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.

Pollination ecology of Agave macroacantha (Agavaceae) in a Mexican tropical desert. I. Floral biology and pollination mechanisms.

In a study of sexual reproduction in long-lived semelparous plants, we observed Agave macroacantha in the tropical desert of Tehuacán-Cuicatlán, Mexico, describing duration of flowering, flower phenology, and nectar production patterns. We also performed two manipulative experiments evaluating (a) the seed production efficiency of different crossing systems (selfing, cross-pollination, apomixis, and control), and (b) the effect of different pollinators (diurnal exposure to pollinators, nocturnal exposure, exclusion, and control) on the seeds produced. Flowering occurred from early May to late July and had a mean duration of 29 days in the individual rosettes. The flowers were protandrous; anthesis occurred in the afternoon of the third day after floral opening, and the pistils matured in the afternoon of the fifth day. The stigmas remained receptive from dusk to the following morning. Pollination was mostly allogamous. Nectar was produced principally during the night, from the first stages of floral aperture until the stigmas wilted and flowering ceased. The flowers were visited during the day by hymenoptera, butterflies, and hummingbirds and during the night by bats and moths. Only the nocturnal visitors, however, were successful pollinators. Agave macroacantha is extremely dependent on nocturnal pollinators for its reproductive success.

Pollination ecology of Agave macroacantha (Agavaceae) in a Mexican tropical desert. II. The role of pollinators.

We did a series of observational studies and manipulative experiments on the guild of nocturnal visitors of Agave macroacantha, including (1) a description of the hourly patterns of visits by moths and bats, (2) an evaluation of the relative contribution of bats and moths to flowering success, and (3) an evaluation of the pollination efficiency of the different bat species. Scapes exposed to moths but excluded to bats yielded ∼50% fewer fruits than those exposed to both pollinator groups. Flowers exposed to the bat species Leptonycteris curasoae showed similar fruiting success to those exposed to Choeronycteris mexicana and to those exposed to the whole nocturnal visitor guild. However, the fruits originated from flowers pollinated by Leptonycteris curasoae yielded significantly more seed than those exposed to Choeronycteris mexicana or to the whole pollinator guild. It is concluded that Agave macroacantha is extremely dependent on nocturnal pollinators for its reproductive success and that bats are especially important for successful pollination. Some of these pollinators are migratory and have been reported to be steadily declining. A continuing decline in the populations of pollinators may impede the successful sexual reproduction of the plant host and may put the long-term survival of this agave species under risk.