Kangaroo Island, 2013.09.30
Many island administrations have instituted systematic killing of cats, under the mistaken belief that this would somewhat help the “good” native animals. Introduced animals (except those of the Homo sapiens species…) being “bad”.
Three commentaries explain why this naive and prejudiced approach is misguided, both ethically and intellectually:
Macquarie Island vegetation devastated because of extermination of its cats, Bergstrom et al. 2009
Domestic cats – wildlife enemy number one or convenient scapegoats ? – Hartwell 2004
In addition to its ethical and intellectual shortfalls, the cat extermination policy is also misguided from a practical point of view.
As a rule, on the planet, areas devoid of cats are teeming with murines (rats and mice), which can reach anywhere (including up trees, but also inside narrow burrows, which are out of reach for cats) and do devour anything.
In Australia, the main diet of feral cats are rabbits and/or murines. On Kangaroo Island, there are no rabbits, but there are murines, including rats.
The two articles mentioned thereunder demonstrate that the island I’m writing from, were it not for its feral cats, could not any more be called Kangaroo Island, but rather should be called Rat Island. Another one…
Rats are mainly nocturnal. Because of the local absence of foxes and the rarity of barn owls (the only other nocturnal bird of prey on the island, the boobook Ninox, is no match for rats), without hunting feral cats at night time, the island would be just another one teeming with rats. As a consequences, it would be largely impoverished in bird and marsupial species.
The case of the invasive black rats on two islands in the Mozambique Channel is most revealing. On Juan de Nova Island, where there are quite a few other introduced species and a relatively complex ecology, the rats typically weigh around 125 g. On nearby Europa Island, which is comparatively pristine and lacks cats, they weigh… 165 g ! Say Russell et al., authors of the study “The island syndrome and population dynamics of introduced rats“: “On Juan de Nova, intrinsic factors such as predation by cats, and to a lesser extent resource competition with mice, are likely to have created selective pressure limiting the body-size of rats”.
Apart from the wildlife aspect of the story, there’s also the domestic one. No farming or housing area can do well, or simply survive, without cats, feral and domestic. Try simply to keep a netted orchard on the island… Mission impossible with the clever rats.
But cats do not only keep mice and rats numbers down. Thanks to their agility, much superior to that of dogs and comparable to that of mongooses, cats also protect a house from snakes, including the poisonous ones. The copperheads and the black tiger-snakes being rather timid animals, they do not hang around a home when they understand there’s a domestic cat on patrol.
So there are reasons to feel grateful for the presence of a cats in both the domestic and natural environment.
From an ecological point of view, there is yet another reason why anti-cats policies are generally misguided, an overwhelming one.
In a smaller ecosystem, omnivorous predators have much more potential for irreversible imbalance than the obligatory carnivorous ones. The reason being that they are much less subject to the population dynamics of the Lotka-Volterra prey-predator model (two oscillating population-curves advancing out of sync but at the same rythm): the omnivorous predators can easily switch to a vegetarian mode, thus avoiding their own population crash which would otherwise have followed their prey’s population crash. In such a situation, the prey population has scant chance to rebuild its numbers, being confronted to still numerous predators while itself having become rarified.
That’s why, ecologically, omnivorous rats and feral pigs are much more hazardous to a smaller ecosystem than any obligatory carnivores such as feral cats, with the partially omnivorous dogs standing in between.
In addition to being largely misguided from an ecological point of view, the cat extermination policy is also misguided from a bioethical point of view. The felids in the wild are the quickest and most efficient killers of preys, with none of the gore typical of pigs, rats, dogs and raptors. Cats are stalking animals, jumping on an unsupecting prey which has no time to get into the terror of being chased. And no adult cat in the wild has time to play with its prey: it dispatches it quickly, with a single powerful bite on the neck; contrary to domestic cats, they simply cannot allow themselves to behave like kittens. For an objective nature watcher and lover, the case is thus clear: no other animals on the planet get close to being such “pain-free” predators as the Felinae.
Adding up to the ecological and bioethical arguments to their favour, last but not least there’s the esthetical aspect: the felines are among the most magnificent animals to be found on this planet. For lovers of style, attitude and beauty, they are the gems of mammalian evolution. Felines have everything for them, but against the acute prejudice of too many humans, the gods themselves contend in vain.
Dr Gabriel Bittar, Kangaroo Island
The island syndrome and population dynamics of introduced rats
Russell at al.
http://link.springer.com/article/10.1007/s00442-011-2031-z
Oecologia November 2011, Volume 167, Issue 3, pp 667-676
Abstract
The island syndrome predicts directional changes in the morphology and demography of insular vertebrates, due to changes in trophic complexity and migration rates caused by island size and isolation. However, the high rate of human-mediated species introductions to some islands also increases trophic complexity, and this will reduce the perceived insularity on any such island. We test four hypotheses on the role of increased trophic complexity on the island syndrome, using introduced black rats (Rattus rattus) on two isolated coral atolls in the Mozambique Channel. Europa Island has remained relatively pristine and insular, with few species introductions, whereas Juan de Nova Island has had many species introductions, including predators and competitors of rats, anthropogenically increasing its trophic complexity. In the most insular environments, the island syndrome is expected to generate increases in body size and densities of rodents but decreases in the rates of reproduction and population cycling. Morphology and reproduction were compared using linear regression and canonical discriminant analysis, while density and population cycling were compared using spatially explicit capture–recapture analysis. Results were compared to other insular black rat populations in the Mozambique Channel and were consistent with predictions from the island syndrome. The manifestation of an island syndrome in rodents depends upon the trophic composition of a community, and may not relate to island size alone when many species additions, such as invasions, have occurred. The differing patterns of rodent population dynamics on each island provide information for future rodent eradication operations.
Near-Complete Extinction of Native Small Mammal Fauna 25 Years After Forest Fragmentation
Gibson et al.
Science 27 September 2013: Vol. 341 no. 6153 pp. 1508-1510 DOI: 10.1126/science.1240495
Abstract:
Tropical forests continue to be felled and fragmented around the world. A key question is how rapidly species disappear from forest fragments and how quickly humans must restore forest connectivity to minimize extinctions. We surveyed small mammals on forest islands in Chiew Larn Reservoir in Thailand 5 to 7 and 25 to 26 years after isolation and observed the near-total loss of native small mammals within 5 years from <10-hectare (ha) fragments and within 25 years from 10- to 56-ha fragments. Based on our results, we developed an island biogeographic model and estimated mean extinction half-life (50% of resident species disappearing) to be 13.9 years. These catastrophic extinctions were probably partly driven by an invasive rat species; such biotic invasions are becoming increasingly common in human-modified landscapes. Our results are thus particularly relevant to other fragmented forest landscapes and suggest that small fragments are potentially even more vulnerable to biodiversity loss than previously thought.
******
Addendum 2015.02.02
A study by Doherty et al. 2015, “A continental-scale analysis of feral cat diet in Australia”, published in Journal of Biogeography, 2015.02.02, DOI: 10.1111/jbi.12469, demonstrates that wherever there are rabbits in Australia, these are the main prey of feral cats, and that any local extermination of rabbits has the consequence of forcing local cats to switch to marsupials and reptiles.
Another reminder that ecosystems have their logic of checks and balance, that it is generally imprudent to tinker with them, and that hell is paved with good intentions.
There are no rabbits on Kangaroo Island, but there are many murines (rats and mice), which are the main prey of the island feral cats. The eradication of murines is not envisageable from a practical point of view, but there are steps being taken for another extermination: to exterminate… the main predators of murines, the feral cats ! Considering that rats are well established about everywhere on the island, and that they have a very high rate of reproduction, it’s obvious that they would gain immediately from any extermination of the island’s feral cats. And considering that rats are predatory, the local birds and marsupials would find themselves worse off on the whole…
Thank you for the sanity.
This morning 24th December, we had a person called ‘Richard’ from Kangaroo Island being interviewed by Narelle Graham.(ABC Local Radio 891). He was advocating the extermination of all ‘feral cats’ on the Island. In the course of the interview he admitted that this so called ‘feral cat’ problem was mainly centered around farms and farm buildings. He also went on to claim that cats on Kangaroo island were the main vector for two diseases that affect lambs. (sorry I am short on the specifics). He did not cite any scientific evidence for this claim.
So, I rang the radio station and voiced my objection, pointing out that cats both domestic and feral played a very significant part in keeping down rat (and mice) populations on Kangaroo island. I also pointed out that other feral species needed to be controlled and that the obsession with feral cats bordered on the irrational.
In reply, another caller, made the claim that….
‘only cats climb trees to eat birds and bird eggs’
and that astonishingly …..
‘there are no rats on Kangaroo Island’.
A later caller (female) also challenged this ‘Richard from Kangaroo Island’ : saying she hoped he had very deep pockets because the job of eliminating ‘feral cats’ would run into the millions of dollars.
I trust you post this : in support of your position.
Its not so much the moggy that is the problem : but the irrational cat haters.
A very interesting article, and I completely agree that it is the irrational cat haters that are causing problems on the island. But, that is not to say that there isn’t a feral cat issue on the island…there is.
Once must ask though, how did to island survive all these rats prior to the introduction of cats?
Hello Anne, thanks for the feedback. I’m not living anymore on the island, being back to Switzerland, but have kept my interest for it.
Some species of rats (murines) were introduced by the English settlers, who also brought in some domestic cats to keep the murine population in check around their settlements. These cats in turn have themselves settled to an independent life close to human settlements. So that answers partially your question.
Other murine species predated these settlements, some of them obviously getting in with the aboriginal peopling of Australia, tens of thousands of years ago, at a time predating the 12000 old global deglaciation which created Kangaroo Island through increased sea level.
In the latter case, it is difficult to figure what role exactly felines played in the Australian ecosystem prior to English settlement, or in the Kangaroo Island one, because of a dire dearth of studies on the matter. For a rather obscure psychological reason, the aboriginal sayings of a feline population at least as old as the canine one (the dingoes) are treated as unserious and not worth of investigating. So the genetic pool of wild felines has not been investigated with this line of reasoning in mind.
Too much prejudice and hatred on this matter, too many just-so stories, not enough open mind and solid data.
As there being any feral animal issue, I beg to differ. Personally, through 16 years of observations, I haven’t seen any feral or non-feral issue with any animal on the island… As I saw it, every animal was part of an evolving ecosystem, and trying to make a living. Including the human species… My problem was and is with some people getting into extermination schemes (and some pretty nasty ones at that) just because they think or believe the ecosystem should be so and so rather than so and so. And every one seemed to have a deep-seated opinion there: some hated corellas, other deers, or seals, or cats, or goats, or wallabies, or kangaroos, or rats, or mice, or possums, or snakes, or ferrets, or wattle birds, you name it. In my opinion, there’s on this matter too much bigotry and lust for control, not enough philosophy of live and let live.
I have two cats, and, a dead rat was brought to me last night. I rest my case. London once had a policy of no cats. This was prior to the Great Fire in 1666. rats everywhere and people dropping dead from bubonic plague.