| Line | In the Sonoran Desert of northwestern Mexico and |
| southern Arizona, the flowers of several species of |
| columnar cacti—cardon, saguaro, and organ |
| pipe—were once exclusively pollinated at night by |
| (5) | nectar-feeding bats, as their close relatives in arid |
| tropical regions of southern Mexico still are. In these |
| tropical regions, diurnal (daytime) visitors to columnar |
| cactus flowers are ineffective pollinators because, |
| by sunrise, the flowers’ stigmas become unreceptive |
| (10) | or the flowers close. Yet the flowers of the Sonoran |
| Desert cacti have evolved to remain open after sunrise, |
| allowing pollination by such diurnal visitors as bees and |
| birds. Why have these cacti expanded their range of |
| pollinators by remaining open and receptive in daylight? |
| (15) | This development at the northernmost range of |
| columnar cacti may be due to a yearly variation in the |
| abundance—and hence the reliability—of migratory |
| nectar-feeding bats. Pollinators can be unreliable |
| for several reasons. They can be dietary generalists |
| (20) | whose fidelity to a particular species depends on |
| the availability of alternative food sources. Or, they |
| can be dietary specialists, but their abundance may |
| vary widely from year to year, resulting in variable |
| pollination of their preferred food species. Finally, they |
| (25) | may be dietary specialists, but their abundance may |
| be chronically low relative to the availability of flowers. |
| Recent data reveals that during spring in the |
| Sonoran Desert, the nectar-feeding bats are |
| specialists feeding on cardon, saguaro, and |
| (30) | organpipe flowers. However, whereas cactus-flower |
| abundance tends to be high during spring, bat |
| population densities tend to be low except near |
| maternity roosts. Moreover, in spring, diurnal cactus- |
| pollinating birds are significantly more abundant in |
| (35) | this region than are the nocturnal bats. Thus, with bats |
| being unreliable cactus-flower pollinators, and daytime |
| pollinators more abundant and therefore more reliable, |
| selection favors the cactus flowers with traits that |
| increase their range of pollinators. While data suggest |
| (40) | that population densities of nectar-feeding bats are |
| also low in tropical areas of southern Mexico, where |
| bats are the exclusive pollinators of many species |
| of columnar cacti, cactus-flower density and bat |
| population density appear to be much more evenly |
| (45) | balanced there: compared with the Sonoran Desert’s |
| cardon and saguaro, columnar cacti in southern Mexico |
| produce far fewer flowers per night. Accordingly, |
| despite their low population density, bats are able to |
| pollinate nearly 100 percent of the available flowers. |
