Martijn L. Vandegehuchte, Valeria Trivellone, Martin Schütz, Jennifer Firn, Frederic de Schaetzen, and Anita C. Risch
Grazing mammals can indirectly influence insects feeding on the same plants. However, these mammals do not just eat any plant. Some plants are nutritious while others are difficult to digest. As the preferred plants get eaten more, less plant tissue remains for the insects. If preferred plants are grazing-tolerant, such as many grasses, they can regrow after grazing, and this fresh regrowth is often nutritious. Other plants mount defenses such as toxic chemicals and become less nutritious. There are also longer-term consequences. If grazing-tolerant, preferred plants are abundant, less preferred plants may not be able to establish themselves. For example, in a grassy meadow that is grazed, it will be hard for a nettle to start growing. However, when the most preferred plants are grazing-sensitive, these plants may be removed by grazers in favor of less edible plants. We therefore expect grazing mammals’ effects on insects to depend on whether the insects feed on plants preferred or not preferred by mammals. We tested this expectation in subalpine grasslands by excluding mammals such as deer, marmots, and mice from plots using size-selective fences. We did this both in short-grass vegetation, with a long history of intensive grazing by mammals, and tall-grass vegetation, which is less intensively grazed. We studied leafhoppers –sap-sucking insects– specialized on different plant types. In the short term, excluding mammals increased the biomass of grasses and other plants preferred by mammals, which increased the number of leafhoppers specialized on these plants. In the long term, sedges, which are less preferred by mammals, became more abundant in the less intensively grazed tall-grass vegetation, leading to higher numbers of sedge-feeding leafhoppers. No effects of mammals on leafhoppers were caused by altered plant quality. Because the sedge-feeding leafhoppers are smaller, this also lowered the average leafhopper body size in the tall-grass vegetation. Our results show that grazing mammals can lower the biomass of preferred and non-preferred plants, and therefore the abundance of their specialized plant-feeding insects, at different timescales. These insights may be useful for mammal population management and help predict the wider effects of extinction or overabundance of mammalian grazers.
Image caption: A few of the more abundant leafhopper species in our samples, feeding on different plant types. A) Verdanus abdominalis, a grass feeder. B) Anaceratagallia venosa, which mainly feeds on legumes, but also on some forbs. C) Eupteryx notata, a species feeding on forbs. D) Kelisia monoceros, a sedge feeder. Photo credit: Gernot Kunz.