I’m not a cat person; I’m not a dog person either; I’m not really a pet person at all. I prefer my animals wild and free, or cooked and on a plate. And so I enter this subject, the impacts of domesticated and feral cats on wildlife, with a slight preference for hearing that cats are a problem and that careless cat-owners are to blame, rather than hoping to read that cats have had no impacts on the natural world.
This book will be slightly uncomfortable reading for many cat-lovers – and so I recommend it very strongly to them, and pretty strongly to everyone else as it is a fascinating story and one that is well written to boot.
The book opens with the story of the extinction of the Stephens Island Wren (not a wren really, but, yes, it lived on Stephens Island, off New Zealand). The last of very few species of flightless passerines to have survived on Earth until a cat arrived on the island, it seems a pregnant cat, and then very soon there were lots of cats and no Stephens Island Wrens.
There are lots of such stories of avian extinctions and taken together they weigh heavily in the litany of evidence against introducing predators into places where they have never previously occurred. Rats and cats have caused lots of problems – of course, it was we who took them to these places and we who released them (no rat or cat has ever sought out predator-free islands and sought deliberately to eat its way through a flightless endemic mammal, bird, reptile or amphibian).
But how about continental systems where there are plenty of predators already, or maybe there used to be before we bumped them off, how do cats perform in those circumstances?
We all have anecdotes on this subject, don’t we? Both my next door neighbours changed last year (was it something I said or didn’t say?). This led to a very noticeable change in cat visits to my garden. On one side I now have an active dog and two active children yapping away in the garden which must make it a less attractive route for passing cats, and on the other side the former cats’ home is now a cat-free and child-rich zone. I see far fewer cats in my garden and my strong impression is that I now see more birds, and the nesting Blackbirds of this year were a first. Coincidence? Maybe, maybe not.
But we can do much better than anecdotes, and this book gathers together a large number of studies, many from North America and rather few from Europe or the UK. This is something of a relief actually, I find it easier to read about the science of feral and domestic cats in Wisconsin in a moderately dispassionate way than I do to read about the issues here at home. Clearly, although we all know that cats kill a lots of small vertebrates, and eat some of them, an awful lot of small vertebrates die every year anyway; if they didn’t then we would soon be knee-deep in frogs, lizards, voles and Great Tits and the world would be a very different place. So the search for the impact of cat predation on wildlife has to tackle the issue of whether the deaths at the paws of cats are ‘additive’ or ‘compensatory’ in the inadequate jargon of science (‘compensatory’ is a particularly inadequate phrase but it’s the one used the most). It’s not enough to say ‘cats kill lots of animals, therefore they must cause declines in animal populations’ because that doesn’t show that the mortality is extra mortality, nor does it, of itself, show that it is enough mortality to cause a population decline.
So ideally, we need lots of data and some clever modelling to assess the likelihood of measured losses of animals to cats being sufficient explanations for changes in animal abundance or more ideally we need experiments where we manipulate predation pressure by cats and see what happens. Introducing cats onto Stephens Island was like a badly designed and badly monitored experiment – but very convincing nonetheless. We know that the Stephens Island Wren was vulnerable to predation by being flightless, small and unevolved to cope with this new pressure; we know that the cats killed and ate the Wrens, and we know that, having survived there for thousands of years, the Stephens Island Wren disappeared from Stephens Island in a very few years after Tibbles the cat arrived. It’s not an experiment but it is convincing. Ideally we would have lots of islands where we introduced cats in different years and saw what happened but that would be rather unethical and practically difficult anyway since there is only one Stephens Island.
My observations in my back garden would be useful if they were more structured (like if I had some numbers to back up my impressions of cat abundance and bird abundance) and if they were replicated over many gardens, over longer time periods, and ideally (again) if cat numbers were experimentally increased after a period to see whether the impacts were reversed.
This book takes you through these areas and you are likely to come out the other side believing that cats can, and do, make a difference to the abundance of other species (their prey species) in a variety of places and situations; sometimes a big difference and sometimes a smaller difference, but often a difference. But if you are very keen on Tibbles then you may find room for quibbles.
We are then taken on a journey through potential routes to solve the problem of roaming cats and these range through keeping them indoors more, through fitting them with bells and bleepers to TNR (trap-neuter-release of feral cats) and on to killing the cuddly critters. All of these approaches are evaluated.
The book ends with a series of passages that could be summarised with the question ‘Why bother?’, which is always asked about restoration of a more natural and usually richer ecology. The authors answer it well. In fact, the authors do everything well in this book. It is a contentious subject which is dealt with very sensibly, and it involves some quite challenging science which is explained well. And it is a good read and not the least bit dry (nor sensational). We move from place to place and we are introduced to a variety of scientists and players in the story.
I enjoyed this book a lot, and rather more than I thought I might.
Cat Wars: the devastating consequences of a cuddly killer by Peter P. Marra and Chris Santella is published by Princeton University Press.
Remarkable Birds by Mark Avery is published by Thames and Hudson.