Plant richness, land use and temperature differently shape invertebrate leaf-chewing herbivory on plant functional groups

Nutrient demands of leaf-chewing invertebrate herbivores change with temperature, which causes shifts in herbivores’ diets. Temperature may act differently on herbivore species, so that factors shaping herbivore species richness may modulate temperature effects on invertebrate herbivory among plant functional groups with different nutrient composition (C:N ratio low to high: legumes, non-leguminous forbs, grasses). Global warming urges a deeper understanding of temperature effects on herbivory among plant functional groups in different habitats and landscapes. This study obtained measures on proportional leaf area loss to leaf-chewing invertebrate herbivores (‘herbivory’) on three plant functional groups on 80 plots of open herbaceous vegetation adjacent to different habitat types (forest, grassland, arable field, settlement) along climate and land-use gradients in Bavaria, Germany. Herbivory was analysed with regard to habitat characteristics (habitat type, plant richness at species and family level, local mean temperature), landscape characteristics (proportion of grassland, landscape diversity; 0.2–3.0-km), climate (multi-annual mean temperature, ‘MAT’) and interactive effects of plant functional group, temperature and habitat or landscape characteristics. Herbivory on plant functional groups changed differently in response to plant richness (family level only) and habitat type, but not to differences in landscape characteristics and temperature – only on grassland plots, multi-annual mean temperature differentially affected herbivory among plant functional groups. Thus, abiotic and biotic factors can differently affect leaf-chewing herbivory on plant functional groups. Under current conditions, plant richness and habitat type more strongly affected herbivory among legumes, forbs and grasses than temperature and landscape-scale land use.

Methods
Leaves at apical, middle and bottom position on a plant were collected from three individuals of three “species” per plant functional group. Morphologically different plants within plant functional groups were assumed to be different “species” but species identity was not determined. Leaves were pressed and dried until digital assessment using gimp for image processing and the BioLeaf App for calculation of proportional leaf area loss by leaf-chewing invertebrate herbivores. Proportional leaf area loss was averaged in sequence per plant individual, per ‘plant species’ and plant functional group. Data points were excluded when data were obtained on less than 10 leaves per plant functional group and plot and when data were not available for all three plant functional groups on a plot (processed data). For details address the manuscript.