Claire Goldsbrough
The ecology of rock-dwelling fauna of the Hawkesbury sandstone
B.Sc.(Hons) 1998
Understanding the processes responsible for retreat-site selection by animals
is important for interpreting their patterns of distribution and abundance,
and may aid the development of strategies for management and conservation for
biological systems. Hawkesbury sandstone areas around Sydney are floristically
and geologically distinctive, but little is known of the ecology of rock-dwelling
fauna in these areas. I conducted a series of field surveys and field and laboratory
experiments to identify the species assemblages that are associated with loose
surface rocks, and to investigate the biotic and abiotic factors involved in
the selection of rock crevices as retreat-sites. Field sampling at Ku-ring-gai
Chase National Park and Muogamarrra Nature Reserve revealed that sandstone rocks
harbour a highly diverse fauna, with spiders (Araneae) and ants (Hymenoptera:
Formicidae) as the two most species-rich taxa. Field survey data indicated that
several species, including coppertail skinks (Ctenotus taeniolatus) and flat
rock spiders (Hemicloea major), were non-randomly distributed among rocks with
respect to rock size, substrate and level of shading. Substrate type was an
extremely good predictor of rock use for both of these animals. The degree to
which the invertebrate assemblages were related to rock characteristics differed
spatially, but consistent patterns emerged. Again, substrate type emerged as
an important cue. Factors that determine thermal regimes in retreat-sites (such
as rock thickness and shading) were less important for these taxa than for the
previously studied broad-headed snakes (Hoplocephalus bungaroides) and velvet
geckoes (Oedura lesueurii). Artificial shading of rocks in the field did not
affect the presence of coppertail skinks or flat rock spiders, although the
results may have been confounded by methodological shortcomings. Retreat-site
selection experiments conducted in the laboratory revealed that the lizards
and spiders select warmer rocks rather than cooler rocks, and that lizards select
sand rather than rock substrate for their nocturnal retreat-sites. Rearing juvenile
flat rock spiders at three different thermal regimes demonstrated that these
spiders grow and develop faster if they occupy warmer retreat-sites. Additionally,
manipulation of spider density and habitat complexity influenced survival, perhaps
explaining the solitary occupation of retreat-sites by flat rock spiders and
their tendency to disperse after hatching. Considered together, the findings
from my study demonstrate that rock-dwelling fauna of the Hawkesbury sandstone
display complex responses to a range of biotic and abiotic cues in retreat-site
selection. Effective management of these habitats for conservation will require
further studies and a more detailed understanding of their ecology.