Extinction debt: Urban biodiversity may be the answer

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Urban biodiversity is crucial to balance endless developments encroaching on wildlife territory (Screenshot via YouTube)

In order to counteract the effects of global warming, we need to consider ways to enhance biodiversity within our cities, writes Dr Peter Fisher.

THE RECENT HEATWAVES in Europe have signalled how hairy things might get here in the antipodes as we head down the straight towards a 3.5–4.0°C world. As David Attenborough testified to a House of Commons Committee last month, Australia is going to be particularly hard hit.

The IPCC’s latest report was no less blunt in highlighting drought, soil erosion and wildfires and diminishing crop yields in the tropics and thawing permafrost near the poles. In short, as Earth's climatic system edges towards uncertain tipping points, the incidence of tinder-dry fire contagions, violent storms and devastating floods is set to rise

A false sense of security

Developments like these might have seemed pretty ordinary to our forebears in the lead-up to the Holocene Epoch, living as they were on the brink of seismic change, amid a series of abrupt climate shifts.

As the archaeologist Steven Mithen wrote in his book, After the Ice:

People were thin on the ground and struggling with a deteriorating climate… massive ice sheets had expanded across much of North America, Northern Europe and Asia. The planet was inundated by drought, sea level had fallen to expose vast and often barren coastal plains. Human communities survived the harshest conditions by retreating to refugia where firewood and foodstuffs could still be found.

Since that point, we’ve been lulled into a false sense of security by the ensuing 10,000-odd years of a peaceful, stable climate. This has allowed us to tame crops and livestock and to come together to form communities, villages and, ultimately, cities. But the calm and tranquil Holocene is now being replaced by the Anthropocene, heralding a return to a volatile and destructive climate.

Overshoot, adapt and recover

Back in 2009, Martin Parry, then co-chair of an IPCC Working Group, undertook some sums as to how much damage would be done by delaying mitigation, adapting and then trying to restore things. He assumed three peak emission points — 2015, 2025 and 2035. As it turned out, 2015 was a no show, 2025 is now shrouded in uncertainty in view of current trends, leaving 2035. For that year, they were planning to adapt to at least 4.5°C of warming.

Ten years on, that figure seems less and less fanciful, at least to the creatures of Western Antarctica experiencing accelerating ice loss. Or, for that matter, Greenland's ice sheet which is melting six times faster than in the 1980s. In this month alone, it lost 10 billion tonnes of surface ice in a single day.

Early onset

Major extinctions around the globe, loss of coastal wetlands and reconfiguring of coastlines are among the woes Parry attributed to warming at the top end of the temperature scale. These are trending now off a climate change lite baseline featuring reduced water availability, increasing species range and wildlife risks, food productivity, more coastal damage from floods and storms and rising deaths from heatwaves. 

As Parry warned at the time:

It will be very expensive to protect against warming at the upper end of the uncertainty range. We therefore will need to make a judgement about what damage is worth avoiding completely (through mitigation) and what we will have to bear. If we make some simple assumptions about the amount of risk we wish to cover, we can identify how much we need to adapt.

A rampageous climatic regime: Bad different

Parry’s counsel gives a lead as to how to proceed into a future that is no longer a projection of the past, a trap authorities and others tend to fall into with so many aspects of city design not the least greening of our urban places. Conditions under a 3.5–4.0°C Australia promise to be horrid as David Attenborough has already noted — big dries, extreme winds, withering heat, deluges, rampant pests and disease affecting plants, loss of pollinator and the ever-present danger of fire/wildfire being but a few.

Meanwhile, historic growing regimes are gone with soil temperatures 2°C higher (detrimental for many species) whilst soil moisture hasn’t returned to '60s levels. Moreover, high winds in combination with deluges (that weaken root anchoring) can promote domino tree falls. Withering heat, too, can cook plants (and have detrimental effects on pollinators). As a result it will be a battle to get new trees beyond the sapling stage.


A possible remedy is to breed species that are more sympathetic to the conditions of climate change — extra pest and disease-resistant, fire-retardant, shade-providing trees less prone to branch fall as well as vegetation that can better resist erosion in coastal zones subject to sea-level rise and strong storm surges. (Of note here is Adelaide’s Waite Arboretum research into tougher street trees.)

How might native birds and animals, including those migrating to our cities in search of food and shelter, fare under a blanket application of a vegetative regime which views heat amelioration as the big one?  .

Adaptors and avoiders

You might have to ask that of our Millennium Drought refugees — little corellas, galahs, sulphur crested cockatoos and crested pigeons. These birds are urban adaptors — creatures who have learned to live with us along with invertebrates.

Still, others are urban avoiders, invariably small birds who live amidst ground cover (not trees). They suffer when their habitats are overwhelmed with loss of bushland and farmland due to urban expansion and are highly susceptible to extinction debt, where the impacts of clearing native vegetation and/or breaking it up into small chunks may not be experienced until a decade or so later. Here in the present, bushfires coupled with decreasing habitat in urbanised landscapes are already taking their toll.

For passerines, the combination of warming, extinction debt and widespread implementation of canopy oriented vegetation could hasten their decline. Meanwhile for the adaptors, urban settings are increasingly evolutionary hot spots — places where speciation is quite rapid. A deeper understanding of habitats (especially their importance to the avoiders), whether as public lands or private gardens, is key to conserving vestiges of that other Australia as desiccation continues to build. 

Work to be done

It’s time for cities to acknowledge that in a climate change afflicted landscape, they’re now significant repositories of biodiversity particularly as inward bird and mammal immigration looks set to ramp up even further. A great deal of thought needs to go into ways of ensuring that all levels of government remain engaged and across that stewardship in the face of explosive population growth.  .

A delayed start on mitigation, such as Parry, will tax planners in balancing this requirement against reducing risk and costs to human populations from things like tree/branch falls, polluted waterways due to un attenuated runoff and, worse, a prospect that wall-to-wall urban forests could set the scene for a catastrophic blaze affecting not only humans but other life species, under a run of 50°C days — think Black Saturday and the 2019 Californian fires.

Grappling with issues like these will sorely test their ingenuity.

Dr Peter Fisher is an Adjunct Professor at the School of Architecture & Built Environment, Deakin University.

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