Ornduff (1987) and Ohlemüller (1997) found that
introduced weed species, mainly annuals, comprised
23.7% and 17.0% of the granite herbfield floras sampled
respectively. These figures are high compared with the ca
9% that weeds represent of the WA flora as a whole
(Green 1985; Keighery 1995). They are even more
significant when considered with granite outcrop
herbfield floras elsewhere on earth, which have far fewer
invasive weeds (Porembski et al. 1997; Wyatt 1997).
Weed growth is most pronounced in full sun on
outcrops, especially where soil has been disturbed and
enriched by rabbit dung or agricultural activity. In these
situations, annual grasses such as Briza maxima, Avena
native annuals throughout much of the south-west.
Weeds are rare only where a dense shrub layer or low
forest of native woody perennials persist. It is clear that a
persistent seed rain of weeds occurs over vast regions of
the south-west, as weeds appear in open areas on
outcrops where little or no disturbance is evident and
native plants dominate.
Hopper (1997) has attributed the high invasibility of
disturbed Western Australian plant communities to the
absence of major glacial soil stripping as an evolutionary
force acting on the flora. Native species are unable to
compete against weeds from habitats where soil
disturbance is a regular pertubation. Further
experimental study of weeds in granite herbfields could
test this hypothesis, and assist attempts at restoration of
Biogeography and Endemism
Western Australian granite outcrops display plant
biogeographic patterns that mirror that of the whole flora
(Hopper 1979, 1992; Hopper et al. 1996); species-richness
and endemism are pronounced in the transitional rainfall
zone (wheatbelt) of the south-west, and attenuate as
rainfall decreases through the pastoral country to the
deserts, Pilbara and Kimberley. For example, the total
number of vascular plant taxa recorded by the senior
author and colleagues on a sample of rocks ranged from
142 (Point Matthew, near Augusta) and 201 (Mt
Frankland) in the highest rainfall forests, 192 (Mt Ney)
and 187 (Yilliminning Rock near Narrogin; Pigott and
Sage 1997) in the transitional rainfall zone, to 85 (Daggar
Hills, near Yalgoo) in the pastoral zone, and 90
(Moolyella Rocks, east of Marble Bar) and 80 (Spear Hill,
west of Marble Bar; Fig 2) in the arid Pilbara.
There are no vascular species shared between
northern (Kimberley, Pilbara) outcrops and those in the
south-west. Moreover, the northern outcrop floras are
virtually identical with that from the matrix of
surrounding terrain, save for outcrop specialists like rock
figs and rock ferns. Walters & Wyatt (1982) similarly
recorded low endemism and little discontinuity between
vascular plants on granite outcrops and adjacent
landforms of the arid Central Mineral Region of Texas.
In contrast, south-western and adjacent pastoral zone
rocks have higher levels of local endemism, especially in
the high rainfall forest region where the outcrops present
the most striking difference in habitat to the surrounding
vegetation matrix (e.g. Wardell Johnson & Williams 1996;
Brooker & Margules 1996).
Biogeographical relationships of outcrop floras across
the south-west are under ongoing study by the senior
author. As a precursor, Hopper & Brown (unpublished)
documented the distribution of 126 orchid taxa on 41
outcrops ranging from the highest rainfall forests
through the transitional zone wheatbelt to the arid zone.
Each rock outcrop was treated as a site in a classification
of the orchid data.
The study highlighted a number of significant trends.
A primary division occurred between the 15 rocks found
in the forested High Rainfall Zone ( >800 mm p.a.) and
the rest ranging from the Transitional Rainfall Zone of
the wheatbelt into the Arid Zone. Subsequent divisions
established as much difference among forest rocks as
among the wheatbelt/arid rocks, even though the forest
rocks were confined to a much smaller area. Moreover,
remarkably, closely adjacent rocks were widely separated
in the classification, indicating significant differences in
their orchids (e.g. 10.3 km Rock and 10.9 km Rock of
Ornduff’s (1987), separated by just 600 m of jarrah forest
on Albany Highway). Conversely, rocks separated
geographically often had similar orchid floras (e.g.
Boyagin Rock and Pingaring Rock on the western and
eastern sides of the south-central wheatbelt respectively).
These patterns suggest significant barriers to orchid
Lists of Western Australian orchid taxa that occur on granite outcrops.
dispersal, particularly between forest rocks, and high
levels of local extinction and stochastic events underlying
the presence of orchids on individual rocks in the south-
west and adjacent arid zone. There have been dynamic
climatic fluctuations across the south-west for several
million years as Australia drifted northwards and arid
conditions overtook much of central Australia (Hopper
1979; Hopper et al. 1996). The diversity of microhabitats
on granite outcrops provided refuge for plants adapted
to both dry or wet conditions as the surrounding matrix
waxed and waned climatically (Marchant 1973; Main
1997). Survivorship in small populations on granite
refuges undoubtedly was a matter of chance in the face
of such repeated climatic turmoil.
Journal of the Royal Society of Western Australia, 80(3), September 1997
Interestingly, the above conditions of small disjunct
outcrop populations undergoing recurrent stresses is
predicted as ideal for genetic divergence and speciation
(Grant 1981). Is this prediction borne out by studies of
Western Australian outcrop plants? Table 1 provides a
recently updated list of 141 orchid taxa recorded from
Western Australian granite outcrops. Of these, 22 (16%)
are more or less endemic. The endemics have
geographical ranges from widespread on outcrops
throughout the south-west (e.g. Spiculaea ciliata; Fig 5) to
highly restricted to a few adjacent outcrops less than 10
km apart (e.g. Caladenia caesarea subsp maritima,
In terms of evolutionary origins, these endemics
display at least three patterns (Hopper and Brown,
• relictual, with no obvious close relatives and
therefore likely to have been on granites for a long
period of time (e.g. Spiculaea ciliata, a monotypic
habitats other than granite (e.g. C. granitora, from
coastal granites east of Albany, sister species to C.
infundibularis of western high rainfall forests and
coastal heaths; C. hoffmanii subsp graniticola, of east
central wheatbelt outcrops, sister to C. hoffmanii
subsp hoffmanii of lateritic loams well to the north-
west in the Northampton region); and
• derived by speciation from allopatric congeners of
other granite outcrops (e.g. C. exstans, from
outcrops east of Esperance, sister to C. integra of
western wheatbelt outcrops).
Thus, the orchid data do indeed support the hypothesis
that conditions on south-west granite outcrops have
facilitated genetic divergence and speciation. Genetic
studies of a few other granite taxa lend further support,
e.g. the herb Isotoma petraea (Bussell & James 1997), and
eucalypts endemic to granite (Hopper & Burgman 1983;
Sampson et al. 1988). Clearly, more work along these
lines is needed.
There is a large number of granite endemics in south-
western Australia, especially among the perennials that
dominate the woody vegetation and herbfields. We have
already shown that 16% of orchids on outcrops are
endemic. For eucalypts, around 24% are endemic
(Hopper, unpublished). The level of endemism for the
whole granite outcrop flora is difficult to determine
without more penetrating research, but there is no doubt
that south-western Australia has higher levels than any
other system documented (e.g. Walters & Wyatt 1982;
Porembski et al. 1995, 1997).
The refugial opportunities offered by south-west
granite outcrops are also evident in species that have
highly disjunct outliers well removed from the main
geographical distribution. These include populations
on wet outcrop sites in much lower rainfall areas than
the main species’ stand (e.g. the Jilakin Rock stand of
jarrah Eucalyptus marginata, the Twine Rock stand of E.
Conversely, arid-adapted species penetrate high
rainfall areas on dry north-facing slopes of granites
(e.g. populations of Eucalyptus drummondii west of
Granite outcrops occupy a very small proportion of
most Western Australian landscapes in which they occur.
Especially in the south-west, the outcrop plant
communities are, therefore, by definition, rare, and likely
to contain rare species.
Hopper et al. (1990) found that endangered granite
outcrop plants numbered 29 (12.2%) of the 238 plants
declared in 1989 as Rare Flora under the Wildlife
Conservation Act. These endangered plants ranged from
large mallees (e.g. Eucalyptus crucis subsp crucis) and
small trees (Acacia denticulosa, Banksia verticillata), through
compact shrubs (e.g. Drummondita hasselii var longifolia,
Verticordia staminosa) and climbers (Kennedia beckxiana, K.
macrophylla) to diminutive herbs (Tribonanthes purpurea)
and annual aquatics (Myriophyllum petraeum). While some
endangered taxa have several populations spread across
a number of disjunct outcrops, some are confined to very
few localities (e.g. Myriophyllum lapidicola, known from
just two gnammas). Accidental destruction of such
populations could be catastrophic. Conservation in the
wild in such cases needs to be backed up by off-site
activites such as germplasm storage and artificial
propagation (underway at Kings Park and Botanic
Garden for M. lapidicola and other critically endangered
granite endemics; Dixon 1994).
Apart from endangered species, the diverse
communities on south-western granite outcrops are
noteworthy in the rapidity with which they change
within and between rocks. They are complex, ever-
changing, and rare in their own right.
While many granite outcrops have been spared direct
clearing due to their unsuitability for agriculture, and
some are included within conservation reserves, most
face threatening processes that need management if the
native biota is to persist. Such processes include
replacement or damage by invasive weeds, feral animals,
grazing, inappropriate fire regimes, clearing, loss of
shrub layer, salinity and dieback disease. We have briefly
addressed the issue of weeds above, and highlighted the
importance of maintaining undisturbed soil and dense
shrub layers to control weeds effectively. Such
restoration activities require an ongoing presence and
commitment to a given outcrop. Local communities are
vital in this context.
Western Australians are fortunate in being custodians
of a unique and diverse suite of granite outcrop plants.
We hope that this brief review will stimulate others to
study and conserve what is a remarkable heritage.
To all those colleagues who helped with field work,
discussion and ideas, our sincere thanks. L Sage assisted with the analysis
of granite orchid biogeography. R Wyatt and R Ornduff hosted SH on
sabbatical in the USA in 1990, and greatly facilitated development of an
international perspective on granite outcrop plants. S Porembski has
recently broadened this international perspective. We are grateful for
their interest and ideas.
Abbott I 1980 The transition from mainland to island, illustrated
by the flora and landbird fauna of headlands, peninsulas
and islands near Albany, Western Australia. Journal of the
Royal Society of Western Australia 63:79-92.
vertebrate fauna of Chatham Island, Western Australia.
Journal of the Royal Society of Western Australia 60:65-70.
Beard JS 1980 A new phytogeographic map of Western
Australia. Western Australian Herbarium Research Notes
Series. Swan. University of Western Australia, Perth.
Beard JS 1990 Plant Life of Western Australia. Kangaroo
Bindon P 1997 Aboriginal people and granite domes. Journal
of the Royal Society of Western Australia 80:173-179.
Brooker MG & Margules CR 1996 The relative conservation
value of remnant patches of native vegetation in the
wheatbelt of Western Australia: I. Plant diversity. Pacific
Conservation Biology 2:268-278.
Brown R 1810 Prodromus Florae Novae Hollandiae et Insulae
Van-Diemen exhibens characteres plantarum quas annis
1802-1805. Taylor, London.
Burgman MA 1987 An analysis of the distribution of plants
on granite outcrops in southern Western Australia using
Mantel tests. Vegetatio 71:79-86.
Bussell JD & James SH 1997 Rocks as museums of
evolutionary processes. Journal of the Royal Society of
Western Australia 80:221-229.
Diels L 1906 Die Pflanzenwelt von West-Australien sudlich des
Wendekreises. Englemann, Leipzig.
Dixon KW 1994 Towards integrated conservation of Australian
endangered plants - the Western Australian model.
Biodiversity and Conservation 3:148-159.
Erickson R 1969 The Drummonds of Hawthornden. Lamb
Erickson R, George AS, Marchant NG & Morcombe MK 1973
Flowers and Plants of Western Australia. Reed Books,
Gaff DF 1981 The biology of resurrection plants. In: The
Biology of Australian Plants (eds JS Pate & AJ McComb).
The University of Western Australia Press, Nedlands, 114-
Grant V 1981 Plant Speciation. Colombia University Press, New
Green JW 1985 Census of the Vascular Plants of Western
Australia. Western Australian Herbarium, Department of
Hopper SD 1979 Biogeographical aspects of speciation in the
south west Australian flora. Annual Review of Ecology
and Systematics 10:399-422.
Hopper SD 1981 Honeyeaters and their winter food plants on
granite rocks in the central wheatbelt of Western Australia.
Australian Wildlife Research 8:187-97.
Hopper SD 1992 Patterns of diversity at the population and
species levels in south-west Australian mediterranean
ecosystems. In: Biodiversity of Mediterranean Ecosystems in
Australia (ed RJ Hobbs). Surrey Beatty & Sons, Sydney, 27-
Hopper SD 1995 Evolutionary networks: natural hybridization
the Role of Networks (eds DA Saunders, JL Craig & EM
Mattiske). Surrey Beatty & Sons, Chipping Norton, NSW, 51-
conservation biology in practice. In: Conservation Biology
for the Coming Decade (eds PL Fiedler & P Kareiva).
Chapman & Hall, New York. In press.
Hopper SD & Burgman MA 1983 Cladistic and phenetic
analyses of phylogenetic relationships among populations of
Hopper SD, Harvey MS, Chappill JA, Main AR & Main BY 1996
The Western Australian biota as Gondwanan Heritage - a
review. In: Gondwanan Heritage: Past, Present and Future of
the Western Australian Biota (eds SD Hopper, JA Chappill,
MS Harvey & AS George). Surrey Beatty & Sons, Chipping
Norton, NSW, 1-46.
Hopper SD, van Leeuwen S, Brown AP & Patrick SJ 1990 Western
Australia’s Endangered Flora. Department of Conservation
and Land Management, Perth.
Houle G 1990 Species-area relationship during primary
succession in granite outcrop plant communities. American
Journal of Botany 77:1433-1439.
James SH 1965 Complex hybridity in Isotoma petraea I. The
occurrence of interchange heterozygosity, autogamy and a
balanced lethal system. Heredity 20:341-53.
Keighery GJ 1995 How many weeds? In: Invasive Weeds and
Regenerating Ecosystems in Western Australia (ed G Burke).
Institute of Science & Technology Policy, Murdoch
University, Perth, 8-12.
Maiden JH 1909 Records of Western Australian Botanists.
Journal of the Western Australian Natural History Society
Main BY 1967 Between Wodjil and Tor. Jacaranda Press,
Main BY 1997 Granite outcrops: A collective ecosystem. Journal
of the Royal Society of Western Australia 80:113-122.
Marchant NG 1973 Species diversity in the south-western flora.
Journal of the Royal Society of Western Australia 56:23-30.
Mares MA 1997 The geobiological interface: Granitic outcrops
as a selective force in mammalian evolution. Journal of the
Royal Society of Western Australia 80:131-139.
Moran GF & Hopper SD 1983 Genetic diversity and the insular
population structure of the rare granite rock species,
of Western Australia 80:87-100.
Newbey KR & Hnatiuk RJ 1985 Vegetation and flora. In: The
Biological Survey of the Eastern Goldfields. Part 3. Jackson-
Kalgoorlie Study Area. Records of the Western Australian
Museum Supplement 23:11-38.
Newbey KR, Keighery GJ & Hall NJ 1995 Vegetation and flora. In:
The Biological Survey of the Eastern Goldfields. Part 11.
Boorabbin-Southern Cross Study Area. Records of the Western
Australian Museum Supplement 49:17-30.
Ohlemüller R 1997 Biodiversity patterns of plant communities
in shallow depressions on Western Australian granite
outcrops (inselbergs). Diplomarbeit zur Erlangung des
Grades eines Diplom-Biologen, University of Bonn.
Ornduff R 1986 Comparative fecundity and population
composition of heterostylous and non-heterostylous species
of Villarsia (Menyanthaceae) in Western Australia. American
Journal of Botany 73:282-286.
Ornduff R 1987 Islands on islands: plant life on the granite
outcrops of Western Australia. Harold L. Lyon Arboretum
Lecture Number Fifteen. University of Hawaii Press,
Ornduff R 1996 An unusual floral monomorphism in Villarsia
(Menyanthaceae) and its proposed origin from distyly. In:
Gondwanan Heritage: Past, Present and Future of the
Western Australian Biota (eds SD Hopper, JA Chappill, MS
Harvey & AS George). Surrey Beatty & Sons, Chipping
Norton, NSW, 212-222.
Pate J.S & Dixon KW 1982 Tuberous, Cormous and Bulbous
Plants. The University of Western Australia Press, Nedlands.
Piggott PJ & Sage LW 1997 Remnant fegetation, priority flora
and weed invasions at Yilliminning Rock. Journal of the
Royal Society of Western Australia 80:201-208.
Pignatti E & Pignatti S 1994 Centrolepidi-Hydrocotyletea alatae, a
new class of ephemeral communities in Western Australia.
Journal of Vegetation Science 5:55-62.