Abstract
1. Understanding the interaction among predators and between predation and climate is critical to understanding the mechanisms for compensatory mortality. We used data from 1999 radio-marked neonatal elk (Cervus elaphus) calves from 12 populations in the north-western United States to test for effects of predation on neonatal survival, and whether predation interacted with climate to render mortality compensatory. 2. Weibull survival models with a random effect for each population were fit as a function of the number of predator species in a community (3-5), seven indices of climatic variability, sex, birth date, birth weight, and all interactions between climate and predators. Cumulative incidence functions (CIF) were used to test whether the effects of individual species of predators were additive or compensatory. 3. Neonatal elk survival to 3 months declined following hotter previous summers and increased with higher May precipitation, especially in areas with wolves and/or grizzly bears. Mortality hazards were significantly lower in systems with only coyotes (Canis latrans), cougars (Puma concolor) and black bears (Ursus americanus) compared to higher mortality hazards experienced with gray wolves (Canis lupus) and grizzly bears (Ursus horribilis). 4. In systems with wolves and grizzly bears, mortality by cougars decreased, and predation by bears was the dominant cause of neonatal mortality. Only bear predation appeared additive and occurred earlier than other predators, which may render later mortality by other predators compensatory as calves age. Wolf predation was low and most likely a compensatory source of mortality for neonatal elk calves. 5. Functional redundancy and interspecific competition among predators may combine with the effects of climate on vulnerability to predation to drive compensatory mortality of neonatal elk calves. The exception was the evidence for additive bear predation. These results suggest that effects of predation by recovering wolves on neonatal elk survival, a contentious issue for management of elk populations, may be less important than the composition of the predator community. Future studies would benefit by synthesizing overwinter calf and adult-survival data sets, ideally from experimental studies, to test the roles of predation in annual compensatory and additive mortality of elk.
Highlights
One of the biggest challenges facing animal ecologists is determining whether mortality by predators is additive or compensatory (Boyce, Sinclair & White 1999)
Two study areas were located in Montana: one in the Garnet Mountains (Raithel 2005; Harris 2007) and another in the Gallatin Valley (Christianson 2008)
We selected the top model(s) using Akaike’s Information Criteria (Burnham & Anderson 1998). Based on their importance in previous studies (Smith, Robbins & Anderson 1997; Smith et al 2006; Harris 2007; Raithel, Kauffman & Pletscher 2007; Barber-Meyer, Mech & White 2008), we considered the following ‘base’ covariates as fixed and included them in all subsequent models: sex, estimated birth weight [sex-specific regression (Smith, Robbins & Anderson 1997)] and estimated birth date (Johnson 1951)
Summary
One of the biggest challenges facing animal ecologists is determining whether mortality by predators is additive or compensatory (Boyce, Sinclair & White 1999). Whether predation is additive or compensatory is expected to be mediated by winter severity (Post et al 1999; Hebblewhite 2005). The effects of summer climate on forage and female elk body condition may mask climate-predation interactions during winter (Cook et al 2004). Compensatory mortality can arise because of competition amongst predators (Williams, Nichols & Conroy 2002; Heisey & Patterson 2006) and need not be restricted to the classic definition of compensation that focuses on hunting and overwinter mortality. Testing for compensatory mortality is further complicated by effects of age structure (Coulson, Gaillard & FestaBianchet 2005; Wilmers, Post & Hastings 2007)
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