The global decline of large carnivores demands effective and efficient methods to monitor population status, particularly using non-invasive methods. Density is among the most useful metrics of population status because it is directly comparable across space and time. Unfortunately, density is difficult to measure reliably, especially for mobile, cryptic species. Recently, efforts have turned to approximating density based on its relationship to more readily estimable indices of occurrence. However, the relationship between density and such indices is contingent on several key assumptions that field studies often violate. Recent research has shown that these relationships are unreliable where sampling units are not independent, as is often the case when estimating density or occurrence of large carnivores. Here, we use the largest data set thus far collected for leopards (Panthera pardus)-88 camera-trap surveys undertaken in 24 protected areas between 2013 and 2018-to explore how density and other population characteristics relate to parameter estimates in occupancy and Royle-Nichols abundance models. We show how home-range size confounds underlying relationships, with larger home ranges inflating the proportion of area used (PAU) and resulting in double counting in abundance models. Relativizing estimates of occupancy and abundance by home-range size improved their relationship with density, but the relationship remained weak and largely uninformative for management. Our findings illustrate the pitfalls of using the PAU or abundance as implicit proxies for density and highlight the challenges of assessing population status for wide-ranging, cryptic species across fragmented landscapes.
Individual specialization, when individuals exploit only a subset of resources utilized by the population, is a widespread phenomenon. It provides the basis for evolutionary diversification and can impact population and community dynamics. Both phenotypic traits and environmental conditions are predicted to influence individual specialization; however, its adaptive consequences are poorly understood, particularly among large mammalian carnivores that play an important role in shaping ecosystems. We used observations of 2,960 kills made by 49 leopards Panthera pardus in the Sabi Sand Game Reserve, South Africa, to quantify the magnitude of individual dietary specialization in a solitary large carnivore, and to examine the proximate and ultimate drivers of this behaviour. We found evidence of individual specialization in leopard diet, with respect to both the species and size of prey killed. Males tended to be more specialized than females, likely because they could access a wider range of prey due to larger body size. Similarly, individuals that encountered a greater diversity of prey tended to be more specialized. Our results confirmed that ecological opportunity was a key determinant of individual specialization; however, contrary to predictions, per capita resource availability (and by extension, intraspecific competition) did not affect the degree of specialization exhibited by individuals. Surprisingly, dietary specialization appeared to disadvantage male leopards. Specialist males overlapped with fewer resident females, had fewer cubs born on their home ranges and had fewer cubs survive to independence on their home ranges than generalist males. This may have resulted from the high degree of environmental stochasticity experienced during our study, as dietary specialization is expected to advantage individuals more during periods of resource predictability. In summary, we showed that a species usually considered to be a dietary generalist was in fact a heterogeneous collection of specialist and generalist individuals. Individual specialization is typically assumed to be maintained by disruptive and/or fluctuating selection; hence, the somewhat paradoxical coexistence of both in the same population might be explained by a dynamic evolutionary equilibrium that exists between specialists and generalists, in which each benefit under different conditions.
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