Feral cats are often portrayed as the scourge of of island ecosystems, killing off or pushing out endemic species at an alarming rate. To an extent such a reputation is deserved, but a new study out in the Proceedings of the National Academy of Sciences reveals that the elimination of a top-level introducted predator can lead to an explosion in the numbers of other predators that were being controlled by the "top cat."
While cats are certainly a problem in many areas where they were introduced, often preying upon adults and chicks in the nest, rats attack the birds from the other side of the reproductive cycle and consume eggs in the nests. Many bird species suffer pressure from both ends, then, but the elimination of cats might actually make things worse for certain species. This is due to something known as "mesopredator release"; when a top-level introduced predator is killed, the populations of a lower-level introduced predator may explode. Such a trend is outlined in the study by Rayner et al., the researchers reviewing 35 years-worth of data from Little Barrier Island off the coast of New Zealand, Cook's petrel (Pterodroma cookii) being the focus of the study. Being that the bird uses burrows and crevices to nest in, its eggs and young can easily been accessed by predators.
The trends observed are very clear; the removal of cats from the island caused an explosion in the number of rats, which caused a steep decline in the breeding success of the birds. When the rats were wiped out in 2004, however, the birds were free from the two major predation factors and experienced a breeding success higher than what was seen when cats and rats were present together. There was one low-altitude site that did not fit the data, however, the birds showing little change in breeding success between when the rats were present and when they were eradicated. What this might reflect is that the rats on the island preferred different foods at warmer low altitudes where invertebrate prey and plant food were more abundant, high altitude rats turning to egg consumption due to a lack of other preferred food sources. Indeed, the rats did not wipe out the birds everywhere but instead hit nests in certain habitats harder than others, animals that breed across temperature gradients and in varying habitats perhaps having more resistance to extinction due to an explosion in a predator population than if all breeding took place at one site or only in one type of habitat. The differences between a low altitude and high altitude nest site might be slight, but apparently they're enough to make a difference.
References;
Rayner, M.J.; Hauber, M.E.; Imber, M.J.; Stamp, R.K.; Clout, M.N. (2007) "Spatial heterogeneity of mesopredator release within an oceanic island system." Proceedings of the National Academy of Sciences, Vol. 104 (52) pp. 20862-20865
- Log in to post comments
I'd accept your title if amended to "Feral cats aren't all that bad on small islands that also have rats."
Please, don't not encourage the wacky feral-cat fanatics here.
Sven -
And the anti-feral-cat advocates are all rational and not the least bit wacky, I suppose? Stories about how bad feral / outdoor cats are (to birds, sea otters, etc., etc.) are always accompanied with all the appropriate caveats? There are never any dubious extrapolations from single cursory studies in specific habitats?
Before you jump down my throat, let me add that I am neither pro- nor anti- feral cats - just an urban ecologist interested in understanding their role objectively. There seems to be far too much emotion when it comes to cats on both sides, based often on very little reliable data. I am therefore glad to see this paper and thank Brian for bringing attention to it. I'll probably blog about it myself soon.
I am reminded at this point about some of the discussions I've had with folks who work with cat welfare groups. It's a very sticky issue, especially since although I love animals, there are times when I do advocate extermination of cats in areas where they are not native and actively threaten the survival of other species.
Also reminds me of that recent case where a bird lover shot a feral cat that had been stalking threatened shorebirds.
As you said, Hai, this is definitely a sticky issue. To tell you the truth I've heard more about exterminating feral cats than keeping them around. This post isn't about the ethics of such things though, just that in this case feral cats kept down the population of another predator and that introduced predators can sometimes be beneficial. Like nearly anything in ecology managing species in a place where introduced animals may threaten indigenous ones is complex, but I think what this shows is that we can't simply show up, kill all the cats, and expect everything to be ok. I just wonder what would have happened if all the rats were exterminated first...
This isn't exactly new science. The elimination of a top predator will OF COURSE lead to the proliferation of the animals it preyed upon, which will in turn effect that prey animal. When reindeer populations up here are high, so are the wolf populations. When the reindeer eat their habitat into the ground (happens every few years) and the population suffers as a result (infant mortality more than anything else), the wolf population declines.
I'm all for getting rid of introduced species when that species negatively effects its new environment. So often our human emotions cloud our duty to protect the natural balance. We live with cats, and we've developed a connection with them. But the world is bigger than that. We have a responsibility.
/rant
How often do introduced species benefit the existing ecosystem?
The only good cat is inside, or dead.
If there are other introductions, deal with controls on them individually.
Zach; The results of the study are intuitive, but as the researchers state in the paper not much research has been carried out on this sort of occurrence. "Mesopredator release" is something that makes sense, like you note, but it is something that needs more study nonetheless.
Caledonian; An introduced species on its own, like cats, foxes, phragmites, zebra mussels, etc. can do a lot of harm, but once they're established in an ecosystem trying to eliminate them altogether might have some unexpected effects. Ecologies don't wait around for us to take action and clean things up; like this study shows, introduced species may provide some benefit to endemic species (even if that benefit is slight).
Wildlifer; Feral cats are problematic and controversial, but I disagree with the statement "the only good cat is inside, or dead." Introduction of cats to islands or new areas should be avoided, but before eradication programs are put in place their role in an ecosystem should be monitored to see if their removal is going to cause a mesopredator release as was seen in this study. Once introduced species become established (especially if they're a predatory species that preys upon other introduced species as well as endemics) we can't expect to just wipe them all out an expect everything to be fine. As I feel this study demonstrates the cats provided a benefit to the bird population until such time as both the rats and cats could both be removed from the island.
"I'm all for getting rid of introduced species when that species negatively effects its new environment.... We have a responsibility."
Probably the introduced species that has most negatively affected its environment is us. Even the cats and rats wouldn't be there if not for us.
Interesting study...
Speaking from a perspective of a minor birder in New Zealand I'm very much for the destruction of the WILD populations of not just cats and rats, but also weasels and possums from New Zealand as all of which were introduced at various points and caused unbelievable damage to the local bird populations.
I wonder if these findings would hold out on mainland New Zealand. Especially the different environmental ranges of predation.
The biggest bird predator and disruptor here on the mainland though are the weasels introduced to kill rabbits. I don't think cats or rats really hunt them much. Efforts to wipe them out from here are probably impossible with a capital IM.
I trap and remove cats, opossum, raccons, red and grey fox, mink from T&E avian breeding habitat on a group of islands. (Nutria as well)
The islands still have fairly healthy snake populations for any introduced rodent problems.
Well said! This is a key observation, but one that is often not understood / appreciated enough by conservationists. Yes, moving species into new ecosystems can be bad, often horrendously so (as seen in many island faunal collapses), and many introduced species need to be removed or substantially controlled. Nevertheless, amid all the emotion about cats and other introduced species, we all too often forget the inherently dynamic nature of ecological communities. When any new species comes into an ecosystem, other species that it interacts with will show some response: both over ecological and evolutionary time scales. The populations of native species may change drastically (go extinct; or perhaps less often, even increase), pushing the ecosystem as a whole either towards a new equilibrium or a new domain. The outcome of any removal exercise then will depend upon where in the course of that trajectory one intervenes; and in some cases, as illustrated by this paper, selective removal can have unintended negative effects on native species.
Just as the cat-fanciers need to do everything they can (keep cats indoors, feed them well, spay them, etc.) to minimize their impacts, the cat-haters need to take a breath and evaluate things a bit more carefully and objectively before advocating removal in every case. What's wrong with a more nuanced case-by-case approach?
First let me clarify I'm no "cat-hater" operating from some emotional bias. I've had indoor cats and have friends who have indoor cats that I often play with.
Cats do not belong outdoors on an island or the (US)continent anymore than Bengal Tigers do. And the only reason "cat-fanciers" object to the removsl of feral cats is because they're not eating their children as the tigers would, even though they're decimating native avian, mammalian and herp species. They have a perverted idea cats are "natural" and hence part of nature. Cats are as natural (and as destructive) as an atomic bomb.
The take-away lesson from the study is to include other control efforts in conjuction with the removal of feral cats.
I love it. Suggest that feral cats need to be controlled and you're an emotionally driven cat hater.
I love cats, as pets. I have 2; they stay indoors. But I agree with wildlifer-cats are better dead than feral. "Mesopredator release" is highly unlikely to be an ecological problem except in very specific cases like the island discussed above. Feral cats mostly eat small mammals, birds, and lizards.
Far from "emotional" motivation, the conservation science is clear on this issue. This is ten years old, but still the best introduction to the issues.
Part of the problem, I think, is that we're talking in very general terms. What the study suggests is that the feral cats on the island provided an ecological benefit until solutions could be found to remove both the cats and the rats. Contra to what wildlifer said, I don't believe that snakes would have been able to control the rat population on the island or in any other place where cats consume introduced rats.
I'm not advocating that we just leave the cats alone or that introduced species that prey on endemic species are a good thing, but rather than many ecologies have suffered introduced predators and we should make sure that the removal of a top introduced predator is not going to cause a rise in population of other introduced species that might do more harm to the endemics. Emotions can run high on both sides, but I think this particular paper opens up an interesting (and important) new aspect of study for conservation and management that has not been extensively studied or entertained (I simply can't accept that it's always better for cats to be dead than feral; better in what context?).
Simply put, this post has nothing to do with advocating that feral cats be killed or that feral cats be left alone; just that once they're introduced into an ecosystem things change and we should be mindful of those changes rather than assuming that we can just kill all the cats and the problem will just go away.
Feral cats used to run around our farm before we purchased it. Nary a wild bird could be seen. Not even hawks bothered to hunt over our land.
We removed the cats and our wild bird population soared (literally and figuratively).
The foxes, coyotes, and hawks take better care of the rodent population than any feral cat or cats and they are native to this area.
Whenever anyone dumps a cat or we find feral cats, we live trap them and take them to the humane society. We let them figure out if they are adoptable or not.
Sven, it is interesting that even Coleman, Temple & Craven described domestic cats, in their own words, as "similar to their wild relatives, and many of their behaviors, such as hunting and other activity patterns, remain ESSENTIALLY UNCHANGED from their ancestral." You are providing those wacky feral cat fanatics more ammunitions. Read this feral fanatic, Nathan Winograd, and his argument on
"Do Feral Cats Have a Right to Live?"
http://www.abolitionist-online.com/article-issue04_feral.cat.rights-nat…
I say let changed the status of feral cats to wild animals. They will lose their domestic animal protection if they are treated equally as wildlife animals. Then, we'll have open season for hunting.
But the point is that they do not belong in North America! The native wildlife--my priority, much higher than the individual lives of kitty cats--has not evolved in the presence of small-cat predators. If I thought that feral cats were only taking house sparrows, starlings, and Mus, I wouldn't care.
Exactly. They don't belong anywhere except Africa. What bothers me is why continue this sickening love affair with cats. Domestic, purebred, stray/feral cats are all the same to me. They're all ruthless predators that need to be eradicated. As for indoor cats, how can we be sure that they will never be let out? Anything happened to its owner, the cat gets out or gets adopted by someone who will let it roam and kill more native wildlife. Audobon's Cats Indoor campaign is just bullshit. If we truly love our native wildlife, we should get rid of all cats. Take a lesson from Aussie people. Many of them will tell you that a good cat is a dead cat. Cat ownership there is rapidly declining.
As I've already said I think most of the comments here are outside the scope of the paper being discussed. I'm in the process of reading through the literature on feral cats on their impacts and I'm not suggesting that we discontinue controlling them and extirpating them when necessary, but I think the main points I've tried to address about changing ecology have been ignored.
It doesn't matter if cats don't "belong" in North America or not; they are here and established, and no matter how hard we try there are always going to be feral cats and people who let their domestic cats outside. This isn't a matter of valuing individual cats lives over the value of biodiversity or a species, but rather about understanding the impacts of an introduced predator that has become established and the impact said predator has on ecologies. Surely there are other dangers to native birds (because that's what most of this debate centers around, it seems) like cowbirds and disease other than cats, and while I'm not saying "We should all just be nice to cats," I think it's important to understand specifics rather than just saying "All cats are bad and should be killed."
As much as it might sicken Lukestra, I love my cats and they will remain indoors. Killing all cats or getting everyone to give up their cats is not going to happen, and removing all cats from North America suffers from the same problem as the idea of bringing elephants and lions here to replace the lost Pleistocene fauna; we have already modified the ecology and we need to know what's going on to best manage it, not kill some creatures, import others, and then expect everything to go back to the way it used to be.
Are cats a problem to native wildlife? They surely can be, but I don't feel that simply exterminating all cats is going to solve problems with native wildlife. What if, as in the study mentioned in the post, the removal of cats actually makes things worse? While it may be counterintuitive given the controversial status of cats I don't think anyone has really looked into such a possibility. Again, I'm not trying to be an apologist for feral cats; I'm only saying that the new study opens up some new possibilities for study that we should try in learn from when trying to help native species recover from predation by introduced predators.
An interesting study would be to take a similar island with similar ecology, and exterminate the rats first. Might the cats then concentrate their predatory efforts on the birds, and become purely a menace?
If this single study suggests anything, it sort of implies that perhaps we should focus our extermination strategies on the cats and the rats equally, rather than concentrating on first one and then the other.