The projected disappearance of small glaciers* worldwide threatens to eliminate the water supply for numerous towns in valleys, such as the Ecuadorian capital Quito, fed by the rivers that flow down from the surrounding mountains. But retreating ice is also a threat to freshwater fauna. According to a study published in Nature Climate Change, the local and regional diversity of mountain aquatic fauna will be reduced considerably if predictions are realised. Until now, the impact of global thawing on biodiversity in watercourses had never been calculated in detail.
Insects, a model group
The research team(1) has focused on the future for populations in streams formed by meltwater in the Alps, Alaska and the equatorial Andes, where the IRD researchers have been working. The IRD team has collected samples from around fifty different sites in the páramos. These highly particular herbaceous ecosystems are typical of Andean summits, situated at over 3,500m in altitude between the limits of the forest and the permanent snowline. Ecologists have calculated the population of macroinvertebrates - mainly insect larvae such as Ephemeroptera, Trichoptera and Diptera. These organisms that mostly live at the base of glacial torrents belong to a group that has already been studied extensively, whose environmental needs are relatively well known, and as such can be considered as a model for scientists. For example, following over a year of regular sampling, over 150 species of invertebrates have been identified living in just one páramo in the Antisana volcano, the 'water tower’ serving Quito.
Extinction between 10 and 40%
In the three regions being studied - temperate, arctic and tropical -, the research team has analysed the reaction of three key elements to the changes in glacial coverage: the local taxonomic diversity (the number of different species present in a stream, for example), or at a regional level (across a hydrographic network, for example), and the variation in this diversity between watercourses. Thanks to this sampling at varying distances from glaciers, the data has revealed that the local diversity increases when moving further downstream. In addition, it seems that the populations in different streams at the same altitude is highly consistent. Within a distance of around one hundred metres, the populations found in two streams that appear the same can be substantially different according to which glacier they drain from. Indeed, the Andean glaciers have diverse actions, melting slower or faster according to their size, which varies greatly, and their exposure to sunlight, for example.
According to these tests, in addition to data from aquatic population studies dating back over 20 years, as soon as the glacial coverage is reduced to the point where it only covers 30 to 50% of the drainage basin, several species begin to disappear. And if the glaciers melted completely, between 11 and 38% of the regional diversity might become extinct, depending on the area being studied, including endemic species.
Fauna unique to these areas
High mountain regions are like isolated islets - migration by new species is restricted and they are favourable to speciation(2). Low temperatures and atmospheric pressure, intense solar rays, irregular rainfall, searing winds, ice... all of these extreme conditions have driven forced the native species to adapt in singular ways, particularly in the tropical Andes. Glacial torrents also impose difficult living conditions on their inhabitants, due to their low mineral content(3) and daily floods(4) that produce major disturbances. A high level of endemism thus characterises the páramos, containing species not found anywhere else on Earth, but also inexorably threatened by extinction if glaciers continue to recede.
A loss in services rendered
This study has demonstrated the crucial role played by glaciers in the creation and dynamism of biodiversity, and thus the impact of their reduction for the preservation of this diversity. But this is not the only blow for the planet. The insects play a major role in the functioning of mountain ecosystems, particularly through decomposition of organic matter that enables soil to be formed. They can also be useful downstream, for the 2 million inhabitants of Quito, for example. For these organisms also function as bio-indicators for the quality of the water that supplies the city and can be used as a tool by those who manage this resource. The threat of extinction of several species has led to fears for the loss of these crucial services provided by the ecosystem.
The ecological role played by the majority of invertebrates under threat in glacial rivers is still not well known. The consequences at higher trophic levels, such as for fish, amphibians, birds and mammals, are difficult to predict. But these exceptional ecosystems seem to be condemned to disappear, before they have even had the offer up all of their secrets to researchers.
* see news sheet no.127 - Small glaciers in the tropical Andes: a forewarning of disappearance (http://www.ird.fr/la-mediatheque/fiches-d-actualite-scientifique/127-petits-glaciers-des-andes-tropicales-une-disparition-annoncee)
(1) These studies have been led by the IRD and the University of Copenhagen, Denmark, in collaboration with the universities of Birmingham and Leeds in the UK, Alaska in the US and Paris-Sud 11, as well as the Pontificia Universidad Católica del Ecuador in Quito, and the CNRS.
(2) An ongoing process leading to the emergence of new species.
(3) Environments less rich in minerals are generally unfavourable for aquatic fauna that draw crucial elements for survival from the water, such as chlorides or sodium.
(4) During the day, the glacier melts and increases the flow in streams, whereas the flows diminish at night.
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