Graeme Caughley

Graeme James Caughley (28 September 1937 – 16 February 1994) was a New Zealand population ecologist, conservation biologist, and researcher.

He combined empirical research with mathematical models, and supported the declining population paradigm.

Caughley was the middle of three children.

He was the only son born to John Norman Caughley and Thelma Caughley.

His father would take him on excursions while his mother encouraged his curiosity.

As a young child Caughley was very inquisitive and he recalled finding a seashell on top of a hill.

At the age of seven he determined that the sea must have once covered the hill and was proud to have resolved the problem.

This event encouraged him to learn more about New Zealand's geology and as Gunn and Walker explain "Ecology had a close call with Graeme Caughley. He almost chose geology at the start of his career..."

Caughley attended Victoria University College in Wellington, New Zealand from 1956 to 1959.

Tyndale-Biscoe writes that "there is no record of any particular lecturers influencing his thinking."

In his last two years Caughley dropped down to part-time and went to work with his friend Thane Riney at the New Zealand Forest Service on feral goat herds.

Caughley continued his education at the University of Sydney (1960–1963) with his advisors Charles Birch (insect population ecologist) and Harry Frith (ornithologist).

Frith was chief of the Australian Commonwealth Scientific and Industrial Research Organisation (CSIRO) and Caughley used the CSIRO sheep station to study the ecology of kangaroos.

Caughley found that kangaroo groups are formed by a random process of members coming and leaving, which was in contrast to what he had seen in red deer.

He interpreted this to mean that random grouping was density dependent with increased grouping at higher densities.

An approximation of density could then be found by taking the average number of individuals per group in a given area.

Continuing his research he found that red kangaroos are much more drought-tolerant than grey kangaroos, which visit water three times more often.

In addition he showed that grey kangaroos prefer areas of higher ground cover than do red kangaroos and that this may be a behavioural relic from when the Tasmanian wolf and Tasmanian devil overlapped in habitat with the kangaroo.

While working for the New Zealand Forest Service, Graeme used his research on the Himalayan tahr (Hemitragus jemlahicus) towards his doctoral work.

He decided to work on tahr because at that time everyone claimed to be an expert on red deer and he thought that he would make a greater contribution working on the lesser known tahr.

Caughley wanted to see if Thane Riney's eruption and stabilisation patterns in deer also applied to tahr.

He used his studies at the Forest Service for his PhD thesis at the University of Canterbury (1962–1967) advised primarily by Bernard Stonehouse (Antarctic and penguin ecologist).

Using three study populations of Himalayan tahr (judged to be in the initial increase, initial stabilisation, and decline stage) Caughley found that tahr follow the pattern that Riney had found in deer.

During 1966 Caughley presented methods by which to determine mortality patterns in mammals.

He looked at mortality rate curves (qx) among ungulates, rats, voles, sheep, and man he found that they followed a common pattern.

This "u" shaped pattern had a high juvenile mortality followed by a decrease and then a steady increase in mortality punctuated by a sharp increase with maturity.

With this mortality pattern it was shown that although age of mortality differs among species as well as cause of death (disease, lack of food, predation) the trend that mammal species follow is similar.

This is important in wildlife management since it shows that regardless of natural mortality factors populations tend to have high juvenile and mature deaths.

While working for the Forest Service and attending the University of Canterbury for his PhD, Caughley modified Lotka and Fisher's equations for birth rates in populations to ones that applied to seasonally breeding populations.

He wrote that most of the animal world had a season for breeding and that if births were treated as occurring at only one point in time then these equations could be used for seasonally breeding populations.

This modification now gave a more accurate estimate of the reproductive value of a population without overestimating births by assuming year around reproduction.

Caughley and Birch (1971) published "Rate of Increase" to point out some of the misuses with this equation.

Originally applied mainly to insects and humans they claim that the questions asked by entomologists are not necessarily those asked by mammalogists and vice versa.

It is this difference in questions that caused misuse of particular equations in the realm of vertebrate studies.

They point out that the rate of increase at a given density for a population of a stable age distribution (rs) is not obtainable when looking at mammals.

This is because the assumptions used to make the age distributions (rs=0) when used to estimate rs cause it to be low.

The second argument is that the maximum rate at which a population with a stable age distribution increases in a given environment (rm= intrinsic rate of increase) can be calculated with the correct data.

It had however, been used incorrectly by mammalogists who thought that vertebrate life table and fecundity data somehow paralleled those of caged insects held at low density.

The correction was to infer what the rate of increase for a given population would be at both the initial density and at a higher density.

In this way they corrected a misapplication of models so that those managing populations would use the right equation for the question that they were asking.