Freshwater ecosystems at risk, say Swiss researchers

June 2010: The decline of biodiversity represents a loss of natural capital for future generations. Freshwater ecosystems are particularly affected, as they harbour disproportionately high levels of biodiversity. But knowledge of the development and decline of diversity in freshwaters remains patchy. So far, measures to protect genetic diversity in rivers and lakes have failed to halt the downward trend. What is widely underestimated, according to scientists at the acquatic institute Eawag, is the extent to which reduced habitat diversity also preHvents species formation, thus accelerating the spiral of decline.

FRESHWATER GENE POOL: Brown Trout Picture: Science Photo Library

Above-average diversity - accelerated loss
A mere 0.3 per cent of the Earth's surface is covered by lakes, rivers and wetlands. Even in Switzerland -Europe's "water tower" - the proportion is barely four per cent. But these habitats harbour a huge variety of species: 40 per cent of the world's 30,000 recognized fish species and more than 100,000 invertebrates are found in freshwaters. Eawag says this diversity is now at risk. Not just relative to the surface area but also in absolute terms, extinction rates are considerably higher for freshwater than for terrestrial and marine species. In Switzerland, for example, 17 of just over 100 known fish species are extinct. More than 60 per cent of all aquatic plants are believed to be threatened. Present-day extinction rates are comparable to those seen during the greatest mass extinction events in the Earth's history.

In addition, ever-fewer new species are being formed, according to Eawag. Evolutionary ecologist Ole Seehausen calls this doubly negative trend a ‘catastrophic biodiversity debt'.

Protection of species formation equally important

Seehausen and his group have demonstrated that changes in the same processes which led to the development of existing species are often responsible for a decrease in the formation of new species -when environmental changes reduce the size or diversity of habitats, for example. Genetic adaptations to ecologically distinct niches are then no longer required, young species merge into a single hybrid form, and the emergence of new species ceases. In the case of the 32 different whitefish species described in Swiss lakes, at least a third have disappeared over the past 50 years. ‘There's not much time left to save the rest,' says Seehausen, who advocates greater cooperation between research and applied conservation.

Evolution - more rapid than expected
The group led by aquatic ecologist Piet Spaak showed that - contrary to the assumptions of traditional conservation science - evolutionary processes can often produce marked changes and adaptations in species within a few generations.

Another remarkable finding emerged from Seehausen's studies of trout: The five types of trout described in Switzerland are evidently adapted to different ecological conditions and can still coexist, without merging, in near-natural rivers. In highly degraded rivers, by contrast, they are displaced by the Rhine trout, which has been widely released. Seehausen notes: ‘There are virtually no coordinated programmes to conserve trout diversity.'

Species loss is attributable not only to the fact that habitats have disappeared or become monotonous, but also to a lack of connectivity. Artificial barriers impede the passage of fish. On the lower reaches of the Töss river in Canton Zurich, for example, Eawag biologists counted 23 fish species below a six-metre-high weir, but only 12 above this barrier.

‘On account of its abundant water resources, its topography and its role as a hinge between different biogeographical regions, Switzerland has a particular responsibility for freshwaters and their biodiversity,' says Eawag researcher Mark Gessner, who is also a member of the Swiss Biodiversity Forum. He compares biodiversity to a broad investment portfolio, offering ‘insurance for the future'. He argues that high levels of species richness and genetic diversity provide greater stability in the face of environmental changes, which in turn ensures the provision of ecosystem services for human populations. These include, for example, fish catches, but also clean water, flood protection or attractive recreational spaces.