Since European settlement in 1788, Australia has had the highest mammal extinction rate in the world. At least one third of Australia’s original or pre-European terrestrial mammal fauna is now extinct or threatened with extinction (Fusco et al. 2017). However, the ongoing recognition of new taxa (e.g., Start et al. 2012, who report the discovery of three new species of rodents that were probably extant when Europeans colonised the Kimberley region of north Western Australia) suggests the extent of Australia’s biodiversity loss is underestimated (Abbott 2000; Woinarski et al. 2014; Abbott & Wills 2016; Burbidge & Abbott 2017). European-settlement-related impacts pose many challenges for the conservation and restoration of Australia’s ecosystems. Landscape modification, associated habitat loss and the introduction of exotic species caused the rapid extinction of numerous taxa and mainland extirpation of at least nine mammals that now survive on land-bridge islands only (Woinarski et al. 2014; Burbidge et al. 2018).
Local extinctions are still happening at an alarming rate, with at least 39 extant mammal species vanishing from >75% of the bioregions they are known to have occupied before European settlement. Some species were deliberately persecuted (e.g., the Thylacine – Thylacinus cynocephalus), some were lost through neglect or apathy (e.g., the recently extinct Bramble Cay Melomys – Melomys rubicola) and others appear to have simply been in the wrong place at the wrong time (e.g., Gilbert’s Potoroo – Potorous gilbertii, though not extinct, Gilbert’s Potoroo is the most endangered marsupial in the world and recently lost 90 per cent of its natural habitat to bushfires).
Our appreciation of diversity loss is further masked by poor documentation of species biogeography by colonists. The distributions of many species were not recorded before they were extirpated or extinguished. The result –– shifting baselines. A baseline in this context is a snapshot of the ecology from a given area at a given time against which biodiversity loss can be measured. Australian conservation managers essentially have a choice between four basic timeframes from which to establish baselines:
employment date of the current park manager;
park proclamation date;
shortly before European arrival (late 18th century);
pre-human arrival (ca. 50,000 years ago) (Hayward 2009).
If timeframes 1 or 2 are adopted, it is almost certain that ‘baseline shift’ will occur. Baseline shift (or ‘shifting baseline syndrome’) transpires when an arbitrary baseline is accepted and used to evaluate change with no consideration of prior human-induced ecological modification (Miller 2005). A growing body of research on Holocene fossil accumulations (Fig. 2) is providing insights into the composition and biogeography of Australian ecosystems prior to European settlement; data that can be used to establish appropriate baselines, measure biodiversity loss and establish the potential for species translocations and reintroductions, particularly to islands (McDowell 2014).
Examples of the differences between pre- and post-European mammal diversity (Table 1) show that if a modern (i.e. post-European) diversity is accepted as the baseline, as much as 80% of a region’s pre-European mammal community may be overlooked. I argue that late Holocene species records, where available, constitute the most appropriate baseline for management and restoration planning. Even if a species has been extirpated from a region, maintenance or restoration of its ecosystem to the highest possible standard could facilitate future reintroduction and may even provide potential for natural repopulation (Falk et al. 2006; Lunt et al. 2013).
Table 1: Summary of Modern and Holocene fossil mammals recorded in various geographic regions of Australia including three Kimberley rainfall zones. NSW = New South Wales; NT = Northern Territory; SA = South Australia; Vic. = Victoria; WA = Western Australia.
Geographic No. modern No. fossil Species Source
Region mammals mammals loss (%)
Kimberley, WA 18 26 30.8 Start et al. 2010
(700–900 mm)
Kimberley, WA 14 26 46.2 Start et al. 2010
(600–800 mm)
Kimberley, WA 8 16 50.0 Start et al. 2010
(500–600 mm)
Pilbara, WA 25 37 32.4 Baynes &
McDowell 2010
Uluru National 23 34 32.4 Baynes & Baird
Park, NT 1992
Jenolan, NSW 17 26 34.6 Morris et al. 1997
Gippsland, 10 28 64.3 Bilney et al. 2010
Vic.
Naracoorte, 19 30 36.7 Macken & Reed
SA 2013
Yorke Peninsula, 82 77 0.4 McDowell et al.
SA 2012
Eyre Peninsula, 5 25 80.0 McDowell &
SA Medlin 2010
Whilst I advocate the use of pre-European faunas as baselines for ecological management, it is important to remember that ecosystems are dynamic and naturally variable. In fact, many Australian ecosystems are characterised by natural variability, which provides long-term resilience to disturbances such as fire, drought and climate change (Walker et al. 2004). However, fossil assemblages indicate that most native mammals persist over thousands of years. Therefore, Holocene fossil accumulations can make significant contributions to management programmes by providing long-term perspectives on ecological systems (Lyman 2006; Froyd & Willis 2008).
When a species is lost from a community the processes and functions they performed are also lost. All mammal species contribute to the maintenance of their community ecology, but few would contribute more than fossorial (digging) species such as bettongs, potoroos and bandicoots. Bettongs are particularly important because most species mainly eat hypogeal fungi (truffles). Consequently, they spread fungal spores wherever they dig. As all Eucalypts form a symbiotic relationship with hypogeal fungus for at least some part of their life, by spreading spores, bettongs perform a fundamental ecosystem service. They also facilitate seedling germination and establishment, soil aeration, incorporation of organic matter and improvement moisture infiltration.
What about rabbits I hear you say? Rabbits are also fossorial but don’t dig anywhere as deeply or widely, and therefore do not appear to contribute to soil improvement anywhere near as effectively as native mammals (Eldridge and James 2009). The almost total loss of native “ecosystem engineers” from mainland Australia has far-reaching implications that may ultimately lead to vegetation succession. I recently described a Bettong (Bettongia anhydra; McDowell et al. 2015; Fig. 3) that was collected alive near the NT-SA border in 1933. Until now it was considered synonymous with its morphologically similar cousin Bettongia lesueur. While its capture by Europeans may be considered unlucky for the individual, it was lucky for us because the species has never been encountered alive since. How many other native mammals have been lost without being recognised or rest in museum cabinets just waiting for the right person to look at them?
Recent breakthroughs in DNA analyses have shown that what were once considered wide ranging species are in many cases species complexes, revealing even more hidden biodiversity. For example, recent molecular research on Antechinus swainsonii has demonstrated it to be a complex of five species. Following this realisation, morphological differences began to emerge from what were previously considered highly variable physical characteristics. So the good news is Australia’s biodiversity is richer than we thought. The bad news is we’re still losing species at an alarming rate. So what can we do to reduce further loss of our unique mammals?
Obviously we need to protect pristine areas that retain most of their pre-European biodiversity, but before attempting any ecological restoration we need to consult the Holocene fossil record to find out which species that area supported in the recent past. Only then can we formulate management strategies that reflect species’ biogeography and ecological preferences, and with a bit of luck some species may recolonise their former range without help from us, like the Plains mouse (Pseudomys australis) did to Arid Recovery, a 123 square km wildlife reserve in the arid north of South Australia.(http://www.aridrecovery.org.au/arid-recovery-news/build-it-and-they-will-come).
All posts are personal reflections of the blog-post author and do not necessarily reflect the views of other DEEP members.
References:
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