RESPONSES IN GROWTH, REPRODUCTION AND LIFE-SPAN TO REDUCED COMPETITION PRESSURE IN THE LAND SNAIL BALEA-PERVERSA

Intraspecific interactions early in an individual's life may lead to reduced reproduction once maturity is attained. Possibly, this could be compensated by increased or prolonged reproduction later in life if competition is relaxed (i.e. after a sudden decrease in population density as the result of a catastrophic event). We tested this compensation hypothesis in the ovoviviparous, self-fertilizing land snail Balea perversa (L.). Juvenile snails were kept for 450 d at different densities on their natural substrate and food (pieces of limestone providing epi- and endolithic lichens). After this period of intraspecific competition, individuals were kept singly under conditions of high food supply for the rest of their life (126-484 d; period of relaxed competition). Age and size at first reproduction, as well as reproductive output, were influenced by the initial experimental density; individuals kept at high density reached sexual maturity late and at a small size. In the relaxation period, reproductive rate did not differ between snails from different initial densities. Individuals initially kept at high population density lived longer during the relaxation period than ones kept at low density. Nevertheless, they did not live long enough to compensate for the earlier loss in fecundity; overall reproductive life span and lifetime fecundity decreased with increasing initial density. On the other hand, B. perversa initially kept at high population density responded with compensatory growth to the more favourable conditions of the later period and became equal in shell size to those kept at low population density. Some snails gave birth to larger offspring than others, but offspring size was correlated neither with the size of the mother nor with the density at which the mother was initially kept. However, offspring size was negatively correlated with the mother's relative growth rate during the relaxation period, indicating a trade-off between offspring size and maternal growth and thus an indirect effect of initial density.