THE POLLINATION BIOLOGY OF TWO PANICULATE

Transcription

1 American Journal of Botany 90(7): THE POLLINATION BIOLOGY OF TWO PANICULATE AGAVES (AGAVACEAE) FROM NORTHWESTERN MEXICO: CONTRASTING ROLES OF BATS AS POLLINATORS 1 FRANCISCO MOLINA-FREANER 2,4 AND LUIS E. EGUIARTE 3 2 Departamento de Ecologı a Funcional y Aplicada, Instituto de Ecologı a UNAM, Apartado Postal 1354, Hermosillo, Sonora C.P Mexico; and 3 Departamento de Ecologı a Evolutiva, Instituto de Ecologı a UNAM, Apartado Postal , México D.F. C.P Mexico Paniculate agaves from tropical deserts depend on nectar-feeding bats for their reproduction, while species from extratropical areas near the northern limit of Leptonycteris curasoae are pollinated by diurnal and nocturnal insects and birds. Agave angustifolia is a paniculate agave with a broad distribution in Mexico whose range coincides with the distribution of L. curasoae, while A. subsimplex has a narrow distribution in Sonora within the range of nectar-feeding bats. We studied the pollination biology of A. angustifolia and A. subsimplex in northwestern Mexico to evaluate the relative importance of bat pollination in a region where L. curasoae is seasonal. Flower visitors included a wide variety of animals, with bats having greater visitation rates in A. angustifolia. A pollinator-exclusion experiment revealed that bats were responsible for most of the control fruit set in A. angustifolia, whereas for A. subsimplex, diurnal and nocturnal visitors were equally effective. Overall, our data indicate that in central Sonora, A. angustifolia depends on nectarfeeding bats for its sexual reproductive success, while A. subsimplex relies on both diurnal and nocturnal pollinators. Given the contrasting distribution ranges of paniculate agaves, our results seem to support the trend from specialization to generalization along the continuum of tropical to extratropical deserts. Key words: Agave angustifolia; Agave subsimplex; nectar-feeding bats; pollination biology; Sonoran desert. Plant pollination systems form a continuum between plants pollinated by hundreds of pollinator species and those pollinated by just one species (Johnson and Steiner, 2000). The concept of the pollination syndrome has the implicit notion that pollination systems tend toward specialization. However, empirical evidence indicates that moderate to substantial generalization is the rule rather than the exception (Waser et al., 1996). Phenotypic models predict specialization to a particular pollinator when the marginal fitness gain exceeds the marginal fitness loss that results from becoming less adapted to other pollinators (Aigner, 2001). Assuming asymmetric fitness tradeoffs, simple models predict that specialized pollination systems will evolve whenever effective pollinators are predictably available in space and time and that a generalized pollination system will evolve when the abundance of effective pollinators varies in space and time (Waser et al., 1996). As angiosperms are thought to occupy virtually every point on the continuum of pollination systems, it is important to understand the ecological forces that have favored generalization or specialization in particular lineages and regions and document the temporal and spatial fluidity of plant pollinator interactions (Waser et al., 1996; Johnson and Steiner, 2000). Paniculate agaves and columnar cacti exhibit a chiropterophilous pollination syndrome (Gentry, 1982; Valiente-Banuet et al., 1996; Eguiarte et al., 2000). Empirical studies on the pollination biology of both groups have shown evidence of regional fluidity in the plant pollinator interaction. For columnar cacti, pollination experiments in tropical deserts showed that bats are the major pollinators (Valiente-Banuet et al., 1 Manuscript received 5 November 2002; revision accepted 27 February The authors thank José Martı nez and Martha Rocha for field and laboratory assistance, Cristina Peñalba for pollen identification, the staff of the Centro Ecológico de Sonora for generous support, and two anonymous reviewers for critical comments. 4 Author for reprint requests (freaner@servidor.unam.mx) , 1997a, b; Nassar et al., 1997). In contrast, pollinationexclusion experiments in extratropical deserts have shown that both bats and diurnal visitors (several species of birds and bees) are effective pollinators (Fleming et al., 1996, 2001; Sahley, 1996). For paniculate agaves in tropical deserts, the available evidence indicates that bats are also the major pollinators (Arizaga et al., 2000a, b), whereas in extratropical areas near the northern limit of nectar-feeding bats, paniculate agaves are pollinated by a wide variety of diurnal and nocturnal insects and birds (Sutherland, 1987; Slauson, 2000). These studies reveal a general geographic pattern: the relative importance of nocturnal vs. diurnal pollinators varies geographically, with paniculate agaves having a relatively specialized pollination within the tropics, where they are dependent on nectar-feeding bats, and moderate generalization outside the tropics, where they are pollinated by a variety of diurnal and nocturnal pollinators (Arizaga et al., 2000a, b; Slauson, 2000). In Mexico, the abundance and species diversity of nectarfeeding bats varies in space and time; thus, their relative importance in the pollination biology of paniculate agaves may change accordingly. Mexico has 12 species of nectar-feeding bats, most of which are associated with tropical and subtropical dry areas (Arita and Santos-del-Prado, 1999). Species richness reaches maximum values along the Pacific versant (the Balsas Basin) and decreases with latitude (Valiente-Banuet et al., 1996; Arita and Santos-del-Prado, 1999). At least two species are migratory (Arita and Santos-del-Prado, 1999); thus, their abundance changes in time. Leptonycteris curasoae Miller is one of the migratory species that is locally abundant and has a widespread distribution (Arita and Santos-del-Prado, 1999). Capture records indicate that this bat species is resident year-round in the tropics, where resources are available throughout the year, and migratory in extratropical areas, where resources are seasonally available (Rojas-Martı nez et al., 1999). Phenological data from paniculate agaves and co-

2 July 2003] MOLINA-FREANER AND EGUIARTE POLLINATION BIOLOGY OF TWO AGAVES 1017 Fig. 1. Geographic distribution of Agave angustifolia, A. subsimplex, and Leptonycteris curasoae in Mexico. Inset shows the geographic distribution of A. angustifolia and A. subsimplex and the location of studied populations and roosting sites of L. curasoae in the state of Sonora. The distribution of A. angustifolia and A. subsimplex were taken from Gentry (1982) and Turner et al. (1995). The distribution of L. curasoae was taken from Arita (1991), Medellı n et al. (1997), and Rojas-Martı nez et al. (1999). Roosting sites of L. curasoae in Sonora were taken from Caire (1978), Cockrum (1991), Cockrum and Petryszyn (1991), Wilkinson and Fleming (1996), Fleming (2000), and F. Molina-Freaner and T. H. Fleming (University of Miami), unpublished data. lumnar cacti suggest that both groups form a nectar corridor for L. curasoae during its migration (Fleming et al., 1993). It is one of the most important nocturnal pollinators of paniculate agaves in the Tehuacán Valley (Arizaga et al., 2000a, b). However, its abundance in the Sonoran desert varies significantly within and among years (Fleming et al., 2001). Thus, its relative importance in the pollination biology of paniculate agaves near the northern limit of its distribution is minor (Slauson, 2000). Valiente-Banuet et al. (1996) suggested that the geographic pattern shown by the pollination systems of columnar cacti reflects year-to-year variation in the abundance and reliability of this nectar-feeding bat at the northern range of its distribution. Bat unpredictability has been suggested as the major ecological factor behind the evolution of the generalized pollination system of columnar cacti from the Sonoran desert (Fleming et al., 2001). Previous studies of the pollination biology of northern paniculate agaves have been conducted in semi-arid regions where L. curasoae is absent or its flowering time barely overlaps with seasonal occupation of this bat near its northern limit (Schaffer and Schaffer, 1977; Sutherland, 1987; Slauson, 2000). However, it is not clear if paniculate agaves from the Sonoran desert that inhabit areas where L. curasoae is clearly seasonal (Rojas-Martı nez et al., 1999) exhibit a generalized pollination system involving bats, birds, and insects, similar to columnar cacti (Fleming et al., 2001). Agave angustifolia Haw. (group Rigidae) and A. subsimplex Trel. (group Deserticolae) are two paniculate agaves with contrasting ranges of distribution. Agave angustifolia ( Agave vivipara, Smith and Steyn, 1999) has the most wide-ranging distribution among agaves in North America (Gentry, 1982). It ranges from Costa Rica in Central America to Tamaulipas and Sonora, in northern Mexico (see Fig. 1). In contrast, A. subsimplex is a small xerophytic agave with a narrow distribution (Gentry, 1982; Turner et al., 1995). It is restricted to a

3 1018 AMERICAN JOURNAL OF BOTANY [Vol. 90 narrow coastal area of Sonora and Tiburón Island (see Fig. 1), and it is a member of the Agave deserti complex whose distribution surrounds the Gulf of California (Gentry, 1982; Navarro-Quesada et al., 2003). Morphologically, A. angustifolia is highly polymorphic through its range, whereas A. subsimplex varies less (Gentry, 1982). Agave angustifolia can sexually reproduce by seeds and propagate vegetatively by bulbils and vegetative shoots, whereas A. subsimplex relies on seeds and vegetative shoots (Gentry, 1982; Arizaga, 1999). However, our knowledge about the relative importance of sexual vs. asexual recruitment in the population dynamics of both species is poor (Arizaga, 1999; Arizaga and Ezcurra, 2002). Arita (1991) detected a close geographic association between the distribution range of A. angustifolia and L. curasoae and suggested that this bat could be its major pollinator. However, the geographic association could be generated by other ecological processes and not necessarily by a close ecological interaction between them. Agave subsimplex occurs in areas where L. curasoae is seasonal (Rojas-Martı nez et al., 1999), and this bat has been observed visiting its flowers (Fleming, 2000). However, our knowledge about the pollination biology of A. subsimplex and the role of nectar-feeding bats is quite poor. In this paper, we study the pollination biology of A. angustifolia and A. subsimplex in the state of Sonora, Mexico. We explore whether L. curasoae is the major pollinator of A. angustifolia (Arita, 1991), assess the relative importance of nocturnal vs. diurnal pollinators, and investigate whether the pollination system of these two species resembles those of paniculate agaves from extratropical regions (Slauson, 2000). Based on current knowledge about the geographic distribution of roosting sites of L. curasoae in Sonora (see Fig. 1), we selected two populations of A. angustifolia at different distances from a known roost to explore the influence of contrasting rates of bat visitation on fruit and seed set. MATERIALS AND METHODS Study sites We studied the pollination biology of A. angustifolia and A. subsimplex at the Centro Ecológico de Sonora (29 01 N, W; elevation 245 m) and at Sierra Kino (28 54 N, W, elevation 50 m), respectively. The Centro Ecológico is located in the plains of Sonora vegetational subdivision of the Sonoran desert (Shreve, 1964), at the southern margin of Hermosillo, Sonora, Mexico. Sierra Kino is located on the central Gulf Coast vegetational subdivision of the Sonoran desert (Shreve, 1964), north of Bahı a Kino (see Fig. 1). The studied population is located 2 km from a known roost of L. curasoae (Horner et al., 1998). The Centro Ecológico de Sonora lies at the western limit of the distribution of A. angustifolia in Sonora (Fig. 1). A large number of rosettes of A. angustifolia were transplanted from a nearby population to the Centro Ecológico during its foundation in The plants have been sexually and clonally reproducing since that time, producing new recruits and becoming established. The Sierra Kino mountain range lies at the southern limit of the distribution of A. subsimplex (Fig. 1). In this area, A. subsimplex is common on rocky slopes near the Gulf of California. Plant phenology During January 2001, we selected for phenological observation 56 plants of A. angustifolia at the Centro Ecológico whose scapes were in different stages of development. Each week, beginning in 3 January 2001, we recorded the phenological status of each selected plant, until all fruits matured. For A. subsimplex, we recorded the phenological status of 28 plants during our occasional (not systematic) visits to Sierra Kino. Floral traits and nectar secretion Agave angustifolia was studied during 2001, and A. subsimplex during During 5 11 March 2001, we studied floral traits and nectar production in A. angustifolia. Thirty flowers from five plants (six flowers per plant) were tagged and bagged when they were opening (day 1). Heights of pistil and stamens were measured on a subset of 10 flowers from three plants. Using a caliper, pistil height was measured as the distance from the base of the flower to the stigma, and stamen height was measured as the distance from base to the point of insertion of filaments into the anther. Measurements were taken daily (1800) until styles wilted. In addition, we observed the condition of anthers and stigmas in each of the 10 flowers. Nectar secretion was measured at 12-h intervals on the 30 flowers from the five selected plants. Sampled flowers were located on umbels in the middle of the inflorescence and excluded from pollinators with nylon mesh netting. Nectar volume was measured with a 1-mL graduated syringe by removing accumulated nectar at 1800 and 600 from day 1 until styles wilted. Sampling was without replacement after nectar removal. Nectar sugar concentration was measured on a subset of 15 flowers from five plants (three flowers per plant). Concentration (percentage sucrose equivalents on a mass basis) was measured on nectar accumulated at 12-h intervals (1800 and 600) using a low-volume field refractometer (0 50%, Bellingham & Stanley, Norcross, Georgia, USA). Air temperature was measured with copper-constatan thermocouples. A sensor was placed under shade and measurements were taken every 10 min and stored in a datalogger (Campbell 21X, Campbell Scientific, Logan, Utah, USA). Agave subsimplex was studied during 7 12 May 2002, using the same methods described for A. angustifolia. In this case, however, sample size was 15 flowers from five plants (three flowers per plant) for pistil and stamen elongation, nectar secretion, and sugar concentration. Floral visitor observations Flower visitation rates of A. angustifolia were measured over 3 d at the Centro Ecológico de Sonora during March 2001 and for A. subsimplex over 2 d at Sierra Kino during May Birds were observed three times during the day using 1-h intervals: , , and Bats were observed three times during the night using 1-h intervals: , , and For diurnal and nocturnal insects, we used 15-min intervals three times during day and night. Flower visits were carefully observed (except bat visits) to determine whether or not stigmas or anthers were touched. During the night of 21 March 2001, we closely (1 m from the flowers) observed 10 flower visits by bats and found that in all cases, visits by bats touched the stigma in pistillate flowers and anthers in staminate flowers. Therefore, we assumed that all bat visits made contact with stigmas and anthers. The unit of observation was a selected umbel and the entire inflorescence. Birds were observed from a point m from the inflorescence, the observer hiding in a shrub and using binoculars. Birds were identified using a field guide (Peterson and Chalif, 1989). Bats were observed from the base of the inflorescence 2 3 m from the flowers. Diurnal and nocturnal insects were observed from a ladder 1 m from the inflorescence. After each visit, we recorded the stage of the visited flower (staminate or pistillate) and time spent per umbel and per plant. For bat identification, we installed two mist nets (6 2 m, Avinet, Dryden, New York, USA) close to inflorescences during two nights and once captured, they were identified using a guide (Medellin et al., 1997), and a sample of pollen was taken from their heads. We conducted an evening-exit census at the Sierra Kino cave on 10 May 2002 (see Horner et al., 1998; Fleming et al., 2001 for methodological details) to estimate the size of the population of L. curasoae that foraged around the studied population of A. subsimplex. Pollination treatments Ten plants representing the spectrum of size variation in the population were selected for the pollination treatments in both species. Five flowers from each plant were included per treatment for a total of 50 flowers per treatment. Treated flowers were selected from 4 6 umbels of the middle section of inflorescences. Six pollination treatments were applied to a total of 300 flowers from March 2001 in A. angustifolia. Previous observations indicated that bat visits produced clouds of pollen that could pass through mesh netting. Therefore, we decided to use a piece of transparent straw placed over the style and closed with cotton at the distal end, in addition to nylon mesh netting to exclude flowers from pollinators in certain treatments (see below). Flowers were selected at the staminate phase for the different pollination treatments. Pollination treatments included hand

4 July 2003] MOLINA-FREANER AND EGUIARTE POLLINATION BIOLOGY OF TWO AGAVES 1019 cross-pollination, self-pollination, pollen exclusion (agamospermy), exclusion of diurnal visitors, exclusion of nocturnal visitors, and a control. (1) Hand cross-pollination. Flowers in this treatment were pollinated with pollen from recently opened anthers of other individuals ( m away) in the population, daily in the early evening from 1900 to 2100 until styles wilted. Pollen was deposited in stigmas by rubbing open anthers against them. Pollen donors changed every day. Flowers in this treatment were not excluded from pollinators. (2) Self-pollination. Flowers in this treatment were excluded from visitors before and after pollination by pieces of transparent straw and nylon mesh netting. Once stigmas were receptive (revealed as the presence of exudates), they were pollinated with open anthers of another flower of the same individual, daily from 1900 to 2100 until styles wilted. (3) Pollen exclusion (agamospermy). Flowers in this treatment were permanently excluded from visitors by pieces of straw and nylon mesh netting until styles wilted. In this case, no pollen was allowed to touch the stigma. (4) Exclusion of diurnal visitors (nocturnal pollination). Flowers in this treatment were excluded from visitors from 0600 to 1830 and allowed any visitor from 1830 to 0600, daily, until styles wilted. Flowers were excluded from visitors using pieces of straw and nylon mesh netting. (5) Exclusion of nocturnal visitors (diurnal pollination). Flowers in this treatment were excluded from visitors from 1830 to 0600 and allowed any visitor from 0600 to 1830 daily, until styles wilted. Flowers were excluded as before. (6) Control. Flowers in this treatment were just tagged and left to the natural agents in the field. Because of lower flower availability per umbel, the six pollination treatments were applied in two time intervals in A. subsimplex, using the same methods and sample sizes described for A. angustifolia. During 7 12 May 2002, we applied the hand cross-pollination, self-pollination, pollen exclusion, and control treatments, and during May 2002, we applied the nocturnal pollination, diurnal pollination, and another control treatment in 10 plants. Fruit set was evaluated 3 wk after the pollination treatments in both species. Fruits were collected in 11 June 2001 in A. angustifolia and 26 June 2002 in A. subsimplex. Once in the laboratory, fruits were opened and used to estimate seed set as the ratio of fertilized, black seeds over the sum of fertilized (black) and unfertilized (white) ovules. Fruit production During June 2001, we estimated fruit production per plant, fruit set on entire inflorescences, and seed set on a set of plants that were not used for pollination treatments in A. angustifolia, while on 26 June 2002, the same parameters were estimated in A. subsimplex. For A. angustifolia, we also visited two additional populations in central Sonora where we estimated fruit production, fruit set, seed set, and the percentage of plants producing fruits and bulbils during June These populations (Saguaral de Empalme and Mirador de San Carlos) were located at contrasting distances from a known roosting site of L. curasoae (see Fig. 1). During June 2002, we visited the same three populations that were studied in 2001 to estimate fruit production, fruit set, and seed set. Data analysis Fruit and seed set values among pollination treatments were analyzed through logistic models using JMP 3.1 software (SAS Institute, 1997). RESULTS Plant phenology During the year 2001, plants of A. angustifolia at the Centro Ecológico de Sonora flowered from early January to late May, with peak flowering time in March (Fig. 2). Developing fruits were detected from mid-february to late April, while mature fruits were observed from mid- May to late in July (Fig. 2). During 2002, plants of A. subsimplex flowered from early April to early June, with peak flowering time in early May (data not shown). Floral traits and nectar secretion Both species have flowers that are protandrous and changed from pre-dehiscent, to staminate, and to pistillate stages over 6 7 d for A. angustifolia (Fig. 3a) and during 5 d for A. subsimplex (Fig. 4a). During Fig. 2. Flowering and fruiting phenology of Agave angustifolia at the Centro Ecológico de Sonora. Temporal changes in the percentage of plants with flowers, developing fruits, and mature fruits (N 56 plants). the pre-dehiscent stage, tepals opened and stamens elongated. The staminate phase started during the evening of day 2 when 4 6 anthers of A. subsimplex or 2 6 anthers of A. angustifolia dehisced and pollen was released. The staminate phase usually ended on day 3 for A. subsimplex, although there was some interplant variation, and in some cases the two remaining anthers opened on day 3 (Fig. 4a). For A. angustifolia, this phase continued through the evening of day 3, when 0 4 anthers dehisced and pollen was presented. The pistillate stage appeared to start on day 3 for most flowers of A. subsimplex and on day 5 for A. angustifolia, when stigmas opened and exudates were released (Figs. 3a, 4a). Flowers were apparently receptive by the evening of days 4 and 6 for A. subsimplex and A. angustifolia, respectively, when stigmas were open and sticky. During day 5, flowers of A. subsimplex started to wilt, while for A. angustifolia, flowers began to wilt on day 7. Nectar secretion started during the pre-dehiscent stage, reached maximum values during the staminate stage, and declined during the pistillate stage (Figs. 3b, 4b). Species exhibited major differences in the volume of secreted nectar (compare Fig. 3b vs. Fig. 4b). Nocturnal accumulation was greater than diurnal in both species, and for A. subsimplex diurnal accumulation was very low. Nectar sugar concentration varied during the lifespan of a flower from 18% to 26% for A. angustifolia (Fig. 3c) and from % to % for A. subsimplex (means 1 SD). Minimum air temperature varied from 5.9 to 12.2 C and from 9.8 to 14.3 C at night, while maximum air temperatures of C and C were recorded among days at the Centro Ecológico and Sierra Kino, respectively. Floral visitor observations Floral visitors to both species were similar and included honey bees, native bees, wasps, hummingbirds, woodpeckers, orioles, sphingids and other moths, and nectar-feeding bats (Fig. 5). However, visitation rates were different between species and sites (Fig. 5). Nocturnal visitation of A. angustifolia, especially by nectar-feeding bats, was greater than diurnal visits and greater than nocturnal visitation of A. subsimplex (Fig. 5). Three female individuals of L. curasoae were captured bearing pollen of A. angustifolia at the Centro Ecológico. In contrast, although 312 individuals of L. curasoae were detected leaving the Sierra Kino cave on

5 1020 AMERICAN JOURNAL OF BOTANY [Vol. 90 Fig. 3. Temporal changes in pistil and stamen height, nectar volume, and concentration during the lifespan of a flower of Agave angustifolia. Values shown are means 1 SD. (A) Temporal changes in filament and style height. 1 pre-dehiscent stage; 2 staminate phase; 3 pistillate phase. Arrows indicate timing of anther dehiscence. Sample size was 10 flowers from three plants. (B) Temporal changes in nectar volume occuring at 12-h intervals. Sample size was 30 flowers from five plants. (C) Temporal changes in sugar concentration in nectar of A. angustifolia. Sample size was 15 flowers from five plants. 10 May 2002, no individual of this species was captured at the study site, presumably due to low visitation rates (Fig. 5). Visitation rate in this site was relatively low during the night (Fig. 5). Fig. 4. Temporal changes in pistil and stamen height and nectar volume during the lifespan of a flower of Agave subsimplex. See Fig. 3 for details. Sample size was 15 flowers from five plants. Pollination treatments In both species, none of the hand self-pollinated flowers set fruit (Figs. 6a, 7a). Similarly, none of the flowers that were excluded from any visitor set fruit (Figs. 6a, 7a). For A. angustifolia, fruit set of the manual crosspollination treatment was (mean 1 SD), while the control treatment had values of (Fig. 6a). Fruit set differences were statistically significant ( 2 9.0, df 1, P 0.002), suggesting limitation by pollinators. The pollinator-exclusion experiment revealed that nocturnal visitors were more effective (fruit set ) than diurnal visitors (0 0). No significant differences were detected between nocturnal pollination and the control treatment ( , df 1, P 0.68), indicating that nocturnal visitors were the most important contributors to control fruit set. Seed set was for the manual cross-pollination treatment, for nocturnal pollination, and for the control (Fig. 6b). No significant differences were detected between treatments ( , df 2, P 0.86). For A. subsimplex, fruit set of the manual cross-pollination treatment ( ) was similar to the control ( ), and no statistical differences were detected (Fig. 7a, , df 1, P 0.27). Similarly, no significant differences were detected in seed set between treatments (Fig. 7b, , df 1, P 0.87), indicating that visitors were effective pollinators. The pollinator-exclusion experiment revealed that diurnal and nocturnal visitors were apparently equally effective pollinators (Fig. 7c), as indicated by the lack of statistical differences between the diurnal pollination treatment and the control ( , df 1, P 0.85) or nocturnal pollination and the control ( , df 1, P 0.30). However, seed set values revealed that neither visitor group achieved the effectiveness of the control (Fig. 7d) as indicated by the significant differences between diurnal pollination and the control ( , df 1, P 0.001) and

6 July 2003] MOLINA-FREANER AND EGUIARTE POLLINATION BIOLOGY OF TWO AGAVES 1021 Fig. 5. Diurnal and nocturnal floral visitors of Agave angustifolia and A. subsimplex at the Centro Ecológico and Sierra Kino, respectively. Minutes of observation are provided in parentheses. For A. angustifolia, diurnal insects included Apis mellifera and one unidentified native bee; birds included Melanerpes uropygialis, Icterus galbula, Calypte costae, and Selasphorus rufus; nocturnal insects included Hyles lineata and one unidentified moth; and bats included only Leptonycteris curasoae. For A. subsimplex, diurnal insects included Apis mellifera, one unidentified native bee, and one unidentified wasp; birds included Calypte costae; nocturnal insects included two unidentified moths; and bats included Leptonycteris curasoae. nocturnal pollination and the control ( , df 1, P 0.007). Fruit production in populations At the Centro Ecolo gico, plants produced on average 78 fruits and 175 seeds/fruit for a total of seeds per plant during 2001 (Table 1). All the plants produced fruits with fruit set values for the entire inflorescence averaging 0.30, and no plants produced bulbils (Table 1). During 2002, fruit set was similar and fruit and seed production slightly declined. The distance from this population to the nearest Leptonycteris roost is unknown. At El Saguaral de Empalme, all plants produced fruits and average seed production was 4809 seeds per plant during 2001 (Table 1). During 2002, fruit set and total seed production per plant was also similar. In this case, the distance to the nearest Leptonycteris roost is 9.9 km and only 5% of the plants produced bulbils during In contrast, the population at El Mirador de San Carlos was 33.6 km from a Leptonycteris roost, and during 2001, only 25% of the plants produced fruits but 30% had Fig. 6. Fruit set (a) and seed set (b) for the different pollination treatments in flowers of Agave angustifolia at the Centro Ecológico de Sonora. Sample size was five flowers per treatment per plant distributed over 10 plants. bulbils (Table 1). In this case, fruit set was only 0.03, with values of five fruits per plant, for a total of 776 seeds per plant. However, during 2002, all plants produced fruits, no bulbils were detected, fruit set was 0.20, and total seed production was 3099 seeds per plant. For the Sierra Kino population of A. subsimplex, fruit and seed set values during 2002 were and , respectively. Individual plants of A. subsimplex produced on average fruits and fruits had seeds per fruit for a total of 5498 seeds per plant. DISCUSSION In this paper we have shown that during peak flowering in central Sonora, A. angustifolia depends on nectar-feeding bats for its sexual reproductive success. Pollinator-exclusion experiments, floral visitor observations, and temporal variation in fruit set values at different distances from a roosting site seem to indicate that L. curasoae is the major pollinator of A. angustifolia, as suggested by Arita (1991). Although floral visitors were observed for a brief period, the available evidence suggest that the pollination system of northern populations of this species is more similar to paniculate agaves from tropical deserts (Arizaga et al., 2000a, b) than to agaves from Arizona (Sutherland, 1987; Slauson, 2000). In contrast, nectar-feeding bats seemed to play a less important role in the pollination biology of A. subsimplex. The pollinator-exclusion experiment

7 1022 AMERICAN JOURNAL OF BOTANY [Vol. 90 Fig. 7. Fruit and seed set for the different pollination treatments in flowers of Agave subsimplex at Sierra Kino. Fruit set (a) and seed set (b) for the control, manual outcrossing, manual self-pollination, and automatic self-pollination treatments. Fruit set (c) and seed set (d) for the control, diurnal, and nocturnal pollination treatments. Sample size was five flowers per treatment per plant distributed over 10 plants. and the limited observations on floral visitors suggest that both diurnal and nocturnal visitors are important. Although more extensive observations on pollinators are needed, the available evidence suggest that the pollination system of this species is more similar to paniculate agaves from Arizona (Sutherland, 1987; Slauson, 2000). Apparently, our results seem to indicate that the trend from relative specialization to generalization in the pollination system of paniculate agaves breaks down in the central Sonoran desert. However, given that A. angustifolia has a tropical distribution and A. subsimplex is a narrow endemic to the Sonoran desert, our results reveal a geographical trend in the pollination biology of paniculate agaves. Future studies with other paniculate agaves from northwestern Mexico with a greater diversity of flowering times (Gentry, 1982; García- Mendoza, 1995; Turner et al., 1995) will reveal if they support this geographic pattern. A comparison of our results with the body of evidence on the pollination biology of agaves shows some striking differences. For instance, in contrast to other studies that found evidence of self-compatibility in both sections of the genus Agave (Trame et al., 1995; Arizaga et al., 2000a; Slauson, 2000; Silva, 2001), our results indicate that both species are selfincompatible. The volume of nectar secreted by flowers of A. angustifolia falls within the range that has been observed for other paniculate agaves (Arizaga et al., 2000a; Slauson, 2000; see Eguiarte et al., 2000 for a review). However, flowers of A. subsimplex secrete relatively lower nectar volumes compared to other paniculate agaves (Eguiarte et al., 2000; Silva, 2001). Similarly, the sugar concentration of both species seems to be slightly higher than the concentration that has been detected in other paniculate agaves (Eguiarte et al., 2000). Several attributes and some observations on A. subsimplex suggest that this species is not dependent on nectar-feeding bats. Although we detected 312 nectar-feeding bats at the Sierra Kino cave, our observations recorded only two bat visits to flowers of A. subsimplex in 420 min. Furthermore, nocturnal nectar secretion was probably low as compared to other paniculate agaves. Agave subsimplex flowers at the same time as other species of columnar cacti that produce flowers with greater nectar volumes in the same area (Fleming et al., 1996, 2001; Fleming, 2000), and L. curasoae feeds preferentially on cactus flowers in the same area where A. subsimplex is sympatric (Fleming, 2000). However, although flower attributes suggest a chiropterophilous syndrome, the low bat visitation rate recorded for A. subsimplex might reflect major differences in nectar availability among species living in the same area or that bat visitation is highly variable for this species in this area. Therefore, more extensive observations (i.e., Silva, 2001) on nocturnal visitors through the distribution range of this species are necessary to evaluate the role of nectar-feeding bats. Several attributes of A. angustifolia showed a close relationship to nectar-feeding bats. Nocturnal nectar secretion was greater than diurnal nectar secretion, and anther dehiscence was nocturnal, indicating adaptation to nocturnal visitors. Our phenological data showed that northern populations of A. angustifolia flower in winter and spring, while capture data from L. curasoae indicate that it is present in latitudes above 29 N during spring and summer (Rojas-Martı nez et al., 1999). We observed bat visitation as early as 22 February 2002, when no other chiropterophilous flower resource was available, indicating that L. curasoae arrives in late winter at the Centro Ecológico (29 00 N), in central Sonora. Bat visitation rates were greater than any other flower visitors, and diurnal pollinators were not effective. Although nocturnal observations

8 July 2003] MOLINA-FREANER AND EGUIARTE POLLINATION BIOLOGY OF TWO AGAVES 1023 TABLE 1. Fruit set and fruit production among populations of Agave angustifolia from central Sonora, Mexico, during 2001 and Data are means 1 SD. Range of variation is provided in parentheses. N number of plants; n number of fruits. Mirador de San Carlos El Saguaral de Empalme Centro Ecológico de Sonora Parameter (4 45, N 17) ( , N 17) n ( , n 13) (0 58, N 20) (0 0.29, N 20) n ( , n 12) (9 56, N 20) ( , N 20) n ( , n 20) (7 64, N 20) ( , N 20) n ( , n 18) (15 111, N 20) ( , N 20) n ( , n 23) (7 237, N 24) ( , N 24) n ( , n 25) Fruits/plant Fruit set Seeds/fruit Seed set Percentage of fruit-producing plants 100% 100% 100% 100% 25% 100% Percentage of bulbil-producing plants 0% 0% 5% 0% 30% 0% Latitude N N N Longitude W W W Distance to the nearest Leptonycteris roost 9.9 km 33.6 km were limited to a brief period, overall, the available evidence suggests that northern populations of A. angustifolia depend on L. curasoae for its sexual reproduction. Our data suggest that the distance to the nearest Leptonycteris roosting site has important reproductive consequences for populations of A. angustifolia. Arizaga and Ezcurra (1995) have shown that under reproductive failure from lack of pollinators or grazing, flowering scapes trigger the production of bulbils that may act as an insurance mechanism that increases the probability of successful propagation of the genet. The population located 9.9 km from a roosting site exhibited fruit set values similar to those observed for other paniculate agaves during 2 yr (Sutherland, 1987; Slauson, 2000). In contrast, the population located at 33.6 km varied significantly among years, probably from differences in bat abundance and visitation rates, and a significant fraction of the plants produced bulbils during 1 yr. Leptonycteris curasoae is known to forage up to km from its roosting site (Horner et al., 1998) and exhibit significant variation in abundance among years (Fleming et al., 2001). Thus, our interpretation is consistent with current knowledge about the foraging behavior and yearto-year fluctuation of this nectar-feeding bat. In this scenario, populations within the foraging range of this bat may be sexually successful, whereas populations at the limits of the foraging range may vary significantly in reproductive output and change from sexual to asexual reproduction (i.e., formation of bulbils), depending on bat abundance. If the probability of establishment of bulbils is greater than sexually derived seedlings (Arizaga and Ezcurra, 2002), vegetative propagation could be more important than sexual reproduction in the maintenance and regeneration of populations near the limits of the foraging range of nectar-feeding bats. Under this scenario, the genotypic diversity of populations of A. angustifolia could vary spatially with bat abundance. A formal study of the genetic and clonal structure of populations at contrasting distances to roosting sites through the distribution range of this species is required to test this hypothesis. We believe that future studies should explore the relative importance of sexual reproduction vs. vegetative propagation in the maintenance and structure of populations of A. angustifolia. LITERATURE CITED AIGNER, P. A Optimality modeling and fitness trade-offs: when should plants become pollinator specialists? Oikos 95: ARITA, H. T Spatial segregation in long-nosed bats, Leptonycteris nivalis and Leptonycteris curasoae, inméxico. Journal of Mammalogy 72: ARITA, H. T., AND K. SANTOS-DEL-PRADO Conservation biology of nectar-feeding bats in México. Journal of Mammalogy 80: ARIZAGA, S Biologı a reproductiva de Agave macroacantha Zucc. en Tehuacán, Puebla. Ph.D. dissertation, Facultad de Ciencias, UNAM, México D.F., Mexico. ARIZAGA, S., AND E. EZCURRA Insurance against reproductive failure in a semelparous plant: bulbil formation in Agave macroacantha flowering stalks. Oecologia 101: ARIZAGA, S., AND E. EZCURRA Propagation mechanisms in Agave macroacantha (Agavaceae), a tropical arid-land succulent rosette. American Journal of Botany 89: ARIZAGA, S., E. EZCURRA, E. PETERS, F. RAMÍREZ DE ARELLANO, AND E. VEGA-PEÑA. 2000a. Pollination ecology of Agave macroacantha Zucc. in a Mexican tropical desert. I. Floral biology and pollination mechanisms. American Journal of Botany 87: ARIZAGA, S., E. EZCURRA, E. PETERS, F. RAMÍREZ DE ARELLANO, AND E. VEGA-PEÑA. 2000b. Pollination ecology of Agave macroacantha Zucc.

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