We currently find ourselves squarely and loudly in the age of misinformation and agenda hysteria where social media cancel culture rules and bad science goes unquestioned. As agendas and narratives become more important that the truth we find it hard to identify the lies when all the information we are provided with are lies. This scourge has clawed itself into science as well and now it is up to everyone to critically evaluate any information they come across. This is especially true for information found on the internet as once reliable sources have become tainted.
In this context I have observed a trend in the environmental and nature conservation community where a group of people are pushing an unfounded narrative about the devastating impact that domestic cats have on the ecosystem and wildlife populations.
Proponents of this movement include sources which I have always considered to be unbiased and reputable, such as Nature and National Geographic. I have been greatly disheartened by my disappointment in their willingness to support a point of view that requires a lot more investigation to be conclusive. Additionally, they publish one-sided popular scientific articles on the topic that are filled with sensation and emotion to distract from the bad science referenced within.
Thus, the following infographic is my contribution to the discussion; to provide balance to the investigation by highlighting good science to the contrary and to show that the role domestic cats play in wildlife populations are as complex as the ecosystems they reside in.
I would like to discuss each of the myths in turn to provide more information about each. However, this is not all that I have to share and I am planning on posting another more comprehensive and in-depth article about the topic in the future. There, I will point out the poor science behind the narrative and highlight how it does more damage than good to both domestic cats and the ecosystems that they are trying to ‘protect’. But I thought it prudent to start with something more bite-sized and visual with the above infographic.
On to the myths busted by science:
Myth 1: Cats are the leading invasive mammal worldwide
Invasion is one of those processes that has a spectrum attached to it. As time goes by; the ecosystem either suffers or benefits from the invasive species. Initially an organism is invasive when it entered an ecosystem in which it is not naturally found and it alters the new environment. Generally, a negative connection is attached to the term invasive, but often invasive species do not survive the new environment and die out. When an invasive species adapts or is more suited to its new environment it can thrive – this can be either to the detriment of the environment (i.e., competition and extinction of endemic species) or the invasive species can become a naturalised alien. Then they contribute to the ecosystem and bring balance to it (i.e., replacing a food source or controlling another species population numbers in the absence of an endemic predator). Therefore, invasion is a term that should be considered within the context of the ecosystem being invaded, the impact it has and whether or not the species has gone from being an invader to a naturalised contributor.
When considering invasive species, mammals are generally well evolved and adaptable to many different environments. Subsequently, they make up the largest proportion of invasive animals. Mammals are also large and easy to observe. This may lead to a bias in their relative representation, simply because they can be readily studied.
However, from the available literature you can observe that rodents take the lead, followed by domestic cats and then the red fox, domestic dogs, Javan mongoose and domestic pigs (REF 1). The species driven to extinction by these mammals are mostly island species, due to the drastic impact of invasion on these isolated and enclosed environments (REF 1). Additionally, island ecosystems have many animals vulnerable to mammalian invaders, such as nesting or ground dwelling sea birds (REF 1). I would also assume that the extinctions are also due to feral mammal populations.
Myth 2: Outdoor domestic cats decimate wildlife populations
It is claimed that outdoor domestic cats are decimating wildlife in the USA. Yet, the main literature used to support this claim has poor experimental design. The article had inconclusive results because it is solely based those results on biased estimates that did not include directly measured wildlife mortality rates for the USA – I will elaborate on this more in my following article. Consequently, I have searched high and low for information that shows the different factors (i.e., wildlife-vehicle collisions, pollution, pesticides, buildings, ecosystem destruction, climate change, entanglement, invasive predation and even wildlife-plane collisions) that may impact wildlife populations and what their relative impact is.
However, I have not been successful in quantifying the relative impacts and mortality rates of these factors. This is due to most available studies being more than 20 years old or there simply isn’t large-scale studies being done (REF 2 and REF 3). The studies to determine mortality rates as a result of cat predation are small (i.e., few hundred owned cats) and isolated to one/two types of environments (i.e., rural, urban or both). Cat predation is estimated from ‘what the cat dragged in’. This means that one cannot distinguish between prey that is already dead (i.e., scavenged or road kill); from those already sick, injured and/or dying; from those that are healthy individuals caught by the cat. Thus, it is concerning to see that domestic cats are being accused of wildlife population destruction and yet no other factors have been properly investigated to make comparisons or to substantiate the claims.
I gathered up as many open-access articles about cat predation as I could; across different environments (excluding islands, since these are a special case as discussed above, and they cannot be compared to mortality rates of mainlands). Mortality rates for wildlife per cat per day ranged from 0.9 kills per cat per year (REF 4) to 21 kills per cat per year (REF 5); form the urban cats in Bristol in the United Kingdom to the rural ‘super predators’ in Finland, respectively. One cannot simply use the average of these two numbers, since cat success rate depends on the hunting environment (urban vs rural) and not all domestic cats are super predators. Although, Erikson (2005) claims that they have a ‘conservative’ estimate of 45 – 365 kills per cat per year for free-ranging cats in rural Wisconsin, USA. Seems Wisconsin cats are all super-super-predators LOL!
When you consider the kill rate of the world’s most successful wild cat, the Black footed cat at 10 – 14 kills per day (Apex predators in the wild: which mammals are the most dangerous?), one can glean a bit more insight into the kill rate of the domestic cat. The hunting success rate of the Black footed cat is 60% in comparison to the success rate of the domestic cat at 32% (although the success rate can increase to 80% in open areas). Now, domestic cats certainly do not have the hunting prowess of a black footed cat and they do not hunt for survival, but more on instinct. Therefore, it is estimated that cats eat less that a third of what they kill. However, this should not affect their relative hunting success rate. Thus, a reasonable range for the domestic cat can be estimated at 1 – 3 wildlife kills per cat per day – here I am considering that hunting success rates include mammal, bird, insect and other prey items. In comparison to the previously measured kill rates for domestic cats (REF 3 – 5), my estimate is several times larger and should account for any other variability in domestic cat kill rates or the true number of prey items brought home.
The total cat population of the United States is 95.6 million (2017, Number of cats in the United States from 2000 to 2017). With 35% of these being indoor cats (2015, Human Attitudes and Behavior on Keeping Cats Indoors), one can estimate the total wildlife kills at 62 – 186 million animals per year. So let’s round up the numbers for ease of comparison to 65 – 190 million kills per year and 170 – 511 thousand kills per day for all the cats in USA.
Fortunately, I managed to find one medium-sized quantitative study on the impact of vehicle collisions on wildlife populations in the United States (REF 4) for comparison to cat predation mortality rates. Using their results of 1.3 roadkills/day/km on a 114 km stretch of road in the United States; I multiplied it with the total km of roads in the USA (6,948,956 km, 2019, List of countries by road network size). The result is a total of 3.3 billion roadkills per year! The authors mentioned that their result may be an under-estimation by up to 10 fold, due to the difficultly of quantifying carcasses of smaller animals and the calculation does not take into account the volume of traffic. Thus, vehicles result in the death of 3 -33 billion animals per year and 9 – 90 million animals per day in the United States alone!
Once we have our ranges and totals for cats and vehicle-wildlife collisions in the United States; let put them into a perspective using a ratio:
|Range per day||Total kills per year||Total kills per day||Ratio Lower Limit||Ratio Upper Limit||Average|
|WVC||1.3 – 13 per km||3 – 33 B|
|9 – 90 M|
|Cats||1 – 3 per cat||65 – 190 M|
|170 – 511 K|
Thus, now we have a comparable scale that indicates for every animal a cat kills; vehicle collisions kill 110 animals. But wait the plot against cats is being unravelled yet!
Myth 3: Outdoor domestic cats prey on imperilled birds
Another claim is that cats prey on birds that are endemic and/or on the ICUN Red List, which almost seem to imply that cats ‘know’ to target these species. No one would argue against the fact that cats who have access to nature reserves or conservation areas containing Red List animals is certainly problematic – but as stated before you cannot take a special case and apply it universally. Thus, I wanted to find a breakdown of the composition of cat prey items, preferably to species level as family level can be misleading.
Once again there are not many studies to document the species composition of cat prey items. Prey identification stays at general classifications, such as ‘Rodents, Birds or Invertebrates’ or they document family level such as ‘Songbird’. I did find three articles, which broke down prey items to species/family level; the one in Finland with the super predators (REF 5), one in Poland (REF 6) and one in the United Kingdom (REF 7). I collected the species from these articles and organised them into family level so that I could establish the relative proportion of each group and then determined whether representatives are abundant/imperilled species.
The most preyed upon animal group was rodents (i.e., mice, rats, voles, moles and shrews). Of these Black rats, Brown rats and local high abundant mice, voles, moles and shrews are the main prey representatives. When we look at the bird composition we see a very similar scenario. There are many cosmopolitan sparrows, martins and finches from the Songbirds family followed by pigeons and doves. The reptile prey are abundant cosmopolitan or local species. Invertebrate prey are a bit tricky so these do not get classified properly. From this I could establish that 80 – 100% of cat prey items were abundant species and that the majority of these were pests.
I found additional cat prey composition breakdowns at higher general levels (i.e., Rodents, Birds and Reptiles) from New Zealand and they show the same trend (REF 8). The majority of prey items are Rodent kills followed by Birds, then Reptiles and Invertebrates. Thus, for relative prey proportions I included those from urban New Zealand to get an average of the proportions, which is summarised in the following table:
Thus, cats seem to serve their traditional role as rodent pest control in urban and rural areas. Their hunting does become problematic in areas with imperilled or ICUN Red List species, but this is not the norm. If you are concerned that you cat may be hunting too many birds; a study from New Zealand found that adding a bell to the cat’s collar reduces the success rate of the cat to catch birds by 50% and 61% for rodents, but to a lesser extent rats, lizards and insects (REF 9). Thus, belled collars is worth considering for outdoor cats who are ‘super predators’ or for cats who hunt many birds without sacrificing rat pest control.
Myth 4: Outdoor domestic cats impact bird populations
This is a follow on from myth 3, where cats are blamed for failing or declining local bird populations. Considering that the cat is a predator, we are all aware that the main role of predators are to remove weak individuals form the population such as the old, sick, injured and/or dying. Additionally, protein is scarce and valuable sustenance. Thus, most predators are not above scavenging already dead animals.
From the investigation above we already notice that rodents make up 60% of a cat’s prey and birds only make up 20%. This is likely due to the fact that birds are hard to catch and it would be reasonable to assume that the most birds brought in by cats would likely have been opportunistic catches or even roadkill. Thus, my next investigation was to determine what the condition of the cat bird prey would be and what proportion of them would be considered healthy reproductive individuals.
Again, little research has been done to establish the ratio of cat prey items of Healthy vs Old vs Sick vs Injured vs Dying vs Scavenged. Most studies quantify and identify cat prey items based off prey brought home by the cat. As a result it is nearly impossible to determine the condition of the cat prey. However, I managed to find one study in the United Kingdom, where owners confiscated any bird prey items and autopsies where performed on prey items by the researchers (REF 7). These birds were compared to the condition of bird-vehicle collision victims using several health indicators.
The results from the comparison were that birds caught by cats were in poor condition. They were underweight with low wing muscle mass and low fat reserves. Vehicle collisions are indiscriminate; killing many different animals, large or small regardless of their condition. Therefore, we can observe that cats are indeed fulfilling their natural predatory role and that the likelihood of reducing bird populations is low, since prey are probably not healthy reproductive individuals that may contribute to the population.
Myth 5: All feral cat colonies are pests & must be eradicated
This is the final myth that I would like to address as I have seen some conservationists promote the idea that feral cat colonies should be eradicated with lethal means since they are pests that destroy local wildlife, particularly birds.
Once more, feral cat colonies and their impact on wildlife is relatively understudied. Moreover, I highly doubt that feral cat colony prey item composition and the health of prey items would differ substantially from those caught by outdoor domestic cats. They would likely catch a larger number of wildlife, and again this would only become problematic for those colonies with access to nature reserves or those on islands.
The best sources I could find for investigating this claim was studies from New Zealand and other islands. New Zealand itself is a large island and feral cat colonies have been problematic for years, even more so on smaller surrounding islands. As mentioned before, this is due to the fact that many islands are nesting grounds for ground dwelling seabirds, which make them especially vulnerable to predation. Thus, it could lead one to falsely assume that eradication of feral cat colonies from islands would always be beneficial.
However, after reading many articles surrounding feral cat colony eradication efforts and their relative successes; this does not seem to be the case (REF 10). Many times feral cat colonies have ‘co-invaded’ the island with rats and/or mice. Since the impact of rats/mice are harder to study; they are generally not considered to be problematic and the focus remains with the main cat predator.
Removing the cat predator with immediate effect using lethal means without considering or investigating their role in the ecosystem, whether this be on an island or in the mainland, can have catastrophic consequences. I will discuss 4 likely scenarios that I have observed in literature.
Mesopredator release is a term to describe how another species (predator or omnivore) can take up the role of the original predator after their removal. In this case, rats and mice become mesopredators once the cat predator is removed. This is because the cat is no longer controlling the rat/mice population and the rats/mice no longer have competition from the cat predator for the prey species – in this case the prey species are usually ground nesting seabirds. For example:
1. On Ascension Island, in an effort to restore sooty tern populations; feral cats were eradicated in 2003 (REF 11). After an initial restoration of sooty tern numbers; the absence of the cat apex predators triggered the mesopredator release of the invasive black rats (Rattus rattus). The rat predation pressure became greater than that of the cat by 2005 after a seven fold increase in their population. Rats alone exerted 97% of the predation pressure of the cat-rat regime. Thus, predation pressure only decreased by 3% after cat predator removal.
2. On Marion Island, cats were introduced to the islands to control invasive house mice (Mus musculus) who were preying on native invertebrates. Feral cats started to prey on the local nesting seabirds and in turn the cats were eradicated in 1991. Years later, in the 2000s mice started to prey on seabird chicks. Larger birds were also injured through scalping, which results in raw, bleeding crowns and necks.
Thus, in both instances we see that cat eradication leads to an initial recovery of the vulnerable species, followed by the mesopredator release years later and then the vulnerable species population starts to decline once more.
Another scenario that is coupled to the cat’s traditional role of pest controller. When the cat is eradicated after becoming a naturalised alien, the herbivorous pest population explodes. Here the herbivore pest species continues to decimate local fauna in the absence of the cat predator. For example:
On World Heritage Island, large-scale local vegetation have been irreversibly damaged by the increase grazing of the European rabbit (Oryctolagus cuniculus). The rabbit populations were mainly controlled by the local cat population (REF 12). The rabbits, like other rodents, are difficult and expensive to eradicate and their conservation control action are estimated to exceed USD $17 million.
Vulnerable Species Restoration
In both scenarios discussed in the MESOPREDATOR RELSEASE section there was an initial restoration of the known vulnerable species. However, with time the ecosystem imbalance became clear. Unfortunately, most examples of ‘cat eradication success’ is tied to initial restoration numbers. I cannot find any long term studies, i.e., 5 – 20+ years after cat eradication to confirm that the removal of the cat alone resulted in species restoration. Thus, I can only provide examples of recent successes, which have not been monitored in the long-term, for example:
On Socorro Island, dual cat and sheep eradication has resulted in restoration of the endemic blue lizard (Urosaurus auriculatus) and local vegetation, respectively (REF 13). Consequently, blue lizard populations recovered by 8 fold, and 11% of the island surface vegetation has recovered. Seeing that the blue lizards made up a large portion of cat prey, it is likely that the lizard population recovery would be sustained in the absence of other multi-predator invaders. The lizard populations have only been monitored for 4 years in this study and long-term observations would confirm whether the eradication has been successful without mesopredator release and/or other unknown indirect effects.
Pest Control Loss
In the MESOPREDATOR RELEASE and HERBIVORE RELEASE sections I have already illustrated the consequences of pest control loss, where pest control was provided as an ecosystem service by the main cat predator.
In urban and rural areas, the eradication of cat predators will likely lead to an explosion in pest populations and with that comes disease, environmental damage and economic loss. For example, rat plagues devastate natural and agricultural habitat along with spreading disease. Uncontrolled rat populations lead to attacks on human infants, however I am unable to find scientific studies on this. What I am about to describe is gruesome, but it is an illustration of the social and public health consequences of uncontrolled rat populations where individual rats become brazen and bold without fear of predation. Rats will feed on infants in their cribs, usually at night, targeting soft tissue around the fingers, toes and particularly the face (i.e., ears, nose and lips). Even more concerning is that the infants don’t cry when being attacked by rats. The result of these attacks is that these infants are scarred for life…
To conclude myth 5, it is clear that cats provide ecosystem services mainly through their traditional role as pest controllers. The lethal and immediate eradication of cat colonies may have severe health, social, environmental and ethical consequences since failed programs do not justify the use of such inhumane control programmes (REF 14).
Alternative control programmes such as trap-neuter-release and management of multiple invasive species along with newer techniques such as exclusion fencing may serve as a long-term sustainable – and more gradual control method. Environments can be monitored, and potential irreversible or damaging effects can be identified sooner and mitigated.
Brief Recap with another infographic
I am certain that I have illustrated a more balanced view on cat predation. Generalisations, one-size-fits-all and extreme approaches only do more harm than good with regards to unethical cat control, failed wildlife conservation attempts and environmental destruction at the expense of the cat. Additionally, we see that the mortality rates of wildlife and how cats contribute to this is relative understudied.
Outdoor domestic cats contribute essential pest control and environmental protection services. Those cats who have access to conservation areas or whose hunting threatens Red List species should be managed with ethical techniques as determined after studying the impact of the presence/absence of cat predation on the ecosystem in question. Harassing cat owners with unfounded lies does not serve to protect the vulnerable species or to promote responsible pet ownership.
Feral cat colonies should be reduced for the sake of animal welfare as well as their potential impact on local wildlife. Trap-Neuter-Release (TNR) programmes improve the well being of the cats, since males are less aggressive and are less prone to fighting. Additionally, females aren’t burdened with litters of kittens one after the other, which only adds to the compounding problem. I am convinced that asking resident pet lovers and cat owners to participate in and/or donating to such TNR programmes will be met with enthusiasm and gratitude.
It will be far more efficient and productive to enlist the assistance of cat owners, animal lovers and other concerned citizens in the effort to conserve the environment, protect wildlife, ensure animal welfare and reducing feral cat populations rather than spreading misinformation and bad science, which aims to prosecute and vilify cats and their owners. Such dangerous claims would inadvertently promote abuse against cats as well.
As cat lovers we are also animal lovers and we are willing to assist with nature conservation efforts and TNR programmes. Thus, I will end off this article with a list of programmes and charities who support TNR; should anyone be interested in donating or volunteering.
Charities, Organisations and Programmes
United States of America
PLEASE FEEL FREE TO SHARE ANY TNR OR OTHER ANIMAL RESCUE LINKS IN THE COMMENTS BELOW!
Science Journal References
- Doherty et al. (2016) Invasive predators and global biodiversity loss. PNAS. 113 (40): 11261–11265.
- Burton and Doblar (2004) Morbidity and mortality of urban wildlife in the midwestern United States. Proceedings 4th International Urban Wildlife Symposium. 171 – 181.
- Erikson, Johnson and Young (2005) A Summary and Comparison of Bird Mortality from Anthropogenic Causes with an Emphasis on Collisions. USDA Forest Service Gen. Tech. Rep. 1029 – 1042.
- Santos et. al. (2016) Carcass Persistence and Detectability: Reducing the Uncertainty Surrounding Wildlife-Vehicle Collision Surveys. PLoS ONE 11(11): e0165608. doi:10.1371/journal.pone.0165608
- Kauhalai, Talvittie and Vuorisalo (2015) Free-ranging house cats in urban and rural areas in the north: useful rodent killers or harmful bird predators? Folia Zool. – 64 (1): 45–55
- Krauze-Gryz, Żmihorski and Gryz (2016) Annual variation in prey composition of domestic cats in rural and urban environment. Urban. Ecosyst. DOI 10.1007/s11252-016-0634-1
- Barker et. al. (2008) Cats about town: is predation by free‐ranging pet cats Felis catus likely to affect urban bird populations? Ibis. 150 (Suppl. 1), 86–99
- Kikillus et. al. (2017) Research challenges and conservation implications for urban cat management in New Zealand. Pacific Conservation Biology, 2017, 23, 15–24 http://dx.doi.org/10.1071/PC16022
- Gordon, Matthaei and Heezik (2010) Belled collars reduce catch of domestic cats in New Zealand by half. Wildlife Research 37(5) 372-378 https://doi.org/10.1071/WR09127
- C.R. Veitch, M.N. Clout, A.R. Martin, J.C. Russell and C.J. West (eds.) (2019). Island invasives: scaling up to meet the challenge, pp. 1 – 752. Occasional Paper SSC no. 62. Gland, Switzerland: IUCN.
- Hughes, Dickey and Reynolds (2019) Predation pressures on sooty terns by cats, rats and common mynas on Ascension Island in the South Atlantic. IN: C.R. Veitch, M.N. Clout, A.R. Martin, J.C. Russell and C.J. West (eds.) (2019). Island invasives: scaling up to meet the challenge, pp. 1 – 752. Occasional Paper SSC no. 62. Gland, Switzerland: IUCN.
- Bergstrom et. al. (2009) Indirect effects of invasive species removal devastate World Heritage Island. Journal of Applied Ecology. 46: 73–81.
- Ortiz-Alcaraz et. al. (2019) Ecological restoration of Socorro Island, Revillagigedo Archipelago, Mexico: the eradication of feral sheep and cats. IN: C.R. Veitch, M.N. Clout, A.R. Martin, J.C. Russell and C.J. West (eds.) (2019). Island invasives: scaling up to meet the challenge, pp. 1 – 752. Occasional Paper SSC no. 62. Gland, Switzerland: IUCN.
- Dorethy and Richie (2016) Stop Jumping the Gun: A Call for Evidence‐Based Invasive Predator Management. Conservation Letters, January/February 2017, 10(1), 15–22