Bird communities in the Swiss Alps are becoming more alike
Bird communities in the Swiss Alps are becoming less and less diverse. Looking at two decades of data, we found that Alpine bird communities have lost some of their variety, both in the kinds of species they hold and in the roles those species play. This trend is likely linked to rising temperatures and changes in how land is used.
Climate change is thought to hit animals and plants in the mountains especially hard, changing where species live, how common they are and how they interact. In Switzerland, some birds, such as the tree pipit and the European pied flycatcher, have moved to higher altitudes over the past two decades. But what this means for whole bird communities, the set of species living together in one place, was not known.
Our new study in Diversity and Distributions, which I led with Thomas Sattler and Hans Schmid of the Swiss Ornithological Institute (Vogelwarte), and Arpat Ozgul, looks into this. It is based on 20 years of data collected by volunteer ornithologists, and the trend points downwards: bird communities in the Swiss Alps are becoming more and more alike. Ecologists call this biotic homogenisation.
Counting species is the simplest way to measure diversity, but species also differ in what they do: what they eat, where they nest, how they use their surroundings. This is called functional diversity. “Not all species play the same role in an ecosystem,” says Vicente. “The loss of a given species can be more detrimental than that of several others.” Specialists such as some woodpeckers are especially important: the holes they make are used by other birds and small mammals for nesting and hiding. To measure functional diversity, we gathered 100 traits for each species, such as body mass, diet and habitat. This let us see how functional diversity changes from the valleys up to the high mountains, and how it has changed over 20 years.
High-mountain specialists on the edge
The tree line acts as a boundary between two quite different groups of birds. Below it live the birds of the farmland in the lowlands up to the high mountain forests, such as the yellowhammer, the common whitethroat and the blackcap. Above it live species adapted to open habitats above the trees. They eat fewer caterpillars, for example, and some, like the rock ptarmigan, nest on the ground. It is these alpine communities whose functional diversity has dropped. “Our study shows that mountain specialists are under increasing risk of becoming misfits,” says Arpat. Their habitat is shrinking, they cannot find good habitat higher up, and generalist species are moving up into it.
The rise of the generalists
Generalists, such as the robin or the European pied flycatcher, can live in many places, and they have become more common. This is probably one reason why bird communities in different places are becoming more similar to each other. The communities are also becoming more redundant: more of their species play much the same role.
When bird communities become more alike, the Alps lose some of their variety, and the mountain specialists come under pressure. Warming and changes in land use, such as shrubs spreading on abandoned pastures and the tree line moving upwards, are likely behind this trend. Protecting these mountain specialists will need urgent conservation and management action.