Tuesday, August 7, 2007

PS 36-184: How to be ornithophilous: Hummingbird pollination of the Sonoran Desert cactus Stenocereus alamosensis

Alberto Búrquez and Enriquena Bustamante. Instituto de Ecología, Universidad Nacional Autónoma de México

Several ornithophilous columnar cacti species have been reported throughout the Americas. In most cases these occur sympatrically with chiropterophilous species; the ackowledged ancestral syndrome of columnar cacti.  We explore the avenues from chiropterophily to ornithophily by studing the reproductive ecology of  Stenocereus alamosensis and S. thurberi two columnar cacti species of the Sonoran Desert. S. alamosensis has a restricted range of distribution. Its showy, red, tubular flowers open at down, produce nectar during the morning, and wither in the afternoon. Exclusion experiments showed that self-pollination and autogamy treatments yielded no fruits, while cross-pollination almost invariably produced fruits indicating an obligate xenogamous reproductive system. Nectar rewards and its timing are well within the expected for hummingbird pollinated species. At the beginning of flowering, it is visited by several species of hummingbirds, but by the end, well into the desert summertime, the main visitor is the resident species Cynanthus latirostris. Given its close phyletic relationship with its sister species S. kerberi, and with S. thurberi, several hypothesis are presented for the evolution of ornithophily  that include shifts in the timing of nectar rewards, changes in flower coloration, and delayed closure of the flowers.