Domestic dogs shape the landscape-scale distribution of a threatened forest ungulate

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Abstract

Domestic dogs are the most abundant carnivores worldwide, primarily due to human support. Food and other subsidies to dogs do not necessarily prevent dog predation on wildlife, particularly where dogs are allowed to range freely. Dog impacts on wildlife are suspected to be significant, yet the nature of dog–wildlife interactions is not fully understood. We tested the hypothesis that the distribution of dogs can significantly influence the space use of potential prey, and that both lethal and non-lethal mechanisms may underlie this interaction. If this is true, then we predicted that (1) evidence of predation and harassment by dogs should be evident where prey and dog activities overlap and (2) potential prey should be less frequent in areas where the probability of dog presence is high. To test these predictions we conducted two related studies. (1) We interviewed dog owners to estimate the probability of dog attack on pudu (Pudu puda), a globally vulnerable deer, and the lethality of these attacks. (2) We conducted a camera-trap survey documenting the landscape-scale distribution of pudu and dogs. Interviews showed that both the probability of dog attack on pudu (>85%) and the lethality of such attacks was high (50%). In occupancy models applied to the camera-trap data, the variable that best explained the distribution of pudus was the probability of dog presence. We tested three alternative explanations for the negative association between pudus and dogs that were not supported. Our findings suggest that dogs are efficient at chasing pudu they detect and that both predation and non-lethal (avoidance) consequences of harassment may be shaping the distribution of pudu. This work brings into focus important mechanisms underlying the threats of domestic dogs to endangered prey.

Highlights

► We evaluated if dogs influenced habitat occupancy by a small deer, the pudu (Pudu puda). ► We found a negative association between dog presence and pudu occupancy. ► In addition dogs frequently attacked and killed pudus. ► Predation by dogs is likely to be among the major threats for pudu and other prey species.

Introduction

Domestic dogs (Canis familiaris) are the most abundant carnivores worldwide with a global population that exceeds 500 million individuals (Vanak and Gompper, 2009b, Wandeler et al., 1993). Although domestic dogs are highly associated with human populations (Vanak and Gompper, 2009b), the free-ranging condition of a significant proportion of them facilitates their interaction with wild animals. These interactions include predation by dogs (Kruuk and Snell, 1981, Silva-Rodriguez et al., 2010b, Taborsky, 1988), interference competition (Silva-Rodriguez et al., 2010b, Vanak and Gompper, 2010), exploitation competition (Butler and Du Toit, 2002) and disease-mediated apparent competition (Laurenson et al., 1998). This broad range of possible interactions, the human subsidy to dog populations, and the global distribution of dogs create adequate conditions for dogs to be recognized as a current global threat for biodiversity conservation (Vanak and Gompper, 2009b, Young et al., 2011).

Despite the paucity of directed studies on this issue (Young et al., 2011), dogs are mentioned as a threat for a large number of species in the IUCN Red List (IUCN, 2011). Most studies of dog impacts on vertebrates relate to disease transmission (e.g., Laurenson et al., 1998) and more recently to interference competition (Silva-Rodriguez et al., 2010a, Vanak and Gompper, 2009b, Vanak and Gompper, 2010), but most species for whom dogs are considered a threat are affected through predation (IUCN, 2011). Early studies often dismissed the potential threat of dog-related predation because dogs were inefficient predators for large-sized North American deer (Causey and Cude, 1980, Scott and Causey, 1973). More recent findings contradict conclusions of low impact by showing that despite low predation rates by individual dogs (but see Taborsky, 1988), dog predation represents an important cause of mortality for diverse species (Corti et al., 2010, Pereira et al., 2010, Taborsky, 1988). Furthermore, the current paradigm in prey–predator interactions suggests that avoiding predation also carries important consequences for fitness (Brown et al., 1999, Laundre, 2010, Laundre et al., 2001). Heightened vigilance (Gingold et al., 2009, Vanak et al., 2009), decreased feeding rates (Vanak et al., 2009), shifts in use of space (Grignolio et al., 2011, Vanak and Gompper, 2010) and decreases in fecundity as a consequence of stress (Sheriff et al., 2009) are costly non-lethal effects of the presence of dogs on diverse vertebrates.

The conflicting goals of interacting prey and predators result in space races where prey attempt to avoid overlap with predators and predators seek to increase the chance of encountering prey (Brown et al., 1999, Laundre, 2010). These dynamic processes introduce spatial heterogeneity in the risk of predation for prey. Spatial variation in predation risk is configured by several factors such as predator encounter risk, predator lethality and effectiveness of vigilance (Laundre et al., 2001). In most prey–predator interactions, predator use of space is constrained to an important degree by the distribution and abundance of prey (e.g., Karanth et al., 2004). However, in the case of domestic dogs, most of their populations are heavily subsidized by human derived food (Vanak and Gompper, 2009a, Vanak and Gompper, 2009b); therefore in general they do not depend on prey (but see Mitchell and Banks, 2005). Hence a key process—density-dependent predation pressure (e.g., Messier, 1994)—underlying many natural occurrences of prey co-existence with predators is thwarted in the dog–prey dynamic by human subsidies to dogs (i.e., dog abundance will not decrease if prey abundance declines). Thus the spatial consequences of interactions between subsidized predators, whose population dynamics, activity levels, and distributions remain unaffected by the abundance and anti-predator strategies of wild prey they hunt, will be distinct from those of unsubsidized predator–prey interactions.

Dog dependence on human resources has important consequences for their distribution in space and therefore for the spatial configuration of predation risk and resultant distribution and abundance of potential prey. Dog populations are maintained by human resources and predation on vertebrates makes just a small contribution to their energetic requirements (Vanak and Gompper, 2009a, Vanak and Gompper, 2009b). Consequently, dogs are strongly associated with human settlements (Silva-Rodriguez et al., 2010a, Vanak and Gompper, 2010, Woodroffe and Donnelly, 2011). This implies that there is a gradient in dog encounter risk as a function of dog density generated by proximity to human houses (Woodroffe and Donnelly, 2011). Dog encounter risk should be associated to the probability that a prey will be killed by a dog and also to the probability that a prey can respond behaviorally to escape or avoid dogs. Considering that dog densities are often orders of magnitude higher than densities of wild carnivores (Vanak and Gompper, 2009b), if dog predation has any measurable lethal or non-lethal impacts on prey, then prey should be scarcer in areas where dogs are more frequent.

In this study we assessed the interactions between domestic dogs and the southern pudu (Pudu puda), a globally vulnerable species that is endemic to the temperate forests of South America (Jimenez, 2010). The pudu is a small sized deer (7 kg) associated with dense understory (Eldridge et al., 1987, Meier and Merino, 2007). Dog predation of pudu is ubiquitous (Jimenez, 2010, Silva-Rodríguez and Sieving, 2011, Silva-Rodriguez et al., 2010b), but the consequences of these interactions on the use of space and distribution of pudu are unknown. Under the hypothesis that dogs have impacts on pudu distribution—either through lethal or non-lethal mechanisms—pudu use of areas where the probability of dog encounter is higher should be greatly reduced. The proposed mechanisms underlying this prediction are that dogs both harass and sometimes kill pudus when they encounter them. However, alternative mechanisms could also reduce pudu densities in areas with higher dog activity. First, similar patterns may be observed if poaching occurs, under the assumption that hunting is more intense in dog-used areas. Second, pudus are associated with forest understory (Eldridge et al., 1987, Meier and Merino, 2007) and human activity can reduce understory density in the South American Temperate region (Diaz et al., 2005); therefore degradation of vegetation density could co-occur in areas where the probability of dog interactions with deer is higher. Third, if pumas (Puma concolor)—the main native predator of pudu (Silva-Rodriguez et al., 2010b)—are associated with the areas highly used by dogs it would be impossible to separate their effects from dogs. We designed our study to test for the presence and action of our predicted mechanism underlying pudu distributions (lethal and non-lethal dog predation) and for these important alternative mechanisms. Utilizing a combination of extensive camera trapping, to determine the patterns of distribution of pudu and its potential predators, habitat assessment around camera trap sites, and interviews with people who are both dog owners and potential hunters in our system, we could discern the relative importance of our research hypothesis versus the likely alternative hypotheses concerning mortality or disruption of space use by pudus.

Section snippets

Study area

The South-American temperate forest located in Chile and Argentina is an important area for conservation of biodiversity (Armesto et al., 1998, Olson and Dinerstein, 1998). This restricted forest biome is threatened by plantations, opening of agricultural areas, logging and the establishment of highways and roads (Armesto et al., 1998, Wilson et al., 2005). The study was conducted in an area located 35 km southwest of the city of Valdivia. The study area encompassed nearly 10,000 ha and included

Dog management and interaction with pudu

Most households in the study area owned dogs (85.6%). The known dog population within our study area was 166 individuals, yielding a density of 3.6 dogs/km2 for the area defined as used by dog. The human to dog ratio was 2.0 and the average number of dogs per dog-owning house was 1.9. Most dogs were male (92.0%), adults (>1 year old; 87.3%) and free ranging (95.3%). Dogs were fed on wheat bran (55.3%), human food leftovers (49.3%), commercial food (26.0%) and home prepared food (14.7%). A high

Dog effects on the distribution of pudu

Anecdotal reports of dog-related mortality are rarely accompanied by studies testing whether dogs influence prey distributions, yet these reports have established dogs as a widespread global threat to the conservation of vertebrates (Young et al., 2011). In this study we identified that harassment and predation of pudus by dogs is not anecdotal, but frequent. This conclusion was clearly supported by reports of local residents providing (a) frequent observations of dogs pursuing or killing pudus

Acknowledgements

We thank local people for participating in this study. L. Branch, M. Christman, R. Fletcher, M. Sunquist and three anonymous reviewers provided insightful comments that helped to improve this manuscript. Our field assistants, A. Silva, G. Ortega and F. Osorio, were fundamental for the success of this project. The Nature Conservancy through Reserva Costera Valdiviana (RCV) provided invaluable logistic support. We thank all the support from A. Almonacid, E. Ovando, D. González, O. Ponce, G.

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