The disappearance of diversity

​(From "Horizons" no. 104, March 2015)Science has a problem. Most biologists are convinced that mass extinction is taking place across the world today, but they have difficulty proving it. Only a few hundred species can be proven conclusively to have died out. One of the biggest problems is the lack of proper figures. We can’t even pretend to have any idea of the number of species in existence on Earth, let alone each individual population size. Today, we know of the existence of some 1.8 million species. But because many areas are difficult to reach and have barely been investigated – such as the deep sea, isolated jungles and many different soils too – the total number of species could actually be ten or twenty times as many. But even in the case of species that have been described by science, it’s more difficult to prove that they’ve become extinct than it is to prove they exist. Especially when it comes to rare species, it’s highly likely that we’ve failed to spot the last few of them that are still alive. So scientists are accordingly hesitant to declare any species as definitively dead.

In lieu of anything else, we resort to estimates. It’s an undisputed fact that man has destroyed species-rich ecosystems on a massive scale, such as rainforests. A common rule of thumb states that the number of species in a habitat sinks to roughly half when its surface area is destroyed by 90%. But even the validity of this ‘rule’ is difficult to prove in reality. For one thing, it can take a very long time before species actually die out. This is called the ‘extinction debt’, which means it can take centuries or even millennia for the final consequences of specific events to take effect and cause species to die out. Furthermore, these rules of thumb were derived from observations carried out on islands. But ecosystems on the mainland differ from those on islands in one crucial matter: it’s far easier for species to migrate. The problem overall is excellently demonstrated by a survey published last year in the journal Science by a team led by Maria Dornelas, a biologist from the University of St. Andrews in Scotland. In a large-scale meta-analysis of time series, Dornelas investigated species development in numerous local ecosystems from the poles to the tropics. Her results have caused quite a stir. The number of species decreased in just some 40% of the habitats analysed. In the remainder, the number of species stayed the same or even increased. This sounds like good news – but it’s only half the truth. The other half is rather more important: on average, some 10% of all species are replaced every decade in a typical ecosystem. We don’t learn any details about this species-exchange from Dornelas’ work, but experience shows that in such processes the immigrant species often displace highly rare, even unique local species. And this is the real threat to diversity.

Immigrants from the south

“If you only work with numbers of species, then you haven’t understood what biodiversity is really about”, says Daniela Pauli. She’s the manager of the Swiss Biodiversity Forum, a competence centre run by the Swiss Academy of Sciences. “The problem is actually the increase in homogenisation. Habitats and species communities are becoming increasingly similar”.

Instead of the ‘extinction of species’, we should rather speak of a ‘shrinkage
of biodiversity’ or the ‘disappearance of diversity’. Biodiversity encompasses far
more than just numbers of species. It’s about the abundance and heterogeneity
of organisms, species communities and gene pools. Switzerland is a good example
of what’s been happening in many other places around the world. We only know
for sure of a few species that have actually died out. The total number of species has
even increased in recent years – primarily thanks to immigration from the Mediterranean regions, such as bee-eaters that can now breed in Switzerland, thanks to global warming. But at the same time, there has been a collapse in the stock of species that were once prevalent. There’s the corn crake bird, whose incessant nocturnal calls annoyed many people just a hundred years ago, but of which just a few dozen are left in Switzerland today. Or there’s the lady’s slipper, a flower that used to be sold by the bundle at markets in the Jura region, but is today an absolute rarity.

And the dwindling continues. “We’re revising the Red List for plants at the moment”,
says Pauli. “We’ve noticed that it’s the numbers of the most endangered species
that have decreased the most – and that’s alarming”. The main problem remains
intensive agriculture, as its fertilisers and irrigation have made many dry grasslands disappear, especially in mountainous regions. And with them, in turn, the plants and animals specifically adapted to that habitat have been disappearing as well. Since 1990, the total surface area of dry grasslands in Switzerland has decreased by
a third, and since 1900, some 95% of them have gone for good. Even the few moorlands left have suffered under the impact of fertilisers, and many are also drying out.

As a result, common or garden plants are spreading, such as sweet grasses. “Unique
species are being lost” says Pauli. “The species composition of lowland moors is becoming more and more like that of normal meadows”. The Biodiversity Action Plan is currently leading a debate about what can be done in Switzerland so as to achieve
long-term sustainability for its diversity (see box).

But why should we even bother to protect biodiversity? We often hear the economic
arguments: a jungle plant is a potential medicine, and a rare whale can be a
tourist magnet. Even an unimposing grass can offer an important service. Botanists
at the University of Basel, for example, have discovered that the root structure of
the Valais fescue (festuca valesiaca) protects against the erosion of unstable mountain
slopes in the Alps and the Caucasus. Also, the use of many a species only becomes
clear when it’s no longer there. For example, Chairman Mao began a major campaign
in 1958 to rid China of its sparrows. Yet just two years later, the Chinese had to
begin importing sparrows from the Soviet Union because its populations of insect
pests had exploded.

But it can be awkward to argue one’s case based on the usefulness of individual
species. What is one to say if an economist works out that this or that animal is of no
use to humans, and so can safely be eradicated?
The true value of diversity can’t be calculated in dollars and cents. Diversity is
of value in and of itself.

An easy target for pests

The advantages of diversity can nevertheless be clearly seen. Monocultures of any
kind are generally vulnerable. In the 19th century the Irish were so dependent on
the potato alone that a million people died when potato blight spread. Bananas sold
today are mostly from the same species and all have the same genome, which makes
them an easy target for pests. These are indeed on the advance, and the consequences are potentially disastrous. Hurricane Lothar in 1999 affected spruce cultures and other monocultures most of all – as did the bark beetle afterwards. Mixed woodlands proved less vulnerable. And let’s have one more example: a higher diversity of mammals in forests is proven to provide protection against Lyme disease, a dreaded tickborne form of encephalitis. Because when other mammals are absent, the ticks primarily attack mice, and these are the principal hosts for Lyme disease germs.

Diversity is a kind of life insurance. Diversity means: adverse events won’t
affect everyone equally badly. Diversity means: there’s always someone there to
take on specific tasks. The world is changing, and no one knows what abilities will
be required in future. If the individual members of a species are different, then
there is a high degree of probability that one of them will be well adjusted to the environmental conditions that will arise in the future – and this is the basis of evolution. And if a community shelters many hundreds of species, then there is a chance that at least some of them will survive even a drastic change. Thanks to them, the ecosystem will remain able to function.

After the volcano on Krakatoa exploded violently in 1883, it took just a year before
botanists found the first grass shoots among the volcanic rocks. By 1886 there were fifteen species of grasses and shrubs on Krakatoa; by 1897 there were already 49,
and by 1928 almost 300. Such an impressive re-colonisation would have been unthinkable without all the specialists possessing the right characteristics at just the right moment. That’s true in every conceivable situation, in every corner of the Earth.
"Biological diversity", wrote the biologist Edward O. Wilson, “is the key to preserving
the world as we know it”.

Mathias Plüss is a freelance journalist who writes about the natural sciences and Eastern Europe

Biodiversity strategy

In 2012, the Swiss Federal Council passed a biodiversity strategy for Switzerland. It comprises ten strategic goals that apply to all levels of biodiversity – such as “[improving] the conservation status of the populations of national priority”, “[ensuring] the conservation of ecosystems and their … genetic diversity” and “[developing] an ecological infrastructure consisting of protected and
connected areas”. Meanwhile, the draft has been presented for the corresponding 2020 Action Plan, on which hundreds of experts have been working. This Plan, along with the measures in it, must now be approved by the Federal Council.

Literature

M. Dornelas et al. (2014): Assemblage time series reveal biodiversity change but not
systematic loss. Science 344: 296–299. T. Lachat, D. Pauli, Y. Gonseth, G. Klaus,
C. Scheidegger, P. Vittoz & T. Walter (eds.) (2010): Wandel der Biodiversität in der
Schweiz seit 1900. Ist die Talsohle erreicht? Haupt Verlag.