Summary1. Demographic changes in response to surgically imposed female sterility were monitored in 12 free-ranging rabbit populations in south-western Australia over a 4-year period. This was part of a research programme aimed at examining the potential for virally vectored immunocontraception to limit the abundance of rabbits (e.g. using a recombinant myxoma virus) and other mammalian pests. Sterility levels were 0%, 40%, 60% and 80% of all females in year 1, with a similar proportion of female recruits sterilized surgically in subsequent years.2. There was a signi®cant decrease in rabbit productivity with increasing sterility level. This was overcome by increased survival of kittens and adults on the highsterility sites, such that the base-level numbers of rabbits were maintained, and mean annual rates of increase (r) were near zero for all treatments in all years. However, in the high-sterility populations this compensation was insucient to overcome the eects of sterility totally, and there was a marked decrease in the seasonal peaks in rabbit abundance for these treatments. 3. Survival and recruitment were dependent upon the level of sterility, and consequently the density of rabbits, with greatest survival of adult rabbits occurring on the 80% sites. Survival of sterile females was greater than that of other adults, probably because of their increased ability to maintain body condition during times of low pasture biomass (summer drought). Thus two density-dependent processes were identi®ed: the ®rst was operating through increased survival of juvenile rabbits, the second through increased adult survival, particularly sterilized females. 4. Because the proportional impact of immigration was greater (i.e. immigrants constituted a greater proportion of the population) and emigration was less, from the 80% sites, the eects of sterility may have been underestimated on these sites. 5. The abundance of European rabbit¯eas, a vector of myxomatosis, was signi®-cantly lower on the 80% sites, but this did not appear to aect the transmission of myxoma. Myxomatosis occurred as an annual epizootic in three of four years, with > 90% of rabbits on site after each epizootic testing positive for myxoma antibodies. 6. To achieve a sustained long-term reduction in rabbit abundance, 60±80% of female rabbits would need to be prevented from breeding. This could be achieved by a recombinant strain of myxoma provided the strain retained good transmissibility and all infected rabbits became sterile for life.
Demographic changes in three free-ranging rabbit (Oryctolagus cuniculus) populations were monitored over 4 years in southern Western Australia. Peak densities followed periods of high rainfall and pasture biomass. The breeding season was prolonged, often extending from at least April to November, with some pregnancies occurring outside this period. Fecundity, determined by the autopsy of pregnant offsite rabbits and the known length of each breeding season, appeared to be relatively high, with the potential for 34–39 kittens doe-1 year-1; however, because not all females are pregnant in all months, the overall productivity of these populations was estimated at 25–30 kittens adult female-1 year-1. Exponential rates of increase varied from 0.13 to 0.30 during the breeding periods and –0.05 to –0.14 during the nonbreeding season. Kitten survival was generally low whereas some adults lived for more than 5 years. Two patterns of myxomatosis were observed: annual epizootics of the disease (3 of 4 years) and an epidemic that slowly spread over many months. European rabbit fleas were most abundant during winter–spring and attained highest densities on adult female rabbits.
Abstract:Top-down control of algal blooms was investigated in the Swan-Canning Estuary, to determine whether the resident zooplankton community was able to substantially reduce phytoplankton biomass through grazing, or whether high phytoplankton biomass effectively inhibited grazing. In situ grazing by zooplankton at two depths in the water column was measured over a diel period on six occasions and examined in relation to the biotic and abiotic habitat conditions. Measured weight-specific ingestion rates varied with phytoplankton and zooplankton species composition, but were significantly greater at the surface than at 5 m, close to the bottom (ranges of 1Ð59 to 252Ð41 and 0Ð51 to 28Ð99 µg phytoplankton C µg zooplankton C 1 day 1 , respectively). There was no evidence of significant diel changes in ingestion rates of zooplankton. Phytoplankton biomass removed by in situ community grazing ranged between 0Ð9 and 45Ð0% of standing stock per hour. The maximum loss occurred when phytoplankton biomass ranged from 249Ð0 to 320Ð8 µg phytoplankton C l 1 . Zooplankton grazing impact was negatively associated with high phytoplankton and low dissolved oxygen in bottom waters. The implications of, and mechanisms associated with, top-down control of algal biomass in this estuary are discussed in relation to plankton succession and the physical environment.
Production of faecal pellets by the herbivorous estuarine copepodGladioferens imparipes, and the rate at which the pellets sink through a water column, were investigated in the laboratory. Pellet production rate, at excess food concentration, varied among food types, with a maximum rate of 3.2 pellets h –1 copepod –1 when copepods were fed on the diatom Skeletonema costatum, and a minimum of 1.5 when fed on the chlorophyte Chlamydomonas globosa. Pellet production rates may have reflected feeding prehistory and related effects on food preference and ingestion rates. Settling rates ranged from 2.0 m day –1 for pellets produced on the thecate dinoflagellate Scrippsiella sp. to 71.8 for pellets produced on S. costatum. The estimate of settling rate was strongly influenced by the method used to measure it and measure pellet density. Application of the laboratory results to average copepod densities in the Swan River estuary suggested a conservative estimate of production of 360 pellets L –1 day –1 , or 8 ˜g C and 1 ˜g N, given appropriate food conditions. With a conservative estimate of settling rate at 6.3 m day –1 , pellets would easily reach the sediments within a tidal cycle in many parts of the Swan River estuary.
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