frost, and triggering the poleward migration of species.15 Welcome to the
Anthropocene16—a geological epoch defined by the pervasive and persistent
signatures left in ice, sediments, and rocks by the activities of humans.17
The pressures on plants and animals as humans consume an ever-larger share of
the planetary pie by putting land to the plough are immense. In 2016, the Royal
Botanic Gardens, Kew, provided the first global assessment of our floras in its State of the World’s Plants report.18 Kew scientists estimate that over 20% of the world’s vascular plant species are at risk of extinction, with a further 4% critically endangered.19
From an estimated pool of over 450 000 species of plants worldwide,20 this means
90 000+ species are at risk. A further 10% of species in the planet’s plant portfolio are classified as ‘near threatened’, which means that without conservation actions they may also face extinction. Over 30% of all cacti, relatively recent additions to our planet’s floras, are facing threats from unscrupulous collectors of live plants and seeds.21 Plants, now more threatened than birds, are facing a similar level of threat to mammals. The biggest threat to plant species is the destruction of habitats caused by deforestation, usually for farming and cattle ranching, closely followed by
selective logging for valuable timber. A major problem is that vulnerable species exhibit highly clumped distributions over the world’s land surface. This creates
regions of exceptionally high floristic diversity—biodiversity ‘hotspots’. Most of these hotspots are located in the tropics where the human population is expanding rapidly, destroying and fragmenting habitats.22
Figure 25 Botanist Hewett Cotrell Watson
(1804–1881).
EdEn undEr siEgE a 175
HEWETT WATSON AND ESTIMATING
SPECIES EXTINCTION
-
In 1859, the same year as the publication of Darwin’s Origin of Species, English botanist Hewett C. Watson (1804–1881) established the mathematical basis
for converting habitat loss into species loss (Figure 25). Born in the South
Yorkshire village of Firbeck,23 Watson was brilliant intellectually and cantan-
kerous. He generously provided Darwin with detailed scientific information
on British plants, and once even turned down the opportunity to meet the
great man, claiming he was too busy. Nevertheless, the two men remained
friends, and with the publication of The Origin of Species, Watson was
among the first to write Darwin a congratulatory note on his extraordinary
achievement.
Watson’s knowledge of British vascular plant floras was unequalled, and
he laid the foundations for one of ecology’s few laws: the species–area rela-
tionship.24 He showed that if you plot the number of vascular plant species in
successively larger sampling areas, ranging from his small garden plot in
Surrey, which was only a few square metres, up to that of all England, the
number of species increased logarithmically. We now know that this pattern
holds for geographical areas larger than England, with the total number of
species increasing at a proportionally faster rate. Ecologists describe such a
relationship with a power-law equation that underpins the theory of island
biogeography. In its modern form, Watson’s species–area relationship offers
a basis for predicting how many species should become extinct as the size of
the ‘island’ of forest in a ‘sea’ of cleared land shrinks.
176 a EdEn undEr siEgE
How many species of plants are likely to be lost through continued habitat
destruction? How long will it take for species extinctions to unfold? Converting
habitat loss into species loss involves combining a modern form of Watson’s spe-
cies–area relationship, which provides an estimate of the number of species in a
given area, with projected rates of habitat destruction. From this, you can estimate how many species may go extinct as the islands of forest shrink. These analyses
show that because of the non-linear nature of the relationship, habitat loss ini-
tially causes little extinction, with the extinctions ramping up as the last remnants are destroyed. Assuming the rate of forest destruction stays the same, the crude
extinction curve peaks in 2050 with an estimated 50 000 species lost per decade.
However, the situation is worse than this analysis implies, because it assumes species are evenly distributed throughout the forests. In reality, this is not the case, many species of plants and animals exhibit highly clumped geographical distributions, often with small ranges. As a result, 44% of vascular plant species are
confined to just 25 ‘hotspot’ regions that comprise no more than 2% of the Earth’s land surface (Figure 26).25 Of these, 17 are in tropical forests, which have already suffered a disproportionate loss of primary habitat, threatening many species
with extinction. Consequently, habitat destruction acts like a ‘cookie cutter’
pressed into poorly mixed dough.26 Those species with small ranges in the
stamped-out area are eliminated, while those which are more widely distributed
survive. If we protect all the remaining habitats in individual hotspots, our species–area curve predicts that 18% of their species will eventually become extinct by 2100. If the higher-than-average rate of habitat loss in these areas continues for another decade, then around 40% of species could be lost by 2100. Immediate
action to protect the remaining unprotected habitats of the humid tropical forest biodiversity hotspots could drastically cut the numbers of species that will eventually be lost.
On top of competition for land to meet our rising demands for food and nat-
ural resources sits the imminent threat of human-made global climate change
to species numbers. Estimates of species loss caused by climate change also
often hinge on Watson’s species–area relationships,27 with the area defined by
the boundaries of the climatic conditions a species of plant or animal encoun-
ters throughout its geographical range. Ecologists call this the ‘climate enve-
lope’ of a species. They represent ‘islands’ of acceptable climate in a ‘sea’ of
Caucasus
Phillippines
Mediterranean
Basin
South-Central
California
China
Floristic
Caribbean
Province
Indo-Burma
Eastern Arc
Polynesia/
and Coastal
Mesoamerica
Micronesia
Forests of
Brazil’s
Tanzania/Kenya
Cerrado
Western
Choco/
Tropical
W.African
Ghats and
Forests
Polynesia/
Darien/
Andes
Sri Lanka
Micronesia
Western
Ecuador
Sundaland
Wallacea
Brazil’s
New
Atlantic
Succulent
Caledonia
Central
Forest
Karoo
Chile
Southwest
Madagascar
Australia
Cape Floristic
New Zealand
Province
Figure 26 Global terrestrial biodiversity hotspots cover no more than 2% of the land surface.
178 a EdEn undEr siEgE
AMAZONIAN BIODIVERSITY
UNDER THREAT
; -
The largest remaining block of the tropical moist forests that once covered
15% of the land surface is the Amazon, where deforestation, logging,
and human-caused fires symbolize the destructive forces of humanity.
Unsustainable logging and burning is degrading and destroying the com-
plex habitats, while road building and mining is fragmenting much of
what remains, especially in the Brazilian Amazon. This extraordinary bio-
diversity hotspot contains an estimated 11 000 tree species.28 The good
news is that over 3000 of these species have sufficiently large populations
of individual trees (at least a million) to ensure they are likely to persist
into the future. The bad news is that up to half of the 5000 or so rare
Amazonian tree species (i.e., those with less than 10 000 individuals) face
extinction. Such small populations of rare tree species with limited gen-
etic diversity are highly vulnerable to extinction by local habitat loss and
climate change. These assessments are uncertain and hampered by incom-
plete knowledge of the biogeography, life history, and environmental
requirements of tropical trees. Here’s the point though. If the trees go
extinct, the complex ecological network of organisms that make up a once
pristine tropical ecosystem collapses along the pathway of diminished
diversity: trees → vertebrates → invertebrates → microbes. So too if the
animals that play a crucial role in pollination, and seed and fruit dispersal,
go extinct, but the direction of causality in the ‘silent forest’ is reversed.29
In Amazonia, some ecologists estimate that 70–80% of extinctions of forest-
dwelling vertebrates caused by rainforest losses are yet to come.30 On the
plus side, this time delay is also an opportunity for restoring habitats or
implementing alternative measures to safeguard species otherwise com-
mitted to extinction.
EdEn undEr siEgE a 179
unsuitable climate prevailing across the rest of the globe. Using this informa-
tion, you can calculate how the geographical envelope of acceptable climate for
different species changes as the climate warms in response to rising atmos-
pheric greenhouse gas concentrations. If the potential geographical range of a
species shrinks, its risk of extinction increases. The approach is irresistible to anyone with access to databases of species distributions, the outputs of climate
models predicting future climate change, and modest programming skills;31
forecasting patterns of extinction in this way has become a cottage industry.
Although many assumptions (e.g., are the soils suitable? what are the effects of
competition? etc.) lie behind these beguiling projections, the basic underlying
idea is probably correct—less habitat, fewer species. What these models hint at,
then, despite the assumptions, is the size and locations of extinctions that future warming might cause.
Extensive analyses assessing the threat to over a thousand species of plants and
animals suggest that a warming of 0.8–1.7°C by 2050 commits nearly a fifth of
species to extinction. This startling figure rises to a quarter of species with 1.8–
2.0°C warming, and over a third with >2.0°C of warming. For plants, 20–50% of
endemic species could be committed to extinction over the next 50 years.
Endemics are highly prized by conservation biologists because they are unique to
a specific region, making their preservation a priority. In projections from this work, warming drives the geographical ranges of many species polewards and
towards higher elevations on mountains.
So much for global analyses, what about the European scene? In Europe, assess-
ment of the climate change threat to plant diversity is flagged as severe, with over half of the 1500 species investigated threatened with extinction by 2080. Species living in mountainous regions of the Northern Mediterranean experience the
greatest projected declines, with up to 80% species loss,32 and it is easy to
understand why. Mountain plant species are specialists in coping with the harsh
conditions. Over time, they have evolved adaptations for exploiting the limited
opportunities to grow and reproduce under montane conditions. Faced with a
future warming climate sweeping across the Alps, Pyrenees, central Spain, French
Cevennes, Balkans, and Carpathians, the narrow cold-climate tolerances of
mountain floras means the only way for species to cool off is to gain elevation.
Many species in alpine and subalpine regions of Europe and North America are
180 a EdEn undEr siEgE
already responding in this way, migrating to higher elevations to escape the past century of climate warming.33 Indeed, analysis of observations and measurements on over 300 European mountains reaching back over a century and half
suggests mountain biodiversity is increasing as warming accelerates and species
migrate upwards and gather towards the summit.34 Does this foreshadow the loss
of species in the near future, as they become pushed off the face of the planet
when they run out of mountain? Perhaps in southern Mediterranean regions,
plants may fare better because the species there are adapted to the hot, dry, summers, tolerating heat and drought, meaning they may cope better with future
climates.
As we can see, some of these alarming predictions are already playing out as
the theorists suggest, but it would be a mistake to accept them unquestioningly
at face value.35 Could, for example, rising atmospheric carbon dioxide concen-
trations partially ameliorate anticipated ecological damage caused by warm-
ing? If the carbon dioxide-rich world expected for 2050 fertilizes plant growth,
it might increase the capacity of some regions to support more species.36 The
idea is that diverse assemblages of plant species are generally more productive,
and higher productivity might generate ecological elbow room for more spe-
cies. Such is the extent of human intervention in eroding biogeographical bar-
riers and modifying species distributions by moving and introducing plants to
new places, the rapid arrival of alien species could conceivably fill the extra
capacity of the environment. In other words, immigration is an important
input term to consider in the biodiversity picture. In many regions of the
world, the introduction of non-native species has greatly increased plant spe-
cies richness, and even generated new species via genome expansion through
hybridization.37
Probably it is too early to draw any comforting reassurances from these twists
to the guessing game on species numbers. None of the pessimistic projections of
doomed diversity yet account for the accelerating pace of future climate change,
its velocity of assault on the natural world. Can plant and animal species reach the escape velocity they need to avoid extinction, or will the changing climate over-whelm the capacity of species to maintain themselves under the stable conditions
they have experienced over the past 10 000 years? Factor velocity of climate
change into the mix and we find that, over 30% of the Earth’s land surface, the
EdEn undEr siEgE a 181
speed at which plant species must migrate to keep pace with a shifting climate in the coming century exceeds their capacity to disperse.38 Those species left behind in the race to keep up with the fast-moving climate envelope may p
erish. Current
projections indicate this is likely to be a common problem confronting plants,
because climates found today on 20–50% of the planet could disappear within a
century.39
Unfortunately, the extinction story does not end there, because our actions
now are leading to an ‘extinction debt’ payable by the natural world at a later
date.40 Extinction debts arise because of the time lag between habitat destruction or climate change and the subsequent terminal decline of populations of plants
or animals. Forecasts of extinction based on species–area curves overlook this
crucial issue, assuming instead that extinctions are instantaneous, reductions in numbers following lock-step with a shrinking habitat area or contraction of an
agreeable climatic envelope. But in reality plant extinction is likely to be a slow-burn phenomenon, and for good reason, as tropical ecologist Dan Janzen noticed
when agriculture replaced native forest in Costa Rica. In the newly created agri-
cultural landscape, surviving patches of forest containing native trees persisted in field margins, and a few pockets of diversity elsewhere, but were unable to
reproduce because the habitat suitable for seedling establishment had been
destroyed. These trees, Janzen wrote, are the ‘living dead’; sitting in the extinction waiting room.41
A clue to the sort of timescale involved in plant extinctions comes from
Napoleon’s St Helena, the remote South Atlantic Island where the tree habit has
evolved with alacrity. When Portuguese navigators discovered the island in 1502,
they introduced goats, which, without natural predators, soon multiplied into
huge herds. Ravaging goats and people destroyed the vegetation, and yet some
plants like the St Helena olive ( Nesiota elliptica), unrelated to the true olive, hung on with remarkable tenacity.42 Despite there being fewer than 10 trees by
1900, the St Helena olive finally went extinct in 2003. Extinction time lags for
woody plants on St Helena are on the order of a century or longer,43 and this may be the rule of thumb for most plants. Individuals of long-lived plants, like trees, for example, may continue to reproduce or, like Janzen’s trees, simply live on
without reproducing, even if climate change or habitat destruction means viable
populations are unsustainable over time. Plants can also regenerate from seed
182 a EdEn undEr siEgE
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