Baseline monitoring of the Tasmanian Glow-worm and other cave fauna: Exit Cave and Mystery Creek Cave – Tasmania

Exit Cave and Mystery Creek Cave in the Ida Bay karst system in south-east
Tasmania contain a diverse and significant cave fauna. The most superlative faunal
feature of these caves is the light displays by the Tasmanian Glow-worm
Arachnocampa tasmaniensis. These displays have been recognised as a world heritage
value under the criterion relating to outstanding natural phenomena. The caves also
provide habitat for several species listed as rare under the Tasmanian Threatened
Species Protection Act 1995; the Blind Cave Beetle Goedotrechus mendumae, the Ida
Bay Cave Beetle Idacarabus troglodytes, and the Ida Bay Cave Harvestman,
Hickmanoxyomma cavaticum.
Although the two caves are protected within the Southwest National Park and are
largely undisturbed by human impacts, the tourism potential of Mystery Creek Cave
has been recognised and ways to improve visitor access have been suggested. Any
changes to the management of these caves need to take into account the ecology of the
glow-worms, and other cave fauna, to avoid the major decline that occurred in a glowworm
population in New Zealand following changes to the cave entrance for the
benefit of tourists.
The present study aimed to establish baseline monitoring data on the Tasmanian
Glow-worm, and other cave fauna, in Exit and Mystery Creek caves that will be
useful in determining changes to their populations as a result of changes in cave
management or other potential threats. The present study also aimed to obtain, for the
first time, information on the ecology of the Tasmanian Glow-worm.
Cave fauna were monitored monthly for 24 months in both Exit and Mystery Creek
Cave. In the Tasmania Glow-worm, a strong seasonal pattern was found, with pupae
and adults most common in spring and summer. The increase in numbers of pupae
and adults coincided with an increase in the number of prey caught in silk threads
produced by the larvae. Larvae were present throughout the year but the number
glowing varied both seasonally and spatially. In Mystery Creek Cave, the number of
larvae glowing was generally highest during summer and autumn and lowest in winter
and early spring. In Exit Cave, there was no consistent seasonal pattern in the number
of larvae glowing among sites, and overall there was less variation between monthly
counts than at Mystery Creek Cave. This difference in seasonal patterns between the
two caves may be due to a difference in climate, with Mystery Creek Cave possibly
experiencing a greater drying out of the cave air in winter than Exit Cave.
The life cycle of the Tasmanian Glow-worm differs to some extent from that reported
for another species of glow-worm, A. luminosa, in Waitomo Cave, New Zealand. In
A. luminosa most life stages of the glow-worm are present throughout the year
although there is an underlying annual cycle. Pupae and adults are most common
during winter and larvae are most common during spring and summer and this is
when the glow-worm display is at its best. The differences between the life cycles of
the two species of glow-worm probably result from differences in latitude and climate
between the two study locations.
Monthly counts of cave crickets and other cave fauna, which were common in Exit
Cave and uncommon in Mystery Creek Cave, revealed few interpretable patterns. The
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only consistent pattern observed was in the part of Exit Cave known as the ‘wind
tunnel’ where cave cricket and cave beetle numbers were high during the warmer
months and low during the cooler months. This is likely to be a response to the winter
effect in that part of Exit Cave.
This study established baseline monitoring data for the Tasmanian Glow-worm in Exit
Cave and Mystery Creek Cave that could be used to compare with any future
monitoring that may be required. On-going monitoring of glow-worms is a low
priority unless a potential threat is identified. Some low level monitoring of glowworms
in Mystery Creek Cave, which has unrestricted visitor access, may be prudent.
The current extent of the glow-worm colonies in Exit Cave and Mystery Creek Cave
should be mapped in both summer and winter to establish baseline data on their
distribution.
Because of the low numbers of beetles, spiders and harvestman recorded during the
present study, a more targeted monitoring program is required for these species to
improve the detection of changes in abundance. This would require a significant
increase in survey effort and is probably not justified at present given the current low
level of threat to these species.