Suchergebnisse

The age-structures of rabbit populations at 4 climatically different sites in South Australia were found to have altered significantly since the introduction of Spilopsyllus cuniculi (Dale) as a vector of myxomatosis. The ratio of young of the year to older rabbits in summer was greatly reduced. There was evidence of decreases in size of all observed populations and (for 1 population) in ability to increase rapidly in favourable years. The ratio of young of the year to older rabbits was a convenient and sensitive index of changes in population structure; for analytical purposes, the log transformation (ln [ratio] or ln [ratio + 1]) was biologically and statistically sound. Since the fleas became established, field-strain viruses may have become more virulent and morbidity rates may have increased, but the factors enabling rabbit fleas to enhance myxomatosis as a cause of mortality are unknown. The apparent virulence may have increased because of selection for viruses of high virulence, which are best suited for transmission by S. cuniculi, and possibly because the dose of virus transmitted by fleas is considerably greater than that by mosquitoes or other agents.

Rabbits given an excess of air-dry food but only small amounts of drinking water had a lower food intake and lost more weight than those given food and water freely. They lost weight because they had less digesta in the gut and less water in the skin, and had catabolized more tissue. Their response to water shortage was similar to that seen in other grazing mammals. With low food intake, gut contents fell and the digestibility of food rose, while the daily mass of and water loss in faeces fell; the moisture content in faeces remaining the same. Dehydrated rabbits produced relatively small amounts of urine, up to 1.9 M urea, which was twice as concentrated as that of rabbits given water freely. The rabbits were not able to reduce their requirement for water to less than 55% of their total intake of food and water. As pasture species in mediterranean-type environments become relatively dry in summer and contain only 10 to 5% water, a shortage of water in natural pastures in those areas may, therefore, limit the amount of food which rabbits can use, and the ability of rabbits to survive in summer would, therefore, depend upon the availability of succulent, drought-resistant perennial vegetation.

Shortage of water in natural pastures led to a sharp decline in a large rabbit population in arid, northeastern
South Australia. The pastures were dry and some rabbits drank at springs and water troughs.
Further from water, rabbits climbed trees and shrubs to obtain succulent leaves and twigs. Rabbits
provided with water maintained their weight and apparently survived better than those which did not
drink. It seems unlikely that the rabbits lost weight because the water shortage reduced the amount of dry
food they could eat. In caged rabbits, water shortage limits food intake but also results in low gut fill;
whereas the wild rabbits had the normal amount of digesta in their guts. It is more likely that, as the
pastures became dry, rabbits ate woody twigs and bark which were moist enough to meet their water
requirements but contained too little digestible energy for maintenance. The water shortage apparently
arose because rabbits were numerous and had eaten out the succulent pasture plants. Normally, it takes a
long drought to reduce arid-zone plants to dry straw, and overgrazing is probably the usual cause of a
lack of adequate water for rabbits.

Rabbits could be controlled by various combinations of warren ripping, poisoning and fumigation, without the need for clearing roadside Eucalyptus spp.

The extent of organochlorine pesticide contamination in 3 species of predatory birds from the central regions of South Australia was investigated. Residues in both eggs and tissue, as well as measurements of shell thickness, were obtained for Falco peregrinus (peregrine falcon), Pandion haliaetus (osprey) and Haliaeetus leucogaster (white-bellied sea eagle). Organochlorine residues in some prey species were also measured. Residues of DDE and DDT in the predatory birds were found to be low to moderate. Concn of total DDT were greatest in F. peregrinus (mean 1.82 mg/kg), followed by the H. leucogaster and P.haliaetus (mean 1.07 and 0.11 mg/kg, resp.). A comparison of F. peregrinus eggs collected before and after 1947 showed significant eggshell thinning (mean 16%). DDT residue levels of concern were recorded for several prey species commonly found in the diet of H. leucogaster and F. peregrinus, particularly in Columbia livia (feral pigeons) (37.46 mg/kg) and Larus novaehollandiae (silver gulls) (3.06 mg/kg). These concn were well above those known to cause reproductive failure in falcons. DDT was also found at high concn in eggs of Pelecanus conspicillatus (pelican) (mean 2.04 mg/kg). Dieldrin was present in most samples, but at low concn. The degree of pesticide contamination detected was considered sufficient to interfere with the reproduction of F. peregrinus in South Australia.

The diet of nestling rainbow bee-eaters, Merops ornatus, was determined by analysis of droppings and
regurgitated pellets collected at one site on Rottnest I., W.A. in the summer of 1982/83, and five sites
in the summer of 1983/84. In total, 2187 insects from 10 families were identified. These comprised:
Hymenoptera (95%), including Scoliidae (14%), Tiphiidae (38%), Sphecidae (l8.5%), Apoidea (1%),
Formicoidea (7.5%) and undetermined Hymenoptera (16%); Coleoptera, Buprestidae (1.5%); Diptera,
Muscidae (<1%); Hemiptera (3%); Odonata (<1%); and Orthoptera (<1%). The relative proportions
of the different prey types varied significantly between sites and between different sampling times at
the same site. There were also site-related differences in the mean length of nestling prey and at three
sites nestlings were fed different sizes of prep during their development. Estimates of prey length based
on wing remains were significantly smaller than those based on head remains at four of the six sites,
and shapes of prey length distributions based on wings and heads were significantly different at all
six sites.

The effect of rabbit control methods on rabbit populations and subsequent changes in vegetation were
assessed over 2 years on a calcareous shrubby grassland in central Australia. Warren ripping and
fumigating decreased mean population levels to four rabbits per kilometre of spotlight transect, and
1080 poisoning decreased mean levels to 9 km-1, compared to the untreated levels of 20 km-l. Release
of European rabbit fleas did not have any significant effect on rabbit populations. In the untreated rabbit
populations, 68% of the fluctuation in numbers was accounted for by total rainfalls in the previous
summer and the previous winter, and by a forage greenness rating. Experimental control of rabbits
combined with exclosures indicated that grazing by high numbers of rabbits reduced the frequency of
Enneapogon spp. at one site whereas levels rose in all other sites during the two years of good rainfall.
The frequency of several species, such as Chenopodium cristatum, Phyllanthus rhytidospermus and
Portulaca oleracea, was highly correlated with rabbit density in the previous year, suggesting that rabbit
grazing promotes their increase. Grazing'by rabbits alone depressed standing biomass in the following
season by 300 kg ha-1 from a maximum 1200 kg ha-1 within the exclosures. Cattle grazing further
decreased biomass by 150 kg ha-1. Seedlings of Acacia kempeana were almost absent in those areas
open to rabbit grazing, but they were reasonably dense where rabbits were excluded. However, the
vegetation response over the 2 years of experiment was due mainly to seasonal changes rather than
rabbit control, and does not justify the expense of rabbit control at this stage. Substantial improvement
in vegetation composition over the longer term may alter this conclusion.