Elevational shifts in the distribution of species can have large consequences to high-elevation ecosystem. More so, when we are talking about herbivores.
In a field experiment, Patrice Descombes and collaborators translocated various species of grasshoppers from medium elevations (1,400 m a.s.l.) to three alpine grassland sites at 1,800, 2,070 and 2,270 m a.s.l. in the Anzeindaz region in the Vaud Alps. They measured how the biomass, structure and composition of the alpine plant communities changed under the influence of the herbivorous insects. They also investigated whether some plant species were more susceptible to herbivory, for instance plants with tougher leaves, or those containing more silica or other constituents such as phenols or tannins.
The grasshoppers’ feeding behaviour had a clear influence on the vegetation structure and composition of the alpine flora. Alpine communities display clear structure in the organisation of the canopy, with plants with tough leaves at the top, and more shade-tolerant plants with softer leaves at the bottom. But this natural organisation was disturbed, because the translocated grasshoppers preferred to feed on taller and tougher alpine plants, that were more similar to their lower-elevation food plants. As a result, the insects reduced the biomass of dominant alpine plants, favouring the growth of small-statured plants. The overall plant diversity thus increased in the short term.
Global warming can disrupt the ecological balance of biotic communities because mobile animals, including many herbivorous insects, can expand their habitat to higher elevations more rapidly than sedentary plants. Herbivorous insects from lower altitudes could therefore have an easy time in alpine habitats with resident plants that are insufficiently prepared to defend themselves against those new herbivores. This could change the current structure and functioning of alpine plant communities as a whole. Climate change would thus have an indirect impact on ecosystems, in addition to the direct consequences of rising temperatures.
Their study has been published in Science. You can find the full article here.
Reference: Descombes, P., Pitteloud, C., Glauser, G., Defossez, E., Kergunteuil, A., Allard, P.M., Rasmann, S. and Pellissier, L. (2020) Novel trophic interactions under climate change promote alpine plant coexistence. Science, 370(6523):1469-1473.
Picture: Research site at 1,800 m in the Chablais region. In the background: Vallon de Nant and the Grand Muveran (photo: P. Descombes)