Aerial surveys 'underestimate' ferals
Aerial surveys could be seriously underestimating the number of destructive feral animals on the loose, say Australian researchers.
They recommend a change in the way these are normally counted to ensure better protection of nature reserves.
Fleming says counting what are often moving targets from a moving helicopter is hard enough to start with.
But to make matters worse, the most common way of counting wild animals ignores the way the brain works, he says.
For example, some surveyors count a group of five, followed by a group of ten, adding these and then remembering the result as you add the next group.
This process continues for 90 seconds before the person counting writes down what they remember.
But, he says, memory is fallible and so it's easy to make mistakes and the longer counting goes on, the more fatigued the counter gets get, the more mistakes are made.
"Sometimes you can remember animals that you don't actually see," says Fleming.
But generally, he says, the evidence is that there is a tendency to underestimate and this is a problem if you are try to stop damage from feral animals like goats and pigs.
Fleming says feral pigs can root up the ground, eat ground-nesting animal and endangered plants, and foul water.
If someone was conducting aerial surveying using the traditional counting method to assess how many feral pigs were in a sensitive area, their underestimate could have serious consequences.
"You mightn't actually be knocking out enough of that pest animal to actually stop their impact," he says.
Fleming says aerial surveyors could be more accurate if they made better use of the way the brain works.
Humans, and some high-level primates, inherently recognise instantaneously groups of 1 to 4 without counting, he says.
He says a method of counting, called subtising, uses this fact.
In subtising, people identify groups of 4 or less without counting, speak these into a recording device and add the recorded numbers up later, says Fleming.
He says this eliminates the error due to poor recollection, but it also takes advantage of something we do instinctively.
"That's what you should do because you're using your brain as it wants to work and you're also likely to be much more accurate," says Fleming.
Fleming set up an experiment to compare the accuracy of counting wild goats from the air using the traditional method of counting versus subtising.
He found that when a group of 160 goats were counted, the traditional method underestimated the number by 35.
When such errors accumulate over a day's work, there can be some serious underestimation, says Fleming.
He says underestimation is less of a problem when it comes to aerial surveying used to set quotas for kangaroo culling.
"Because it's an underestimate, that means they're being conservative and that's a good thing. They'll never kill too many kangaroos," says Fleming.