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The comparative ecology of the burrowing bettong (Bettongia lesueur) and European rabbit (Oryctolagus cuniculus)

Robley, Alan Joseph (1999) The comparative ecology of the burrowing bettong (Bettongia lesueur) and European rabbit (Oryctolagus cuniculus). PhD thesis, Murdoch University.

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The comparative ecology of Burrowing bettongs (Bettongia lesueur) and European rabbits (Oryctolagus cuniculus) were studied on Heirisson Prong, Shark Bay between 1995 and 1998. Burrowing bettongs were once widespread across mainland Australia, but now survive only on three islands off the west coast of Western Australia and as a reintroduced population on Heirisson Prong, a 1200-hectare peninsula in Shark Bay, Western Australia.

The general question investigated by this thesis is whether rabbits usurp resources essential to bettongs, limiting their population fitness and growth, particularly during extended dry periods. The comparative approach taken resulted in a quantitative comparison of foraging strategy, home range, diet, spatial distribution, habitat use, and population dynamics of the two species.

Investigating the foraging activities of bettongs and rabbits can indicate the potential for competition over food resources. Both bettongs and rabbits were observed foraging in a 30 x 40-metre enclosure of native vegetation over several nights in October 1997. The degree of overlap in foraging activities of bettongs and rabbits was only moderate (O = 0.54). Foraging ratios indicated that bettongs allocated most of their time (63%) to gathering food. Of the total time bettongs were observed feeding 61% was spent browsing in shrubs. The majority of the remainder being spent digging and searching for subterranean food. Rabbits spent the majority of their time acquiring food (81%). Repeated measures MANOVA showed that rabbits spent considerably more time grazing (F1,40 = 21.152, P = 0.004) than bettongs.

It is possible that the ranging behaviour of bettongs is influenced by interspecific competition with the European rabbit. Home ranges of burrowing bettongs and European rabbits were measured in January and March 1998. Seven adult bettongs and seven adult rabbits were tracked within an area of 2.25 km x 2 km.

The mean home range of bettongs based on the 100% minimum convex polygon was 95.2 hectares, with males having slightly larger ranges than females. The home range of rabbits was 8.5 hectares. Bettongs travelled a mean distance between observations of 403 m compared 175 m for rabbits. Bettongs and rabbits used space differently with these differences suggesting a difference in the distribution and/or use of food resources.

Rabbits may exploit food resources that are essential to the survival and fitness of bettongs, reducing the availability of these resources to a level that bettongs are unable to harvest them. The degree of dietary overlap between the two species was ascertained by comparing the identifiable particles in the scats of bettongs and the stomachs of rabbits over two winters and two summers. The diets of bettongs and rabbits were significantly different in both summer (Mantel t = 2.4, P < 0.05) and winter (Mantel t = 12.0, P < 0.001). The main components of the bettongs diet were fungi, browse, fruit, and seed. There is an indication that they may have scavenged rabbit cadavers during summer. Rabbits were restricted to grasses, forbs and shrubs. This result, combined with morphological and physiological differences, suggests that bettongs and rabbits do not compete directly for food resources.

Shelter may be an important resource for which competition occurs as both species use warrens. Fifty-one bettong warrens were located by radio tracking 22 free-range bettongs in June and July 1996. These were compared with 141 rabbit warrens, recorded from seven transects walked in the same months. Nine variables were measured for each bettong and rabbit warren. Several rabbit warrens and two bettong warrens were excavated to investigate their internal structure.

Bettong warrens had more active entrances (mean = 2.43 ± 0.24) than rabbit warrens (mean = 1.69 ± 0.09). Bettong warren entrances were wider (mean = 207.0 ± 8.5 mm) and taller (mean = 166.0 ± 4.3 mm) than rabbit warrens (mean = 166.0 ± 4.9 mm and 146.0 ± 3.3 mm). Bettong and rabbit warrens differed in the frequency with which they occurred on various aspects (χ24 = 23.3, P < 0.001.). Bettong warrens were found under shrubs with more cover (mean = 81%) compared to those used by rabbits (mean = 69%, F1,189 = 8.56 P = 0.004). Bettong warrens were located more on the bottom/middle slopes and less on the plain than would be expected if both rabbits and bettongs warrens were evenly distributed throughout the landscape (%23 = 8.45, P = 0.038). Bettongs use deep well-ventilated, multi-entrance warrens throughout the year. Rabbits are unable to remain in warrens during the hot summers and shelter above ground under shrubs.

Apart from historical and natural history observations, no data exists on the impact of rabbits on the use of warrens by bettongs. Warren use, reproduction, and body condition of bettongs were assessed in the absence of rabbits and then subsequently with successively higher densities of rabbits in two experimental yards between November 1997 and January 1998. At the end the three months rabbits were removed, and the condition and warren use of bettongs was assessed.

Despite the doubling of the rabbit density in each yard each month the incidence of recorded cohabitation between bettongs and rabbits was low. Six rabbits shared six warrens with six bettongs for a total of 42 days. The number of times that bettongs moved warrens due to the presence of a rabbit in the warren on the previous night approached significance (χ21 = 3.608, P = 0.058). Cohabitation of warrens by bettongs and rabbits occurred but with no evidence of a decline in body condition or reproductive output of bettongs. The observed change in warren use by bettongs coincides with female bettongs using fewer warrens and remaining stationary in selected warrens for longer as pouch young neared the end of pouch life.

Changes in bettongs rate of increase, reproductive output, body condition, and recruitment were investigated between January 1996 and January 1998 during which time rabbit numbers increased from a moderate density reaching a ten-year high (approximately 45,000 rabbits) before declining to very low numbers. The study period was also characterised by above average winter rainfall and decreasing plant biomass through dry summers.

The free-range bettong population increased steadily from five individuals in January 1996 to 71 in May 1998. The observed rate of increase for bettongs between January 1996 and January 1998 was r = 1.9. In the period between January 1996 to January 1997 the observed rate of increase for bettongs was r = 0.9. Female bettongs reproduced all year, with an average of 66% of females carrying pouch young. Changes in rabbit density had no effect on the body condition of male or female bettongs. Recruitment averaged 11%, peaking in May and July of each year. Changes in rabbit abundance were not related to fluctuations in recruitment rate.

The results of this comparative study suggest that the niche of rabbits and bettongs at Heirisson Prong is sufficiently different to allow co-existence. Aspects of the bettongs' ecology may enable them to withstand habitat changes wrought by the rabbit. The most striking is a broad and adaptable diet, with a digestive physiology capable of digesting a wide range of foods. In addition, they are able to move over large areas in search for high quality food, and to live in complex warrens in large social groups. Consequently, the potential impact of the rabbit on the population dynamics of the bettong may well be restricted by these behaviours.

Item Type: Thesis (PhD)
Murdoch Affiliation: Division of Science and Engineering
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: Thank you.
Supervisor(s): Bradley, Stuart and Short, Jeff
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