03. Scott Hocknull - Dinosaur Man
Mt Etna DATING FAUNAL SUMMARY
Graph showing changes in species abundance over time from a hydric (water-abundant) habitat, through to a xeric (arid) one and then to a mesic (medium water level) habitat. Image: Scott Hocknull, Queensland Museum, Creative Commons licence BY-NC-SA.
Extinction of the rainforest
Throughout its history, the rainforests at Mt Etna have been relatively stable, even during times of glacial and inter-glacial cycles. Therefore, the peaks and troughs of climate don’t seem to affect the diversity and stability of this fragile ecosystem as much as something else. This other factor caused a major change in animal species and the extinction of rainforest in the region.
Something happened between 280 and 205ka that changed Mt Etna forever. There was a loss of over 80% of small to medium-sized mammalian species. New species replaced those that died out with about 75% replacement. Frog species richness decreased with new forms appearing such as Neobatrachus and Cyclorana, the water-holding frog. Lizard species numbers increased with the appearance of new xeric-adapted species.
These rainforests were replaced by an arid-adapted fauna and since this time several other faunal turnovers have occurred. Over the youngest middle Pleistocene (205–170ka) and Holocene fauna, frog and lizard species remained relatively stable. Three agamid (dragon) lizard species were replaced with three new ones: the bearded dragon; frilled-neck lizard; and the two-lined dragon.
Looking at this faunal turnover from a regional scale, things began to change from about 350,000 years ago. Thresholds were reached where flora, fauna and climate shift to a completely different, drier and more unpredictable climate. The presence of arid-adapted species indicates a marked drop in rainfall during this faunal turnover at Mt Etna. This was probably a greater drop than what had occurred during previous glacials.
Climate changed dramatically, perhaps trying to find a new equilibrium that had dominated the past 4-5 million years. Ecosystems have to respond to these phase shifts or go extinct. The rainforest at Mt Etna simply reached a precipice. It could not come back from it, thus became extinct.
Compare this to the fossil story from Naracoorte Caves in South Australia, where over the same period ecosystems changed. However, here species were quicker to recover and less extinctions occurred.
The climate had started to change much earlier in the south. The Naracoorte fauna had already stabilised and was able to respond to these new climatic extremes. The trend towards aridity started about 600-500ka in southern regions and as early as 4-2 million years ago in central Australia. This arid trend did not start until 280ka at Mt Etna.
The striking difference between the Mt Etna and the Naracoorte records is that the Mt Etna non-megafaunal animals were driven extinct during this arid event. At Naracoorte, the fauna was more stable throughout the middle Pleistocene to the present day. This may reflect a longer time to this earlier onset of climate change. In contrast, the fauna at Mt Etna did not have as much time to adapt to the changed climate. This may be the reason for the greater number of extinctions.
The major faunal turnover at Mt Etna from 280-205ka was associated with this trend to increasing aridity. This may have been due to a possible shift towards an El Nino effect and decreasing monsoons. This would have led to increased aridity throughout northern Australia. As climate change occurred over a shorter time period, northern species did not have as much time to adapt. Species could not recover, leading to more extinctions.
What then does this mean for our present day rainforests?
- Are we now on the verge of another phase shift but instead of it being natural, it’s human-made?
- Is the short period of climate change the factor that will force some of our endangered species further down the road to extinction?