Consumption rates and food preferences of slugs in a calcareous grassland under current and future CO₂ conditions

Abstract 

This study explored consumption of a generalist herbivore feeding on leaf tissue of various plant species of a calcareous grassland, and tested whether consumption levels and preferences changed when plants were exposed to 5 years of in situ CO₂ enrichment. The first part of this experiment tested whether the consumption patterns of slugs (Deroceras reticulatum) observed in single-species feeding tests were altered when slugs were given a choice of food sources. Overall consumption increased 270% when slugs were given a choice, and they preferred having a choice of food sources more than they preferred having any one food source. Surprisingly, slugs consumed fewer legumes and grasses and more non-leguminous forbs when given a choice. In the second part of this experiment, feeding behaviors of slugs in response to elevated CO₂ were investigated by feeding them leaves of two legumes, one grass, and a non-leguminous forb (Trifolium medium, Lotus corniculatus, Bromus erectus, and Sanguisorba minor, respectively) in two or four species combinations. In the leguminous species mix, the non-leguminous species mix, and the combined mix (legumes and non-legumes), neither overall consumption by herbivores nor species preference was significantly altered by long-term CO₂ enrichment. In the combined species mix, slugs preferred legumes to non-legumes (P=0.012) and exhibited a weak functional group preference shift from non-legumes to legumes (P=0.089) in response to CO₂ enrichment. This is the first time such a shift has been observed, and provides evidence that there may be multiple herbivore responses to rising atmospheric CO₂ concentrations. Numerous single-species feeding tests using insects have shown that consumption by herbivores may increase when herbivores are fed plants grown in enriched CO₂ atmospheres. This study clearly demonstrates the limited applicability of non-choice feeding trials to generalist herbivores in species-rich communities.