A study from the Swiss mountains shows how complex ecosystem disturbances can be as a result of climate change: The alpine ecosystems are threatened by grasshopper species, which can migrate to higher and higher altitudes in the course of rising temperatures, is shown by experiments. The plant communities, which up to now have had to arm themselves less against the voracious insects, could get completely mixed up, say the scientists.
Climate change is changing the conditions in many regions of the world, sometimes at a rapid pace: It is getting warmer, precipitation is changing and complex new patterns are emerging in the environment. While the heat expands globally towards the poles, in the mountains it migrates up the slopes. Many studies have already shown that some animal and plant species are following this trend. If they colonize new habitats, however, they could seriously disrupt the ecological balance that has developed there over a long period of time.
Weakly armored flora
This is especially true for alpine areas, explain the scientists working with Patrice Descombes from the Swiss Federal Institute of Technology in Zurich. An important aspect here is that there are usually fewer herbivorous insects at high altitudes because they cannot cope well with the conditions there. That is why the plants in these areas had to produce fewer defense strategies compared to species from lower regions – such as spines, thorns, hair or toxic substances. Against this background, Descombes and his colleagues have now experimentally investigated what could happen if herbivores from central locations – in this case locusts – migrate to higher alpine meadows and encounter the local plant communities.
To this end, the scientists transferred various species of locusts from medium altitudes (1400 meters) to three alpine grassland locations at 1800, 2070 and 2270 meters above sea level under controlled conditions. The grasshoppers ancestral there were previously removed from the test areas. In the course of the experiment, the researchers recorded how the biomass, structure and composition of alpine plant communities changed under the influence of the alien herbivorous insects. The researchers also examined the characteristics of the plants found at the test site in the Anzeindaz region in the Vaudois Alps.
Gnawed ecosystem
It turned out that with their feeding behavior, the grasshoppers clearly influenced the vegetation structure and composition of the alpine flora. As the researchers explain, the plant communities are usually clearly structured: the top layer of vegetation is made up of plants with relatively strong structures, with more shade-tolerant species with softer leaves growing underneath. This natural structure was disrupted by the introduced locusts, the evaluations showed: The insects therefore preferred to eat the higher-growing plants. Due to their leaf structure, nutrient content or growth pattern, these were similar to the lowland plants that the locusts normally eat, the scientists report.
As a result, the insects disproportionately reduced the biomass of the actually dominant alpine plants, and this in turn favored the development of small-stature species that herbivores avoided. “Immigrant herbivores only eat certain plants in their new habitat, which changes and reorganizes the competitive conditions between the various alpine plants,” says Descombes. Global warming could disturb the ecological balance because mobile animals such as herbivorous insects can expand their occurrence to greater heights faster than sedentary plants. This could change the current structure and functioning of alpine plant communities as a whole, the researchers sum up.
The results of the study thus illustrate how climate change affects ecosystems not only directly due to the increase in temperature, but also indirectly due to changed relationships between herbivores and plants.
“So far, climate impact research has primarily examined the direct effects of temperature on ecosystems. The new interactions between organisms that migrate to new habitats could cause important structural changes. They are important drivers of changing ecosystems in an ever warmer climate, ”says co-author Loïc Pellisier from ETH Zurich.
Source: ETH Zurich, specialist article: Science, doi: 10.1126 / science.abd7015