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Preface
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Predatory Behaviour of Domestic Cats - based on scientific research Nature has made the domestic cat a predator superbly adapted mentally and physically to prey on small animals, like rodents and birds. Cats have a passionate desire to hunt and a few cats are obsessive hunters; even when well fed the urge to hunt can remain strong (as with some people), so they continue to hunt given the opportunity. Predators, being at the top of the food web, tend to be rare; they cannot be more numerous than the prey they feed on. However, domestic cats are unlike any other mammal predator (with the possible exception of domestic dogs) in being counted in their millions. Humans have made them super-abundant and spread them all over the globe.
Disinclined to Read All This Section? The Big QuestionSummary for this Section The often repeated claim that domestic cats are responsible for the extinction of at least 33 bird species is false, based on wrong information. The original publication for the claim is not 33 bird species but 18 bird species, of which ten might be extinct. What killed them off can only be surmised. They lived on small remote islands where people introduced cats. But cats are only part of a wider human facilitated extinction problem, not the cause of it. Given the abundance of domestic cats and the determination of some cats to hunt, it may be surprising that it is only in the last several years that scientists have begun seriously to explore their predatory behaviour. In particular they want to understand what effect domestic cats have on wildlife. The last few years some of the cat researchers have made certain claims such that: 1. Domestic cats have exterminated species 2. Domestic cats kill billions of animals 3. Domestic cats are harmful to prey by their mere presence What is the research behind these claims and are the claims true? These are important questions because to understand cats and their wildlife prey we need accurate data on which to base reliable conclusions. 1. Domestic Cats Have Exterminated Species? The Claim A much publicised and often repeated assertion about domestic cats is: Feral cats are responsible for the extinction of at least 33 bird species. (Nogales et al 2004)This quote is from a group of scientists reviewing the effectiveness of the eradication of feral domestic cats by humans. The claim has been repeated so often that many people accept it as fact without checking it. The American Wildlife Society and the University of Nebraska, Lincoln, are just two among several claimants that cats exterminate species: Domestic cats have tremendous impacts on wildlife and are responsible for the extinction of numerous mammals, reptiles, and at least 33 bird species globally. (The American Wildlife Society 2011) While loss of habitat is the primary cause of species extinctions, cats are responsible for the extinctions of at least 33 species of birds around the world. (Hildreth et al)The claim that cats exterminate species has serious consequences: it drums up public hate for domestic cats. Nor are professional researchers immune to cat hate. Nico Dauphine, a bird researcher at the National Zoo, Washington DC, announced in her review of free-ranging cats at a bird conference: Historically, cats have been specifically implicated in at least 33 bird extinctions, making them one of the most important causes of bird extinctions worldwide.And added that, �feral animal removal should become a permanent, regular feature of wildlife management. (Dauphine et al 2009).�Removal� in wildlife conservation is a euphemism for killing. Dauphin� came to hate cats, tried to poison them in her neighbourhood, was caught and given a suspended sentence in 2011 for attempted cruelty to animals. But how true is the claim that cats exterminate species? Let us examine it in a little detail. Source of the Claim Nogales and colleagues state their claim comes from a book by Christopher Lever (1994): Naturalized Animals: the ecology of successfully introduced species. Lever states in his book that, According to Jackson (1977), naturalised predators have collectively been responsible for the extermination throughout the world of no fewer than 61 avian taxa, the principal culprits being feral domestic Cats which have caused 33 extinctions, rats 14, and the Small Indian Mongoose nine. (Lever 1994:91)So where did Jackson get his information? The information comes from a talk at a symposium about alleviating problems encountered by endangered birds. His talk was published a year later in the book Endangered Birds (Jackson 1978). In this book Jackson presents a bar graph showing that domestic cats have exterminated 33 species of birds and that the �data are summarized from Ziswiler (1967).� Ziswiler (1967) is a book entitled Extinct and Vanishing Animals. Ziswiler lists in the Appendix a number of extinct bird and mammal species. Next to each species he marks the reasons for their extinction, such as �hunted for meat or fat�, or �hunted for trophies or souvenirs�; or through �destruction of the forest�, or through �feral dogs�, feral cats�, feral pigs�, �rats� or �foxes�, and so on. Ziswiler does not cite evidence for these judgements but states they are based on four sources. Ziswiler�s source for bird extinctions is a book by J C Greenway, Extinct and Vanishing Birds of the World, published in 1958, itself based on information gleamed from yet earlier works. Of the other sources Ziswiler names, one is IUCN Bulletins for 1964; these are announcements or statements so cannot provide detailed information. The two other sources are books about mammals, which do not shed light on bird extinctions (Extinct and Vanishing Mammals of the Western Hemisphere by G M Allen published in 1942 and Extinct and Vanishing Mammals of the Old World by F Harper published in 1945). Ziswiler�s Appendix Reappraised Of all these publications it is the list in Ziswiler�s Appendix that is the most compact source of information on bird species that domestic cats are reputed to have exterminated. In the Table below I reappraise his list regarding cats in light of modern data, mainly from IUCN (www.iucnredlist.org) and Bird Life (www.birdlife.org). Information about species and their precise taxonomic identity can be vague, so I have given modern scientific names in brackets for some of the species.
Extermination by Cat? But were any of these species, even including the sub-species and sibling species, really exterminated by cats? Nogales and co-researchers assert they were: Feral cats are directly responsible for a large percentage of global extinctions, particularly on islands. (Nogales et al 2004)a) Islands This is a vague, sweeping and outrageous claim, made by supposedly objective scientists. The 33 species purportedly exterminated by cats have now jumped to �a large percentage of global extinctions�, for which cats themselves are blameable. All the birds (except the extant Eyrean Grass-wren of central Australia) lived in small populations on small remote islands. This predisposed them to the dangers of going extinct. A tiny compact population is less able to withstand heavy predation or colonise wider regions. Domestic cats did not originally exist on any of these islands but people introduced them. Put a domestic cat in a room with nothing else but a few birds, leave them for a week and what can you expect to find on your return? People made many changes to these islands and introducing cats was only one factor in a complex web of relationships. Just one example illustrates: Bird Life state in their species factsheet for the Samoa Wood Rail, Pareudiastes pacificus (number 6): Cats, rats, pigs and dogs have no doubt contributed to its disappearance, and hunting may also have been a factor as it was formerly a favoured food of the human... Slash-and-burn cultivation threatens remaining areas of upland forest� Wild cattle and pigs have browsed the understorey and ground-cover along the main range�So the reality of �extinction by cat� is not as clear cut as some people wish to make out. b) Circumstantial Evidence Too many people place too much emphasis on anecdote and indirect evidence to implicate domestic cats in bird extinctions. The real reasons for extinction or extirpation of these bird species have never been proven. This is highlighted by islands where domestic cats have or have not been introduced. For example, according to Taylor (1979), Europeans discovered the sub-Antarctic Macquarie Island in 1810 and, even though people had introduced cats and other predators, the Macquarie Parakeet (or Kakariki) (number 10 in the Table) stuck around for the next 70 years. However, rabbits were introduced in 1879 and Taylor conjectures that this increased the cat population - the rabbits being a vital food supply for the cats � which led to greater predation on the Parakeets until the birds died out. So domestic cats seem not to have killed of the Parakeets categorically. A mixture of humans, cats, rabbits and a small bird population with nowhere else to colonise are implicated. Or were these coincidental to something else that really killed off the Macquarie Parakeet? c) Loose Language: do cats exterminate or extirpate? Badly chosen terminology used by researchers in their publications make some readers believe domestic cats have exterminated a species. Exterminate means make extinct: a whole species is killed off; no living individual of that species exists anymore anywhere in the world. Extirpate means a species is killed off in a region, such as an island or part of a continent, but the species survives elsewhere. Woods et al (2003) provide an extinction versus extirpation mix-up: �the Socorro dove Zenaida graysoni has also been driven to extinction primarily by cats�European sailors found the small Socorro Island in the 16th century and the Socorro Dove lived only on that island, west of Mexico. As well as introducing themselves, people introduced domestic cats, rodents and sheep. However, although no longer living on the island, about 200 doves survive in captivity and are being bred for reintroduction. So the Socorro dove was extirpated on Socorro but is not extinct. d) Sloppiness Researchers do not always thoroughly read (or even read) the research published in their field that they build on and refer to in their own publications. This may be because they are pressed for time or cannot get a copy of a research paper (sometimes they might only read an abstract). At other times they might rely on an author�s assertion in a published paper as most likely to be true, so do not check it before referring to it in their own paper. Thus otherwise reputable researchers may repeat a claim that is without foundation. Nogales repeated a claim by Lever, who repeated a claim by Jackson, who repeated a claim by Ziswiler, who got his information from an even earlier claim... Sometimes researchers just do sloppy research that should never be published. Ziswiler�s Mammals What of the eight mammal species in Ziswiler�s Appendix that domestic cats are said to have exterminated? The same muddle of extinction applies to them as it does to the birds. Conclusion Claims that domestic cats have exterminated 33 bird species is wrong. It is based on vague and erroneous information. All statements about cats exterminating bird species are false: there is no scientific evidence that domestic cats have exterminated any species, bird or otherwise. If there was good evidence that cats have exterminated species then it would be used instead of this weak false claim. The moral is that scientists must do their scientific research thoroughly, otherwise they will get a reputation for carelessness. Furthermore, we must read claims by scientists (and anyone) with an open mind, especially if their claims are not supported with good evidence. 2. Do Domestic Cats Kill Billions of Animals?
Disinclined to Read All This Section? Cats are certainly an abundant predator. Yet the large numbers of prey they take went largely unnoticed. A benefit of the following research papers is that they helped make domestic cats a species of scientific curiosity, especially regarding the effects they have on their wildlife prey.Summary for this Section Domestic cat predation research is at a preliminary stage. It is far from complete and open to critical analysis and better research. Therefore sweeping claims about cats are unhelpful and likely wrong. We must study domestic cats as part of a complex of interacting species and not as though they live in isolation from their prey. Rather than global figures, we need reliable data about predator/prey populations in their local habitats. To understand how cats affect their prey populations we must know to what extent cats take hapless survivors (those who will breed) or doomed surplus (those who will die anyway). Domestic cats are the same species as the extirpated European wildcat and have taken over their ecological niche; they are not an invasive species in Europe. Humans are a significant force in the evolution and maintenance of domestic cats so must be included in the study of cats and the resolution of perceived problems. 1) Churcher & Lawton (1987) An early stimulus for studying domestic cat predation was the publication of a science paper with the picturesque title Predation by Domestic Cats in an English Village. The village is Felmersham, near the River Great Ouse, in Britain. Once a week for a year (1981-1982) the researchers called on the cat owners and recorded the prey the 70 odd cats who lived in the study area brought home. Is basing research on �bring-back� prey an effective means of counting cat prey? The number of ways of counting cat prey is limited. The �bring-back� prey methods is nifty and ample compared with the customary but laborious and doubtful alternatives of prodding about in faeces or poking around in the smelly contents of a dead predator�s guts. The researchers discovered the cats brought back over a thousand prey animals, on average about 14 prey per cat: roughly one third birds to two thirds mammals, mainly rodents, plus unidentified prey. They concluded that the cats were a major predator in the village. 2) May (1988) Then another scientist let the �cat out of the bag�. Having read the Felmersham data, his question was: how much prey might domestic cats catch nationally? He simply multiplied the average number of prey caught per cat by the number of domestic cats in the whole country (14 x six million - roughly the number of domestic cats thought to live in Britain at the time). He arrived at the astonishing total that, in round terms, cats in Britain catch 100 million birds and small mammals per year. Thus he helped set off a trend, generalising from a small study of cat predation to extrapolating to cats over the whole country and prompting whether we should be concerned about the effects of cats. 3) Woods et al (2003) Another key work on domestic cat predation was a survey across Britain a few years later by Woods and collaborators. Patrons with cats were sought through the office of the Mammal Society and by appeals via the national media. Questionnaires were sent to just over 600 cat owners and for five months in 1997 the researchers documented the number of prey the nearly 1,000 cats brought home. The cat owners logged over 14,000 prey, an average of 17 prey per cat during this period. Like May, the researchers then extrapolated from their data to what they thought the British domestic cat population of nine million cats (it had apparently grown by this time) might bring back over the same five month period. They worked out that the British population of cats would bring back about 27 million birds, 57 million mammals and five million herpetofauna (reptiles and amphibians). They conclude that cat abundance makes cats a major predator of wildlife throughout the country. If we scale up the Wood�s figures to 12 months of the year, for the purpose of making comparisons with other studies, British domestic cats would be catching an annual average of 65 million birds, 137 million mammals and 12 million herpetofauna, or an average of 7 birds, 15 mammals and one herpetofauna per cat per year or 24 prey per cat per year. 4) Loss et al 2013 Finally an American study of domestic cat predation, that has gathered some notoriety. It is by Loss and co-workers at the Smithsonian�s Migratory Bird Center and at the US Fish and Wildlife Center. The authors reviewed annual estimates of cat predation from several small scale studies published over the years in scientific journals. Based on this they made the startling assertion that the 84 million cats in conterminous US kill billions of prey animals every year. According to their analysis, cats in America kill an average of around two billion birds, 12 billion mammals and 650 million herpetofauna. Doing some simple arithmetic on the researchers� data this would make an average of 23 birds, 143 mammals and eight herpetofauna per cat per year. Lumping all these prey types together comes to 174 prey animals per cat per year, or 17 times the number of prey British cats were calculated to bring home in the Woods study. It seems that American cats are hunting-mad compared with their lazy British counterparts! Criticisms The first three cat studies above are straightforward. Churcher and Lawton do not exceed their evidence. May�s conclusion is revealing, alerting us to the fact that lots of domestic cats kill lots of prey, something that many people did not appreciate, and worthy of further research. Woods and co-workers took domestic cat predation research further, acknowledging various shortcomings of their work and do not make sweeping claims. However, Loss and colleagues go beyond mere reporting. They pronounce that domestic cats not only kill billions of animals every year but are the single greatest source of human-caused bird and small mammal mortality in the US, that domestic cats �threaten wildlife in the United States and globally� and that we need to do something about it. What are we to make of the claims of Loss and co-workers? Many people take the published assumptions, statements and judgements about cats by scientists as indisputable truths. And the Loss paper has become a notorious banner for ailurophobes: that domestic cats as unremitting killers of wildlife driving species to extinction. This attitude has consequences for domestic cats, some of it lethal. A popular gut reaction by the American Bird Conservancy, far from alone in its denouncement of cats, is several Web pages devoted to the �evils� of cats. The headline of one page declares in a massive font: Outdoor Cats: Single Greatest Source Of Human-Caused Mortality For Birds And Mammals, Says New StudyThe page builds on the pronouncements of Loss and co-workers, makes the old assumption that cats have exterminated species, that they will exterminate more in future, and that cats are a threat to the very health of nature: This study, which employed scientifically rigorous standards for data inclusion, demonstrates that the issue of cat predation on birds and mammals is an even bigger environmental and ecological threat than we thought.And that, Every time we lose another bird species or suppress their population numbers, we're altering the very ecosystems that we depend on as humans.The American Bird Conservancy develops several pages along these lines, topped by pictures of suitably evil-looking cats with lifeless birds hanging from their maws. The American Bird Conservancy is a non-profit organisation founded in 1994 and dedicated to conserve wild birds and their habitats throughout the Americas. The society no doubt does good work for birds. But it is not difficult to see a questionable side to the body. The more dangers to birds it whips up, the more public anger about domestic cats and the more members and donations it gets. �Avicidal� cats are good for business. So do we need do something about domestic cat predation, as Loss and co-workers express? Research on cat predation suggests there may be a problem. But before we leap into the dark we must consider clearly where we are going. a) Studies Are Preliminary Research on domestic cat predation is at a preliminary stage. Research results must be open to critical analysis and further research that may confirm or refute the results; research should not be taken as undisputed truth. Therefore it is misleading to overstate results and conclusions and wrong and harmful to make sweeping statements. Research papers published on any subject usually have flaws in their methods. Cat research is not an exception and the Loss (2013) paper has been criticised for several serious statistical failings. Among the problems is the uncertainty of how the data on cat predation rates were acquired; that extrapolating from small samples of cats is likely to give imprecise results when applied to all cats nationally; statistical use of the data could have been made better and in conclusion the paper is unsuitable for publication, even if it had a major revision (Matthews 2013). b) We Need Reliable Detailed Data Before we consider doing something about any perceived problem, we must have reliable and accurate data on which to base potential actions. A comment on the Loss (2013) paper is: �computing gross estimates of mortality is a critical first step, but it quickly becomes apparent that the true parameter of interest is not the headline-grabbing total, but the species-by-species mortality estimate. (Machtans et al 2014)In other words global figures obscure local facts, where the detail of the predator-prey battles are fought. For effective conservation action we must move away from grand totals to the effects of domestic cat predation on individual species (Machtans et al 2014). Indeed, we have to zoom in for detailed knowledge about what is happening to different populations of species in various environments, from cities to wilderness. Obviously, bird species differ from each other, so we can expect they will not fall equal prey to cats. The ground foraging house sparrow (Passer domesticus) seemed particularly prone to cats in the village of Felmersham, studied by Churcher and Lawton (1987). House sparrows were also relatively highly predated among bird species in Bristol city, a conjecture being that fresh sparrows were migrating in to top up the population from the surroundings (Baker et al 2005). Focusing on the death rates of populations in different habitats is more revealing than coming up with a single global figure, that gives no indication of where a problem might lie or were a solution may be found. c) Cats Do Not Live in Isolation Cats live as part of a complex of interrelating communities. This was illustrated by Charles Darwin when he spotted a relationship between clover, bumble bees, mice and cats. Observing the English countryside he saw that clover grew in profusion around villages, that it was spread by bumble bee pollination, and that field mice, who destroy bumble bee nests, were relatively scarce around the villages. He concluded that the clover owed its abundance to the cats of the villages who hunted the mice and kept their numbers down (Darwin 1859). What then is the point of studying domestic cats as if they lived isolated lives? A scientist would answer that preliminary research must be relatively simple or it will get bogged down in a complicated mess. The fact is that we are still in the early stages of cat predation research, so cat research tends to be kept simple, such as taking a look at what prey cats bring home. Loss and his colleagues are therefore viewing a one sided picture - about which it is dangerous to make grand sweeping statements. Basing their ideas on so little data they are likely to be wrong. A scientist wit ventured an additional chain of events as a jocular retort to Darwin�s observation. That since clover was an essential food of cattle, and the staple diet of British sailors was bully beef, then the British Empire, which was dependent on its navy, ultimately owed its fortune to domestic cats (recounted in Vandermeer et al 1988). But would such a conclusion be unescapably true? d) Hapless Survivors or Doomed Surplus? Millions of animals are born every year, but no species can increase without limit, so millions of animals die every year. They die because the environment cannot support them all. Say a population of one million birds starts to breed. The animals pair off and produce two offspring per pair. That now makes two million birds. But if there is only enough food and shelter for one million birds, then a million birds will perish (assuming there is nowhere new to colonise). The breeding population has been called the �hapless survivors� and the dying population the �doomed surplus�. Animals of the doomed surplus might tend to be weaker or sickly than the hapless survivors and die from starvation, disease, predation and so on. Are domestic cats preying on hapless survivors or doomed surplus? If cats take only the doomed surplus their predation will make no difference to the size of the bird population. The doomed surplus birds will die whether killed by cats or some other means. Only if cats kill the hapless survivors will the population decline because fewer animals will breed. But we do not know what effect domestic cat predation has on prey populations. A little evidence suggests that cats tend to prey on the doomed surplus. For example, Baker et al (2008) found that cat-killed birds in Bristol city had significantly lower physical fitness than individuals of the same species who were killed by �accident�, such as collisions with cars or windows. Finding evidence that cats are preying on hapless survivors or doomed surplus is a difficult challenge; researchers must study not only cats but their prey populations. But we do know that cats have not exterminated any species. So, generally, it seems possible that cats eat into the doomed surplus, not the hapless survivors. What of the individual prey? If they are going to die then they are going to die. Is an unexpected sudden snatch by a predator, made near painless by shock, better than a long death from starvation, a weakened immune response and attendant illness? e) Cats Have Always Lived Here European wildcats inhabited Europe before people destroyed them from most regions and replaced them with domestic cats. But the two cats are variations of the same species. Domestic cats evolved naturally in the Middle East from wildcats at least 10,000 years ago to fill a niche created by human settlements (Driscoll et al 2009). Domestic cats differ superficially from wildcats in coat pattern, gut length and some aspects of behaviour. Science formerly made separate species of them for the sake of convenience. Now science recognises them as variations of the same species and reclassified them as such; the European wildcat is Felis silvestris silvestris and the domestic cat is Felis silvestris catus (IUCN: Felis silvestris). This means three things in Europe. First, domestic cats are not an invasive species in Europe, as often erroneously alleged (as though being an invasive species is always necessarily bad, which it is not). Second, it is likely that the wildlife prey of the cats treat both variants in exactly the same way; thus wildlife evolved to cope with small cats. Thus roaming domestic cats Europe are natural, they have taken over the ecological niche of the extirpated wildcat. People are therefore wrong when they say domestic cats should not exist or should not hunt. The problem is the human-made extirpation of the wildcat and the very large human-made domestic cat population. The situation in North America is not the same as in Europe as the wildcat never lived there. The ecological counterpart of the wildcat in North America is the bobcat (Lynx rufus). Bobcats once roamed much of North America and are still the most common North American wild cat. Bobcats are twice as heavy as a domestic cat and their prey overlap at the small end; they both eat rodents, birds and rabbits, although bobcats sometimes take larger prey like deer. Thus the domestic cat in North America could be looked upon as a smaller version of the bobcat and not as unnatural an introduced species as some people would make out. f) Human Power Humans have ramped up the domestic cat population and keep it high. Humans have had great influence on island species by habitat change and introducing alien species including cats. How does the wholesale degradation of natural ecosystems influence cat predation, indeed predation by any predator? To understand cat predation properly we have to see it against its human perspective. Until this receives the attention it should the story of domestic cat predation will be largely untold. To be continued� References
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