Lowe, Wheeler & Twigg: Impact of rabbits 
Figure 9. The clear 'rabbit-graze line' within the bush remnant (with rabbits) behind the rabbit-proof fence compared to the lack of 
impact in the surrounding pasture with no rabbits. 
n = 4) with 0.5 ha to 3 ha (mean 1.75 ha) of crop lost. 
Losses were lower in lupin crops during the study, and 
ranged from $50 to $297 (mean $195, n = 3) or 0.25 ha to 
1.5 ha (mean 0.98 ha) of affected crop. Obviously, rabbit- 
proof fencing is relatively permanent, and if properly 
maintained is likely to last for at least 15 years. Thus the 
associated costs would need to be discounted against the 
benefits obtained over such a time period. There are also 
tax benefits (e.g. depreciation) to landholders. The fence 
cost approximately $5 000 per km (materials, $4 000; 
labour, $1 000), and there is also a small on-going 
maintenance cost. Depending upon the shape and size of 
the area protected, the use of rabbit-proof fencing would 
cost between $250 and $500 per hectare of protected 
'crop' in the first year (2001 $AUD). However, this outlay 
would be discounted in subsequent years. 
Discussion 
Short-term effects of confining rabbits 
The abundance of rabbits within the fenced vegetation 
was always considerably less than that which occurred 
within the unfenced bush remnant. The dung counts in 
November 2000 (end of breeding season), for example, 
indicated a relatively large increase in rabbit numbers in 
the unfenced remnant vegetation which was not seen in 
the fenced remnant. This suggests that the rabbits within 
the fenced vegetation only had access to less palatable/ 
nutritious native vegetation and this may have restricted 
their reproductive output. Conversely, rabbits in the 
unfenced remnant vegetation had ready access to a 
canola crop of higher nutritional value potentially 
enhancing their reproductive output. Significant 
breeding by rabbits depends on the provision of green 
feed of adequate nutritional quality (King & Wheeler 
1985; Williams et al. 1995; Twigg et al. 1998). In the 
unfenced remnant vegetation, where the increase in 
rabbit numbers occurred, there seemed to be a greater 
decline in the abundance of seedlings and reshoots (re¬ 
sprouting vegetation). This was accompanied by a 
decline in the percentage cover of sedges and grasses 
that was in contrast to the fenced area, where the 
percentage cover of sedges and grasses had increased. 
The number of seedlings and reshoots was similar 
between the open (grazed) and exclosure (ungrazed) 
plots within fenced and unfenced remnant vegetation 
after the 14-month monitoring period (Fig 3). There are 
four possible causes for this similarity, and these are not 
necessarily mutually exclusive. Firstly, rabbit numbers 
within the fenced vegetation were low and may have 
been below the threshold level required to cause obvious 
environmental damage. Conversely, because the rabbits 
in the unfenced remnant vegetation had ready access to 
other foods (eg crops and pasture), there may not have 
been a great demand for these rabbits to feed within the 
remnant vegetation as they always had access to an 
adequate food supply within the surrounding paddocks. 
Hence their impact on the remnant vegetation was 
minimal. This may occur irrespective of rabbit density. 
Thirdly, the seed bank/species richness of the remnant 
vegetation was depauperate, and hence the vegetation 
was unable to respond. We do not favour this option as 
our study sites had a diverse range of plant species, and 
we did observe a difference in the response between the 
exclosures and the open plots in the long-term 
experiment. Finally, because the numbers of rabbits 
ultimately confined within the fenced remnant vegetation 
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