Journal of the Royal Society of Western Australia, 86(2), June 2003 
areas burnt in the 1997 fire but not enclosed against 
grazing, almost a quarter of the area was still denuded 
after 4 Vi years. The heath within enclosures after the 1997 
fire appeared rejuvenated, and resembled the more 
diverse heath described in the 1950s (Storr 1959), while 
that surrounding the burnt area resembled the more 
degraded heath described in the 1980s (Hesp et al. 1983). 
In the 1950s there were Acanthocarpus preissii with 
Austrostipa flavescens, and two less common tussock 
grasses Poa caespitosa (probably Poa poiformis ) and 
Danthonia caespitosa (probably a misidentification of 
Austrodanthonia occidentalis - M Hislop, WA Herbarium, 
personal communication). Acacia rostellifera, the sedges 
Lepidosperma gladiatum, L. pubisquameum and Carex preissii 
and Conostylis candicans. Dicotyledons were uncommon, 
with Thomasia cognata, Guichenotia ledifolia and Trachymene 
coerulea in disturbed areas. By the 1980s heath was 
described as the most widespread community on the 
island, covering about one-third of the island's area and 
the species list was reduced to Austrostipa , Acanthocarpus , 
Thomasia , Guichenotia and Conostylis but with the notable 
addition of the introduced Trachyandra divaricata (Hesp et 
al. 1983). In 2001 these were the species found in the 
heath surrounding the burnt site. 
The native heath appears to be resilient, as Trachyandra 
divaricata was the only introduced perennial species 
noted in the Research Site after 180 years of settlement 
and the introduction of many other alien species. It is a 
primary coloniser of disturbed and unvegetated areas, 
and is known to be toxic to horses and unpalatable to 
quokkas (J Dodd, Department of Agriculture, WA, 
personal communication). Trachyandra was present on 
Rottnest in the 1960s but seldom occurred away from 
coastal dunes (Storr 1962); White & Edmiston (1974) 
expressed concern that the heath was being replaced by 
Trachyandra , and by 2001 it was an important component 
of the surrounding unbumed heath in the Research Site. 
The species appears to persist with continued 
disturbance but to give way to competition from native 
species if disturbance ceases. 
Alternative stable states 
Vegetation dynamics may be usefully represented in 
terms of alternative stable states (Sutherland 1974; May 
1977; Laycock 1991; Law & Morton, 1993; Hobbs 1994; 
Noble 1997). Transitions between states occur if there are 
specific changes in prevailing conditions. Melaleuca/ 
Callitris forest, or Acacia rostellifera scrub or Acanthocarpus 
heath, can dominate on Rottnest Island depending on the 
circumstances (Fig 9). 
If there were occasional hot fires, then low Melaleuca/ 
Callitris forest would dominate. The dense shade of this 
forest has little understorey, so the quokka population 
would remain comparatively small. Fires would tend to 
be intense and infrequent but widespread, fuelled by 
accumulated litter so that parts of the island would be 
free of quokkas for a time after burning, and palatable 
tree species would escape grazing until they were large 
enough to withstand it. 
Under conditions of frequent fires and little grazing. 
Acacia scrub would dominate; no shrub on Rottnest 
recovers as well after fire as Acacia rostellifera (Storr 1963). 
Frequent fires eliminate Melaleuca/Callitris locally, as two 
fires within about 10 years could kill mature trees and 
then their offspring before they reached reproductive 
age, leaving no propagules to germinate. 
With heavy grazing, Acanthocarpus heath would 
MELALEUCA/CALLITRIS 
Low forest 
Figure 9. Alternative stable states model for Rottnest Island. 
58 
