In any discussion of predator control, cats are the elephant in the room, but ignoring them may be the worst option.
In October 2019 we posted comment about hedgehogs being “The Forgotten Predator”. Now it is time to consider if feral cats are “The Ignored Predator”. That cats are missing from New Zealand’s Predator Free 2050 aspiration is an anomaly that needs to be addressed. Is it a lack of ecological data, a lack of know-how or a lack of political will that is behind this exception?
A lack of ecological data cannot be the reason behind this omission because in 2016 a paper authored by Australian and New Zealand based research ecologists was published in the Proceedings of the National Academy of Sciences that clearly showed the impact that feral cats have on the loss of global biodiversity. Of contemporary vertebrate extinctions, invasive predators were implicated in almost 60%, and feral cats contributed to 60 (42%) out of those 142 extinctions.
Therefore, feral cats were directly connected to a quarter of the recent extinctions of vertebrate species. These numbers are also probably underestimates as the authors found a further 23 critically endangered species that were listed as “possibly extinct”.
So if that is a synopsis of the past then what might be in the future? The authors went on to identify a further 596 species of birds, mammals and reptiles classed as vulnerable, endangered or critically endangered. Almost two thirds of this number were birds and over 60% of this grand total were imperiled by feral cats. The authors concluded in their popular article, published in “The Conversation”, that feral cats were, “arguably the most damaging invasive species for animal biodiversity worldwide.”
The highest number of endangered species threatened by invasive predators occur on islands and of the world’s key island groupings, the islands of Micronesia, Melanesia and Polynesia have the highest number of birds and reptiles that are vulnerable to extinction driven by invasive predators. New Zealand runs a close second for vulnerability to bird extinctions.
We should not be surprised to learn that feral cats are a primary causative factor in the extinction of vertebrates that are yet to occur. The authors “expect that the number of species affected by invasive predators will climb as more knowledge becomes available.”
So if feral cats as invasive predators are not overlooked as a result of a lack of ecological data then might it be a lack of know-how?
Hauturu / Little Barrier Island is a steep and rugged island on the northern boundary of Auckland’s Hauraki Gulf. Cats reached the island around 1870 and are blamed for the total extinction of the last population of the North Island snipe, the local extinction of North Island saddleback / tīeke and serious population declines of burrow nesting seabirds including grey‐faced petrel / ōi , Cook's petrel / tītī and black petrel / tāiko. Tuatara and other lizards were also significantly reduced.
Whereas cat control was carried out sporadically for 80 years from 1897, in 1977 the New Zealand Wildlife Service began a cat eradication programme using leg-hold traps, cage traps, toxins and dogs. Although it was only leg hold traps and toxins that proved effective, cats were eradicated over a three year period and a total of 151 individual cats were known to have been removed. When complete, the 2,817 hectare Hauturu / Little Barrier Island was the largest landmass from which cats had been removed. Cats have now been removed from more than 80 islands worldwide, the largest of which, Dirk Hartog Island in Western Australia, is 22 times the size of Hauturu / Little Barrier Island.
So with a clear understanding about the ecological impacts of feral cats as invasive mammalian predators, and effective and proven techniques to eradicate populations of feral cats from large areas, is it a lack of political will that will see feral cats remain throughout New Zealand as the ignored predator to continue to prey on our vulnerable native wildlife?
Pet cats as companion animals evoke strong emotions in people, however there are marked differences in the potential ecological impacts of different cat populations. There is no doubt that pet cats prey on wildlife, however many of those prey species are not native and most are not endangered. While this does not diminish the need to reduce pet cat impacts on wildlife, programmes to increase people’s awareness of the impacts their pet cat may be having on wildlife and greater regulation around cat ownership both have potential to reduce the wildlife impacts of companion cats.
Colony cats live in the grey zone between being domesticated and being wild. Often these cat populations are supported by humans and in many areas there are programmes to manage downwards the number of colony cats using desexing to reduce birth rates. New colonies of cats can be expected to be formed so responsibility for managing these cats must be explicit. Local authorities have mechanisms through their pest management responsibilities to achieve this and they should be mandated to achieve this.
With feral cats, ignoring their control and management has the potential to cause perverse consequences for biodiversity conservation. Should rat eradication be successful at the landscape level then the removal of these rodents will remove key prey species for feral cats and prey switching by feral cats can be expected to occur. This would result in greater cat predation pressure on birds, reptiles and large bodied invertebrates.
A devil’s advocate might argue that increased cat predation would never fully compensate for rodent predation. However, predation pressure on prey species would change and possibly different life stages, such as breeding adults, would be at greater risk compared with rodent predation.
Feral cats also place significant population regulation on rabbits so a significant reduction in feral cat numbers would be expected to result in higher rabbit numbers. As rabbits are major agricultural pests in many areas there would be significant resistance to anything that might lead to even higher numbers. The flip side of this is that feral cats harbour and can transmit toxoplasmosis to livestock and for sheep this pathogen can lead to abortion storms that can greatly reduce lambing percentages.
Therefore, might the lack of will to prosecute feral cat management actually be tied to silent agricultural interests rather than to vocal urban pet interests? If so, then might the development of truly effective rabbit management also be the key to unlock the conundrum of feral cat control to reduce the toll these apex predators are taking on our wildlife?
What's in a Name?
The familiar saying "A bird in the hand is worth two in the bush" warns about the risks that come with trying to achieve more by challenging the status quo.