Science Education News (SEN) Journal 2017 Volume 66 Number 4 December 2017 | Page 45

GENERAL ARTICLES
Swimming in Sand – Frogs and Sand-Mining at Smiths Lake (continued)
that fresh sand deposits were no longer being exposed and the
release of salt and lime decreased markedly, however these
substances have been deposited in the low-lying areas near the
dune and are yet to be cleared from the surface water there.
Our current understanding of the influence of pH on amphibian
physiology is largely limited to laboratory studies conducted
under artificial conditions. Most frog species are tolerant of
relatively narrow pH ranges (Barth and Wilson 2010). Laboratory
studies suggest a correlation between pH tolerance and inter-
specific variation in distribution, with species inhabiting acidic
environments exhibiting greater tolerance to low pH ( Freda and
Dunson, 1986; Pierce and Wooten, 1992; Picker et al., 1993).
Sustained low pH was found to impede tadpole growth and
increase developmental time; how this affects later stages in the
life cycle of frogs is unknown (Freda and Dunson, 1985; Griffiths
et al., 1993). The effect of increasing pH is likely to be equally
disruptive on frog life cycles. Most micro-geographic changes
in the distribution and abundance of species are the result of
subtle influences of the abiotic and biotic environment, rather
than direct mortality effects (Freda and Dunson 1985).
The exposure of large tracts of elevated dune sand during
mining appears to have led to the deposition of large amount
of dune sands in the nearby low-lying areas, including the dune
pond and swamp. Both sites are considerably shallower than
before mining, and water depth will be a factor in the types of
aquatic vegetation that ultimately re-establishes in these sites.
Unfortunately, the only measurement of sedimentation was water
depth, so little information is available about other changes to the
geochemistry of the water. There have been substantial changes
in the dissolved oxygen content of the water and the oxidation-
reduction potential indicative of other geochemical changes in
water quality.
Meyer et al. (2009) found that altered pH can seriously impact the
development of tadpoles of Litoria fallax, a widespread species
that was present in both the dune and swamp site at Bridge Hill.
L. fallax was abundant in the dune site and remained abundant
throughout the course of the study despite the changes in pH.
In the swamp site, L. fallax was initially a rarely encountered
species but became more commonly encountered over time as
the pH's became less acidic in the swamp. Barth and Wilson
(2010) found that Australian frogs that occur in wallum habitats
(such as the swamp site at Bridge Hill) are not only sensitive to pH
changes but also respond in unpredicted ways to the presence
or absence of the organic acids in the water. Changes in pH in
wallum water alter the solubility and activity of organic acids, and
these changes affect the survivorship of frogs in the area.
As the salinity and pH of surface water in the dune pond and
swamp are still well above levels recorded pre-mining, and 34
years have now elapsed, it must be concluded that the elevated
salinity and pH levels must be determining factors in the re-
colonisation of these sites by frogs. The longer the salt and pH
levels remain high the longer it will take for the frog communities
to re-establish themselves.
Most of the water quality variables measured show the greatest
changes during and immediately following the mining phase.
Current water conditions fluctuate less dramatically and have
returned to levels more similar to pre-mining levels. However, the
combined effects of the initial changes wrought by mining, and
secondary changes caused by the subsequent modifications
to the environment, have impeded the ability of frogs to re-
colonise these sites. Little is documented about the ability of
frogs to respond to transient and permanent habitat changes
to the condition of their habitats. The only studies in Australia
that have considered their ability to re-colonise areas after major
disturbances have concentrated on recovery after wild fires
rather than after mining (Andrew 2011, Bamford 1992, Bamford
and Roberts 2003, Driscoll and Roberts 1997, Gillespie and West
2012, Lemckert et al. 2004, Penman et al. 2006). All of these
studies indicated that if the frogs survived the immediate burning
phase there was a high rate of re-colonisation of habitats, provided
their habitats had not been significantly altered by excessive ash
accumulation or repeated burning that led to vegetation changes.
It appears that sand mining-activated releases of salt and
calcium carbonate through the surface water may be enough
to explain the subsequent changes in frog communities. The
changes in salinity and pH would have an immediate impact
on tadpoles (which are unable to move out of the hydrological
impact area), and to a lesser extent on frogs (since they can
move out of the impacted area). The changes in salinity and pH
will then promulgate changes in the vegetation in the affected
area, compounding the effect of altered water geochemistry.
Recovery of Frog Habitats
The changes to frog habitats in areas close to the mine path
appear to have occurred in two phases: there was an immediate
impact associated with the release of salt and lime from the dune,
and then there appears to have been a second change later that
appears to be associated with increased sedimentation of the
areas below the dune (Tables 3 and 4). Sand-mining finished
at Bridge Hill in 1983 and re-vegetation works were carried out
for another two years afterwards. The cessation of mining meant
Fox and Fox (1984) suggested that 20 years would sufficient
time for small mammal re-colonisation of sand-mined areas,
Twigg and Fox (1991) suggested a similar lag time for re-
colonisation by reptiles. At Bridge Hill, some re-colonisation by
frogs has occurred, but the frog communities have not returned
to pre-mining species composition or abundances. Clearly the
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SCIENCE EDUCATIONAL NEWS VOL 66 NO 4