Rippey, Hislop & Dodd: Vascular flora of Rottnest Island 
islets. Dyer and Green Islands. A criticism of the theory 
of island biogeography (Sauer 1969) is the implication 
that equilibrium in the total number of species represents 
stability and characterises the community structure of an 
island. On Rottnest Island, the equilibrium between 
immigrations and extinctions masked the dynamic nature 
of the flora and considerable change in terms of species 
composition. 
The vegetation of Rottnest Island contained a large 
proportion of exotic species, comprising 42% or 83 of a 
total 196 species in 1998-2001. The comparable figure for 
the Perth region was 27%, or 547 aliens in a total 2057 
species (Marchant et al. 1987), and for Western Australia 
as a whole 11%, or 1052 in a total of 9640 (Paczkowska & 
Chapman 2000). The turnover rate of 2 species per 
annum (1.0% pa) between the two surveys was high, with 
exotic species proving particularly mobile. 
Rottnest Island is fairly large (1900 ha) with varied 
habitats, but its flora is depauperate compared with the 
adjacent mainland. A grassy Acanthocarpus heath of low 
diversity is now the most widespread of the vegetation 
communities on the island, a situation not recorded 
before the 1950s. The greatest changes to the island's flora 
probably took place over the first 100 years of human 
settlement, when there was clearing, burning, tree 
cutting, and farming with the introduction of crop 
species and their attendant weeds and grazing by farm 
animals. The quokka population was suppressed by 
hunting. Storr's (1962) survey recorded the plant species 
on the island after this time, when farming had ceased, 
the quokka population had recovered, and the island had 
roads and a reliable water supply. Major disturbance 
factors had been absent for a decade, with the notable 
exception of the widespread fire of 1955. 
Very few of the species introduced with the farming 
enterprises survived to the late 1950s. Storr (1962) 
recorded no agricultural annuals (their palatability and 
lack of dormancy presumably ensured their early 
demise) but the perennials Ricinus (Ferguson 1986) and 
Agave (Keighery 1988) persisted, as well as some long- 
lived fruit trees planted beside Garden Lake. Agricultural 
weeds presumably were abundant during farming 
operations, but few of these survived the harsh climate 
and quokka grazing. Some, such as Avena barbata, 
Arctotheca calendula and Centaurea melitensis, were 
recorded by Storr (1962), and were still present on the 
island in our survey but were rare. The tendency for 
annuals to give way to perennials is classic succession, 
following disturbance. However the present low 
prevalence of annuals may be due to their greater 
vulnerability to quokka grazing. 
The vegetation of Rottnest Island has not had the same 
intensity of disturbance since Storr's (1962) census but 
conditions have not returned to the pre-European 
situation. First, there has been increasingly heavy traffic 
between the island and the mainland, attendant upon 
thousands of visitors each year (currently around 500 000 
pa), which provided many opportunities for the 
introduction of new weed species. Secondly, wildfires 
that could revitalise the vegetation and temporarily 
reduce grazing by quokkas, have been infrequent and 
quickly controlled. Thirdly, the quokka population 
presumably has become so large that it dominates the 
makeup of the flora, in effect preventing the regeneration 
of palatable species, including the tree species (Pen & 
Green 1983). 
Many of the plant extinctions on Rottnest Island may 
have taken place shortly after Storr's (1962) census in the 
aftermath of more recent fires and disturbance events. 
Certainly 29 of the 44 species that became extinct were 
known to have been rare in the late 1950s. Storr (1962) 
recorded 25 as such, mentioning that two of them 
(Dodonaea aptera and Portulaca oleracea) were known from 
only one specimen. A further 4 species were not recorded 
by Storr (1962) at all, but were known to have been on 
Rottnest Island from single WA Herbarium specimens 
collected in the late 1950s (Bromus madritensis, 
Heliotropium curassavicum, Polypogon tenellus and 
Zygophyllum billardicrei). Significantly the majority of the 
species that became extinct were noted by Storr (1962) as 
being grazed or heavily grazed by quokkas. 
Exotics made up an increasing proportion of the flora, 
and showed a particularly rapid turnover. These 
introduced species were concentrated in the developed 
areas, where they dominated the flora, but some were 
widespread across the island. The 35 exotic trees and 
shrubs listed in Appendix 4 include relics of nineteenth 
century market gardens and orchards, shading 
ornamentals for the benefit of twentieth-century tourists, 
and reafforestation species. These are of historic and 
landscaping or ornamental significance, as well as being 
potential sources for future naturalisations. Nine of the 
species that had been cultivated on the island became 
naturalised between the late 1950s and 1998-2001; 
specifically Agave attenuata, Caesalpinia gilliesii , Casuarina 
glauca , Eucalyptus utilis, Ficus rubiginosa, Iris sp, Melia 
azedarach, Narcissus tazetla and Nerium oleander. These are 
included in the comprehensive list of Rottnest flora 
(Appendix 2). 
A number of introduced species were considered a 
threat to the vegetation of the island and have been 
targeted for eradication, including Zantedeschia aethiopica, 
Euphorbia paralias, Ricinus communis, Rhamnus alaternus 
and Nicotiana glauca. Gomphocarpus fruticosus is a noxious 
weed that was eradicated from all but one site, where it 
was retained because it was the only food plant on the 
island for the Wanderer butterfly (Danaus plexippus) and 
the native Lesser Wanderer (Danaus chrysippus) (Hay et 
al. undated). 
Some of the species that immigrated or became extinct 
merit comment. Pelargonium littorale ssp littorale was 
'widespread and abundant' in the 1950s according to 
Storr (1962), although subject to heavy grazing. This was 
not collected in 1998-2001; possibly it had been eaten out 
by quokkas. Malva australiana, which was recorded by 
Storr (1962) as common on the islets, could not be found, 
and has apparently been supplanted by the European 
tree mallow, Malva dcndromorpha. M. australiana is now 
rare on the islands off Perth, persisting only on Carnac 
(Abbott et al. 2000) and Middle Shag Islands (Rippey et al. 
2002). M. australiana and M. dcndromorpha are palatable to 
quokkas and so do not occur on Rottnest itself, except for 
a clump of M. dcndromorpha on the islet in Lake Baghdad. 
M. dcndromorpha occasionally hybridises with M. 
australiana, an example of genetic pollution. The native 
Hydrilla vcrticillata was collected in 1955 from a small pool 
on the south east edge of Government House Lake and 
was identified by Storr (1962) as Elodea canadensis, an 
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