Making Eden

by David Beerling

  dormant in the soil (until the numbers of seeds are exhausted) and can reproduce

  by making clones, meaning the last plant could still produce occasional, if genetically restricted, offspring. Past and ongoing climate change and habitat destruction is creating an extinction debt that will take a century or longer to play out. This raises the thorny question of how we identify the ‘living dead’ in the extinction waiting room.

  Apocalyptic forebodings of uncertain future extinctions resonate because in

  spite of the acknowledged uncertainty in the numbers and time lags, plant extinc-

  tions are already upon us. Although hard to pin down, rates of plant extinction

  during the Anthropocene are about 10- to 100-fold higher than the natural back-

  ground rate.44 How does this measure up against past mass extinction events,

  and what can we learn about the prospects for life’s survival and recovery?

  Palaeontologists are always keen to point out that 99% of all species that have

  evolved on Earth over the past 3.5 billion years have gone extinct.45 Viewed over the immensity of geological time, extinction is the norm, with species having an

  average lifetime of 5–10 million years, depending on the group of organism;

  mammal species are less durable than plants, for instance. Yet sitting far above the background level of extinction are five well-documented so-called mass extinction events that occurred during the past 540 million years since complex plants

  and animals populated the planet. Sharp spikes in extinction rates far exceeding

  background levels characterize the so-called ‘Big Five’ mass extinctions, each representing the loss of 75% or more species. These past catastrophes in the history of life on Earth offer three important cautionary lessons for our current crisis.46

  Lesson 1. Draw up a list of causes for each of the Big Five mass extinctions and it soon becomes apparent that a synergy of environmental factors pushed life over

  the edge, rather than any single event. The end-Permian mass extinction, 250 mil-

  lion years ago, wiped out nearly all life on Earth and coincided with vast episodes of volcanic activity in Siberia that injected huge amounts of carbon dioxide

  into the atmosphere, triggering global warming, and noxious ozone-destroying

  sulphurous gases.47 The end-Triassic mass extinction, 201 million years ago,

  coincided with the birth of the Atlantic Ocean, as North America and Africa

  slowly drifted apart. The tectonic activity resulted in volcanism belching out huge

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  amounts of carbon dioxide into the atmosphere. As the atmospheric carbon

  dioxide concentration soared, the hot, parched planet that followed wiped out

  biodiversity on land and acidified the oceans, to extinguish life in the seas.48 Today, current rates of increase in the atmospheric carbon dioxide concentration, climate change, and ocean acidification far outstrip those experienced during any

  past extinction event. Humans have added to the mix with deforestation, habitat

  loss and fragmentation, expansion of agriculture, over-fishing, over-hunting, and pollution. Lesson one from Earth history is this: human activities are creating

  the perfect storm for the sixth mass extinction, priming the engine of species

  extermination.

  Lesson 2. Rigorous meaningful comparisons between the magnitude of extinction events from hundreds of millions of years ago and the present are difficult.

  The fossil record is patchy, with much of the evidence for depletions in

  biodiversity obliterated, making rates of extinction hard to accurately pin down.

  Nevertheless, careful estimates suggest that in terms of magnitude, the Big Five

  trump current losses, with each losing between 75% and 90% of species.49 In the

  1990s, the World Conservation Monitoring Centre listed 592 plant species as

  having gone extinct since 1600. In 2016, The International Union for the

  Conservation of Nature Red List of Threatened Species listed fewer than 150

  extinct species of plants. Granted the strong suspicion of undocumented extinc-

  tions, these ballpark estimates of magnitude for plants do not yet qualify as

  mass extinctions in the palaeontological sense. It is on the question of rates that things become concerning, with the best estimates of plant species extinction

  for the Anthropocene far exceeding those calculated for the Big Five, if we scale past mass extinction rates to the same window of time.50 Lesson two from Earth

  history is that unless we relieve the pressures that are pushing today’s species to extinction, we will propel the world towards the sixth mass extinction within a

  few generations. For vertebrates—mammals, birds, fish, amphibians, and others—

  it may already be underway, with recent declines in their diversity unprecedented in the past 65 million years.51 Plants have so far fared better, but may be sitting in the extinction waiting room for longer than animals, for reasons we have

  already discussed.

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  Lesson 3. Extinctions from the past provide a rough guide to the recovery time of the biosphere, following catastrophic losses. On average, it takes anything from

  hundreds of thousands of years to tens of millions of years for ecosystems to

  recover, reorganize, and bounce back to pre-extinction levels of biodiversity.

  After the end-Permian mass extinction, tropical forest and coral reef diversity

  remained depleted for ten million years or so, and global biodiversity took a hundred million years to recover.52 After the end-Cretaceous mass extinction event, 65

  million years ago, that famously saw off the dinosaurs, the diversity of life in the seas took millions of years to rebound.53 Lesson three from Earth history is that if our actions drive life towards a sixth mass extinction, we are likely sealing the fate of biological diversity for millions of years.54

  By 1992, nations of the world had recognized the urgency of the situation and

  signed up to the UN Convention on Biological Diversity. By 2002, 193 parties had

  pledged to lower rates of biodiversity loss substantially by 2010. Endorsed by the World Summit on Sustainable Development, this goal became incorporated into

  the UN Sustainable Development Goals for 2030. But 2010 came and went, and

  despite the political posturing over bold commitments to biodiversity conserva-

  tion, the world saw no significant slowing of biodiversity decline between 1970 and 2010.55 Prospects for the diversity of plant and animal species continue to remain grim. A chink of light appeared in October 2010 when the Aichi Targets of the

  Strategic Plan for Biodiversity 2011–2020 became enshrined in the UN’s Convention on Biological Diversity to halt biodiversity loss.56 One of these targets sought formal protection for 17% of the terrestrial world. In the same year, the UN updated the Global Strategy for Plant Conservation with the aim of protecting 60% of plant species. Are these two aspirational goals compatible? The good news is that we are currently protecting ~13% of the global land area. The bad news is that the protected areas are not in the right places to protect 60% of plant species, with a bias towards land in cool, high, or dry places or places far away from people and with little economic value.57 Conservationists urge preservation of tropical and sub-tropical islands, moist tropical and sub-tropical forests, and Mediterranean eco-

  systems that hold high numbers of endemic plant species. Other approaches to

  conserving biodiversity are necessary and under development.

  The continued lack of co-ordinated action to halt biodiversity decline, despite

  the repeated setting of these international targets,58 is a great tragedy of our time.

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  The voice of conservationists has
no particular standing with politicians, and few seem to take notice of species disappearing each year, or the rapidly climbing rate of their disappearance. Curves depicting rising global temperatures and atmospheric carbon dioxide concentrations over the past fifty years seem to have clarity in demonstrating the reality of climate change; tabulated lists of threatened

  species appear somehow to be less compelling.59 We are continually losing

  opportunities for the future, and committing biodiversity to extinction during

  the course of this century. As the Harvard evolutionary biologist Edward Wilson

  points out, this is a sin, for which future generations are least likely to forgive us, because there is no way to get them back once they are consigned to the dustbin

  of history. The pioneering genome scientist J. Craig Venter and his team may be

  on the verge of synthesizing new life forms by assembling and inserting a syn-

  thetic genome into a bacterial cell,60 but, ironically, we are failing to preserve the biodiversity we already have.

  Yet saving the world’s floras and fauns from the threat of climate change is far

  from straightforward and these simple statistics hide the complex challenges

  facing conservationists. A good example is the Californian Floristic Province

  (CFP). The CFP is a stunning world-renowned biodiversity hotspot with a dis-

  tinctive flora composed of over 5000 native plant species, with more than 2000

  endemics. Species–area relationship analyses, together with the projected changes in climate over the next century, predict that some 60% of species could experience drastic contractions in their ranges. Yet California’s varied terrain, with mountains and desert areas, means that species may be forced to move in very different directions, causing a break-up of the composition of present-day floras. Quite

  what might happen will be sensitive to actual rates of plant migration, and raises the question of whether we should intervene to help plant dispersal as part of a

  conservation strategy. If not, weedy species with short lifetimes and easily spreading seeds are poised ready to fill the gaps in California, and those created elsewhere in the world. This presents a dilemma to conservationists: should they prioritize areas, refugia, where we expect plant diversity to peak in the face of future climate change? These ‘future’ refugia may contain a disproportionately high number of

  threatened species that could persist, offering valuable opportunities for conservation. Or should conservationists devote time and effort to protecting mountain-

  ous areas harbouring species threatened by drastically shrinking ranges? In the

  end, the number of species projected to survive climate change may depend on

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  their ability to disperse into suitable habitats and climates. In a world with increasingly fragmented habitats, landscape connectivity for allowing species to migrate is crucial.

  Another conservation perspective that shifts emphasis away from the overrid-

  ing preoccupation with preserving rare species in threatened diversity hotspots

  comes from thinking about evolutionary history. The great Russian-born evolu-

  tionary biologist Theodosius Dobzhansky (1900–1975) may have been indulging

  in some shameless self-promotion when he famously proclaimed that ‘nothing in

  biology makes sense except in the light of evolution’, but few scientists would

  disagree with the sentiment. Now conservationists are also embracing the need to

  recognize the importance of the evolutionary history of species, the accumulated

  baggage of their past. History matters in the delicate game of surviving threats

  like climate change and habitat fragmentation because where you sit on the green

  tree of plant life can determine your risk of extinction. Your position on the tree also says something about the amount of evolutionary history your extermination

  will wipe out. Consider that, if a species is at risk of extinction, then its close relatives with similar ecological and physiological characteristics will also have a

  higher than average chance of being at risk too. The extinction of species clustering together on the green tree of life could jeopardize the existence of entire

  families and genera of plants.61

  Saving evolutionary history itself might be a valuable focus for the conserva-

  tion movement. We could target areas preserving patterns of biodiversity and the

  evolutionary processes that generated the patterns, bringing pre-emptive conser-

  vation strategies within reach. Combine the evolutionary histories of species with their geographical distributions and you can discover the ‘cradles’ of diversity

  (regions where diversity is generated), and can distinguish them from ‘museums’

  of diversity (regions where it persists).62 Take the floras of the Cape Province of South Africa, for example. It is an area of less than 90 000 km2 and, like the Californian Floristic Province, represents another extraordinary biodiversity hotspot, containing more than 9000 plant species, 70% of which are endemic.63 Most of the threatened

  plant species belong to relatively young, fast-evolving lineages making up the tips of the green tree of life.64 Put another way, rapidly diversifying groups of plants contain the more vulnerable species. Because much of the present-day plant diversity in the Western Cape results from recent speciation events, plant extinctions will erase relatively little of the evolutionary history of this biodiversity hotspot. This is not to

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  reject areas in the region with recently diversifying floras as conservation targets.

  Rather, it points to the need for including other regions in conservation schemes to maximize the potential for preserving evolutionary history.65

  As conservationists face these sorts of complex issues, the sceptic, and indeed

  the heretic, might ask—does it matter? To which we surely offer the indignant

  reply that we have a profound moral imperative and ethical obligation to preserve Earth’s rich biological diversity for future generations. After all, the estimated 7–12 billion species of organisms (other than bacteria or viruses) with which we

  share the planet are astonishing products of the same evolutionary process that

  gave rise to us. There is also another, pragmatic response to this question to consider, that of preserving essential ‘ecosystem services’ provided by nature. Here, in this utilitarian view of nature, plants often take centre stage. They underpin high-profile ecosystem functions, including the things we take for granted such as

  food, carbon sequestration, water, timber, fuel, soils, nutrient cycling, pollination, and pharmaceuticals.

  Over ten years ago, Robert Constanza and colleagues controversially attempted

  to estimate the value of ecosystem services worldwide.66 Acknowledging substan-

  tial uncertainties in such an exercise, Constanza put a crude initial price on the

  ‘services’ provided by the biosphere as a whole of around $33 trillion per year. As he points out, ‘The economies of the Earth would grind to a halt without the services of ecological life support systems, so in one sense their total value to the economy is infinite’. Putting a price on nature is an unnervingly anthropocentric view of the world, mistakenly implying nature is here to serve us. Are we comfortable with reversing over 500 years of scientific thinking from the greats like

  Copernicus, Galileo, Hutton, and Darwin who each played a role in the gradual

  disassembly of the seemingly special and central position of Earth-bound humans

  in the Universe? Probably not, but the pragmatic view is that without the bold

  step of valuing ecosystem services, we cannot begin to engage policymakers,

  engineers, and other scientists whose decisions threaten the sustainability of the
planet. According to economists, the price to pay for the erosion of nature’s ecosystem services could be in the region of $2–5 trillion per year.67

  Not surprisingly, biodiversity underpins the ecosystem services on which we

  depend for our very existence. If it falls below some safe threshold or limit, then the wide range of services which are central to our health and well-being, such

  as crop pollination, and regulation of the global carbon and water cycles, are

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  critically threatened. Framed in this context, the most comprehensive analysis of biodiversity change to date suggests that 58% of the world’s land surface has fallen below the safe threshold.68 Hold on a minute, you might cry. If such a large proportion of land has already passed below the safe threshold for biodiversity loss, why have we not already noticed negative effects on human society and well-being? The Stanford ecologists Paul and Anne Ehrlich offer an engineering meta-

  phor by way of explanation in their book Extinction: The Causes and Consequences of the Disappearance of Species. Published in 1981, decades before the conceptualization and economic valuing of ecosystem services, they neatly illustrate how uncertainty in biodiversity loss relates to the functioning of ecosystems by likening it to the loss of rivets from an aeroplane wing. How many rivets are we comfortable with popping out before refusing to fly? ‘Ecosystems, like well-made airplanes, tend to

  have redundant subsystems and other features that permit them to continue

  functioning after absorbing a certain amount of abuse. A dozen rivets, or a dozen species, might never be missed. On the other hand, a thirteenth rivet popped

  from a wing flap, or the extinction of a key species involved in the cycling of nitrogen, could lead to a serious accident.’69 In this context, we should recall American conservationist Aldo Leopold’s (1887–1948) great quote, ‘the first rule of intelli-gent tinkering is to save all the cogs and wheels’.

  Another way to think of how our destruction of nature degrades ecosystem

  services is the ‘ecological footprint’ concept. Conceived by Mathis Wackernagel

  and William Rees, an ecological footprint provides a metric for calculating the

  human pressure on planet Earth.70 The idea is that natural ecosystems generate a