Insurance against reproductive failure in a semelparous plant: bulbil formation in Agave macroacantha flowering stalks.

Bulbils are small aerial rosettes that occur on the flowering stalks of semelparous Agave plants and in related families, and that are capable of acting as clones of the parent plant. We hypothesized that bulbil formation was inversely related to fruiting success in the flowering stalk, and we explored this hypothesis in A. macroacantha, a species from South-Central Mexico. Forty randomly chosen plants were divided amongst three treatments: (a) elimination of all floral buds, (b) exclusion of pollinators, and (c) control. We also studied 22 plants in which the flowering stalk had been felled by goat grazing. Between September and November 1991 we kept a record of the numbers of bulbils and capsules produced in each flowering stalk. Significant (P<0.0001) differences between treatments were found in the proportion of plants hearing capsules and bearing bulbils. The control treatment had the highest proportion of plants producing capsules, treatment a had the highest proportion of individuals bearing bulbils, while treatment b showed an intermediate response. In the goat-grazed group, 45% of the plants failed to produce any propagative structure after the stalk was cut, and half of all plants produced bulbils on the remaining stump. A significant inverse relationship between the numbers of capsules and the numbers of bulbils per plant was found for the three randomly assigned treatments. Our results suggest that once the production of the flowering stalk has been triggered and the death of the rosette is irreversible, bulbils may act as an insurance mechanism that increases the probability of successful reproduction of the genet.