Ecosystems are complex. In order to understand the role of the domestic cat in respect of predation, it is important to recognise that the cat is one of a large group of predators some of which are native and some introduced (non-native species).
Other introduced species such as rats and mice can have an impact upon birds, mammals and amphibians. There are many factors which affect the impact of the domestic, stray and feral cat on prey species. Such factors include:
- The population density of the cats,
- the habitats of native species,
- the presence of other predators,
- the fecundity (fertility) of native species.
The relationship between different predators and a range of prey species is complicated. Therefore – and this is the point I wish to make – the removal of cats (and I’m talking primary about feral cats) may have a much more widespread effect than is immediately obvious to advocates who promote the elimination of feral cats as a solution to the predation of native species.
A mathematical model worked out in a study illustrated this point. The model included birds, rats and cats and it showed that by removing all cats there would be a surge in rat numbers which in turn would result in the extinction of bird species (the prey). The study was conducted in 1999 by Courchamp and others.
The same scientist and his team did a second study in 2000. The study examined the relationship between birds which were the prey, rabbits (an introduced prey species) and cats which were the predator. The study took place in an island setting. The rabbits provided food for the other predators and were the primary diet of the cats. This resulted in a larger population of cats than would normally be sustained if the rabbits weren’t there or were scarcer. When the rabbit populations were reduced the cats were able to change over to other prey species such as birds. For people engaged in speciesism (favouring one species over another) greater predation on birds over rabbits would be seen as a catastrophe.
A classic case of the interaction of species and the complexity of ecosystems was noted on Marion Islands, situated in the Indian Ocean off South Africa. The eradication of cats made no impact upon the population of the Lesser Sheathbill. The population of this bird was lower than on a neighbouring island, Prince Edward Island.
The differences in the population sizes of this bird species on these two islands was due to a complex interaction of species and I’ll quote the author MR Slater:
“These differences were believed to be due to a decrease in the birds’ macro-invertebrate prey (especially weevils and flightless moths), which may have been due to increases in house mice as a result of the cat eradication, decreases in borrowing petrels (which promote invertebrate species), and climate warming, which also increases mouse population.”
You can see, immediately, the complexity of ecosystems in that assessment by Mr Slater. It also illustrates that the removal of cats may not result in the recovery of a threatened species which is the prey of the cat.
I am sure that the Australian authorities are aware of this although, judging by their proclamations about their desire to exterminate feral cats, they don’t appear to.
Primary source: The Welfare of Cats pages 163-164 by MR Slater.