Sophie L. Gilbert

Dead before detection: addressing the effects of left truncation on survival estimation and ecological inference for neonates

Summary 1. Neonate survival is a key life history trait, yet remains challenging to measure in wild populations because neonates can be difficult to capture at birth. Estimates of survival from neonates that are opportunistically captured might be inaccurate because some individuals die before sampling, resulting in data that are left truncated. The resulting overestimation of survival rates can further affect ecological inference through biased estimates of covariate effects in survival models, yet is not addressed in most studies of animal survival. Here, we quantify the effects of left truncation on survival estimates and subsequent ecological inference. 2. Vaginal implant transmitters (VITs) enable capture of ungulates at birth, yielding data without left truncation. The effects of left truncation on survival estimation were quantified using age-dependent survival models for VIT and opportunistically captured neonatal deer. Differences in daily survival rates (DSRs) and cumulative survival probability were calculated for the first 70 days of life. In addition, left truncation was simulated by removing fawns that died during the first 1 or 2 days of life from the VIT-caught sample, isolating the effect of left truncation. 3. Cumulative probability of survival during the first 70 days of life was overestimated by 7–23% for fawns caught opportunistically compared with those caught by VIT, depending on model design. Differences in DSRs were large at age 1 day, but had converged by age 30 days. Simulated left truncation resulted in overestimates of survival of up to 31%. Model selection and covariate coefficients were strongly affected by left truncation, producing spurious ecological inference, including changes to sign and/or magnitude of inferred effects of all covariates. 4. We recommend (i) every effort be made to capture neonates; (ii) consistent capture methods, using at least in part non-truncating techniques, be implemented across years and study areas; and (iii) exclusion of left-truncated data from survival estimates until DSRs converge with those calculated from non-truncated data. This work emphasizes the importance of accounting for left truncation in survival estimation for any species with strong age-dependent survival in order to prevent biased conclusions produced by sampling method rather than true ecological effects.

Re-Evaluating Neonatal-Age Models for Ungulates: Does Model Choice Affect Survival Estimates?

New-hoof growth is regarded as the most reliable metric for predicting age of newborn ungulates, but variation in estimated age among hoof-growth equations that have been developed may affect estimates of survival in staggered-entry models. We used known-age newborns to evaluate variation in age estimates among existing hoof-growth equations and to determine the consequences of that variation on survival estimates. During 2001–2009, we captured and radiocollared 174 newborn (≤24-hrs old) ungulates: 76 white-tailed deer (Odocoileus virginianus) in Minnesota and South Dakota, 61 mule deer (O. hemionus) in California, and 37 pronghorn (Antilocapra americana) in South Dakota. Estimated age of known-age newborns differed among hoof-growth models and varied by >15 days for white-tailed deer, >20 days for mule deer, and >10 days for pronghorn. Accuracy (i.e., the proportion of neonates assigned to the correct age) in aging newborns using published equations ranged from 0.0% to 39.4% in white-tailed deer, 0.0% to 3.3% in mule deer, and was 0.0% for pronghorns. Results of survival modeling indicated that variability in estimates of age-at-capture affected short-term estimates of survival (i.e., 30 days) for white-tailed deer and mule deer, and survival estimates over a longer time frame (i.e., 120 days) for mule deer. Conversely, survival estimates for pronghorn were not affected by estimates of age. Our analyses indicate that modeling survival in daily intervals is too fine a temporal scale when age-at-capture is unknown given the potential inaccuracies among equations used to estimate age of neonates. Instead, weekly survival intervals are more appropriate because most models accurately predicted ages within 1 week of the known age. Variation among results of neonatal-age models on short- and long-term estimates of survival for known-age young emphasizes the importance of selecting an appropriate hoof-growth equation and appropriately defining intervals (i.e., weekly versus daily) for estimating survival.

Behavioral plasticity in a variable environment: snow depth and habitat interactions drive deer movement in winter

In seasonally varying environments, animals should alter habitat selection through time to avoid the harshest conditions. Winter severity is limiting for many ungulates in high-latitude ecosystems, and quality of habitat is an important determinant of winter survival. Previous studies in Southeast Alaska indicated that Sitka black-tailed deer (Odocoileus hemionus sitkensis) selected old-growth forest that provides both snow interception and forage, but with great variability among studies, years, and geographic areas. Clearcut timber harvest has greatly reduced the extent and quality of old-growth forest. The value of 2nd-growth and old-growth forest types to deer likely depends on snow depth, which is highly variable in space and time. We measured selection for vegetation classes, landscape features, and forage biomass by monitoring 56 GPS-radiocollared adult female deer from 1 January to 1 April between 2011 and 2013. Simultaneously, we measured snow depth across deer home ranges daily. We determined that snow depth had a strong effect on selection for vegetation classes. During periods of low snow, deer selected young 2nd growth but avoided old 2nd growth and high-volume old growth. As snow depths increased, young 2nd growth was avoided and deer selected old 2nd-growth and productive old-growth forests. The composition of vegetation classes within the landscape influenced selection, with deer selecting locally abundant habitats. These behaviors suggest that the widespread distribution of forest patches that provide snow interception and forage biomass may be critical to fulfilling the energetic requirements of deer during winters with snow. Such context-dependent habitat selection is likely widespread among wildlife species in variable environments and should be incorporated into study design and analysis.

Tweet success? Scientific communication correlates with increased citations in Ecology and Conservation

Science communication is seen as critical for the disciplines of ecology and conservation, where research products are often used to shape policy and decision making. Scientists are increasing their online media communication, via social media and news. Such media engagement has been thought to influence or predict traditional metrics of scholarship, such as citation rates. Here, we measure the association between citation rates and the Altmetric Attention Score—an indicator of the amount and reach of the attention an article has received—along with other forms of bibliometric performance (year published, journal impact factor, and article type). We found that Attention Score was positively correlated with citation rates. However, in recent years, we detected increasing media exposure did not relate to the equivalent citations as in earlier years; signalling a diminishing return on investment. Citations correlated with journal impact factors up to ∼13, but then plateaued, demonstrating that maximizing citations does not require publishing in the highest-impact journals. We conclude that ecology and conservation researchers can increase exposure of their research through social media engagement and, simultaneously, enhance their performance under traditional measures of scholarly activity.

Density-dependent signaling: An alternative hypothesis on the function of chemical signaling in a non-territorial solitary carnivore

Brown bears are known to use rubbing behavior as a means of chemical communication, but the function of this signaling is unclear. One hypothesis that has gained support is that male bears rub to communicate dominance to other males. We tested the communication of dominance hypothesis in a low-density brown bear population in southeast British Columbia. We contrasted rubbing rates for male and female bears during and after the breeding season using ten years of DNA-mark-recapture data for 643 individuals. Here we demonstrate that male brown bears rub 60% more during the breeding than the non-breeding season, while female rubbing had no seasonal trends. Per capita rub rates by males were, on average, 2.7 times higher than females. Our results suggest that the function of rubbing in the Rocky Mountains may not only be to communicate dominance, but also to self-advertise for mate attraction. We propose that the role of chemical communication in this species may be density-dependent, where the need to self-advertise for mating is inversely related to population density and communicating for dominance increases with population density. We suggest that future endeavors to elucidate the function of rubbing should sample the behavior across a range of population densities using camera trap and genotypic data.

Bald Eagle Predation on Sitka Black-Tailed Deer Fawns

Bald Eagles (Haliaeetus leucocephalus) are known predators, but records of predation on large mammals are limited and highly seasonal. During winter, Bald Eagles are known to scavenge but not predate large mammals (Ewins and Andress 1995; Elliott and others 2011). In contrast, while records of active predation are rare even during nesting season, several instances have been documented (Vermeer and Morgan 1989; Hayward 2009). Diets of nesting Bald Eagles are diverse, and include large mammals such as deer (Vermeer and Morgan 1989; Watson 2002), in addition to other dietary items ranging from anthropogenic trash to dead and live fish, marine invertebrates, seabird eggs and young, and other mammal species (Hansen and Hodges 1985; Vermeer and Morgan 1989; Watson 2002; Anthony and others 2008). While consumption of large mammals by Bald Eagles typically occurs when they scavenge dead animals, evidence is accumulating that Bald Eagles actively predate the young of large mammals during nesting season. For example, Bald Eagles have been documented preying on Harbor Seal pups (Phoca vitulina; Hayward 2009) and Sea Otters (Enhydra lutris; Anthony and others 2008) during summer. In addition, Bald Eagles have killed young ungulates during summer, including fawns of White-tailed Deer (Odocoileus virginianus; Duquette and others 2011), lambs of domestic sheep (Smith 1936; McEneaney and Jenkins 1983), and likely Woodland Caribou calves (Rangifer tarandus caribou; Environment Canada 2011). Southeast Alaska is home to the largest population of Bald Eagles in the United States (Hodges 2011), where abundant marine resources are important for breeding success and survival (Anthony 2001; Elliott and others 2011). In this ecosystem, Sitka Black-tailed Deer (Odocoileus hemionus sitkensis) are the dominant large ungulate (Hanley 1993; Alaska Department of Fish and Game 2009). Neonatal fawns are small and highly vulnerable to predation (Gilbert and others 2014), and are thus a potential source of prey to nesting Bald Eagles on Prince of Wales Island. Here, I report the likely predation of 2 Sitka Black-Tailed Deer fawns by Bald Eagles. These are the 1st recorded incidents of Bald Eagle predation on deer fawns in this ecosystem and, to my knowledge, of Odocoileus hemionus; however, there have been 2 previously recorded predation events on White-Tailed Deer fawns (Line 1961; Duquette and others 2011). Between 2010 and 2012, I radio-collared 154 neonatal deer on central Prince of Wales Island, the largest island in the Alexander Archipelago of Southeast Alaska (Fig. 1). All animal capture and handling was carried out in accordance with the University of Alaska Fairbanks Institutional Animal Care and Use Committee (IACUC #136040-14) regulations. Fawns were captured as part of a study of deer habitat and predator-prey relationships in the temperate rainforest ecosystem of Southeast Alaska. Fawn capture methods are described in detail in Gilbert and others (2014). Following capture, I monitored fawns in early morning and late afternoon of each day during the summer (from birth until 1 August), and investigated mortalities as soon as possible when a mortality signal was received from a radio collar. As a result, I investigated all mortalities within 24 h, and typically within 12 h, allowing accurate identification of cause of death based on site and carcass conditions. In addition, this twice-daily monitoring schedule allowed for classification of diurnal versus nocturnal mortalities. Prince of Wales Island is known to support the highest density of nesting Bald Eagles ever recorded, with average distances between neighboring nests of 3.1 km (Anthony 2001). In addition, Anthony (2001) recorded extremely low nest success and number of fledglings per nest, and suggested food limitation during early nesting season, before the arrival of anadromous salmon in streams, as a probable cause. Bald Eagles were common in the study area, and GENERAL NOTES