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 subordinate 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.
We used the mathematics of the mark-recapture model to derive a factor correcting counts of emus surveyed from the air. The emus were neither marked nor recaptured, the correction factor being derived from the number of emu groups counted independently by two observers simultaneously scanning the same transect. The analysis suggests that about 68% of emu groups on the transect are counted by a given observer during a standard survey, and that his counts must therefore be multiplied by 1.47 before they estimate true density of groups. Having determined independently the mean size of emu groups as 3.75 at that time of the year, we applied this factor to counts from a survey of 1480000 km2 in Western Australia. Overall density was 0.074 emus km-2, being highest in the pastoral zone at 0.103 km-2 and lowest in unoccupied land at 0.008 km-2. The difference probably reflects availability of drinking water.
Red and western grey kangaroos were surveyed from the air in Western Australia during the winter of 1981. The area covered, 1 528 000 km2 or 61% of the State, excluded only the Kimberleys in the north and the Gibson and Great Sandy Deserts of the interior. Hence almost all kangaroo range within the State was surveyed, to provide an estimate of 980 000 reds and 436 000 greys. Densities were much lower than those of the eastern States. Red kangaroos were most abundant in mulga shrubland, chenopod shrubland and tussock grassland, and least abundant in hummock grassland. Densities were associated strongly with land-use categories, being high in areas used for extensive sheep grazing and low in vacant Crown Land and arable land. In contrast to reds the western grey kangaroos were confined to the south and west of the state, their distribution being related more directly to climate than to vegetation or land use. They live in the winter rainfall zone. We suggest that their restricted breeding season results in peak nutritional demands associated with lactation, and hence energy requirements, being synchronized with the spring flush of pasture following winter rains. Approximately 14% of the red kangaroo and 8% of the western grey kangaroo populations in Western Australia were harvested legally in 1981.
The distributions of the two species of grey kangaroo, Macropus giganteus of eastern Australia and M.fuliginosus of southern Australia. overlap in a zone of 0.68 X 10*6 km2 in south-eastern Australia, the zone including parts of South Australia. Victoria, New South Wales and Queensland. The species' boundaries determining the overlap zone are mapped and the geographic trends in density within it are determined for each of the two species. Whereas the western boundary of M. giganteus has moved inland this century, possibly as a response to an increased number ofwatering points, there is no strong indication that M. fuliginosus has extended its range significantly. The recent discovery of the latter species in Queensland over an area of 100 000 km2, reported here for the first time, need reflect nothing more than the difficulty of differentiating the two species in the field.
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