Wednesday, 28 February 2018

What do moggies munch?

In a recent debate on a Facebook page for our community I made a comment about feral cats massacring native wildlife.  This led to some criticism and a demand that I produce scientific evidence.  Here we go.

A starting point is a fact sheet put out by the Australian Government Department responsible for the Environment and many other things:  This says
“They generally eat small mammals, but also catch birds, reptiles, amphibians, fish and insects - taking prey up to the size of a brush-tail possum. In pastoral regions, they feed largely on young rabbits, but in other areas feral cats prey mainly on native animals. “  
It isn’t clear where is meant by pastoral regions: the choices could vary from
  1. Areas  not climatically suitable for regular cropping; or
  2. Areas where grazing is a major industry.
They continue:
” On the mainland, they are identified as a threat to 35 species of birds, 36 mammals, 7 reptiles and 3 amphibians. Cats have probably contributed to the extinction of many small to medium-sized mammals and ground-nesting birds in the arid zone, and seriously affected bilby, mala and numbat populations. In some instances, feral cats have directly threatened the success of recovery programs for endangered species.”
Overseas the Smithsonian - a very eminent organisation - has reported  that
“... between 1.4 to 3.7 billion birds lose their lives to cats each year in the United States. Around 33 percent of the birds killed are non-native species (read: unwelcome). Even more startlingly, between 6.9 to 20.7 billion small mammals succumb to the predators.” 
 An article on the ABC website includes the views of a range of Scientists, advocates and wildlife managers.  It outlines the paucity of definitive information but indicates that the cats are a serious problem for a wide range of wildlife.

This  report from The Conversation  is one example of the most widely reproduced recent writing on the topic.  It says
“Feral cats are estimated to eat tens of millions of native animals each night in Australia. But what kinds of wildlife are they eating? In research published today in the Journal of Biogeography, my colleagues and I show that cats kill hundreds of different kinds of animals, including at least 16 species considered globally threatened.”
The research linked in that article is work by a number of scientists from Australian Universities and State Government agencies.  They have accessed a number of data sets covering research into feral cat diet.  It says the feral cat is
“... feeding mainly on rabbits when they are available, but switching to other food groups when they are not. “  
They go on to hypothesise that rabbit control strategies may lead to the cats switching to other prey.  The link between prey switching  and rabbit control is addressed further in this paper by a researcher from Biosecurity SA   That paper includes the following (emphasis added ):
“In fact, Catling (1988) reported similar dietary changes in successive years, only one of which was drought, and described it as an annual prey cycle associated with the breeding cycle of rabbits. That study makes it abundantly clear that wherever rabbits remain a dietary staple of feral cats, alternative native prey will be repeatedly exposed to seasonal episodes of prey-switching, often when simultaneously exposed to additional stressors of summer drought.
 The abstract of the paper by Catling (a scientist from the CSIRO Division of Wildlife and Ecology)  cited above includes the following:
“After the rabbit breeding season, diet changed to other prey and resulted in an annual prey cycle which was similar for foxes and cats.”
“Invertebrates, birds, reptiles and small mammals were supplementary prey for cats with carrion opportunely eaten.” 
I did not find any articles which said that feral cats are not a risk to native wildlife.

As my original comment was in an exchange which started with a discussion of a particular wandering feline (and the exchange then seemed to include a second mooching moggie) I was also challenged to prove that the beast(s) in question were involved in massacres.  As I don't know the beast(s) in question that is not possible but the article by the Smithsonian  contrasts owned and unowned (ie feral and stray) cats.  A major conclusion is (emphasis added):
“Although our results suggest that owned cats have relatively less impact than un-owned cats, owned cats still cause substantial wildlife mortality; simple solutions to reduce mortality caused by pets, such as limiting or preventing outdoor access, should be pursued.”
It is thus very good that the owner of the second cat above said that she was going to be confining her cat in the future.

The Smithsonian article is reporting in a lay-friendly form published research by two researchers from the Smithsonian and a third from the US Fish and Wildlife Service.  A telling line in the abstract of their paper is:
"Our findings suggest that free-ranging cats cause substantially greater wildlife mortality than previously thought and are likely the single greatest source of anthropogenic mortality for US birds and mammals."
It is possible that some other anthropogenic processes (eg habitat clearing) play a relatively greater role in Australia but clearly cats are seen by Australian scientists as major threats to wildlife across both countries.

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