Omnipresence of leaf herbivory by invertebrates and leaf infection by fungal pathogens in agriculturally used grasslands of the Swiss Alps, but low plant damage

Abstract

Agriculturally used grasslands in the Alps are characterised by a trade-off between high fodder production in some and high plant species richness in others. In contrast to plant species richness and production, however, little is known on the relevance of biological interactions between plants, invertebrate herbivores, and fungal pathogens for grasslands in the Alps. At the time when the vegetation was fully developed, but prior to agricultural use, we examined whether leaf damage by herbivory and fungal pathogen infection, and their diversity, are affected by plant functional group, land use, and altitude. Moreover, we studied whether extent and diversity of leaf damage are related to each other, to plant species richness, and to standing crop. We recorded the leaf area damaged by ten types of herbivory and five types of fungal pathogen infection on 12,054 plant leaves of legumes, other forbs, and graminoids collected in 215 grassland parcels in 12 valleys in the Swiss Alps. With 83 % of all leaves infested, herbivory and fungal pathogen infection were omnipresent. However, only 2.7 % leaf area was damaged by herbivory and 1.2 % by fungal pathogens. Damage by herbivory was highest on legumes, and damage by fungal pathogens was highest on graminoids. More leaf damage by herbivory occurred in traditionally mown sites and at lower altitudes, while damage by fungal pathogen infection was independent of land use and altitude. Most types of herbivory were found on legumes and on leaves from fertilised sites, whereas the number of fungal pathogen types was highest on graminoids and in unfertilised sites. Larger standing crop was associated with higher leaf damage and diversity of herbivory types per leaf. Neither damage by herbivory nor by fungal pathogens was correlated with plant species diversity. In more plant species rich parcels, the number of herbivory types was lower at the leaf level, but tended to be higher at the parcel level. Our results highlight the omnipresence of plant-herbivore and plant-pathogen interactions. They suggest that current land use changes from mowing to grazing or to abandonment decrease the diversity of herbivory, and that fertilisation decreases pathogen diversity. As our results did not reveal conservation conflicts between diversities of plants, herbivores, and fungal pathogens, and as the damage from herbivory and pathogens is generally low, we conclude that for protecting the high diversity of plant-herbivore and plant-pathogen interactions a diverse low-intensity land use should be maintained.

Insect herbivory in alpine grasslands is constrained by community and host traits

“…These results are largely in accordance with observations in semi‐natural grasslands at various altitudes in the Swiss Alps (Fischer et al. ). The study of Fischer et al.…”

“…However, the pattern we observed matches those documented by Fischer et al. (), who also observed less herbivore damage in monocots but higher damage in legumes as compared to other herb species in the Swiss Alps. Similarly, Scherber et al.…”

“…However, the degree of damage was low in more than 96% of observations. These results are largely in accordance with observations in semi-natural grasslands at various altitudes in the Swiss Alps (Fischer et al 2012). The study of Fischer et al reports an even higher incidence of herbivory (83% of leaves) but leaf area loss was only 2.7%.…”

“…This is in line with the negative association of altitude and leaf herbivory found in managed grasslands of the Swiss Alps (Fischer et al. ; Pellissier et al. ).…”

“…However, empirical evaluation of these predictions is scarce (Galen 1990;Blumer & Diemer 1996). Although altitudinal effects on herbivory intensity were shown in montane species (Scheidel & Bruelheide 2001), we know of only a single study (Fischer et al 2012) that provides data on the incidence of herbivory from above the tree line beyond a very local spatial scale. Furthermore, Blumer & Diemer (1996) revealed higher rates of herbivory by grasshoppers in high-alpine grassland communities than at lower altitudes.…”