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

This study explored consumption of a generalist herbivore feeding on leaf t issue of various plant species of a calcareous grassland, and tested whethe r consumption levels and preferences changed when plants were exposed to 5 years of in situ CO2 enrichment. The first part of this experiment tested w hether the consumption patterns of slugs (Deraceras reticulatum) observed i n 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 le gumes 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 g rass, and a non-leguminous forb (Trifolium medium, Lotus corniculatus, Brom as erectus, and Sanguisorba minor, respectively) in two or four species com binations. Ln the leguminous species mix, the non-leguminous species mix, a nd 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 no n-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 i s the first time such a shift has been observed, and provides evidence that there may be multiple herbivore responses to rising atmospheric CO2 concen trations. Numerous single-species feeding tests using insects have shown th at consumption by herbivores may increase when herbivores are fed plants gr own in enriched CO2 atmospheres. This study clearly demonstrates the limite d applicability of non-choice feeding trials to,generalist herbivores in sp ecies-rich communities.