SHOOT SURVIVAL UNDER INTENSE GRAZING FOR 2 BROAD-LEAVED HERBS WITH DIFFERENT CHEMICAL DEFENSE SYSTEMS

Shoot survival of a toxic herb, Actaea spicata, and a herb with a dige stibility-reducing defense, Geranium sylvaticum, was studied in an exp erimental set-up where microtine rodents were allowed to graze freely. It was hypothesized that the defense system of Actaea would work in a risk-reducing way, while the defense system of Geranium would have co st-reducing properties. If so, shoot survival of Actaea would be highe r at all times than for Geranium, and shoot survival would also be hig her for Actaea when grazed by a more generalized herbivore (Microtus a grestis) than by a more specialized one (Clethrionomys rufocanus). Usi ng failure-time analyses, it was shown that Actaea indeed had a signif icantly higher shoot survival than Geranium in all treatments but one, and that survival was also higher when grazed by the generalist than by the specialist. It was concluded that the defense system of Actaea had stronger characteristics of a risk-reducing defense than that of G eranium.