This may be surprising, but many millions of birds die naturally every year, mainly through starvation, disease or other forms of predation. There is evidence that cats tend to take weak or sickly birds. We also know that of the millions of baby birds hatched each year, most will die before they reach breeding age. This is also quite natural, and each pair needs only to rear two young that survive to breeding age to replace themselves and maintain the population.
It is likely that most of the birds killed by cats would have died anyway from other causes before the next breeding season, so cats are unlikely to have a major impact on populations. If their predation was additional to these other causes of mortality, this might have a serious impact on bird populations. Those bird species which have undergone the most serious population declines in the UK such as skylarks, tree sparrows and corn buntings rarely encounter cats, so cats cannot be causing their declines.
Research shows that these declines are usually caused by habitat change or loss, particularly on farmland. Populations of species that are most abundant in gardens tend to be increasing, despite the presence of cats. Blue tits, for example, the second most frequently caught birds, have increased by more than a quarter across the UK since Of the birds most frequently caught by cats in gardens, only two house sparrow and starling have shown declines in breeding population across a range of habitats during the last six years.
Gardens may provide a breeding habitat for at least 20 per cent of the UK populations of house sparrows, starlings, greenfinches, blackbirds and song thrushes four of which are declining across the UK. For this reason it would be prudent to try to reduce cat predation as, although it is not causing the declines, some of these species are already under pressure.
Cat predation can be a problem where housing is next to scarce habitats such as heathland. It could potentially be most damaging to species with a restricted range such as cirl buntings or species dependent on a fragmented habitat such as Dartford warblers on heathland. Martin Harper Blog. How nature can help protect our homes Following the floods this winter, watch how one area is using nature as a natural protector.
Some argue that cats are natural predators and should be allowed to roam and kill prey as they please. But, according to Piazza, the fight is not fair. This argument overlooks several important factors. As the predator population dwindles, the prey population has time to repopulate. Domesticated cats disrupt the cycle. Her goal, instead, is to unite them.
Not only do these practices protect other animals, they keep cats safe and healthy, too. National Geographic National Geographic. By Cordilia James. Published 24 Sept , BST. Photograph by Jak Wonderly. The problem with outdoor cats Cats have contributed to the extinction of 63 species of vertebrates, most of them birds, says Peter Marra , director of the Georgetown Environment Initiative.
Where cats get their own staircases. Existing estimates of mortality from cat predation are speculative and not based on scientific data 13 , 14 , 15 , 16 or, at best, are based on extrapolation of results from a single study In addition, no large-scale mortality estimates exist for mammals, which form a substantial component of cat diets.
We conducted a data-driven systematic review of studies that estimate predation rates of owned and un-owned cats, and estimated the magnitude of bird and mammal mortality caused by all cats across the contiguous United States all states excluding Alaska and Hawaii.
This magnitude of mortality is far greater than previous estimates of cat predation on wildlife and may exceed all other sources of anthropogenic mortality of US birds and mammals.
After excluding studies that did not meet a priori inclusion criteria designed to increase the accuracy of our analysis, we developed probability distributions of predation rates on birds and mammals. We generated an estimated range of bird and mammal mortality caused by cat predation by incorporating the above distributions—including separate predation rate distributions for owned and un-owned cats—and running 10, calculation iterations. We augmented US predation data by incorporating predation rate estimates from other temperate regions Supplementary Table S1.
For birds, we generated three US mortality estimates based on predation data from studies in: 1 the United States, 2 the United States and Europe and 3 the United States, Europe, and other temperate regions primarily Australia and New Zealand. Owing to a lack of US studies of un-owned cat predation on mammals, we estimated mammal mortality using data groupings 2 and 3.
We based all other probability distributions on US studies distribution details in Table 1 ; data in Supplementary Table S2. We focus interpretation on the estimate generated using US and European predation data because it is the lowest value. Furthermore, this estimate is more likely to be representative of the US than the estimate based on incorporation of data from Australia and New Zealand, where the wildlife fauna and climate are less similar to the United States.
We estimate that cats in the contiguous United States annually kill between 1. The predation estimate for un-owned cats was higher primarily due to predation rates by this group averaging three times greater than rates for owned cats.
Our estimate of mammal mortality was robust to the choice of predation data as evidenced by a 1. We focus interpretation on the lower estimate, which was based on United States and European predation data and US values of other parameters. We estimate annual mammal mortality in the contiguous United States at between 6.
The estimate that incorporated European data but not data from Australia and New Zealand may be slightly lower because wildlife across much of Europe were historically exposed to predation by a similarly-sized wild cat Felis sylvestris and, therefore, may be less naive to predation by domestic cats.
However, it is unlikely that European wildlife have fully adapted to the unusually high densities of domestic cats in much of this continent 9. For both birds and mammals, sensitivity analyses indicated that un-owned cat parameters explained the greatest variation in total mortality estimates Fig. Amount of variation in estimates of wildlife mortality in the contiguous United States contributed by each parameter in the cat predation model percentages represent adjusted R 2 values from multiple regression models.
Our estimate of bird mortality far exceeds any previously estimated US figure for cats 13 , 14 , 16 , as well as estimates for any other direct source of anthropogenic mortality, including collisions with windows, buildings, communication towers, vehicles and pesticide poisoning 13 , 15 , 16 , 17 , 18 , 19 , 20 , Systematic reviews like ours, which includes protocol formulation, a data search strategy, data inclusion criteria, data extraction and formal quantitative analyses 22 , are scarce for other anthropogenic mortality sources.
Nonetheless, no estimates of any other anthropogenic mortality source approach the value we calculated for cat predation, and our estimate is the first for cats to be based on rigorous data-driven methods.
Notably, we excluded high local predation rates and used assumptions that led to minimum predation rate estimates for un-owned cats; therefore, actual numbers of birds killed may be even greater than our estimates. Free-roaming cats in the United States may also have a substantial impact on reptiles and amphibians.
However, US studies of cat predation on these taxa are scarce. To generate a first approximation of US predation rates on reptiles and amphibians, we used the same model of cat predation along with estimates of cat predation rates on these taxa from studies in Europe, Australia and New Zealand. Reptile and amphibian populations, and, therefore, cat predation rates, may differ between the regions where we gathered predation data for these taxa and the United States.
Furthermore, reptiles and amphibians are unavailable as prey during winter across much of the United States. Additional research is needed to clarify impacts of cats on US herpetofauna, especially given numerous anthropogenic stressors that threaten their populations for example, climate change, habitat loss and infectious diseases and documented extinctions of reptiles and amphibians due to cat predation in other regions 4 , The exceptionally high estimate of mammal mortality from cat predation is supported by individual US studies that illustrate high annual predation rates by individual un-owned cats in excess of mammals per year 6 , 25 , 26 , 27 , 28 and the consistent finding that cats preferentially depredate mammals over other taxa Supplementary Table S1.
Even with a lower yearly predation rate of mammals per cat, annual mortality would range from 3—8 billion mammals just for un-owned cats, based on a population estimate of between 30 and 80 million un-owned cats.
This estimated level of mortality could exceed any other direct source of anthropogenic mortality for small mammals; however, we are unaware of studies that have systematically quantified direct anthropogenic mortality of small terrestrial mammals across large scales. Native species make up the majority of the birds preyed upon by cats.
For mammals, patterns of predation on native and non-native species are less clear and appear to vary by landscape type. In densely populated urban areas where native small mammals are less common, non-native species of rats and mice can make up a substantial component of mammalian prey Further research of mammals is needed to clarify patterns of predation by both owned and un-owned cats on native and non-native mammals, and across different landscape types.
Sensitivity analyses indicate that additional research of un-owned cats will continue to improve precision of mortality estimates. Our finding that un-owned cat population size and predation rate explained the greatest variation in mortality estimates reflects the current lack of knowledge about un-owned cats. No precise estimate of the un-owned cat population exists for the United States because obtaining such an estimate is cost prohibitive, and feral un-owned cats are wary of humans and tend to be solitary outside of urban areas.
In addition, human subsidized colonies of un-owned cats are maintained without widespread public knowledge. Population size estimates can be improved by incorporating observations of free-ranging cats into a wildlife mortality reporting database Context for the population impact of a mortality source depends on comparing mortality estimates to estimates of population abundance of individual species. However, continental-scale estimates of wildlife population abundance are uncertain due to spatio-temporal variation in numbers.
For mammals, clarification of the population impacts of cat predation is hindered by the absence of nationwide population estimates. For all North American land birds, the group of species most susceptible to mainland cat predation Supplementary Table S3 , existing estimates range from 10—20 billion individuals in North America A lack of detail about relative proportions of different bird species killed by cats and spatio-temporal variation of these proportions makes it difficult to identify the species and populations that are most vulnerable.
The magnitude of our mortality estimates suggest that cats are likely causing population declines for some species and in some regions. Threatened and endangered wildlife species on islands are most susceptible to the effects of cat predation, and this may also be true for vulnerable species in localized mainland areas 5 because small numbers of fatalities could cause significant population declines. Threatened species in close proximity to cat colonies—including managed TNR colonies 11 , 12 —face an especially high level of risk; therefore, cat colonies in such locations comprise a wildlife management priority.
Claims that TNR colonies are effective in reducing cat populations, and, therefore, wildlife mortality, are not supported by peer-reviewed scientific studies Our estimates should alert policy makers and the general public about the large magnitude of wildlife mortality caused by free-ranging cats.
Structured decisions about actions to reduce wildlife mortality require a quantitative evidence base. We provide evidence of large-scale cat predation impacts based on systematic analysis of multiple data sources. Future specific management decisions, both in the United States and globally, must be further informed by fine scale research that allows analysis of population responses to cats and assessment of the success of particular management actions.
We are not suggesting that other anthropogenic threats that kill fewer individuals are biologically unimportant. Virtually nothing is known about the cumulative population impacts of multiple mortality sources. Furthermore, comparison of total mortality numbers has limited use for prioritization of risks and development of conservation objectives. Combining per species estimates of mortality with population size estimates will provide the greatest information about the risk of population-level impacts of cat predation.
Although our results suggest that owned cats have relatively less impact than un-owned cats, owned cats still cause substantial wildlife mortality Table 2 ; simple solutions to reduce mortality caused by pets, such as limiting or preventing outdoor access, should be pursued. Efforts to better quantify and minimize mortality from all anthropogenic threats are needed to increase sustainability of wildlife populations.
The magnitude of wildlife mortality caused by cats that we report here far exceeds all prior estimates. Available evidence suggests that mortality from cat predation is likely to be substantial in all parts of the world where free-ranging cats occur. This mortality is of particular concern within the context of steadily increasing populations of owned cats, the potential for increasing populations of un-owned cats 12 , and an increasing abundance of direct and indirect mortality sources that threaten wildlife in the United States and globally.
We initially focused this search on US studies, but due to a limited sample of these studies, we expanded the search to include predation research from other temperate regions. We also searched for studies providing estimates of cat population sizes at the scale of the contiguous United States and for US studies that estimate the proportion of owned cats with outdoor access and the proportion of cats that hunt wildlife.
We checked reference lists of articles to identify additional relevant studies. Lead authors of three studies were also contacted to enquire whether they knew of ongoing or completed unpublished studies of cat predation in the United States.
We grouped studies based on the ranging behaviour of cats investigated. We defined owned cats to include owned cats in both rural and urban areas that spend at least some time indoors and are also granted outdoor access.
We defined un-owned cats to include all un-owned cats that spend all of their time outdoors. We did not classify cats by landscape type or whether they receive food from humans because the amount of time cats spend outdoors is a major determinant of predation rates 33 , 34 and because predation is independent of whether cats are fed by humans 6 , 34 , Studies were only included if: 1 they clearly reported cat ranging behaviour that is, a description of whether cats were owned or un-owned and whether they were outdoor cats or indoor-outdoor cats , and 2 the group of cats investigated fit exclusively into one of the two groups we defined above that is, we excluded studies that lumped owned and un-owned cats in a single predation rate estimate.
For some studies, we extracted a portion of data that met these criteria but excluded other data from cats with unknown ranging behaviour.
We only included mainland and large island New Zealand and United Kingdom predation studies, because cat predation on small islands is often exceptionally high 36 , 37 and focused on colony nesting seabirds For a list of all included and excluded studies, see Supplementary Table S1.
Most studies report an estimate of cat predation rate that is, daily, monthly or annual prey killed per cat or present data that allowed us to calculate this rate. When studies only reported predation rate estimates for all wildlife combined, we calculated separate predation rates by extracting taxa-specific prey counts from tables or figures and multiplying the total predation rate by the proportion of prey items in each taxon.
If taxa-specific counts were not provided, we directly contacted authors to obtain this information. For studies that presented low, medium and high estimates or low and high estimates, we used the medium and average values, respectively. For studies that presented more than one predation estimate for cats with similar ranging behaviour for example, owned cats in rural and urban areas , we calculated the average predation rate.
This assumption results in coarse predation rate estimates, but estimates from this approach are even more conservative than those from the first assumption because many stomachs and scats undoubtedly included more than one bird or mammal. Predation rate estimates from many studies were based on continuous year-round sampling or multiple sampling occasions covering all seasons. However, seasonal coverage of some studies was incomplete.
To generate full-year predation rate estimates in these cases, we adjusted partial-year predation estimates according to the average proportion of prey taken in each month as determined from year-round studies reporting monthly data birds and mammals 8 , 33 , birds only 7 , For partial-year estimates from the northern hemisphere, we offset monthly estimates from southern hemisphere studies by 6 months.
The final annual predation rate estimates for all studies are presented in Supplementary Table S1. The year-round studies we used represent different geographical regions for birds—England, Kansas US , Australia and New Zealand; for mammals—England and Australia with varying climates and slightly varying seasonal patterns of predation. For both birds and mammals, averaging across full-year studies resulted in higher proportions of predation in the spring and summer compared with fall and winter, an expected pattern for much of the United States.
The reference studies we used, therefore, provide a reasonable baseline for correcting to full-year mortality estimates. This approach greatly improves upon the assumption that mortality is negligible during the period of the year not covered by sampling. We estimated wildlife mortality in the contiguous United States by multiplying data-derived probability distributions of predation rates by distributions of estimated cat abundance, following Quantification was conducted separately for owned and un-owned cats and for birds and mammals.
However, we only used studies from the contiguous United States to construct all other probability distributions listed below. From the probability distribution of each parameter see Table 1 and Supplementary Methods for details about the specific probability distributions used , we randomly drew one value and used the above formulas to calculate mortality.
Random draws were made using distribution functions in Programme R rnorm and runif commands for normal and uniform distributions, respectively. We conducted 10, random draws to estimate a potential range of annual predation on each wildlife taxa.
We used multiple linear regression analysis to assess how much variance in mortality estimates was explained by the probability distribution for each parameter. We used adjusted R 2 values to interpret the percentage of variance explained by each parameter. How to cite this article: Loss S.
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