Essay 9. Clearing the Future: Ecosystems, Agriculture, & A History of Land Clearing In Queensland

golden-tailed-gecko-jt-sml
This Golden-Tailed Gecko (Strophurus taenicauda) is just one of many threatened species living in the ever decreasing woodlands of the Brigalow Belt.

Since early in human history one of our greatest survival tools, aside from communication, has been our ability to modify the environment to suit our needs (1). This might range from simply changing the things around us in form, such as creating structures for shelter or modifying and manipulating timber, stone, or other materials for tool making and so on, through to large scale changes like fire-farming, vegetation clearing, soil tilling/earthworks, even changing landscape hydrology by redirecting or damming water flow, and more (2). As the most intelligent species to have evolved on this planet, our capacity to conceptualize and, through planning & coordination, manifest significant changes to our surroundings is often overlooked. How commonplace building a new roadway must seem to the passers by, zipping past the construction fencing on adjacent, previously designed streets & motorways. Yet no other organism we know can affect change on this scale, let alone for it’s own, pre-planned, (hopefully) well thought-out purposes.

Such powers come, as always, with caveats. We depend on our environment for survival, yet one of humanity’s greatest survival tools is the taming and managing of our environment. In doing so, we cause a lot of damage to the environmental processes around us, known as ecosystem services, that we depend on (3; 4; 5). Some such services are obvious, like O2 production and CO2 capture through photosynthesis by plants and phytoplankton, essential for producing and maintaining a breathable atmosphere. Wetlands and their plants offer important bio-filtration services for water sources. Aside from the obvious bounty of vegetable and meat products which many of us take for granted we also rely on both an abundance and diverse community of animals, including a wide variety of insects, for pollination services(6; 7). These busy ecosystem workers are constantly spreading genetic material across the landscape, on both day shift and night shift, from flower to flower, giving rise to the fruits and seeds of the future. Without them much of our food production ceases.

This leads to an obvious question: in what ways do human activities interfere with these ecosystem services? Let’s briefly run through the main impacts, keeping in mind that more examples can be drawn. According to the Millennium Ecosystem Assesment, a United Nations led appraisal of human environmental impacts involving over 1,300 worldwide experts (4), ecosystem services can be placed in four categories. Provisioning Services are our sources of goods such as food, fibers, & fuels. Regulating Services manage our environment, including climate, mitigating disease outbreaks, water purification, even the aforementioned pollination management. Ecosystems provide a wide range of Cultural Services, not just for those with spiritual or traditional values, but for recreation, aesthetic appreciation, general and mental well being, as well as the financial/educational benefits of ecotourism. Supporting Services, which humans don’t interact with in principle, provide the basic support for all other ecosystem functions and services, including basic energy provision through photosynthesis, or primary production. A further consideration, all of these services, as well as their oversight and management, provide careers for many, and a significant portion of the global economy (4; 5; 7)

Our impacts on the Provisioning Services have been, numerically, both positive and negative (4). An increase in overall production through improved efficiency of the main sources of most western diets (crops, aquaculture, & livestock) comes with a decrease in wild production, unsustainable water use and habitat clearing, and reduction of diverse ecosystems to single species mono-cultures (6; 8). Resulting local extinctions are irreversible losses of genetic diversity, as is the reduction of crop varieties. Medically inclined readers might also note the loss of possible plant medicines and bio-pharmaceuticals, their genetic recipes replaced, perhaps, by a single bland root vegetable (fuck turnips). Turning to Regulating Services, one need only look at urban air pollution and the associated health risks to see the dangerous effects of a declining ability of our environment to cleanse the air (9). Globally, climate change is already causing large environmental and social issues (10; 11). We’ve also managed to increase soil erosion, hinder water purification systems, make it easier for pests and diseases through our environment, reduce the number of pollinators, and altered our mangroves and coastlines’ ability to withstand natural disasters. All Cultural Services suffer from the decreased extent and health of our natural spaces, resulting in losses of sacred sites and species. While recreation and ecotourism is expanding, it is destined to occur in increasingly degraded sites, less valued by tourists and commerce. In total, 60% of ecosystem services assessed were degraded (4). This trend continues despite extensive efforts in green education and awareness, which seem outpaced by our ability to consume and clear forests (12; 13).

These impacts are largely the result of over-harvesting and competition for space and resources (4). Our excessive reliance on a polluting, finite energy source such as fossil fuels and our continuing extensive clearing of forested land are both leading causes of environmental decline (8). Here in Australia, land clearing rates have trebled since 2009 after recent reforms were removed and we are now, of eleven globally recognized deforestation fronts, the only one in a developed nation (14). Aside from the lost potential for carbon sequestration and the release of greenhouse gasses through decomposing vegetation and soil disturbance during land clearing (8), habitat loss is a major threat to many species both locally and globally, threatening many Australian species such as the Koala with extinction if changes aren’t made soon (5; 15).

While land care and conservation groups continue the hard work of raising awareness and combating both fossil fuel developments and unsustainable clearing of habitat, the loss of and degradation of our natural systems continues. Of course, there are plenty who argue that current rates of tree clearing, gas, coal, and other mining developments are in fact sustainable, even necessary for local economies and job security (16). As conservationists, it is often a difficult task to reconcile environmental and social necessities. That said, this is precisely the task at hand, and if we are to achieve sustainability as a society our management decisions must include consideration the needs of both the environment and all our fellow inhabitants. To any readers who might believe that slowing tree clearing is less important than the jobs that come from those industries involved, may this serve as my sincere attempt to persuade you otherwise.

As they say, think local etc. Having lived here for most of my life, let’s look at the history of land clearing in Queensland, Australia, and how our current practices might be impacting the state of things today. The overall picture for Australia, despite our high biodiversity values and rare, endemic fauna and flora, is not good. In 1999, Australia had the fifth highest land clearance rate globally, beat only by Brazil, Bolivia, Indonesia and the Democratic Republic of Congo (17). Based on the Native Vegetation Inventory Assessment (NVIS), the area of estimated remaining vegetation in Australia in 2004 (compared to pre-1750 vegetation) is 87%. While this sounds promising, the losses have been uneven, with 34% of rain forest destroyed, while only 2% of Acacia-dominant open woodland has been lost. Additionally, remote sensing satellite data from the Australian Geospatial Intelligence Organization shows a troubling disparity with the NVIS, with only 33% of the area defined as ‘woody vegetation’ appearing to contain woody vegetation. This potentially leaves a mere 26% of woody vegetation remaining in Australia as of 2004, a stark contrast to the 78% shown in the NVIS.

Turning to Queensland specifically, hows does the history of land use compare to the rest of Australia? Unfortunately, Queensland has long history of land clearing since white settlement in 1825 as a penal colony for the southern states’ more troublesome convicts, forming government as a separate colony in 1859 (18). In an attempt to encourage new settlers The Alienation of Crown Lands Act of 1860 and the Unoccupied Crown Lands Occupation Act of 1860 soon followed, treating the bulk of Queensland as ‘Wastelands of the Crown’ despite some of the largest populations of indigenous people in the country. With 25,000 new colonial arrivals over the next three years, the state began a period of expansion resulting in more clearing and development, particularly along the east coast, along with further atrocities against indigenous people and immigrants of colour. These included the removal of ‘natives’ to government defined sanctuary areas to allow for white development, the adoption of the White Australia policy along with the state’s formation in 1901, and the massacre of at least 60,000 indigenous people during the early settlement period, often at the hands of a state run “Native Police Force” (19). This systematic oppression did not end until the amendment of various discriminatory and racist Acts and the passing of new legislation recognizing indigenous rights throughout the 1970s, though discrimination and social inequality continues in many ways to this very day.

While the east coast underwent a rapid growth of colonial settlements, it is the more central parts of the state which bore the brunt of tree clearing (20). During the early stages, clearing focused on grasslands and open alluvial Eucalyptus woodlands, considered to have greater pastoral value, with some landholders owning 100,000 hectares or more of unfenced sheep property. As sheep counted for up to 80% of the Queensland economy, tree clearing was initially slow. As of 1880 there was 97-98% forest cover remaining, although the hard-hoofed, grazing livestock likely had a greater impact on grassland ecosystems. This period was followed by several reforms in legislation regarding property size in an attempt to further increase population, and the rapid spread of invasive Prickly Pear cactus (various species of Opuntia) throughout the landscape, eventually tamed by release of the Cactus Moth (Cactoblastis cactorum) (21). Tree clearing rates for dairy and agriculture varied, increasing in the 1930s following the Great Depression as governments subsidized unemployed labour to improve production.

As the population expanded so did the size and reach of settlements, with an increase in sheep, cattle, and agriculture (20). With this began the rapid clearing of the semi-arid zones and more western regions through the 1950s, particularly the Brigalow Belt. Running north from inland northern New South Wales (22), between coastal forests and the drier semi-arid regions further west and reaching the coast at it’s most northern point near Townsville, this wide band of low, dense woodland and forest often features hardy species of Acacia including the Brigalow tree (A. harpophylla). While it can reach 25 meters in height, much of the Brigalow bushland is today often characterized by smaller, denser, bushier growth and suckers from the root systems nearby. When cut down, Brigalow has an amazing capacity to regrow multiple shoots and suckers from the roots (24). While these suckers are certainly a feature of natural systems, the density of bushy growth is a consequence and testament to the difficulty of clearing in the past.

In 1953, it was determined that the Brigalow bioregions represented “the greatest potential of any land for development in this State” (20). This was followed in 1962 by The Brigalow and Other Lands Development Act and Brigalow Development Scheme, designating massive expanses of Brigalow to be cleared for agriculture (20; 24). Despite it’s resilience, the arrival of modern technology was a turning point in the clearing of Brigalow. Until the mid-1940s, clearing was generally achieved by “ringbarking”, cutting or constricting the trees outer layers at the base to stop the flow of nutrients and killing the tree. The introduction of bulldozers and military vehicles massively increased the rate of clearing by allowing removal or killing of the Brigalow root systems, becoming commonplace by the 1950s. With commencement of the Brigalow Development Scheme in 1962, tractor-chain clearing was heavily employed, as was blade ploughing where tractors hauled an underground blade to sever the Brigalow from it’s roots, preventing any regrowth. While landholders were to aim for 10% vegetation retention, the vast fires lit when burning dead Brigalow often meant little or no vegetation remained. Within 5 years, 30% of the Brigalow Belt had been destroyed.

It wasn’t until the ’80s that scientists raised concerns about the impacts on biodiversity, but clearing continued with little opposition until recently with increasing awareness and recognition of the value of Queensland’s ecosystems, as well as the unique species and communities of the Brigalow (20;25). Despite the massive reduction in landcover and seemingly inhospitable, dry conditions, remnant and regrowth Brigalow ecosystems harbour a surprisingly large variety of fauna (26). Reptiles are especially reliant on Brigaow landscapes, which contain some of the highest reptile diversity of any forest type in Australia, home to 148 species of which 13 are endemic, making up a significant portion of habitat for 14 more, including multiple threatened species such as the Ornamental snake (Denisonia maculata), Golden-Tailed Gecko (Strophurus taenicauda), Yakka Skink (Egernia rugosa), and Brigalow Scaly-Foot (Paradelma orientalis) (27; 28; 29). With recognition of the Brigalow’s biodiversity values and the critical state of Queensland’s forests came The Vegetation Management and Other Legislation Amendment Act 2004, aiming to end broad scale tree clearing by 2006. Nonetheless, by 2009 it was estimated that the extent of remnant Brigalow was down to a mere 8% or so, now protected and endangered (30).

Most recently, changes to the Vegetation Management Act by the Newman LNP Government in 2012 led to a return of broad scale clearing, particularly in Queensland’s north. According to the latest Statewide Land Cover and Tree Study (SLATS) report, 296,000 hectares of woody vegetation was cleared between 2014-15, almost doubling since the changes in 2012 (31; 32). Accordingly, the current Queensland State Government recently attempted to pass amendments to protect vegetation from further broad scale clearing. Opponents blocked it in parliament, fearing losses to economic development in Australia’s north, claiming the current plans provide adequate protection for both vegetation and the reef (33). This is despite over 100,000 hectares cleared in the Great Barrier Reef catchment basin in the previous year, potentially harming the already declining water quality of the reef, even risking it’s standing on the UNESCO World Heritage List (32).

And so we come to a familiar crossroad. At this critical time for the planet’s climate, and considering the history, former extent, and current tree clearing rates in Queensland, it seems obvious to me that a change is needed to slow down the destruction, increase the amount of carbon offset through new growth, and help native ecosystems recover. Such rhetoric might seem useless to those who depend on land clearing for their livelihood in the remote lands of Australia’s north. What kind of impact on people’s livelihood would slowing down tree clearing rates have?

Lucky for us we need only look back a decade or so, as the current changes would simply reinstate protections passed in the Vegetation Management Act of 2004. Opponents claim these changes would make the state’s agriculture and business less competitive, closing down new developments in the area (16). This is no small issue, as agriculture makes up a significant portion of Queensland’s economy; we’re Australia’s largest producer of beef, pork, and vegetables with estimated sales of around $3.26, $2.10, and $1.21 billion respectively in 2013-14 (34). Agriculture is also the main driver of clearing, with additional pressure from urbanization and natural resource extraction (4). Nonetheless, between 2004-14 the agricultural sector grew by $2 billion in profitability showing that both environment and agriculture can be balanced (although some would say more protections were necessary). Land clearing continued, albeit under stricter guidelines to minimize the damage.

A key indicator of agricultural economics is the “farmers’ terms of trade”, a ratio of prices farmers received for produce over prices paid for supplies such as fertilizer. Despite ongoing claims that such acts would economically damage farmers, the terms of trade were not affected by the legislation (35). Furthermore, there is strong statistical correlation between farmers terms of trade and tree clearance with an approximate one year lag time. Increasing terms of trade is followed a year later by an increase in clearing, and the same for decreasing terms. Wealth and prosperity allows for greater expenditure on development and clearing, in the hopes of a greater return on capital investment. Assuming, for now, that terms of trade are a good metric for the economy (36) it seems the state of the agricultural economy dictates the rates of agricultural tree clearing, not the other way around. This flies in the face of claims that further tree clearing legislation would hurt farmers. More likely, such restrictions would hinder corporations with projects in extraction and mining, or anyone with unsustainable tree clearing as business models, perhaps explaining their vested interests in Queensland politics and the continuing claims of corruption (37; 38).

An important consideration is how these changes will affect the Queensland’s indigenous population. With much of the clearing occurring in the north, it is sometimes assumed that indigenous people will be most affected, living in these remote northern areas. Nonetheless, while northern communities are certainly the people facing the clearing front, demographic data shows that indigenous people, just like the all of humanity, are increasingly moving to urban centers (39). In fact, the majority of Queensland’s indigenous peoples are already widespread throughout the population, rather than in remote areas.

While traditional owners of these northern land seek the right to develop agriculture should they so chose, the government claims this would still be permitted under permits. Financial compensation would also be provided for any loss of clearing rights to indigenous land, the money to be used for “Aboriginal economic empowerment and development” (40). Despite the caveats and compensations, opponents claim these restrictions would cause “Aboriginal Social and Economic disadvantage”, a serious implication (41) which, in light of Queensland’s history of indigenous oppression and disenfranchisement, must be considered thoroughly, though perhaps by someone more versed in indigenous matters than yours truly. For now I’ll say that while I don’t believe the changes proposed would adversely affect the overall economy (as we saw in 2004), nor traditional owners seeking to start agribusinesses, neither should indigenous people, potentially ideal custodians of the environment (42), be forced to relinquish any land rights. Surely compromise can be found if level heads come to the table. This seems, sadly, an increasing impossibility in the world of modern politics and bi-partisanship where the focus is to beat the other team or make profits, rather than serve the greater good.

With all this in mind, we should remember that we face an uncertain future. We’ve expanded parts of our ecological Provision Services at the expense of many others, while damaging or altering the Regulatory and Cultural Services our ecosystems provide. We’ve decimated central Queensland and the Brigalow Belt, expanded coastal urban sprawls, and continue to clear the northern and central parts of the state at unprecedented rates (22). Global changes in land use from 1997 to 2011 resulted in an estimated loss of $4.3-$20.2 trillion per year in ecosystem services, a figure considered conservative by the authors (5). The Brigalow Belt region is still losing woody vegetation at around 130 000 ha/year, over 40% of total statewide clearing (31), yet we continue broadscale clearing in Queensland, killing wildlife and destroying their habitat in the process, releasing carbon through burning or decomposing timber and damaging soils, often for high greenhouse impact beef. Climate change affects everyone, whether in remote communities or cities. Farmers and producers will often be among the worst to feel the impacts; radically changed weather patterns can cause droughts, disease, and crop failures, severely impacting farming families (43). Urban areas will suffer from mass migration away from failing rural areas, greater risk from sea level rise due to coastal preferences, and ever lower supply despite growing demand for produce leading to higher and higher prices (10; 11). Greater climatic variability and reduced capacity of our ecosystem to recover from severe weather events adds further pressure on humanity’s future prospects.

Although the wealthy and influential seem determined to keep the status quo despite our current destructive course, such a bleak future is only one possible outcome. The Brigalow Belt and other woodlands continue to grow back, and while regrowth needs time to age, it can eventually support similar communities to remnant forest fragments (22; 29; 44). While time might heal a lot of wounds, it is the young who must live with the consequences of their parents’ long-term environmental impacts, a truly unfair prospect if the generations before look like an unending series of exploitation and greed. It seems clear that we need a more sustainable, low impact, renewable based economy, yet how we achieve this transition will also be critical for our democracy and inclusiveness as a society. Since more adults are firmly integrated into societal norms and status quos, young people are more likely to be have positive environmental associations and challenge authority for environmental injustices (45). An awareness of issues and a desire for greater environmental education (46) suggests youth across the globe appear set to take up the challenge of environmental protection. Let us hope they act fast, for the machinery and greed of the old world show no sign of stepping down with dignity, instead obstinately digging their heels in and doing whatever they can to eek out some final profits before the curtains finally close on their environmentally destructive industries.

I also hope ‘kids these days’ like to read long essays…

Ciao,

Janne

References

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  9. Si, Q., Cardinal, B.J. (2017) The Health Impact of Air Pollution and Outdoor Physical Activity on Children and Adolescents in Mainland China. The Journal of Pediatrics. 180: 251 DOI: 10.1016/j.jpeds.2016.10.016

  10. Bradbear, C., Friel, S. (2013) Integrating climate change, food prices and population health. Food Policy. Volume 43, Pages 56–66

  11. Araos, M., Berrang-Ford, L., Ford, J.D., Austin, S.E., Biesbroek, R., Lesnikowski, A. (2016) Climate change adaptation planning in large cities: A systematic global assessment. Environmental Science & Policy. Volume 66, Pages 375–382.

  12. Fien, J., Teh-Cheong Poh Ai, I., Yencken, D. et al. (2002) Youth environmental attitudes in Australia and Brunei: implications for education. The Environmentalist. 22: 205. doi:10.1023/A:1016571526997.

  13. Deniz, D. (2016) Sustainable Thinking and Environmental Awareness through Design Education. Procedia – Environmental Sciences. Improving Sustainability Concept in Developing Countries (ISCDC).Volume 34, Pages 70–79.

  14. newsroom.unsw.edu.au/news/science-tech/australia%E2%80%99s-land-clearing-rate-once-again-among-highest-world

  15. abc.net.au/news/2016-05-07/koala-extinction-imminent-in-southern-queensland-report-warns/7388912

  16. couriermail.com.au/goqld/law-change-will-make-queensland-less-competitive/news-story/d6124bfe57088c7160707998ca313b46

  17. Beeton, R.J.S.,Buckley, K.I., Jones, G.J., Denise, M., Reichelt, R.E., Trewin, D. (2006 Australian State of the Environment Committee), Australia State of the Environment 2006, Independent report to the Australian Government Minister for the Environment and Heritage, Department of the Environment and Heritage, Canberra.

  18. qld.gov.au/about/about-queensland/history/creation-of-state/

  19. law.uq.edu.au/files/1263/Queenslands-Frontier-Killing-Times-Facing-up-to-Genocide-Baldry-McKeon-McDougall-2015.pdf

  20. Seabrook, L., McAlpine, C., Fensham, R. (2006) Cattle, crops and clearing: Regional drivers of landscape change in the Brigalow Belt, Queensland, Australia, 1840–2004. Landscape and Urban Planning. Volume 78, Issue 4, Pages 373–385

  21. Freeman, D.B. (1992) Prickly Pear Menace in Eastern Australia 1880-1940. Geographical Review. Vol. 82, No. 4, pp. 413-429 DOI: 10.2307/215199

  22. Lucas, R.M., Clewley, D., Accad, A., Butler, D., Armston, J., Bowen, M., Bunting, P., Carreiras, J., Dwyer, J., Eyre, T., Kelly, A., McAlpine, C., Pollock, S., Seabrook, L. (2014) Mapping forest growth and degradation stage in the Brigalow Belt Bioregion of Australia through integration of ALOS PALSAR and Landsat-derived foliage projective cover data. Remote Sensing of Environment, Volume 155, Pages 42-57.

  23. Johnson, R.W. (1964) Ecology and Control of Brigalow in Queensland. Queensland Department of Primary Industries, Brisbane.

  24. qhatlas.com.au/content/brigalow

  25. Bailey, A. (Ed.) (1984) The Brigalow Belt of Australia. The Royal Society of Queensland, Brisbane

  26. Peeters, P.J., Butler, D.W. (2014) Brigalow: regrowth benefits management guideline. Department of Science, Information Technology, Innovation and the Arts, Brisbane.

  27. qmdc.org.au/module/documents/download/52

  28. Covacevich, J.A., Couper, P.J., McDonald, K.R. (1998) Reptile diversity at risk in the Brigalow Belt, Queensland. Memoirs of the Queensland Museum. 42(2):475-486

  29. Bruton, M.J., McAlpine, C.A., Maron, M. (2013) Regrowth woodlands are valuable habitat for reptile communities. Biological Conservation. Volume 165, Pages 95–103

  30. Accad, A., Neldner, V.J.,Wilson, B.A., & Niehus, R.E. (2012) Remnant vegetation in Queensland. Analysis of remnant vegetation 1997–2009, including regional ecosystem information. Brisbane Queensland Department of Science, Information Technology, Innovation and the Arts.

  31. Queensland Department of Science, Information Technology and Innovation (2016). Land cover change in Queensland 2014–15: a Statewide Landcover and Trees Study (SLATS) report. DSITI, Brisbane.

  32. abc.net.au/news/2016-08-07/tree-clearing-report-queensland-laws-jackie-trad/7698474

  33. abc.net.au/news/2016-08-19/queensland-parliament-tree-clearing-laws-fail-unesco-fears/7765214

  34. publications.qld.gov.au/storage/f/2014-07-02T05%3A08%3A03.269Z/state-of-queensland-agriculture-report-june-2014.pdf

  35. environment.gov.au/system/files/pages/63b569ff-ae63-4d7b-be54-16f2e79900e0/files/nga-factsheet3.pdf

  36. abc.net.au/news/2015-06-03/retrospective-approval-referred-queensland-corruption-watchdog/6518332

  37. bea.gov/papers/pdf/measuring_the_effects_of_terms_of_trade_reinsdorf.pdf

  38. tai.org.au/content/greasing-wheels

  39. Dugdale, A.E. (2008) Where do Queensland’s Indigenous people live? Medical Jouranal of Australia. 188 (10): 614.

  40. theaustralian.com.au/national-affairs/state-politics/treeclearing-laws-unfair-for-aborigines-says-land-council/news-story/72aeaa808f645749378a778ffbdd96a9

  41. cylc.org.au/files/9714/6232/5672/20160504_Upholding_the_right_to_develop-CYLC_Submission.pdf

  42. Maclean, K., The Bana Yarralji Bubu Inc. (2015) Crossing cultural boundaries: Integrating Indigenous water knowledge into water governance through co-research in the Queensland Wet Tropics, Australia. Geoforum. Volume 59, Pages 142–152.

  43. Ellis, N.R., Albrecht, G.A. (2017) Climate change threats to family farmers’ sense of place and mental wellbeing: A case study from the Western Australian Wheatbelt. Social Science & Medicine. Volume 175, Pages 161–168

  44. McAlpine, C.A., Bowen, M.E., Smith, G.C., Gramotnev, G., Smith, A.G., Lo Cascio, A., Goulding, W., Maron, M. (2015) Reptile abundance, but not species richness, increases with regrowth age and spatial extent in fragmented agricultural landscapes of eastern Australia. Biological Conservation. 184 174–181

  45. Boeve-de Pauw, J., Van Petegem, P. (2010) A cross-national perspective on youth environmental attitudes. The Environmentalist. 30; 2. pp 133–144

  46. Fien, J., Teh-Cheong Poh Ai, I., Yencken, D., Sykes, H., Treagust, D. (2002) Youth environmental attitudes in Australia and Brunei: implications for education. 22: 205. doi:10.1023/A:1016571526997

Essay 8. A Taste of Toad: Conditioned Taste Aversion vs Invasive Cane Toads

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A lace monitor (Varanus various), one of the many native predators at risk from invasive toad ingestion, giving the writer/photographer a defensive display  🙂

Among the plethora of man-made environmental issues currently challenging global biodiversity, invasive species are increasingly recognised as a serious problem. While the loss, fragmentation, & modification of natural habitats are still the main drivers of biodiversity decline, particularly in the context of future climate change (1), studies on the impact of non-native species have recently demonstrated the significant impact they’re having on native biodiversity worldwide (2,3). While non-native predators such as cats are have a significant impact on fauna through hunting, other invasives may cause impacts in a less direct manner.

Simple spatial competition is often a major factor. Invasive plants, for instance, are a major factor in biodiversity declines globally (4). As an example, the Water Hyacinth (Eichhornia crassipes) from Brazil now occupies 50 countries and 5 continents, Australia included (5). Once established it can grow in thick mats, out-competing natives, obstructing water flow and choking waterways, obscuring sunlight while the rotting vegetation and roots decrease oxygen levels in the water, all dramatically reducing biodiversity (6). Locally, us south-east Queenslanders need only go for a short walk around almost any natural area, particularly near a roadway, to find dense, thorny, impenetrable thickets of Lantana camara (7). This central & south American perennial has established invasive populations in 50 countries, including around 4 million hectares throughout Australia’s Great Dividing Ranges. It is also capable of releasing ‘allelopathic’ chemicals into the soil to hinder the growth of other species, quickly dominating an area. Since resources such as water and nutrients are rather ephemeral and patchily distributed in Australian ecosystems, species tend to form intricate mutualistic relationships over time, and spatial competition from invasives can upset this balance rather quickly.

With a long history of relative geological and climatic stability, Australian fauna has had many generations to adapt to our more ephemeral conditions and resources, often adopting slow but efficient modes of reproduction and resource use. They are thus easily out-competed by fast-breeding introduced species from places abundant in resources, like European foxes & rabbits (8). While foxes & cats hunt our native mammals, rabbits out compete native herbivores, stripping valuable vegetation from the land which might have been an important foraging ground for natives, potentially spreading diseases, damaging the soils, & more. Introduced ungulates (hoofed mammals such as goats, cattle, pigs, horses, & deer) not only graze heavily on native vegetation, reducing plant diversity as well as habitat and food for native fauna, but also cause severe erosion with their hard, penetrating hoof-falls (9), as opposed to the pad-footed, elastic bouncing of our largest native herbivores, the Macropods. Even in areas with endemic native ungulates, populations of introduced species can outpace the natives and overpopulate an ecosystem (10), eroding riverbanks while denuding them of stabilizing riparian vegetation by feeding and wallowing, potentially even causing saltwater infiltration in coastal floodplain systems (9). The spread of weeds and invasive plants is also often further aided by invasive species, transporting seeds in their fur, feathers, and feces (11).

Aside from these competitive & predatory impacts, invasives can present another threat to our native fauna; poisoning. Toxic plants are common and often successful invaders; Lantana camara for example can cause illness in livestock when ingested (7). Toxic animals, such as the Cane Toad (Rhinella marina), are also a potential threat to many native predators. Since its introduction to Australia in 1936 in an attempt to control the agricultural impact of the cane beetle on sugar cane crops, the cane toad has successfully established invasive populations and spread far beyond the area originally intended (12). Additionally to competing with native frog species for resources, these invaders possess toxins produced in the skin and parietal glands along the neck. This includes the eggs and tadpoles, which are also poisonous to some degree. With no poisonous frogs native to Australia our native predators often have a dietary fondness for them (13), and many species will happily pounce on a poisonous introduced toad before realising their mistake.

Luckily, populations in nature often demonstrate remarkable adaptive responses to novel environmental influences such as invasions. In the case of the cane toad, despite initial declines in Quolls, varanid lizards, blacksnakes, death adders, & more, many Australian species appear to be adapting to this new challenge in a variety of ways (reviewed by Shine in 12). Some snakes, for instance, have experienced morphological shifts in response to their presence. Red-bellied blacksnakes and common tree snakes in areas with a long history of toads appear to have a decreased ratio of head size to body length (14). Imagine the pre-1970, toad-naïve population of blacksnakes in Cairns, a population with a range of body sizes & shapes at least partially determined by inheritable genetic variation. It appears that those snakes capable of consuming a lethal dose were quick to die out of the population when toads arrived on the scene. Surprisingly, these are generally smaller individuals with larger heads. As a snake’s body grows, it gains length more than breadth, and thus larger snakes have a smaller head size relative to their body than do smaller individuals. Those larger individuals were less capable of ingesting a lethal dose of toad poison, passing on their genes for longer body size, perhaps even smaller head size, or over time for improved immune response or kidney function & thus greater resistance to Bufotoxin (15).

While morphological and even physiological responses are important, behavior also plays an essential role in how our natives are dealing with the invasion. Australian bird’s have been remarkably successful in this regard, either consuming toads without harm or ignoring them altogether (16). A further strategy involves a greater degree of intelligence and manipulation of the prey item; avoiding the poisoned parts. Certain species have been known to eviscerate toads and consume only the tongues and internal organs, avoiding the skin and glands which produce toxins (16,17, 18).

Another behavioural solution is simple; learning not to eat toads. Many species have demonstrated an ability to learn rapidly the dangers of ingesting poisonous toads. In fact, many snakes seem to be far less at risk than we previously thought, perhaps learning early on that these lumbering toads do not make the best prey items (19). Whatever the mechanism, snakes seem to be weathering the impacts quite well! This may be due partly to their hunting behavior; snakes are quite often specialists, with senses tuned for a specific set of chemical and physical cues such as the smell and image of their ideal prey items. This specificity perhaps makes them less likely to feed on something as unfamiliar as a giant South American toad.

Generalist species which feed on a greater variety of prey, particularly if that includes anuorphagy (feeding on frogs), are more likely to feed on invasive toads and are at much greater risk (12; 20). This includes species like Quolls and monitor lizards which tend to feed on practically anything that they can catch, including carrion and road killed toads. As such, while certain snakes and other specialized predators seems to be rather immune to the presence of toads, or recover rapidly after arrival, both northern Quolls and a variety of monitor lizards are suffering population level impacts in parts of northern Australia. These species are also more vulnerable to the impacts of toads as they can ingest larger quantities, tearing off pieces to consume a large, adult toad which would present a swallowing difficulty to many snakes.

The situation is further complicated by a recently recognized evolutionary process called “spatial sorting” documented at the cane toad invasion front (reviewed by Rollins et al. in 21). As it happens, larger, longer legged toads are more adept at crossing long distances than slower, smaller toads, and thus larger individuals will generally be more prevalent at the invasion front. Those larger individuals at the front of the invasion wave are also more likely to come across similar, large individuals when reproducing, sharing genes for larger, longer legged toads with each other at the front of the invasion. Let’s imagine a deck of cane-toad playing cards, capable of replicating with a 50% chance of passing their value on, where face cards can travel further than others. Now lets place a stacked deck on the edge of a long table and start spreading them, then replicating, and repeating until the table is covered. The leading edge will eventually be dominated by face cards, having left behind the slower deuces, mixing with only other face cards as they expand. After several decades of expansion, this process has resulted in much larger, longer legged cane toads at the forefront of the invasion than at the source. Furthermore, as the individuals at the invasion front are more likely to experience predators, there’s increased selection pressure for larger toxin glands and greater protection, thus more likely to cause mortality for whoever eats it.

Why do snake populations seem to recover so rapidly after an initial decline following introduction? Aside from morphological shifts in body shape to hopefully increase the chance of sub-lethal dosing, could it be that without the ability to tear and ingest large quantities of toad, snakes simply were more likely to ingest smaller individuals with a sub-lethal dose? Or perhaps when attempting to subdue large individuals, the extruded Bufotoxin gets into mucous membranes, again in less than lethal quantities? In either case, seems likely that these creatures can learn from their near fatal experience and not to make such rash dietary judgments in the future. Having (repeatedly) poisoned my younger self with certain liquors, I now feel a visceral illness at the slightest smell of those specific, repellent beverages! This is known as Conditioned Taste Aversion (CTA), and it is believed that it may aid greatly in the battle against cane toads.

We know that CTA occurs in nature; various organisms produce distasteful substances in an effort to stop becoming food, such as certain “poisonous” feed plants and their deterrent effect on animals; from livestock, to lizards, guinea pigs, & more (22; 23; 24). In the US, aversion to the toxic taste of fireflies has been demonstrated in both eastern fence (Sceloporus undulatus, a small iguanian) and broad-headed lizards (Plestiodon laticep, formerly “Eumeces laticep, Heilprin, 1888”) (25). Considering that many species, including a variety of reptiles, appear capable of CTA, the question becomes: can CTA enhance survival of native populations when toxic invaders are involved? Do native animals learn aversion from non-lethal toad encounters in the wild? Alternatively, can we actively teach toad avoidance to susceptible predators like varanids and dasyurids to bolster their survival chances?

These questions are currently being answered in Australia’s top end in a valiant effort by scientists studying a toad vulnerable species. This includes the northern Quoll (Dasyurus hallucatus), one of the more susceptible predators with cane toads now occupying most of it’s native rage, driving local populations to extinction and forcing the establishment of a breeding program at the Territory Wildlife Park, as well as an effort to translocate wild Quolls to two toad-free islands in 2003 (26). Monitors (Family: Varanidae) and other larger lizards like the Tiliqua genus, the blue-tongued skinks, are similarly at risk due to their fast metabolism, strong prey drive/food requirements, recorded anurophagy, powerful jaws, and wide gape capable of ingesting large toads/carrion even while youngsters (27,28). This is where CTA comes in.

In an effort to stem the decline of native predators due to cane toads, scientist have recently been trialing both lizards and Quolls as candidates for population level CTA intervention. Briefly, the idea is to see if feeding predators small toads with a survivable but unpleasant dosage of toxin, often enhanced with a nausea-inducing chemical, teaches avoidance behavior. After observing naturally occurring CTA in the red-cheeked Dunnart (Sminthopsis virginiae), a small predatory marsupial in the same family as Quolls, the Dasyuridae (29), it was hypothesized that northern Quoll populations which were taught toad-aversion using a sausage of minced toad tissue, or even small toads, prior to being released on the mainland would have better survival rates. After confirming CTA behaviour in captivity through feeding trials and subsequent toad-avoidance training sessions, 31 CTA trained or “toad-smart” Quolls and 31 control subjects, part of a captive breeding conservation program, were fitted with radio collars and released into the wild (26). “Toad-smart” males survived five times longer on average, females around twice as long. It seems that mammals, or at least the Dasyurids, strongly benefit from this kind of CTA training. Mammals however seem much more adept at learning than many other taxa. Furthermore, results in these captive bred individuals might not represent wild populations accurately. While further trials continued with mammals, with a significant amount of success (see link 30 for some developments in the news), the rush was on to see if reptiles, particularly wild-caught individuals, could also learn to avoid this toxic invader.

As the first interaction can be very important for learning aversion (first impressions and whatnot) trials in reptiles began with the blue-tongued lizard (Tiliqua scincoides) populations in the Northwest, ahead of the invasion front, ensuring subjects were all toad-naive initially (28). After determining that lizards can develop an aversion to Lithium Chloride (LiCl) injected cane toad sausage in a lab setting, toad-trained lizards were released back into the wild, 17 CTA trained (8 treated with a low dose of LiCl, 9 with a higher, nausea-inducing dose) and 18 control lizards. As the toad front advanced, the lizard’s behaviour, health, & survival recorded over the next several months by radio-tracking & capture every 1-7 days. Although 8 lizards lost their trackers prior to toad arrival and a further 13 were lost for unknown reasons, 22 subjects were monitored as the toad front arrived, with 12 surviving the full period of the study. The results were promising; of the 10 lizards lost during toad arrival, 3 died from natural causes (predation or injury) while the remaining 7 died from attempts to feed on toads. 3 of these deaths were from the untrained “control” group, another 4 from the low-dose treatment group. All 9 lizards treated with high enough does of LiCl to induce regurgitation of the prey item survived the full trail period. A good spew is, after all, a thing to remember!

After this success in blue-tongued lizards, conservationists turned their attention to the Varanidae, the monitor lizards, which can suffer up to 90% population declines upon the arrival of toads (27). Also of interest was the long-term survival prospects of wild, toad-smart animals; does CTA training carry on into the future in wild populations? To discover the answers, herpetologists headed to the Kimberly region on northwest Australia for a long term study, again ahead of the toad-front (31). Three months prior to the arrival of the toads (over the two wet seasons of the study period, between November 2013 and May 2015), 66 yellow-spotted monitors (Varanus panoptes) were tracked by radio-telemetry and offered juvenile toads as food on multiple occasions. 22 successfully bit the offerings, and were highly unlikely bite a second time, suggesting an effective CTA response. Over the next two wet seasons, the toad-front arrived and spread throughout the area.

Tracking continued throughout the 2013-2015 toad invasion, showing that indeed this learning is retained over some time. In the toad-heavy southern study location, half the trained yellow-spotted monitors were still alive at the end of the study, while only a single untrained lizard out of 31 survived longer than 110 days, eventually dying from a toad on day 183. Critically important, lizards trained to avoid eating juvenile toads also avoided adults, indicating a generalized aversion to toads rather than just the juvenile stimuli. Further studies with toad-sausage proved less successful for monitors, possibly due to the lack off a visual stimuli (32). Monitor lizards have incredible eyesight and utilize vision heavily to hunt down fast moving prey, while the blue-tongued lizards are a slower, smaller, robust omnivore, perhaps using taste and smell to a greater degree. Additionally, due to their varied diet including plants and fungi, blue-tongues might be generally more likely to ingest toxic food than carnivorous montior lizards. Might an evolutionary history of tasting and avoiding a variety of toxic plants or insects perhaps make them somewhat pre-adapted for CTA? Following on, are omnivorous reptiles less at risk, achieving CTA more readily than carnivores due to greater ability to differentiate smells rather than sights?

These questions and more remain to be answered as conservationists continue the struggle against exotic invaders. For now, behaviourally immunizing some individuals with young toads appears to carry over at least two breeding seasons (32). With some luck, determination, and diligence, many native animals will avoid an untimely end, remembering their horrible toad-eating experience for many years. Hopefully, having survived the large individuals invading their territory, their young will have the next generation of juvenile toads to taste next season, learning from wild toads what their parents were taught by people.

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