. Horses have no effective predators, and unmanaged populations can double in 4–5 years and triple in 6–8 years. In order to meet the multiple-use paradigm for managing public rangelands, the Bureau of Land Management (BLM) has limited horse population growth through the periodic capture and removal of animals. While the WFRHBA mandates disposal of captured horses through placement into private ownership and prompt destruction of any excess animals, administrative restrictions have prohibited the destruction of healthy horses for nearly the entire history of the management program. This has led to an ever-increasing number of unwanted horses maintained in captivity, which has required increasing Congressional appropriations. There are currently 44,000 horses in long-term captivity at an annual cost of approximately $50 million. Recently, Congress has denied requests from the BLM for further funding increases to support continued growth in the number of horses in long-term maintenance, obligating the BLM to dramatically curtail population management. Horse numbers on public rangelands are now rapidly increasing, and if left minimally managed will exceed the capacity of rangeland resources, resulting in serious degradation of these public lands for all other uses and eventually will result in large numbers of horses dying of starvation and dehydration. Horse advocates suggest this management crisis can be solved with the aggressive use of contraceptive technologies. Limitations in efficacy and the logistics of administering contraceptives indicate that contraceptives can only slow population growth rates, but alone cannot decrease numbers. The BLM and other stakeholders are pressing for authorization to destroy excess horses but are facing public and Congressional opposition, with the potential that the status quo continues. A sustainable wild horse and burro (E. asinus; WHB) management program could be achieved by a combination of reducing the on-range population and treating adequate numbers of horses remaining on rangelands with contraceptives to reduce subsequent population growth rates. Under this scenario, the freeroaming horse population would produce a modest annual increment of horses, which could be removed and readily placed into private ownership. It has taken nearly half a century for the wild horse problem to reach this critical point, and any transition to a sustainable program will take time and additional resources. The fundamental challenge to developing a sustainable program will be solving the problem of the fate of excess horses. The policy decisions confronting us are historic, challenging, and controversial with a real danger of not finding the resolve to chart a new course for the WHB Program. If we fail and continue with the current policies, then horses, native wildlife, all stakeholders, and our public rangelands will pay a heavy price.
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