This file was created by scanning the printed publication. Errors identified by the software have been corrected; however, some errors may remain. Factors Affecting the Distribution, Pollination Ecology, and Evolution of Agave chrysantha Peebles and A. palmeri Engelm. (Agavaceae) Liz Slauson1 Abstract.-Taxonomy, biogeography and pollination ecology of two closely related taxa of the Madrean Archipelago, Agave chrysantha Peebles and A. palmeri Engelm. are reviewed. Several questions remain regarding the reported obligate mutualism between A. palmeri and an endangered species of bat, Leptonycteris curasorae, and the role bats and other animals may play in the pollination of A. chrysantha. Preliminary evidence suggests that diurnal visitors play an important role in pollination of A. chrysantha and A. palmeri, and bats appear to have a more faculative role in pollination than previously thought. The seasonal, variable and migratory nature of Leptonycteris seems unlikely to support a tight mutualistic relationship with agaves. Plasticity in pollinator species may be most adaptive in agaves with large geographic ranges (A. palmen) and variable habitats (A. chrysantha). "Bat-adapted" traits may be just as advantageous to insects and other animals in hot and arid climates where peak activity is near dawn and dusk. INTRODUCTION morphology was often grossly altered due to the unnatural conditions of cultivation in green­ Agaves are perennial leaf succulents consist­ houses of Europe, and as a result, most taxa ing of a basal rosette where water and described during this era are unrecognizable by carbohydrates are stored. Plants are monocarpic; their written descriptions alone (Gentry 1982). Al­ they require 10-50 years to reach maturity, then though botanists became more familiar with the initiate an'inflorescence, flower and die. Although genus during the early 1900's, they rarely ob­ the genus Agave (Agavaceae) is an important served agaves in habitat and failed to realize the vegetation component in the biotic communities large degree of leaf variability that can exist both of the Madrean Archipelago, reproductive, eco­ within and between populations. Floral characters logical and speciation processes are poorly were largely ignored, and the concentration on understood. Taxonomic problems have long con­ vegetative differences resulted in a large degree of tributed to a large degree of confusion. During the taxonomic "splitting" at the species level (Gentry early to mid-nineteenth century, agaves from the 1982). Other factors contributed to a poor under­ New World were imported to Europe as ornamen­ standing of agaves as well, Few botanists have tal novelties. Early taxonomists in Europe collected specimens due to the presence of teeth, attempting to name these cultivated species gen­ spines, caustic juices, and the difficulty in process­ erally had no information regarding their origin, ing specimens. Consequently, few specimens of provided no preserved or type specimens, no il­ each taxon have been available for study, many of lustrations, and used vegetative rather than which were poorly prepared and lacked floral or reproductive characters to diagnose species. Leaf other taxonomically significant characteristics. Numerous populations exist in rugged and inac­ cessible terrain which has resulted in limited 1Desert Botanical Garden, 1201 N. Galvin Parkway, Phoenix, AZ distributional data. The long time span required 85008. by members of this genus to reach reproductive 194 maturity makes ex-situ reproductive biology stud­ flowers with red to brownish tepal apices of A. ies or rapid evaluation of offspring difficult palmen), 4} more congested umbels, 5} shorter (Gentry 1982). Thus, large gaps in knowledge ex­ panicles, 6} broader and shorter lanceolate leaves ist with respect to ecology, reproductive biology, with large teeth, and 7} undulate to repand leaf cytology and genetics. margins (Gentry 1982). Agave chrysantha has been Gentry's (1982) more recent and comprehen­ recognized as a subspecies of A. palmeri (Little sive monograph of Agave emphasized 1943) and as a distinct species (Gentry 1982). Gentry comparative morphology, particularly with re­ (1982) has suggested that A. chrysantha may be a gard to floral characters. Species comprehension geologically young species which has not yet was greatly enhanced by his understanding of reached a stabilized or isolated condition, possibly morphological variability of populations and eco­ originating "through introgression with A. palmeri types, reproductive biology, introgression, andA. parryi." Alternatively, A. chrysantha may hybridization and polyploidy. Both polyploidy represent the northern end of a cline of A. palmeri and hybridization are common in Agave (x = 30), that has developed by primary or secondary inter­ and appear to be important mechanisms in the gradation with A. palmeri. evolution of the genus (Pinkava and Baker 1985). The pollination ecology of the Ditepalae is of This trend towards reticulate evolution appears to interest as several recent studies have suggested be due to a lack of complete reproductive isola­ that bats of the genus Leptonycteris are obligate tion between taxa recognized at the species level pollinators of A. palmeri (Howell 1979, Schaffer (Gentry 1967, Burgess 1979, 1985). The climatic and Schaffer 1979, Howell and Roth 1981). Howell fluctuations in the southwestern U.S. during the and Roth (1981) proposed that reported declines glacial-interglacial cycle of the Pleistocene may in Leptonycteris popUlations could potentially se­ have resulted in repeated periods of range expan­ verely impact sexual reproduction in paniculate sion and genetic interchange between species, agaves. This hypothesis was based on a decline in followed by periods of range contraction and iso­ seed set of herbarium specimens over a 30 year lation in small, disjunct populations (Burgess period and low fruit and seed set (approximately 1985). This repeated contact may have been suffi­ 25% and 20% respectively) in A. palmeri popUla­ cient to prevent development of reproductive tions where bats were absent. Fruit and seed set barriers. were high (81% and 700/0) where bats were pre­ The genus Agave is generally thought to have sent. No data were presented documenting visitor evolved in the mesic habitats of central" Mexico or visitation rates. Sutherland (1982, 1987) has (Gomez Pompa 1963), however, many species documented that mean fruit set for paniculate have successfully radiated northward into the agaves is generally low (20-25%), however, fruit more arid environments of northern Mexico and and seed set may be highly variable between the southwestern United States. The Group branches. The high fruit set reported by Howell Ditepalae within the genus Agave is composed of and Roth in bat-pollinated popUlations may be a 13 taxa primarily centered in the Sierra Madre Oc­ result of inadequate sample size, while the "low" cidental of Mexico, but two closely related results may be more representative of normal fruit members of this group, A. chrysantha Peebles and set. Cockrum and Petryszyn (1991) have ques­ A. palmeri Engelm., extend into central and tioned reported declines in Leptonycteris southern Arizona respectively, representing the numbers based on the fact that few observers northernmost distribution of the group. Agave have understood the variability and seasonality of palmeri occupies grama grasslands and oak Leptonycteris movement in the northern part of woodlands of northern Mexico and southern por­ its range (resulting in reports of absent, declining tions of Arizona and New Mexico, whereas Agave and low popUlation numbers). Herbarium speci­ chrysantha is found in desertscrub, chaparral, ju­ mens cited by Howell and Roth were reviewed by niper woodland and the fringes of pine-oak Cockrum and Petryszyn who obtained different woodland communities of central and southern estimates of fruit and seed set. They also noted Arizona. Both species may be found on granitic, that specimens cited by Howell and Roth were volcanic and limestone mountain slopes. Agave from localities which were either near the edge or chrysantha appears to be the nearest relative of A. beyond the known range of Leptonycteris. palmeri based on floral and other morphological Gentry (1982) postulated that other members characteristics, and differs from A. palmeri by 1) of the Ditepalae may have mutualistic associa­ its smaller flower size, 2) shallower tube, 3) clear tions with nectar-feeding bats due to their similar yellow perianth (versus the pale greenish-yellow flower structure. He proposed that a "wave of 195 nectar flow" exists from spring to winter, pro­ tance. Nectar production in bat-pollinated species viding food for bats as they migrate south. such as A. palmeri is nocturnal with peak pro­ Starting in the south with the March bloom of A. duction from 2000-2200 hours (Howell 1979). colorata in Sonora, this "nectar flow" moves Pollen protein content of bat-pollina'ted agaves north to Arizona for the summer with the flow­ tends to be high (44% in A. palmerl) while nec­ ering of A. palmeri, then extends south through tar sugar concentration is relatively low the Sierra Madre Occidental via A. shrevei in (11-20%) (Howell 1972). Bee-pollinated species southern Sonora, to A. durangensis in Chihua­ generally exhibit the opposite trend. Pollen pro­ hua, and finally to A. wocomahiin Durango and tein content is generally low (8-16%) (Howell Zacatecas which bloom into December. Arita 1972),