Effect of fertility control on a population's productivity

The effect of a sterilising agent upon the productivity of vertebrate pests, such as feral horses, feral dogs, wild rabbits or fruit-eating birds, depends upon the population's social structure and mating system. We investigated the theoretical effect on productivity of three forms of dominance, two effects of sterilisation on dominance, and four modes of transmission. Seventeen of the possible 24 combinations are feasible but lead to only four possible outcomes. Three of these result in lowered productivity. The fourth, where the breeding of a dominant female suppresses breeding in the sub-ordinate females of her group, leads to a perverse outcome. Productivity increases with sterilisation unless the proportion of females sterilised exceeds (n- 2)/(n- 1) where n (> 2) is the number of females in the group. A knowledge of social structure and mating system is therefore highly desirable before population control by suppressing female fertility is attempted or even contemplated

Does Dingo Predation Control The Densities of Kangaroos And Emus?

The density of red kangaroos in the sheep country of the north-west corner of New South Wales is much higher now than it was last century. It is also much higher than the present density across the dingo fence in the adjacent cattle country of South Australia and Queensland. The picture is similar for emus. Farther east, about halfway along the New South Wales-Queensland border, no difference in density between the two States could be detected for red kangaroos, grey kangaroos and emus. We examine and discard several hypotheses to account for the density contrasts in the west and the lack of them further east, deeming it unlikely that the pattern reflects environmental gradients, or differences in plant composition and growth, hunting pressure or availability of water. Instead, we favour this hypothesis: that the past and present patterns of density are attributable directly to predation by dingoes, which can hold kangaroos at very low density in open country if the dingoes have access to an abundant alternative prey

Surveys of the Distribution and Density of Kangaroos in the Pastoral Zone of South Australia, and their Bearing on the Feasibility of Aerial Survey in Large and Remote Areas

Kangaroos were censused form the air in 1978, and again in 1979, within the pastoral zone of South Australia, in an area of 242,000 km2, three and a half times the size of Tasmania. Logistical problems were minimal despite the remoteness of much of the area, but more then half the time in the air was spent in flying to and between transects. Red kangaroos occurred throughout the zone at the mean density of 4.62km-2. Western grey kangaroos averaged a density of 1.22 km-2 over the whole area but were restricted to its southern half. Although the area was sampled at the low intensity of 1.3% the estimates were reasonably precise, that for red kangaroos having coefficient of variation of 7% at each survey, that for Mgrey kangaroos, 13%. Estimated numbers did not differ significantly between years. Maps of density and distribution are given for each species. The cost of such a survey is around 6c per km2 for each 1% sampling intensity

Kangaroos and climate: an analysis of distribution

(1) The distributions of three partially sympatric kangaroo species, Macropus giganteus (Shaw), M. fuliginosus (Desmarest) and M. rufus (Desmarest), were analysed to determine their climatic characteristics. (2) M. giganteus occupies areas only where rainfall either has little seasonal trend or where rainfall in summer exceeds winter rainfall. (3) M. fuliginosus is found in areas of uniform or winter rainfall. (4) Seasonality of rainfall has little influence on the distribution of M. rufus. Instead, its distribution reflects interaction between mean annual precipitation and mean annual temperature. (5) The extent of sympatry and allopatry appears to be determined by the independent reaction of each species to specific and differing climatic stimuli rather than by biological interaction between species

The Australian Kangaroo Populations, 1984

Between the first Australian-wide aerial survey of kangaroos in 1980-82 and the second in 1984, indices of relative abundance determined by the same methods show that red kangaroos declined by 24% and western and eastern greys by 35%. The declines are attributed mainly to drought in the eastern half of the continent, partly offset by increases in the western half. We estimate numbers in 1984 at 6.3 million red kangaroos (cf. 8.3 in 1980-82), 1.2 million western greys (1.8) and 5.8 million eastern greys (9.0), a total of 13 millions of all three species compared with 19 millions in 1980-82. This is an overall drop of about 30%. Because recent work suggests that the sightability of both species of grey kangaroos is lower than that for reds, estimates of grey kangaroos will be less than the real numbers and should, therefore, be regarded as indices of relative abundance rather than as absolute estimates

A Double-Survey Estimate Of Population Size From Incomplete Counts

We sought to estimate the number of crocodile nests on the Liverpool River, Northern Territory, Australia. Two methods of survey were available, aerial survey and ground survey, in each of which sightings could be mapped. Hence, those nests which were detected by both methods could be identified. These counts were used to demonstrate the method by which an estimate of total number could be calculated. The assumptions and limitations are discussed