CATS and RATS: Back of the Envelope Calculations

by John Ogden

In going to into battle it is always good to know the size of the opposing army. The data are usually difficult to get, but some estimate is required. The tables below are very much ‘first approximations’ and will need much improvement if the Island is to tackle the predation problem in the future.

Table 1 documents that 1151 feral cats (958+193) were destroyed by conservation efforts in two areas of Great Barrier over a ten year period. (At least 300 feral cats were shot by Alan Gray at Awana during the same interval, and others were trapped/shot elsewhere). Using mean annual cat kills in the two locations, converting them to cats killed per hectare, and multiplying by the total coverage of the vegetation type on the whole of Great Barrier gives an estimate of 854 (641 minimum to 1067 maximum) for the whole-island feral cat population. The true figure may be closer to the upper limit, as minimizing assumptions have been made in the calculations.

Table 2 shows some very approximate estimates of the total rat population. These estimates are made with inadequate data and do not allow for large fluctuations throughout the annual cycle. At Windy Hill, rat trapping in forest and scrub over 4 years (2000-2004) on c. 450ha caught 11,548 rodents, equivalent to 6.4 rodents/ha/year. Similar calculations from trapping in mostly scrub and grassland at Awana gave 31 rodents/ha/year. Both these figures must underestimate the numbers actually present as they are based only on rats actually caught. However, based on these figures and multiplying by the areas of similar vegetation types on the Island, as a first approximation it appears that there are at least a quarter of a million rats on the Island. To judge from annual fluctuations at Awana and Windy Hill, this could rise to a million during the autumn peak.

What are a thousand feral cats and a quarter of a million rats eating every day? The annual totals are staggering! Based on the carefully researched figures and the minimizing assumptions of John Innes and others, in 2012 I concluded that on Great Barrier at least 86,500 birds are lost to predation by rats and feral cats every year. (see: Environmental News 28, 2012, based on: Innes, J.,Kelly, D.., Overton, J. M. & Gillies, C. 2010. “Predation and other factors currently limiting New Zealand forest birds”. New Zealand Journal of Ecology 34(1):86-114. View at: www.newzealandecology.org.nz/nzje/ ). That huge figure for mortality is based on the average number of nests per hectare of forest and scrub (4 nests with average 2 eggs each = 8 potentially new birds per ha per annum), the area of forest /scrub (24,294 ha); the percentage failure rate (73%) and the percentage of those failures due to predation, mainly by rats (61%).

Thus: ((((24294 x 4) x 0.73) x 2) x 0.61) = 86,545

As I said in 2012 the actual figure means diddley-squat, but even if the true figure is only 10% of that estimate, there would still be nearly nine thousand birds lost to predation every year. This is why so many of our bird species are facing extinction, and also why bird populations seem to recover so dramatically when predators are eliminated by toxin drops.

Another way of thinking of the figure is to say that if my estimate of rat numbers (251,000) is correct, then on average one rat needs to eat less than one egg (0.34) per year to eliminate eighty-six thousand potential new birds on Great Barrier every year. Again, even if my estimate of rat numbers is way off – say 90% too big – each rat only has to destroy three birds (eggs or chicks) per year to have that effect. Thus, even with very minimum estimated numbers for rats, the figures are staggering, they explain what is happening, and they are surely cause for concern.

Cats pose another problem because individuals have a tendency to ‘specialise’ in rats, or lizards, or rabbits, or birds. The bird-specialist-cats can be particularly devastating for ground nesting birds, such as the black petrels on Hirakimata. Some data on diets from cats at Okiwi showed that mammals (rabbits, rats) were their main diet (rather than brown teal). The data on Great Barrier domestic cats, collected in the Petscan Survey in 2000, can be used to give an idea of cat diets on the Island.

Table 3 is based on an assumption that domestic and feral cats have similar prey. This is unlikely to be correct, but it provides a first approximation to the slaughter caused by feral cats. Note in particular that the estimate of birds killed by feral cats (1049 + 747 = 1796 per annum) is probably a large underestimate, as feral cats are presumably better at this form of hunting than domestic cats. Moreover, as noted, specialization on particular ‘easy prey’ species can have devastating effects.

Feral cats must catch all their own food, and feed their kittens. Even if their predation is the same as the domestics (and it is almost certainly greater), they would be killing over 1000 native birds/year, and even more reptiles. By the same calculations they would also account for possibly 19,652 rats , which seems like a lot, but is only 7 or 8% of the estimated total rat population.

Many studies show that the rat population must be consistently reduced to less than 30% tracking tunnel visits per night to protect native birds. (The Dept. of Conservation uses a figure of 5% tracking tunnel visits as the guideline for bird re-introductions). Without rat control tracking tunnel percentages at Windy Hill range from a low of 40% to a high of 100% (average c. 76%). A 7% reduction in the rat population by cats may be exerting some control on rat numbers, but will not get tracking-tunnel percentages down to a level at which birds benefit. In any case there is also direct predation on birds by cats. Simple observation and ecological theory indicate that carnivorous mammals rarely reduce their prey by more than 10%, so these figures are much as expected. Cats do eat a lot of rats, but nowhere near enough to benefit birds. Please don’t conclude that this means we need more cats!

Environmental News Issue 35 Summer 2016