Section 2 provides an overview of current knowledge regarding the ways in which domestic cats impact wildlife; the resulting effects on native species’ populations and ecosystems; and available strategies for responding to these issues. It does so in sufficient detail to enable a meaningful analysis of the law in light of this knowledge. Section 3 then addresses the central concern of this paper, which is ‘what the law requires’ in this regard, by presenting and discussing the main results of our legal analysis. Following that analysis, Section 4 identifies and evaluates factors that can influence the application of relevant legal obligations. Section 5 offers concluding observations.

The geographical scope of the study is global, and we combine general discussions of applicable legal instruments with concrete illustrations. Substantively, the scope is confined to nature conservation law (alternatively referred to as wildlife law or biodiversity law), that is, legal instruments expressly aimed at the conservation and/or restoration of native fauna and flora and their habitats. Thus, we do not analyse public health law, tort law, animal welfare law, urban and land use planning law, or any other area of law of relevance to domestic cats. Likewise, the study does not address the impacts of free‐roaming cats on human interests, such as the transmission to people of potentially serious diseases like toxoplasmosis (Aguirre et al., 2019 ; Gerhold & Jessup, 2013 ), the killing of pets, and nuisances such as defecation in playgrounds and private yards and the (often nocturnal) noise produced by cats in heat.

In this desktop study, we first identified the ways in which domestic cats impact wildlife, the degrees to which they do so, and available remedial measures, based on a review of existing research published in the scientific literature. We have analysed wildlife law in light of these facts, using standard legal research methodology. This involved identifying relevant legal instruments and provisions therein, whereby we largely limited our analysis to international law; interpreting these provisions in accordance with the applicable rules of interpretation (for international law, these are the rules codified in the 1969 Vienna Convention on the Law of Treaties); and applying these provisions to domestic cats and their impacts on wildlife. We then proceeded to identify and assess potential factors influencing the application of the law, including feasibility, scientific uncertainty, and the (perceived) interests of domestic cats themselves and of their owners.

Around the world, domestic cats Felis catus —from free‐ranging pets to feral cats—impact wildlife in various ways, and to various degrees. Impacts include predation, competition, disturbance, disease transmission and hybridization (see Section 2.1 below). Much legislation exists, at national and international levels, aimed at the conservation and restoration of wildlife and biodiversity. The application of these nature conservation laws to domestic cats and their impacts is the focus of this paper. The paper emphasizes the obligations of governmental authorities and the actions imposed by existing international law with regard to domestic cats. This analysis is timely because scientific evidence has grown rapidly over the past 15 years and now clearly documents cats’ large‐scale negative impacts on wildlife (see Section 2.2 below). Notwithstanding this growing awareness of their negative impact on wildlife, domestic cats continue to inhabit a place that is, at best, on the periphery of international wildlife law. No doubt, there are political, sociological and psychological explanations for this regulatory oversight but the implications for wildlife conservation are profound. This article speaks directly to this legal ‘blind spot’, highlighting the urgent need for a more consistent and focused application of international wildlife laws to this issue.

2 DOMESTIC CATS AND WILDLIFE

Domestic cats descend from wildcats Felis silvestris, and since their domestication in the Near East (and perhaps Egypt) approximately 10,000 years ago, they have travelled with people to virtually all corners of the world (Driscoll et al., 2007; Ottoni et al., 2017). Domestic cats inhabit all continents except Antarctica, and feral populations presently exist in even the remotest archipelagoes (Courchamp, Chapuis, & Pascal, 2003). Cats are popular as pets and mousers. In addition, stowaway, runaway and intentionally released cats have established free‐ranging cat populations in numerous places, and many of these have become ‘feral’ in the sense that they are fully independent of people. Domestic cats are very adaptive in terms of food, habitat and climate, and are intensive breeders—they reach reproductive age between 7 and 12 months and can have up to three litters a year. Pet cats generally receive food, shelter and health care from their owners, and stray cats also often receive food and other forms of care (e.g. vaccinations) from humans. These ‘subsidies’ ease or remove the constraints of limited food availability, intraspecific competition and disease, and enable domestic cats in many areas to reach high densities, far higher than those of similar‐sized wild predators such as wildcats (Beutel, Reineking, Tiesmeyer, Nowak, & Heurich, 2017; Coleman & Temple, 1993; Crooks & Soulé, 1999; Legge et al., 2017; Sims, Evans, Newson, Tratalos, & Gaston, 2008). Worldwide, at present domestic cats are likely to be orders of magnitude more numerous than all individuals of all wild cat species added together (Hunter, 2015).

There is a continuum of human control over the food provisioning, reproduction and movement of domestic cats, from fully indoor to fully feral cats (Crowley, Cecchetti, & McDonald, 2019). In this study, we are concerned with all domestic cats which spend part or all of their life outdoors beyond the full control of humans. We use the term ‘free‐ranging’ as covering all such cats—from owned pets allowed to roam outdoors, through barnyard cats and colonies of stray cats receiving food from people, to feral cats living completely independently of humans. All such free‐ranging domestic cats have the potential to impact wildlife.

2.1 Ways in which domestic cats impact wildlife The most direct way in which domestic cats influence wildlife is through predation. This is perhaps also the most significant way, given the high numbers and densities of cats in many areas, coupled with their hunting instinct, which can be strong even in well‐fed pet cats (Coman & Brunner, 1972). Cats are opportunistic hunters, and prey items include a wide range of animals, including birds, mammals, reptiles, amphibians, fish, and invertebrates like butterflies and dragonflies. To illustrate, the 14,370 prey items brought home by a sample of 986 British pet cats in a 5‐month survey period in 2003 included 20 mammal species (e.g. mice, voles, shrews, squirrels, stoats, rabbits and bats), 44 bird species, four reptile and three amphibian species and some invertebrates (Woods, McDonald, & Harris, 2003). To provide another example, a recent citizen science survey in Italy rendered records of 2,042 animals killed by 145 cats, involving no less than 207 different species (Mori et al., 2019). Whereas most cat prey tends to consist of small animals, domestic cats have been documented to prey on medium‐sized animals weighing up to 4 kg (e.g. Fancourt, 2015). Of all free‐ranging domestic cats, feral cats hunt the most as hunting is their sole source of food. However, it is estimated that the great majority of other unowned cats also hunt (over 80% according to Loss, Will, & Marra, 2013), and around 50%–80% of owned cats which are allowed outdoors (Loss, Will, Longcore, & Marra,2018; Loss et al., 2013; Loyd, Hernandez, Carroll, Abernathy, & Marshall, 2013). Studies confirm that typically only a fraction of hunted prey is brought back to the house or the farm, for instance 23% (Loyd et al., 2013) or 10% (Krauze‐Gryz, Gryz, & Żmihorski, 2019). Factors that may influence owned cats’ predation rates and species caught, other than the location of their home, include the cats’ age and condition, the extent they are fed and cared for, the use of bells and other anti‐predation devices, the time of day cats are allowed outside, and the time of year (see, e.g. Barratt, 1998; Kays & DeWan, 2004; Krauze‐Gryz, Zmihorski, & Gryz, 2017; Silva‐Rodríguez & Sieving, 2011; Van Heezik, Smyth, Adams, & Gordon, 2010; Woods et al., 2003). An indirect way in which prey species can be affected by free‐ranging domestic cats is through disturbance or fear effects caused by the cats’ mere appearance, presence or scent. As documented in a range of studies reviewed by Loss and Marra (2017), such fear or intimidation effects can influence foraging and defence behaviours, stress responses, energy income and body condition, vulnerability to other predators, and reproductive investment and output. For instance, Bonnington, Gaston, and Evans (2013) found that even briefly confronting blackbirds Turdus merula with a taxidermied cat near their nest reduced subsequent feeding of the young by one‐third, and significantly increased the risk of subsequent nest predation by corvids. Another indirect impact is competition, which occurs when domestic cats exploit the same food, space and/or shelter as other species. For example, every mouse eaten by a cat cannot be eaten by a hawk (George, 1974). Domestic cats can, furthermore, impact wildlife through disease transmission. A broad range of vertebrates can be affected by cat‐transmitted diseases like toxoplasmosis, rabies or feline leukaemia (Dubey, 2002; Gerhold & Jessup, 2013; Hartley & Dubey, 1991; Loss & Marra, 2017; Work et al., 2000). Yet another way of domestic cats impacting native species conservation is hybridization, which can result when domestic cats mate with wildcats or other wild cat species (Macdonald et al., 2010). Hybridization can result in the extinction of native species both directly and indirectly. Of particular significance to this analysis, hybridization can lead to ‘genetic swamping’, where interbreeding with domestic cats produces hybrid populations in which no remaining individuals can properly be described as the native, wild cat species (Todesco et al., 2016). Incidentally, blurring taxonomic distinctions between wild cat species and domestic cats can undermine the application of species‐based conservation laws, where such distinctions determine whether a species is an appropriate focus for conservation action (Fitzpatrick, Ryan, Johnson, Corush, & Carter, 2015, but see Chan, Hoffman, & Oppen, 2019).

2.2 Impacts on individuals, populations, species and ecosystems The range of impacts that domestic cats have on wildlife of the kinds described above, and which often act in combination, are increasingly well documented at multiple ecological scales: from individuals and populations of species through to ecological processes and ecosystems. Section 2.1 illustrates the breadth of different impacts that domestic cats have on wildlife. The current section analyses the implications of these different impacts from domestic cats on native wildlife. There remains some scientific uncertainty about these implications, particularly at broader, ecosystem scales and in the context of complex interactions with other species. However, the research synthesized below demonstrates that, even in relation to the more complex questions of population impacts, ecosystem health and ecological interactions, there is growing evidence of negative impacts from domestic cats. Studies in various countries have quantified cat predation on individuals of several species groups. In Canada, domestic cats—from pets to ferals—are estimated to kill between 100 and 350 million birds per year (Blancher, 2013). Even at the lowest end of 100 million, this makes predation by domestic cats ‘probably the largest human‐related source of bird mortality in Canada’ (Blancher, 2013; also Calvert et al., 2013). In Australia, feral and pet cats together are estimated to kill an average of 377 million birds per year, that is, a million birds per day (Woinarski et al., 2017); as well as an average of 649 million reptiles, with cat predation reported for 258 reptile species (Woinarski et al., 2018). Many more cats roam the United States, and their aggregate predation tally runs into the billions, with an estimated 1.3–4.0 billion birds, 6.3–22.3 billion mammals, 258–822 million reptiles and 95–299 million amphibians killed by free‐ranging domestic cats each year (Loss et al., 2013). Again, this makes domestic cats the top source of direct human‐related mortality for birds and small mammals in the United States, easily eclipsing other sources such as mortality from poisons and pesticides and collisions with structures and vehicles (Longcore et al., 2012; Loss et al., 2013; Loss, Will, & Marra, 2015). Similar studies in Europe reiterate the negative impacts of cat predation on individuals within populations of native species. For example, one study estimated that owned cats in the United Kingdom, in a 5‐month survey period, brought home 57 million mammals, 27 million birds and five million reptiles and amphibians, implying they killed several times these numbers (Woods et al., 2003). A Dutch report estimated that 141 million animals are predated by cats on average in the Netherlands per year, with pet cats responsible for almost two‐thirds of this number (Knol, 2015). Another study used data from bird ringing programmes in Belgium and France to gauge cat predation on garden birds, noting that such predation was a leading cause of death reported by observers, on par with window collisions, and that cat‐related mortality had increased by 50% between 2000 and 2015 (Pavisse, Vangeluwe, & Clergeau, 2019). An assessment of predation by farm cats in Poland estimated an average of 136 million birds and 583 million mammals are killed around Polish farms annually (Krauze‐Gryz et al., 2019). As Crowley et al. (2019, p. 19) summarize the evidence, ‘even when killing behaviour is not universal, large numbers of cats inevitably kill large numbers of wild animals’. Of course, even very low numbers of individuals lost to predation can amount to a severe impact on wildlife in small populations or fragile ecosystems. For example, a modest number of domestic cats is held responsible for the extinction of a species of small, flightless passerine, the Stephens Island wren Traversia lyalli, on a New Zealand island—although the popular account that this extinction was caused by a single cat owned by the lighthouse keeper is probably oversimplified (Galbreath & Brown, 2004). Impacts other than predation have been studied to a much lesser extent, but this of course does not imply a lesser influence on biodiversity from the other impacts described in Section 2.1. Consider, for instance, all the billions of prey items consumed by domestic cats which are not available to native mammalian, reptilian and avian predators (Loss & Marra, 2017). Similarly, cat‐transmitted diseases like toxoplasmosis are likely to be a significant cause of mortality for a range of vertebrate species, including threatened species (Dubey, 2002; Gerhold & Jessup, 2013; Hartley & Dubey, 1991; Work et al., 2000). Furthermore, different direct and indirect impacts from cats will often act in tandem on populations of native species (Loss & Marra, 2017). For example, many bird and mammal species will simultaneously undergo direct predation and indirect fear effects (e.g. Mahlaba, Monadjem, McCleery, & Belmain, 2017). The fosa Cryptoprocta ferox, Madagascar's top native predator, appears to suffer from ‘considerable competition’ through consumption of shared prey by free‐ranging domestic cats (Merson, Dollar, Tan, & Macdonald, 2019) and is also at risk from toxoplasmosis (Rasambainarivo, Farris, Andrianalizah, & Parker, 2017). Wildcats are subject to the same combination of competition and disease, with hybridization added to the mix. Concerns over hybridization with domestic cats also exist for some other species, for example, the rusty‐spotted cat Prionailurus rubiginosus in India and Sri Lanka (Kittie & Watson, 2014). Hybridization, disease, competition, disturbance and predation by domestic cats do not just affect individual animals but also whole populations of species, in some cases to the point of extinction. Uncertainty remains regarding the exact magnitude of domestic cats’ impacts at these population levels. Challenges for determining the population‐level effects of cats include, inter alia, the difficulty of determining what proportions of cat‐caused mortality are compensatory (affecting animals that would have died anyway) and additive (where mortality due to the impacts of domestic cats adds to overall mortality); and the general challenge of disentangling causes and effects in ecological systems (Baker, Molony, Stone, Cuthill, & Harris, 2008; Beckerman, Boots, & Gaeston, 2007; Hackländer, Schneider, & Lanz, 2014; Loss & Marra, 2017). The above considerations, however, including the sheer numbers mentioned, distinctly suggest the prevalence of such population‐level impacts for both island and mainland wildlife populations, and many such impacts have already been documented—not only involving birds and mammals but also, for instance, lizards (Li, Belasen, Pafilis, Bednekoff, & Foufopoulos, 2014; Stokeld et al., 2018; Woinarski et al., 2018). Domestic cats have also been implicated at broader scales, in the global extinction of at least 63 species—40 birds, 21 mammals, two reptiles—which is to say 26% of all known contemporary extinctions in these species groups (Doherty, Glen, Nimmo, Ritchie, & Dickman, 2016). Likewise, domestic cats currently endanger at least a further 367 species which are at risk of extinction (Doherty et al., 2016). In a ranking of alien species threatening the largest numbers of vertebrates worldwide, domestic cats came in third—only rats (Rattus spp.) and the chytrid fungus Batrachochytrium dendrobatidis that is wiping out amphibians around the world, are ahead of them (Bellard, Genovesi, & Jeschke, 2016). Cats have played a particularly significant role in native species loss on some continents, on many islands, and among populations of certain species groups. For example, they are a principal cause of the declines and extinctions of many of Australia's unique mammal species (Woinarski, Burbidge, & Harrison, 2015). To illustrate, one study showed that feral cats caused 65% of mortality for woylies Bettongia penicillata, a rare marsupial (Marlow et al., 2015). Another study used enclosures to assess the impact of feral cats on long‐haired rats Rattus villosissimus, finding that these native rodents went extinct in areas frequented by cats but persisted in areas surrounded by cat‐proof fences (Frank et al., 2014). At least 13 further studies demonstrate similar predation impacts on populations of other mainland vertebrates in New Zealand, Europe and North America (see Loss & Marra, 2017). Several of these studies revealed that predation of various bird species at study sites in the United Kingdom and the United States was so severe that the studied populations are likely to act as ‘sinks’, requiring immigration from areas with fewer cats to persist (Baker et al., 2008; Balogh, Ryder, & Marra, 2011; Smith, McKay, Richardson, Shipley, & Murphy, 2016; Thomas, Fellowes, & Baker, 2012; see also Loss & Marra, 2017). A recent Italian study provided further ‘strong evidence that free‐ranging domestic cats may seriously affect the conservation of [various] wildlife species, which are already suffering from population declines due to other causes, e.g. habitat loss’ (Mori et al., 2019). As the latter quote indicates, predation by cats also interacts with and exacerbates the effect of other threats to wildlife, increasing the risk of regional or global extinctions. For example, feral cats in Australia have been observed congregating outside their usual hunting territory in areas recently burned by wildfire because fires reduce ground cover for small native mammals which cats then hunt far more efficiently (McGregor, Legge, Jones, & Johnson, 2016). The impact of this behaviour may be increasingly devastating for small endemic mammals in areas where wildfires are becoming more severe and more common as a result of climate change (McGregor et al., 2016). Whereas disturbance or fear effects are even harder to quantify with precision than direct predation effects, the aforementioned blackbird research (Bonnington et al., 2013) and several other empirical studies provide concrete indications that cat‐caused fear effects are adversely influencing inter alia the foraging, space use and reproduction of impacted species’ populations (Balbontín & Møller, 2015; Freeberg, Book, & Weiner, 2016; Tryjanowski et al., 2015). Indeed, the available evidence indicates that fear effects can exercise an even greater influence on prey populations than predation itself (Loss & Marra, 2017; Preisser, Bolnick, & Benard, 2005). As one study illustrated, even when urban songbird predation mortality from domestic cats is as low as 1%, fear effects from those same cats can still reduce bird abundance by 95% (Beckerman et al., 2007). A documented example of a disease impact is the death of five members of the endangered puma (Puma concolor) subpopulation known as Florida panthers, caused by an outbreak of feline leukaemia virus which was traced back to a single domestic cat (Brown et al., 2008). Population impacts from cat‐borne diseases appear likely for a further range of species (Loss & Marra, 2017). For example, significant mortality from toxoplasmosis has been documented for marsupials, Neotropical primates and even marine mammals (Gerhold & Jessup, 2013). Regarding the latter, Toxoplasmosis gondii oocysts from domestic cat scat can reach the marine environment through freshwater runoff from cities, and has been identified as a major cause of death in southern sea otters Enhydra lutris nereis off the California coast—both through direct mortality from the disease and increased vulnerability of infected otters to shark attacks—and as hampering the otter population's recovery (Conrad et al., 2005; Kreuder et al., 2003). Domestic cats can also pose a real risk to wildcat conservation through hybridization, especially when wildcat densities are low, as documented for Hungary (Pierpaoli et al., 2003) and Scotland (Beaumont et al., 2001; Hubbard et al., 1992; Macdonald et al., 2010). Finally, predation, disturbance, competition and other impacts of domestic cats can have broader impacts on ecological processes and ecosystems. Impacts of domestic cats are especially and notoriously strong on islands, particularly those islands where the native—and often endemic—fauna is unadapted to mammalian carnivores (Medina et al., 2011; Nogales et al., 2013). A meta‐analysis of dozens of cat diet studies showed that on 40 islands around the world, at least 248 different species had been preyed on by feral cats—113 birds, 27 mammals, 34 reptiles, three amphibians, two fish and 69 invertebrates (Bonnaud et al., 2011). A significant proportion of the cat‐induced extinctions of, and current risks to, species referred to above involve island species (Bellard et al., 2016; Doherty et al., 2016; Medina et al., 2011). Species extinctions caused by cats in fragile and often‐remote island environments can cause ecological processes such as seed dispersal or pollination for native plants to break down (Medina et al., 2011). Furthermore, ecological functions that are lost when a species becomes extinct through cat predation or other impacts may not be replaced, as few species can independently return to island environments once a population is locally extinct. As a result, the transformation of island ecosystems triggered by the impact of cats may, without direct human intervention, be irreversible. However, adverse ecological impacts of domestic cats on biodiversity are certainly not limited to islands. Given cats’ large numbers, subsidized high densities and other traits mentioned above, their impacts can also be significant on ‘mainlands’, that is, continents and large islands (such as Madagascar, New Zealand and the UK), with a recent review concluding that there is ‘overwhelming evidence demonstrating that cats affect mainland vertebrate populations’ (Loss & Marra, 2017, p. 507). Highly disturbed or degraded mainland ecosystems, including areas heavily burnt by wildfire, may be particularly vulnerable to species impact and ecosystem impact by domestic cats (May & Norton, 1996; McGregor et al., 2016). Of course, human impacts on biodiversity from overexploitation and land clearing are significantly more damaging than the harm caused by domestic cats (Maxwell, Fuller, Brooks, & Watson, 2016). Nevertheless, given the scale and diversity of their impacts on wildlife populations and ecosystems, it seems not to be an overstatement to say that domestic cats are among the ‘most ubiquitous and environmentally damaging invasive predators on Earth’ (Loss & Marra, 2017, p. 502).