Evaluating apparent competition in limiting the recovery of an endangered ungulate

College of Forestry and Conservation, University of Montana, Missoula, MT, 59812, USA, .
Oecologia (Impact Factor: 3.09). 07/2012; 171(1). DOI: 10.1007/s00442-012-2397-6
Source: PubMed


Predation can disproportionately affect endangered prey populations when generalist predators are numerically linked to more abundant primary prey. Apparent competition, the term for this phenomenon, has been increasingly implicated in the declines of endangered prey populations. We examined the potential for apparent competition to limit the recovery of Sierra Nevada bighorn sheep (Ovis canadensis sierrae), an endangered subspecies under the US Endangered Species Act. Using a combination of location, demographic, and habitat data, we assessed whether cougar (Puma concolor) predation on endangered bighorn sheep was a consequence of their winter range overlap with abundant mule deer (Odocoileus hemionus). Consistent with the apparent competition hypothesis, bighorn sheep populations with higher spatial overlap with deer exhibited higher rates of cougar predation which had additive effects on adult survival. Bighorn sheep killed by cougars were primarily located within deer winter ranges, even though those areas constituted only a portion of the bighorn sheep winter ranges. We suspect that variation in sympatry between bighorn sheep and deer populations was largely driven by differences in habitat selection among bighorn sheep herds. Indeed, bighorn sheep herds that experienced the highest rates of predation and the greatest spatial overlap with deer also exhibited the strongest selection for low elevation habitat. Although predator-mediated apparent competition may limit some populations of bighorn sheep, it is not the primary factor limiting all populations, suggesting that the dynamics of different herds are highly idiosyncratic. Management plans for endangered species should consider the spatial distributions of key competitors and predators to reduce the potential for apparent competition to hijack conservation success.

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Available from: Heather E. Johnson, Jun 06, 2014
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    • "This can be especially true in cases where predator numbers are determined by primary prey densities, but the predator selectively preys upon a secondary prey species, causing a disproportionate effect on secondary prey (Messier 1994, Sinclair et al. 1998). Disproportionate predation on secondary prey by cougars has been documented in systems where mountain caribou (Rangifer tarandus caribou; Wittmer et al. 2005), Sierra Nevada bighorn sheep (Ovis canadensis sierrae; Johnson et al. 2012), mule deer (Robinson et al. 2002, Cooley et al. 2008), and porcupine (Erethizon dorsatum; Sweitzer et al. 1997) served as secondary prey species for cougars. We conducted a 3-year study to document kill rates, prey use, and prey selection of a sample of adult cougars in a multiple-prey system. "
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    ABSTRACT: Cougars (Puma concolor) are a primary predator of mule deer (Odocoileus hemionus) and elk (Cervus elaphus) throughout western North America. Effective management of predator-prey systems requires a solid understanding of kill rates, prey use, and selection. We implemented a 3-year study in northeast Oregon to investigate cougar diet, kill rates, and prey selection in a multiple-prey system to assess the degree to which patterns in cougar predation may be generalizable across systems and to identify selective predation patterns of cougars that may affect ungulate populations. We marked 25 adult cougars with global positioning system (GPS) collars and monitored predation sequences for 7,642 days to identify kill sites. In field investigation of kill sites, we identified remains of 1,213 prey items killed by cougars, of which 1,158 (95.4%) were native ungulates. On average, cougars killed 1.03 ungulates/week (95% CI = 0.92–1.14), but kill rates varied by season, sex, and reproductive status of cougars. Cougars killed ungulate prey 1.55 (95% CI = 1.47–1.66) times more frequently during summer (May–Oct) than winter (Nov–Apr). Kill rates were higher in summer because juvenile ungulates were the most frequently killed prey item and were smaller than prey killed in winter. Female cougars with kittens >6 months old killed prey more frequently than males, solitary females, and females with kittens <6 months old likely in response to the increased energetic burden of raising kittens. Male cougars killed larger prey than females, which likely explains why males killed at similar rates as solitary females, despite the larger body size of males. We documented patterns in prey selection influenced by season and demographic classification of cougars. Diets of male cougars included roughly equal amounts of elk (52.2%) and deer (47.8%), whereas diets of females were dominated by deer (74.6%). Male and female cougars displayed strong patterns of selection for elk calves during summer. During winter, female cougars selected deer fawns and males selected elk calves. Female cougars with kittens >6 months old demonstrated little selection for any age class or species of prey, highlighting an opportunistic foraging strategy to maximize energy gains while feeding young. Across all cougars, we observed a pattern of selection for adult male deer during winter but not summer and did not observe patterns of selection for adult elk according to sex. Our results strongly supported the hypothesis proposed by [Knopff et al. (2010) Journal of Wildlife Management, 74: 1435–1447] that cougar predation is influenced by season and demographic classifications of cougars and our results provide strong evidence that this hypothesis should be generalizable to other areas. The patterns of selection for juvenile elk and deer suggested wildlife managers should consider the potential negative effects of cougars on ungulate populations in areas where juvenile recruitment has been chronically low. © 2014 The Wildlife Society.
    Full-text · Article · Sep 2014 · Journal of Wildlife Management
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    • "This has important implications for predator-prey modeling and subsequent management of both pumas (and sensu other large carnivores) and their ungulate prey. Given that puma predation is a significant cause of mortality for many North American game species (e.g., bighorn sheep, Ovis canadensis [Johnson et al. 2013]; mule deer [Forrester and Wittmer 2013]) "
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    ABSTRACT: Bioenergetic modeling is employed to estimate the energetic demands of many cryptic carnivores and their kill rates needed to meet their energetic requirements. We tested two prevalent assumptions driving energetic modeling of predator kill rates: (1) morphological and physiological information (weight, energetic demands of activity patterns) of individual predators are sufficient to accurately predict their kill rates, and (2) kill and consumption rates are equivalent (meaning that carnivores consume all of what they kill). We did this by testing whether two independent energetic models accurately predicted puma (Puma concolor) kill and consumption rates in three study systems in North and South America with variable ecology, including climate and prey assemblages. Our results demonstrated that current puma energetic models drastically underestimate actual puma kill rates quantified through intensive field monitoring. We concluded that puma energetic models more realistically estimate puma consumption rates needed to meet metabolic requirements. Puma kill rates determined from field efforts were not explained by puma weight (in kg) or activity patterns (in distance traveled), which were the variables used in energetic models. Our kill rates in kg/day determined from field investigations of GPS clusters were the highest reported to date and statistically equivalent across three distinct ecosystems, a range of puma characteristics, variable lengths of monitoring, variable daily distances traveled, and across systems with 1–3 ungulate prey. In contrast, puma kill rates in ungulates/ week differed across study areas, suggesting that kill rates described in kilograms per day are better suited for comparing puma kill rates across systems while kill rates in terms of ungulates per unit time are better suited for modeling predator-prey dynamics for a particular ecosystem. Based on these results we concluded that energetic models using morphological and physiological variables alone were insufficient to predict kill rates, and proposed that rather than focusing future research on refining current energetic models, future research should be directed at understanding the behavioral ecology driving carnivore kill rates.
    Full-text · Article · May 2014 · Ecosphere
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    • "Growth of bitterbrush, the primary winter forage for mule deer in the Sierra Nevada (Kucera 1997, Pierce et al. 2004), was influenced largely (r 2 ¼ 0.65, P ¼ 0.001) by the water content of the snowpack from the preceding April (Fig. 2; Pierce et al. 2012). Accordingly, seasonal nutrition was influenced by snowfall from the preceding year; however, per capita availability of forage was determined by the relationship of snowpack and number of individuals present in the subsequent year (Pierce et al. 2012, Monteith et al. 2013). Therefore, we calculated a density-dependent proxy to forage availability based on the quotient of the water content of the snowpack during the preceding April, and the estimated number of females for that year (per capita snowpack; cm/female). "
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    ABSTRACT: Vital rates of large herbivores normally respond to increased resource limitation by following a progressive sequence of effects on life-history characteristics from survival of young, age at first reproduction, reproduction of adults, to adult survival. Expected changes in life history characteristics, however, should operate through changes in nutritional condition, which is the integrator of nutritional intake and demands represented primarily by the deposition and catabolism of body fat. Elucidating seasonal patterns of nutritional condition and its relative influence on individual and population performance should improve our understanding of life-history strategies and population regulation of ungulates, provide insight into the capacity of available habitat to support population growth, and allow assessment of the underlying consequences of mortality on population dynamics. We acquired longitudinal data on individual female mule deer (Odocoileus hemionus), and linked those data with environmental and population characteristics. Our goal was to provide a nutritional basis for understanding life-history strategies of these large mammals, and to aid in the conservation and management of large herbivores in general. We studied a migratory population of mule deer that overwintered in Round Valley on the east side of the Sierra Nevada, California, USA, and was subject to a highly variable climate and predation from a suite of large carnivores. We intensively monitored nutritional and life-history characteristics of this population during 1997–2009 as it recovered from a population crash, which occurred during 1985–1991. Deer in Round Valley migrated to high-elevation summer ranges on both sides of the crest of the Sierra Nevada (Sierra crest), where a rain shadow resulted in a mesic and more forested range on the west side compared with xeric conditions east of the Sierra crest. Average survival of neonatal mule deer to 140 days of age during 2006–2008 was 0.33 (SE = 0.091), but was lower for neonates on the west side (0.13, SE = 0.092) compared with those on the east side (0.44, SE = 0.11) of the Sierra crest. Birth mass and nutritional condition of mothers had a positive effect on survival of young; however, those effects were evident only for neonates born east of the crest where predation pressure was less intense compared with the west side. Black bear (Ursus americanus) predation was the main cause of mortality for west side young (mortality rate = 0.63, SE = 0.97) compared with canid and felid predation for east-side young (0.29, SE = 0.076). Mean autumn recruitment of young during 1997–2008 was lower for females on the west side (0.42, SE = 0.037) than for females on the east side (0.70, SE = 0.041) of the crest, and was affected positively by March ingesta-free body fat (IFBFat) of individual females. At the level of the population, ratios of young to adult females (1991–2009) were highly variable and strongly related to March IFBFat of adult females during the current and preceding year. Reproduction by yearling females was sensitive to per capita availability of forage during summer (as 1-yr-old individuals), thereby influencing whether a sufficient body mass for ovulation was obtained. Litter size remained high (1.69, SE = 0.027) during the study, but was influenced positively by forage availability, negatively by summer temperature, and was greater for females that resided on the west side of the Sierra crest during summer than those on the east side. In contrast, pregnancy rates remained unchanged across years of study (0.98, SE = 0.005). Survival of prime age (2–9-yr-old) females was 0.90 (SE = 0.021) in summer, 0.94 (SE = 0.012) in winter, and 0.87 (SE = 0.025) annually. Although relatively stable across years, both winter and summer survival were influenced positively by the preceding April snowpack relative to the density of the population. Mean IFBFat of adult females was 7.2% (SE = 0.077) in March 1997–2009 and 9.7% (SE = 0.23) in November 2002–2008. Nutritional condition offered a mechanistic link between factors that influence resource limitation and population performance, because condition of adult females in autumn and late winter was sensitive to the nutritional history of individual animals as related to forage growth, population density, migratory tactic, reproductive costs, and nutritional carryover. Nutritional condition of adult females in March also was the most parsimonious predictor of finite rate of population growth (λ) during the forthcoming year. The relative magnitude of effect of nutritional condition on survival and reproduction was mostly in accordance with the predicted changes of vital rates in response to resource limitation for populations of large herbivores. Our results indicated that management and conservation of large herbivore populations could be improved by integrating indices of nutritional condition into current monitoring and research programs. We offer a method to estimate the proximity of a population to nutritional carrying capacity (NCC) that is based on nutritional status of the population relative to population performance (termed animal-indicated NCC). The proximity of the population to animal-indicated NCC represents the short-term capacity of the environment to support population growth. A nutritional approach to monitor and manage populations offers a direct link to the capacity of the habitat, and reduces the need to estimate population abundance or set goals according to population size. We also propose that the consequences of mortality (degree of additive or compensatory mortality) on population dynamics can be assessed by comparing the estimated nutritional capacity for survival and recruitment of young to that measured empirically, because more young are produced than what the habitat can support when nutrition is limiting. Our approach is useful for quantifying effects of predation, and provides a basis for determining the efficacy of predator control to enhance ungulate populations.
    Full-text · Article · Apr 2014 · Wildlife Monographs
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