ReviewA review of the interactions between free-roaming domestic dogs and wildlife
Highlights
► Domestic dogs Canis familiaris are a globally abundant domestic carnivore. ► We review documented interactions between wildlife and free-roaming domestic dogs. ► Globally, free-roaming dogs cause issues for species conservation and human health. ► Dogs negatively interact with wildlife, mainly by predation and disease spread. ► Interdisciplinary collaboration between experts is needed for effective solutions.
Introduction
Introduced species are one of the major causes of concern for conservation biologists (Diamond, 1989, Macdonald et al., 2006). As the human population has spread so has the purposeful and accidental introduction of many species into a variety of habitats and ecosystems. Negative impacts from domestic animals pose particular issues for conservation as they are intimately tied to the economic, social and political values of the local people and therefore require interdisciplinary cooperation for successful outcomes (Lindenmayer and Hunter, 2010). Dogs (Canis familiaris) and cats (Felis catus) are the most common domesticated carnivores and are prevalent throughout the world, living in strong association with human communities (Ferreira et al., 2011). Domestic cats have been shown to impact wildlife populations negatively through predation (van Heezik et al., 2010), hybridization (Yamaguchi et al., 2004) and disease (Fredebaugh et al., 2011). Consequently, the effect of free roaming domestic cats on wildlife has drawn much attention from biologists (Calver et al., 2011) and they are listed as one of the top 100 worst invasive species (Lowe et al., 2000). However, different perspectives on the value and effect of domestic cats have sometimes led to wide division between conservation biologists, animal welfare groups and the public. Division is mainly centred around control methods and whether Trap–Neuter–Return or removal are preferable (Longcore et al., 2009, Loyd and DeVore, 2010) but this division has affected the actions taken to control cat colonies (e.g. Meyers, 2010) and will have consequences for the local wildlife. Comparatively, the impact of domestic dogs is only just starting to attract wide attention from conservation biologists (e.g. Young et al., 2011). In order to avoid similar division between groups, as found concerning cats, and to facilitate cooperation, comprehensive review and analysis of the ways dogs affect wildlife species is required.
Domestic dogs and humans have had a close relationship for over 15,000 years (Driscoll and Macdonald, 2010, Vila et al., 1997). The dog population has expanded around the globe alongside the human population and, in 1993, the most recent global population estimate was made at 500 million dogs (Wandeler et al., 1993). Dogs have diverse and complex roles in human communities, for example as pets and family members, sacred icons, or working animals used for protection and waste disposal, with varying levels of integration with the human community (Serpell, 1995). While in some places people see fit to keep dogs in their handbags, beds or houses, in others they may be chained outside or left to roam. Furthermore, all dogs in a single population may not be treated in the same way – for example, a companion or prized dog may be kept indoors permanently, while a valuable working dog may be intermittently chained up and mongrels left free. Dog populations can, therefore, be an amalgam of sub-populations including, for example, dogs that share homes, are completely dependent on humans and considered members of the human family, free-roaming dogs only reliant on human communities for food and shelter, and dogs requiring no human contact (often described as feral), with each group exhibiting different demographic and behavioural traits. Cross-cultural differences in human attitudes towards dogs, perhaps particularly between western views and others, mean the status afforded dogs in many cultures and societies may possibly be obscure or seemingly contradictory to the manner in which they are kept (Serpell, 1995).
Domestic dogs can be a considerable problem to humans and wildlife, (Feldmann, 1974, Young et al., 2011) with a sizeable economic cost – estimated at $620 million annually in the US (Pimentel et al., 2005) – and can cause conflict with humans through direct bites, attacking livestock and transmission of diseases, particularly rabies. Dogs are the cause for 99% of the 55,000 human fatalities due to rabies reported worldwide (Knobel et al., 2005), predominantly in Asia and Africa, and, globally, post-exposure preventatives are given annually to 15 million people, averting around 327,000 deaths (WHO, 2011). In addition more than 60 other zoonoses are associated with dogs (Matter and Daniels, 2000).
As with cats, dogs may impact native wildlife through predation, competition, disturbance, hybridization and disease transmission (Young et al., 2011). Direct predation of wildlife is probably the most obvious interaction to observe: for example, Young et al. (2011) recorded 28 previous studies of interactions between dogs and wildlife, primarily through predation and harassment but also competition, and documented dogs harassing Mongolian gazelle (Procapra gutturosa), saiga (Saiga tatarica mongolica) and argali (Ovis ammon) in Mongolia. However, research also shows that the more complex and subtle interactions, for example transmission of diseases such as rabies and canine distemper virus, may also severely impact wild species. Indeed, the highly endangered Ethiopian wolf (Canis simensis) has suffered a number of rabies epidemics over the last 20 years that reduced the population by around 75% (Randall et al., 2006). For many decades people have recognised the importance of diseases and health in wildlife conservation, however it is only since the turn of the century that this critical issue has really begun to draw attention from the wider conservation community (Deem et al., 2001, Riordan et al., 2006).
Free-roaming dogs, those that are not permanently restrained or under human control and may be owner or unowned, are thought to account for about 75% of the global dog population (WSPA, 2011). As awareness about free-roaming dog populations has increased, primarily in urban locations, concern has risen for their welfare. These dogs may suffer from high mortality, malnutrition, disease, parasitism, starvation and abuse (Pal, 2001, Sowemimo, 2009). When a problem is identified, for example a threat to public health, local and national governments or communities may attempt to eradicate dogs in a variety of ways (Dalla Villa et al., 2010), with attendant welfare concerns, while owners may abandon or destroy puppies and adults they cannot afford (Hsu et al., 2003). Consequently, animal welfare organisations, human health organisations and other interested parties working together investing time, money and resources on management projects in various countries in an attempt to improve human health and the welfare of free-roaming dog populations by reducing the prevalence of rabies and reproduction rates (Jackman and Rowan, 2007, Tasker, 2009). Current guidelines for dog population management, drawn up through collaboration between animal welfare and human health agencies, recommend a combination of vaccination and sterilisation (OIE, 2011, WHO and WSPA, 1990). These guidelines aim primarily to reduce human health issues, the spread of rabies and dog population growth. The World Health Organisation (WHO) and World Society for the Protection of Animals (WSPA) advise that the killing of free-roaming dogs is not successful in achieving these goals (WHO and WSPA, 1990). Wildlife conservation concerns are not considered in these guidelines, however the projects and research being carried out to manage dog populations provide an opportunity for conservation biologists to pool resources and collaborate with people already working in the field.
The interaction between dogs and wildlife species may be arguably most likely to occur in rural locations. In rural locations the human:dog ratio is generally lower than in urban areas, with the number of dogs sometimes exceeding the number of humans (Wandeler et al., 1993). Dogs in rural landscapes may move between human-dominated areas where they get feed and shelter, to the surrounding landscape where they may encounter wild species (Macdonald and Carr, 1995), not only potentially causing problems for wildlife but also connecting wildlife and the local human population. Through a comprehensive review of published literature documenting the interactions between free roaming dogs and wildlife in rural areas, this paper aims to: update the estimated size of the global dog population, assess the common types of interaction between dogs and wildlife, report the scale and geographic spread of dog-wildlife interactions, identify the wildlife species involved, review any solutions proposed and, overall, extend the work of previous reviews. By investigating how free-roaming dogs and wild species interact we aim to provide a clear background of knowledge on the conservation issues that may arise, informing people of problems, providing a platform for future research and basis for collaboration between the conservation, welfare and human health agencies.
Section snippets
Global domestic dog population
To derive information on the global domestic dog population size, data were gathered on human:dog ratios or dog population sizes, from published scientific literature, pet food organisations and animal welfare groups, for six continental regions. Dog population sizes were combined with projections of human population sizes in the same year available from the US Census Bureau (2011) and a human:dog ratio calculated. These ratios were used to calculate a dog population in 2012 based on projected
Global domestic dog population
Wandeler et al. (1993) estimate the global domestic dog population at 500 million. This appears to have been calculated by assuming a conservative human:dog ratio of 10:1 and using the human global population figure of 5.5 billion in 1993 (US Census Bureau, 2011). Using the same method of estimating dog population from more recent figures of human:dog ratios from different continents, we recalculate the global population. A change in the population estimate may, therefore, show the effects of
Discussion
Our study has estimated the global population of domestic dogs to be over 700 million and demonstrated that they can cause a variety of species conservation problems. This review of 69 papers concerning the interaction between dogs and wildlife in rural situations has shown that free ranging domestic dogs interact with several wildlife species (64 recorded), from a variety of taxa, in many countries (29) throughout the world. The problems recorded were primarily caused by predation (50% of the
Acknowledgements
Our thanks to A.N. Rowan for inspiration for this research, and to Tom Moorhouse and Rosalind Shaw for comments on the manuscript. JH was supported by a grant from HSUS and WSPA.
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