Positive and negative effects of herbivory on the population dynamics of Senecio jacobaea L. and Cynoglossum officinale L.

Herbivore effects were studied on populations of the biennial plant species Senecio jacobaea and Cynoglossum officinale. During a three year period (1985-1988) population characteristics (herbivory, number of seedlings, rosettes and flowering plants) were compared in-and outside exclosures, as well as parameters reflecting vegetation cover. In S. jacobaea, a strong negative effect of Tyria jacobaeae was found on seedling establishment, rosette growth and flowering. On the other hand, vertebrate herbivores (mainly rabbits) had an indirect positive effect by limiting the development of the surrounding vegetation (esp. grasses). The increasing vegetation cover in protected populations caused a reduction in germination, seedling- and rosette-growth. Herbivory on C. officinale was low (<10%), no direct effects of herbivores on plant populations were shown. Indirect effects of herbivory through an increasing vegetation were even more pronounced as in S. jacobaea. Therefore, although both plant species may first benefit from herbivore-exclusion, their populations are dependent on rabbits eating other plants (esp. grasses) and reducing competition.