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Factors Affecting the Distribution,
Pollination Ecology, and Evolution of
Agave chrysantha Peebles and
A. palmeri Engelm. (Agavaceae)
Liz Slauson 1
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 greenhouses of Europe, and as a result, most taxa
described during this era are unrecognizable by
their written descriptions alone (Gentry 1982). Although botanists became more familiar with the
genus during the early 1900's, they rarely observed agaves in habitat and failed to realize the
large degree of leaf variability that can exist both
within and between populations. Floral characters
were largely ignored, and the concentration on
vegetative differences resulted in a large degree of
taxonomic "splitting" at the species level (Gentry
1982). Other factors contributed to a poor understanding of agaves as well, Few botanists have
collected specimens due to the presence of teeth,
spines, caustic juices, and the difficulty in processing specimens. Consequently, few specimens of
each taxon have been available for study, many of
which were poorly prepared and lacked floral or
other taxonomically significant characteristics.
Numerous populations exist in rugged and inaccessible terrain which has resulted in limited
distributional data. The long time span required
by members of this genus to reach reproductive
Agaves are perennial leaf succulents consisting of a basal rosette where water and
carbohydrates are stored. Plants are monocarpic;
they require 10-50 years to reach maturity, then
initiate an'inflorescence, flower and die. Although
the genus Agave (Agavaceae) is an important
vegetation component in the biotic communities
of the Madrean Archipelago, reproductive, ecological and speciation processes are poorly
understood. Taxonomic problems have long contributed to a large degree of confusion. During the
early to mid-nineteenth century, agaves from the
New World were imported to Europe as ornamental novelties. Early taxonomists in Europe
attempting to name these cultivated species generally had no information regarding their origin,
provided no preserved or type specimens, no illustrations, and used vegetative rather than
reproductive characters to diagnose species. Leaf
1Desert Botanical Garden, 1201 N. Galvin Parkway, Phoenix, AZ
85008.
194
maturity makes ex-situ reproductive biology studies or rapid evaluation of offspring difficult
(Gentry 1982). Thus, large gaps in knowledge exist with respect to ecology, reproductive biology,
cytology and genetics.
Gentry's (1982) more recent and comprehensive
monograph of Agave emphasized
comparative morphology, particularly with regard to floral characters. Species comprehension
was greatly enhanced by his understanding of
morphological variability of populations and ecotypes, reproductive biology, introgression,
hybridization and polyploidy. Both polyploidy
and hybridization are common in Agave (x = 30),
and appear to be important mechanisms in the
evolution of the genus (Pinkava and Baker 1985).
This trend towards reticulate evolution appears to
be due to a lack of complete reproductive isolation between taxa recognized at the species level
(Gentry 1967, Burgess 1979, 1985). The climatic
fluctuations in the southwestern U.S. during the
glacial-interglacial cycle of the Pleistocene may
have resulted in repeated periods of range expansion and genetic interchange between species,
followed by periods of range contraction and isolation in small, disjunct populations (Burgess
1985). This repeated contact may have been sufficient to prevent development of reproductive
barriers.
The genus Agave is generally thought to have
evolved in the mesic habitats of central" Mexico
(Gomez Pompa 1963), however, many species
have successfully radiated northward into the
more arid environments of northern Mexico and
the southwestern United States. The Group
Ditepalae within the genus Agave is composed of
13 taxa primarily centered in the Sierra Madre Occidental of Mexico, but two closely related
members of this group, A. chrysantha Peebles and
A. palmeri Engelm., extend into central and
southern Arizona respectively, representing the
northernmost distribution of the group. Agave
palmeri occupies grama grasslands and oak
woodlands of northern Mexico and southern portions of Arizona and New Mexico, whereas Agave
chrysantha is found in desertscrub, chaparral, juniper woodland and the fringes of pine-oak
woodland communities of central and southern
Arizona. Both species may be found on granitic,
volcanic and limestone mountain slopes. Agave
chrysantha appears to be the nearest relative of A.
palmeri based on floral and other morphological
characteristics, and differs from A. palmeri by 1)
its smaller flower size, 2) shallower tube, 3) clear
yellow perianth (versus the pale greenish-yellow
flowers with red to brownish tepal apices of A.
palmen), 4} more congested umbels, 5} shorter
panicles, 6} broader and shorter lanceolate leaves
with large teeth, and 7} undulate to rep and leaf
margins (Gentry 1982). Agave chrysantha has been
recognized as a subspecies of A. palmeri (Little
1943) and as a distinct species (Gentry 1982). Gentry
(1982) has suggested that A. chrysantha may be a
geologically young species which has not yet
reached a stabilized or isolated condition, possibly
originating "through introgression with A. palmeri
andA. parryi." Alternatively, A. chrysantha may
represent the northern end of a cline of A. palmeri
that has developed by primary or secondary intergradation with A. palmeri.
The pollination ecology of the Ditepalae is of
interest as several recent studies have suggested
that bats of the genus Leptonycteris are obligate
pollinators of A. palmeri (Howell 1979, Schaffer
and Schaffer 1979, Howell and Roth 1981). Howell
and Roth (1981) proposed that reported declines
in Leptonycteris popUlations could potentially severely impact sexual reproduction in paniculate
agaves. This hypothesis was based on a decline in
seed set of herbarium specimens over a 30 year
period and low fruit and seed set (approximately
25% and 20% respectively) in A. palmeri popUlations where bats were absent. Fruit and seed set
were high (81% and 70 0/0) where bats were present. No data were presented documenting visitor
or visitation rates. Sutherland (1982, 1987) has
documented that mean fruit set for paniculate
agaves is generally low (20-25 %), however, fruit
and seed set may be highly variable between
branches. The high fruit set reported by Howell
and Roth in bat-pollinated popUlations may be a
result of inadequate sample size, while the "low"
results may be more representative of normal fruit
set. Cockrum and Petryszyn (1991) have questioned reported declines in Leptonycteris
numbers based on the fact that few observers
have understood the variability and seasonality of
Leptonycteris movement in the northern part of
its range (resulting in reports of absent, declining
and low popUlation numbers). Herbarium specimens cited by Howell and Roth were reviewed by
Cockrum and Petryszyn who obtained different
estimates of fruit and seed set. They also noted
that specimens cited by Howell and Roth were
from localities which were either near the edge or
beyond the known range of Leptonycteris.
Gentry (1982) postulated that other members
of the Ditepalae may have mutualistic associations with nectar-feeding bats due to their similar
flower structure. He proposed that a "wave of
195
nectar flow" exists from spring to winter, providing food for bats as they migrate south.
Starting in the south with the March bloom of A.
colorata in Sonora, this "nectar flow" moves
north to Arizona for the summer with the flowering of A. palmeri, then extends south through
the Sierra Madre Occidental via A. shrevei in
southern Sonora, to A. durangensis in Chihuahua, and finally to A. wocomahiin Durango and
Zacatecas which bloom into December. Arita
(1991) has shown that the ranges of L. curasoae
and L. nivaJis significantly coincide with those
of A. angustifolia, A. salmiana, and A.
tequilana, although this relationship may be due
to the fact that both bats and agaves occur in
arid areas.
The distribution of the majority of Ditepalae
members also coincides with the range of Leptonycteris, although A. palmeri is the only
member with documented Leptonycteris pollination (Howell 1972, Howell and Roth 1981).
Southern populations of A. chrysantha are potentially within the range of Leptonycteris,
however, Baker and Cockrum (1966) have suggested that Leptonycteris probably never
extended very far north of the Santa Catalina
Mountains. Schaffer and Schaffer (1977) noted
that a population of A. palnleri (? A. chrysantha)
with bright yellow flowers on the north side of
the Santa Catalina Mountains probably depended on large bees for pollinatiqn. Anther
dehiscence and peak nectar production in A.
chrysantha occurs at night, and flowers have a
"ripening fruit" odor, all of which may attract
bats or moths. However, flowers are yellow in
color and have pollen and nectar available during the day to attract diurnal pollinators.
Howell (1979) has suggested that bat-pollinated
agaves are derived from insect-pollinated species, and that A. chrysantha is an intermediate
form between primitive insect-pollinated and
advanced bat-pollinated species. On the other
hand, insect pollination may be secondarily derived in A. chrysantha from bat-pollinated
Ditepalae (Le., A. palmerl). Sutherland (1987)
noted that A. mckelveyana, another paniculate
form of Aga ve, also has nocturnal anther dehiscence and nectar production, but is primarily
pollinated by insects. He postulated that agave
floral characters are conservative and retain batadapted traits despite predominate pollination
by insects.
Nectar production, nectar sugar concentration and pollen protein analysis may also
provide important clues to pollinator impor196
tance. Nectar production in bat-pollinated species
such as A. palmeri is nocturnal with peak production from 2000-2200 hours (Howell 1979).
Pollen protein content of bat-pollina'ted agaves
tends to be high (44 % in A. palmerl) while nectar sugar concentration is relatively low
(11-20%) (Howell 1972). Bee-pollinated species
generally exhibit the opposite trend. Pollen protein content is generally low (8-16%) (Howell
1972), although nectar sugar concentrations are
more variable (18-68%) (Schaffer and Schaffer
1977). Nectar production amount and pattern,
nectar sugar concentration and pollen protein
percentage are unknown in A. chrysantha.
Factors other than pollinators may be important with regard to fruit set and reproductive
fitness in agaves. Sutherland (1982) found no
significant difference in percent fruit set per inflorescence in A. chrysantha when pollinated
naturally, pollinated naturally plus hand pollinated, or hand pollinated alone, suggesting that
fruit set in agaves is not pollen or pollinator limited, but rather resource limited. Udovic (1981)
obtained similar results in a study evaluating
the effect of pollinators on fruit production in
the semelparous species Yucca whipplei
(Agavaceae). However, in a comprehensive
study of fruit set in eight species of yucca, Addicott (1985) found that pollinator limitation was
the factor that most often controlled fruit set.
Genetic load may be a more important general
factor in reproductive success than both pollen
and resource limitation. Weins (1984) and Weins
et al. (1987) have shown that preemergent reproductive success in outcrossing species is not
linked to resource or pollen availablity, but
rather to genetic load (any lethal mutation or
allelic combination) as a result of meiotic recombination. In a recent review considering the
roles of pollen limitation, resource limitation
and genetic load in fruit and seed set, Burd
(1994) found significant pollen limitation occurred in the majority of species evaluated, and
this pollen limitation generally varied among
times, sites or years. This suggests the pollination environment can be quite stochastic,
resulting in frequent pollination limitation and
limited female success at the level of individual
flowers, entire plants or populations.
The purpose of this study was to investigate
the pollination ecology of A. chrysantha and A.
palmeri, and to determine the importance of
various diurnal and nocturnal pollinators with
regard to fruit and seed set.
METHODOLOGY
Study Sites
Research took place at four study sites in central and southern Arizona. Agave chrysantha
study sites were located at the northern edge of its
distribution in the Sierra Ancha Mountains above
Parker Creek (Parker Mesa Site, Gila County, T5N
R13E Sec. 24, elevation 1400 m) and near the
southern edge of its range in the Santa Catalina
Mountains above Peppersauce Canyon (peppersauce Site, TION R16E Sec. 21, elevation, 1450 m).
Study sites for A. palmeri were located in the foothills of the Santa Rita Mountains (Santa Rita Site,
Pima County, T19S R16E Sec. 10, elevation 1520
m) and the foothills of the Mustang Mountains
(Mustang Site, Santa Cruz County, 31°43'13.6" N
latitude, 110°30'52" W longitude, elevation 1500
m). Research was conducted at the Parker Mesa
Site during July 1993 and at the Santa Rita Site
during August 1933. Studies were conducted at
the Pepper sauce and Mustang Sites during July
and August 1994 respectively.
Flower and Pollinator Observations
Twenty flowers were chosen and labeled prior
to dehiscensce. Time of pollen dehiscence,..length
of exerted style and condition of filaments, tepals
and stigma were noted each day until stigmas
wilted.
Observations of pollinators were conducted
over 3-4 days at all study sites, except the Santa
Rita site where pollinators were only observed
one day due to stormy weather. Pollinators were
determined by field observations of floral visitors
who appeared to transfer pollen to receptive stigmas. Night vision goggles were used for
nocturnal studies. Birds and bats were surveyed
separately from insects with the observer situated
approximately 10 m away from the inflorescence.
Visitation rates of birds were measured by recording the number of visits to open flowers on an
inflorescence for one hour shortly after dawn,
during mid-day and approximately one hour
prior to dusk. Bat visitation rates were determined by scanning a clumped group of agaves for
approximately one hour after dusk, near midnight, and before dawn. Visitation rates of insects
(primarily bees and moths) were determined by
scan sample: the number of insects active on an
umbel were counted for ten minutes every two
hours during the day (approximately 0500-1900
hours) for diurnal visitors and three times during
the night (approximately 2100, 2400 and 0430
hours) for nocturnal visitors. Insects were observed from a ladder approximately 1.5 m from
the study umbel. Predominate flower stage(s)
(predehiscent, dehiscent, post-dehiscent and pistillate) of each observed umbel was recorded.
Identity of visitor, visitation behavior and environmental conditions were also noted. Insects
were captured and mounted for later identification.
Pollen and Nectar Studies
Predehiscent flowers were observed during
the night to determine time of dehiscence. Nectar
production was measured on three replicates each
of 20 predehiscent, dehiscent, postdehiscent and
pistillate flowers every three hours from 2100 to
0600 (no nectar is produced during the day) with
a tuberculin syringe and needle. Experimental
flowers were located on umbels positioned in the
middle section of the inflorescence and exclosed
from pollinators with a fine nylon netting (Howell
1979). Standing nectar crop was measured at dusk
and dawn on two replicates each of 20 predehiscent, dehiscent, postdehiscent and pistillate
flowers. Nectar sugar concentration of standing
nectar crop flowers was measured at dawn in the
field with a hand-held refractometer.
Pollination Studies
Plants with inflorescences that were centrally
located within a population were chosen at each
site for pollination experiments. Test umbels were
chosen from the middle section of inflorescences,
and 15 plants each were randomly assigned to one
of the following treatments: 1) control umbels
available to both diurnal and nocturnal visitors, 2)
umbels available to only diurnal visitors (umbels
bagged at sunset and unbagged at sunrise), 3) umbels available to only nocturnal visitors (umbels
bagged at sunrise and unbagged at sunset), and 4)
umbels bagged, but liberally hand pollinated
daily during stigma receptivity (3-4 days) with
fresh pollen collected from different individuals
within the population (Pepper sauce and Mustang
sites only). Umbels were covered with nylon net
bags that excluded any animals greater than 1.5
mm in size. Umbels were bagged prior to stigma
receptivity (generally after anther dehiscence),
197
and bagging continued until all styles were wilted
(4-5 days). Umbels of experimental and control
plants from the Parker Mesa, Santa Rita and Peppersauce populations have been collected thus far,
and percent fruit set was determined. Data were
analyzed for each site by chi-square analysis.
RESULTS
Flower and Pollinator Observations
Agave flowers are protandrous, gradually
changing from a male to female (pistillate) state
over a 5-6 day period. On the first day of flowering (pre-dehiscent stage), the tepals open and the
filaments and anthers are exerted. Flowers remain
in this condition until the evening of Day 2 (dehiscent stage) when anthers dehisce. Time of
dehiscence differs between taxa; anthers of A. palmeri open between 2000 and 2200, generally
shortly after sunset, while A. chrysantha anthers
dehisce later, between 2400 and 0200 of Day 3.
Stigmas are tightly closed at dehiscence, although
styles are beginning to elongate. Agave chrysantha styles are exerted 0-15 mm the morning of Day
3 (post-dehiscent stage), while styles in A. palmeri
are exerted 15-35 mm above the tepals. The morning of Day 4 (early pistillate stage), the tripartite
stigmas are usually closed, but are generally
slightly moist and open by evening. Thus, flowers
become receptive approximately 48 hours after'
dehiscence. Filaments are beginning to wilt at this
time and styles are exerted 16-27 mm in A.
chrysantha and 25-40 mm in A. palmeri. By the
morning of Day 5 (pistillate stage) stigmas are
open and sticky, and filaments are wilted by the
end of the day. Styles are exerted from 21-30 mm
and 25-48 mm in A. chrysantha and A. palmeri/
respectively. Tepals have generally wilted by the
morning of Day 6 (late pistillate stage), and stigmas may be widely parted and moist to dry and
slightly wilted. Styles are completely wilted by
Day 7.
Flower stage composition of umbels was
noted to be different between A. chrysantha and
A. palmeri. Flowers on A. chrysantha umbels
tended to be predominately in only two stages
throughout the flowering period of the umbel, although larger umbels might exhibit more flower
stages. For example, an umbel might be composed
mostly of buds and pre-dehiscent flowers, progressing the next day to pre-dehiscent and
dehiscent stages, etc. However, A. palmeri umbels
198
tended to have all flower stages present during
the nlajority of time the umbel was blooming with
a smaller numbers of flowers open in each stage.
Floral visitors included a diverse range of animals: honeybees (introduced), bumblebees,
carpenter bees, hummingbirds, orioles, hawkmoths, butterflies, wasps, moths, and a variety of
small solitary bees. Bats were not observed at any
site despite over 15 total hours of periodic observations. Although visitors varied in composition
and numbers between sites, honeybees (Apis melli/era) were the dominant visitors and consumers
of pollen and nectar at all sites (Table 1). Bumblebees (Bombus sonorus) and carpenter bees
(Xylocopa cali/ornica arizonensis) were generally
the next most common visitors. Although present
at the Peppersauce site, carpenter bees had low
visitation rates as they tended to approach and
then avoid the observed umbels, possibly bothered by the observer. Small moths and
hawkmoths were the predominate nocturnal visitors, however, diurnal visitor frequency was
greater than that of nocturnal visitors at all sites
(Table 2). Peak visitation occurred at dawn and a
smaller burst of activity took place prior to dusk.
Both honeybees and bumblebees foraged most actively in the early morning, first actively
gathering pollen, and once the majority of the
day's crop was harvested, nectar was collected.
Carpenter bee activity tended to peak later in the
morning and continue through the afternoon.
These results are similar to those obtained by
Schaffer et al. (1979). Moths were most active
shortly after dusk, however haw kmoths were active near dawn as well. Some evidence of
aggression between foragers was observed. Hummingbirds seemed to be bothered by the presence
of bumblebees at the Peppersauce site, and would
pull away when bees approached. Carpenter bees
were quite aggressive at the Mustang site, often
chasing each other away from umbels. Hawkmoths and bumblebees were observed to knock
one another off flowers during their interactions
at dawn and dusk at the Santa Rita site.
Pollination was a very haphazard event, and
only 36 potential pollination events were noted in
over 30 hours of observation at all sites (bird and
bat observation time excluded). The small size of
the majority of visitors, allowed them to "rob"
nectar by entering flowers above the tepals and
avoiding the exerted, receptive stigmas. Honeybees were large exploiters of floral rewards who
performed little pollination due to their small size
and foraging habits. Honeybees tend to gather
pollen from a single inflorescence (McGregor et al.
Table 1.-BebYIen-!ite variltion of maigr floral visitgrs of A. ch!Jl.santha and A. l1.almeri (visitslminlumbel}.
A. chrysantha
Parker Mesl
Visitor
Honeybee
Bumblebee
Hummingbird
Moths
Hawkmoth
Carpenter bee
Scott's oriole
Blk swallowtail
Bat
Peln!erSauce
Total min
observed
Visits/min
480
480
360
60
60
480
360
480
120
0.34
0.26
0.13
0.16
0.03
0.02
0.02
0.01
0.0
Visitor
Honeybee
Moths
Wasp
' Carpenter bee
Bumblebee
Hummingbird
Housefly
Sm blk bee
Hornet
Bat
Total min
observed
Visits/min
690
100
690
690
690
1.38
0.25
0.08
0.06
0.04
0.03
0.02
0.01
0.01
0.0
480
690
690
690
120
A. palmeri
Mustang
Santa Rill
Visitor
Honeybee
Bumblebee
Hawkmoth
Moths
Sm bees
Bat
Total min
observed
Visits/min
Visitor
90
90
10
10
90
120
2.4
0.92
0.05
0.05
0.05
0.0
Total min
observed
Visits/min
Honeybee
Carpenter bee
Wasp
Sm blk bee
Hummingbird
Moths
Small green bee
Hawkmoth
Sm bees
Other wasps
Brown wasp
Blkwasp
Bat
300
300
300
300
240
90
300
90
300
300
300
300
570
1.42
0.52
0.32
0.31
0.12
0.18
0.13
0.10
0.06
0.03
0.02
0.01
0.0
Table 2. -Between-site variation of diurnal and nocturnal visitors to A. chrysantha and A. palmeri (visits/min/umbel).
A. chrysantha
Parker Mesa
PeRl2ersauce
Visitor
Visits/min
Total min
observed
Visitor
Visits/min
Diurnal
Nocturnal
Birds
Bats
0.65
0.20
0.15
0.0
480
60
360
120
Diurnal
Nocturnal
Birds
Bats
1.58
0.25
0.03
0.0
Total min
observed
690
100
480
120
A.pa/merl
Santa Rita
Mustang
Visitor
Vlslta/mln
Total min
observed
Diurnal
Nocturnal
Birds
Bats
3.46
1.00
90
60
0.0
120
Visitor
Diurnal
Nocturnal
Birds
Bats
199
Visits/min
3.09
0.30
0.21
0.0
Total min
observed
300
90
540
570
1959, Alcorn et al. 1961), so that if contact with
receptive stigmas occurred, fertilization was not
likely (agaves are primarily self-incompatible).
Larger animals such as hummingbirds and hawkmoths were observed to hoover and avoid
touching stigmas unless foraging in the middle of
a moderate to large-sized umbel. Stigma contact
generally occurred when insects landed awkwardly on umbels and touched stigmas, or while
foraging on freshly dehiscent anthers with erect
filaments, crawled over adjacent receptive stigmas. Bumblebees and carpenter bees were
observed to most frequently come in contact with
receptive stigmas (Table 3), their intermediate size
making it more difficult for them to avoid exerted
stigmas while foraging.
0.7-r---------~--------__,
o.e
0.5
1
i
04
•
I
0.3
0.2
0.1
Day
Figure 1.-Dally nectar production of A. chrysantha and A. palmeri.
Pollen and Nectar Studies
Mean daily nectar production of flowers is
presented for A. chrysantha (Peppersauce site)
and A. palmeri (Mustang site) in figure 1. Nectar
prod uction curves were similar, although nectar
amounts were greater in A. palmeri due to the
larger size of flowers. Nectar production was observed to decrease at a greater rate in A. palmeri
after Day 3. As flowers moved from a male to
female stage, nectar production decreased. Hourly
nectar production differed somewhat between
taxa (fig. 2). Nectar production in A. chrysantha
began at dusk and nectar was produced at a fairly
steady rate until 0300 when production slowly declined. Agave palmeri nectar production rose
rapidly from dusk until 2400 when it peaked, then
decreased steadily until dawn. This production
curve was similar for all flower stages in both A.
chry.t;antha and A. palmeri except for pistillate
flowers (evening of Day 5-morning of Day 6)
which produced a small amount of nectar from
dusk until 2100 with negligible production afterwards.
Standing nectar crop production versus total
hourly production and nectar sugar sugar concentration for Peppersauce and Mustang sites are
prest~nted in Table 4. Standing crop nectar
Table 3.-0bserved stigma contact by flora' vl.ltor. to A. chrysantha and A. palmeri.
A. chryuntha
fIPPI[IIYCI
fl[lnnMIH
Visitor
Stigma
contacts
Total min
observed
Blk Swallowtail
5
480
Bumblebee
4
480
V'.'tor
Bumbleb..
Honeybee
Carpenter bee
2
480
Housefly
Wasp
1
480
Stigma
contacts
Total min
observed
3
690
690
690
A.palmerl
Mulling
Sloll Bill
Visitor
Hawkmoth
Stigma
contacts
Total min
ob.erved
VI.'tor
Carpenter b..
90
Total min
observed
11
300
Bumblebee
5
300
Honefbee
5mb..
2
300
2
300
Blk/Wh ite bee
200
Stigma
contacts
300
0.3
amounts, although generally less than total daily
production, were similar to total daily values.
Thus, the majority of nectar produced was available for diurnal visitors at dawn. Variation in
nectar production was greater in Days 4 and 5 as
flowers aged and production decreased. Mean
nectar sugar concentrations were similar in both
taxa, and ranged from 13-21% in A. chrysantha
and 13-25% in A. palmeri.
Visitation rate varied as a function of nectar
production (Table 5). Umbels which were predominately
pre-dehiscent/ dehiscent
and
dehiscent/post-dehiscent had the highest visitation rates while pistillate umbels had low
visitation rates.
0.25
0.2
0.15
0.1
0.05
21
24
Hour
Figure 2.-Hourly nectar production of dehlscent to post dehlscent
flowers in A. chrysantha and A. palmeri,
Table 4.-Standing nectar crop (ml), nectar sugar percentage and total daily nectar (ml) production of A. chrysantha (Peppersauce
site) and A. palmeri (Mustang site). Data are mean values, numbers in parentheses are SE)
A. chrysantha
NectiU Sygl[ %
Standing~
Dusk
Day 1
Day 2
0.093
Dawn
Total
0.254
0.35
17.6
(1.77)
(0.05)
(0.07) ..
(0.09)
0.104
0.184
0.228
(0.049)
(0.081 )
(0.112)
0.47
(0.11 )
0.247
(0.124)
0.256
0.003
0.187
0.191
15.5
(0.007)
(0.065)
(0.064)
(1.77)
Day 4
0.0
0.007
0.007
14.0
(0.022)
(0.002)
(N = 1)
Day 5
0.0
0.0
0.0
Day 3
Totll DliI~
Nectar
(0.124)
0.087
(0·.088)
0.001
(0.002)
A. palmeri
~Ictar
Stlndlng~
Sugar %
T2tal DII~
~
Dusk
Dawn
Total
Day 1
Day 2
Day 3
Day 4
Day 5
0.390
0.532
(0.121)
(0.123)
0.061
0.471
0.532
(0.034)
(0.146)
(0.163)
(3.04)
(0.114)
0.083
0.282
0.366
15.87
0.341
(0.040)
(0.177)
(0.191)
(3.16)
(0.181)
0.008
0.005
0.013
14.50
0.002
(0.011)
(0.016)
(0.020)
(N = 3)
(0.007)
201
17.0
0.623
0.141
(0.081)
(4.1)
19.22
(0.231 )
0.677
Pollination Studies
100 - , - - - - - - - - - - - - - - - - - - - - - - - ,
90 - f - - - - - - - - - -
In preliminary studies at the Parker Mesa site,
fruit set of A. chrysantha umbels exposed to only
nocturnal pollinators was significantly lower
(17.2%) than diurnally pollinated umbels (22.8%)
(0.001<P(X2 = 19.02/1 df» and controls (24.1%)
(0.001< p(X2 = 43.1/1 df» (fig. 3, Table 6). Night
pollinated umbel fruit set in A. chrysantha at the
Peppersauce site was also significantly lower
(2.2%) than day pollinated umbels (16%) (O.OOl<P
(X2 = 283.6/1 df). Fruit set was 19% for controls
(significantly higher than day-pollinated plants,
0.001 <P(X2 = 19.15/1 df» and 51 % for hand pollinated plants ,(significantly higher than controls,
0.001 <P (X2=319.61/1 df». In A. palmeri (Santa
Rita site), nocturnally pollinated umbels had significantly lower (10.3%) fruit set than
day-pollinated treatments (14%) (0.001 <P(X2 =
18.596/1 df» and controls (16.5%) (0.001<P(X 2 =
96.4/1 df» (fig. 4, Table 6).
Mean total fruit set (entire inflorescence) of A.
chrysantha at the Parker Mesa site was 25.9% in
experimental plants (night and day pollinated
plants) and 24% in controls, similar to previous
findings of Sutherland (1982). However, fruit set
was quite variable throughout the popUlation (experimental
plants
10.8%-50.9 %,
controls
12.1%-39.8%). Mean umbel number per inflores-
70 / - - - - - - - - - -
60 f - - - - - - - - - t:
~it!
~
~
50 f----------/7':m---
40~---
304--------
20 4----------=
10
Peppersauce fruit set %
Peppersauce fruit abort
Parker Mesa fruit set %
Parker Mesa fruit abort
Figure 3.-Percent fruit set of A. chrysantha as determined by
pullinatlon treatment.
cence was 14.8 (range 11-20) in A. chrysantha with
highest fruit set observed on umbel 6. Mean total
fruit set in the experimental A. palmeri plants was
somewhat lower (14.6%) than the control population (19%), and fruit set was also highly variable
between individuals (4-40% in experimental
plants, 5.3-42.7% in controls). Mean umbel
number was 17.5 umbels/inflorescence (range 1126) with the largest number of fruits occurring on
umbel 10.
Table S.-Mean dally nectar production (mO vs. diurnal
visitation (visits/min) in A. chrysantha (Peppersauce site)
and A. palmeri (Mustang Site).
Table 6.-Effect of pollination treatment on fruit set % in A.
clJrysantha and A. palmeri. (Data represents mean number
of fruits/number of fruits + number of aborted
fruits/umbel. Numbers in parentheses represent SE)
A. chrysantha
Flower Stage
Nectar Amt
Visits/Min
Pre-dehiscent
0.47
4.7
Parker Mesa
Dehiscent
0.24
3.96
% Fruit Set
Post-Dehiscent
A. chrysantha
. pepper,auce
% Fruit Set
0.25
3.56
Pistillate
0.09
2.34
Night pollinated
17.2
(18.6)
2.2
(3.49)
Late pistillate
0.001
0.13
Day pollinated
22.8
(17.4)
15.7
(14.46)
Control
24.1
(15)
18.8
(16.62)
51.0
(36.42)
A. palmerl*
Hand Pollinated
Flower Stage
Nectar Aim
Pre:.cfehiscent/dehis.
0.623
4.5
Santa R.ita
Dehiscent
0.623
3.5
% Fruit Set
Post-dehiscent
0.677
Pistillate
0.341
Late pistillate
0.002
Visits/Min
A. palmeri
1.27
Night pollinated
0.75
*Flower stage listed was the predominate stage, although flowers
were present in previous and latter stages.
202
10.3
(16.24)
Day pClllinated
14.3
(18.67)
Control
16.6
(14.03)
DISCUSSION
rather primitive method of pollination can be
achieved by a variety of animals which mayor
may not include bats.
The arrangement of floral stages within umbels appears to impact pollination as well.
Pollination was most frequently observed when
insects, attracted to flowers with freshly dehisced
pollen or large amounts of nectar, would rather
haphazardly touch adjacent flowers with receptive stigmas. Consequently, as flowers aged and
floral rewards decreased, visitation rates decreased and pollination events were less likely.
Umbel size and position on the inflorescence may
also effect fruit set. Umbels located in the middle
of the inflorescence were the largest in size, and
had the highest fruit set. The higher number of
flowers and floral stages at anyone time in large
umbels presumably increases pollination success.
Flowering of individual agaves within a population is asynchronous, so that umbels positioned
in the middle of the inflorescence of an early or
late blooming plant may have a better chance of
receiving pollen from another individual than
those umbels on the bottom or top of an inflorescence.
Diurnal pollinators appear to contribute significantly to pollination ~d subsequent fruit set
in A. chrysantha and A. palmeri. Control and day
pollinated umbel fruit set percentages were near
20% and are similar to results of natural fruit set
(Sutherland 1982). Outcrossing, hermaphroditic
plants commonly have low frUIt set (Sutherland
and 'Delph 1984), and observed pollination rates
of 20% suggest adequate pollination occurred.
The similarity of fruit set in control and day-pollinated umbels imply that nocturnal pollinators are
not critical for sufficient fruit set. Night pollinators contributed little to fruit set (2.2% ) in the
Peppersauce population, and approximate reported self pollination fruit set rates (1.580/0)
(Sutherland 1982). Although the presence of bats
may increase fruit set of nocturnally pollinated
flowers, their presence does not appear to be vital
for adequate fruit set to occur. Further studies are
needed to deterqrine whether populations pollinated by bats have significantly higher fruit and
seed set than populations where bats are not present. If agaves are resource limited as Sutherland
(~982) suggests, then mean fruit set in any populahon would not be expected to be significantly
higher than 20-25%. Hand pollination was very
effective in increasing fruit set above control and
treatJnent percentages, indicating some pollinator
limitation existed. Further studies are required to
evaluate the percent seed set per fruit.
Results of this study suggest that several differences exist between A. chrysantha and A.
palmeri with regard to their reproductive biology.
Time of pollen dehiscence, flower stage composition of umbels and peak nectar production were
consistently different between taxa. When differences
in floral
color
and phenology
(yellow-flowered A. chrysantha blooms from late
May-early August, A. palmeri flowers from late
June-early September and flowers are cream-colored with reddish tepal apices) are also taken into
account, separation at the species level appears
appropriate despite the lack of complete reproductive isolation.
Pollinator observations suggest that diurnal
insects appear to play an important role in pollination of A. chrysantha and A. palmeri, despite
several characteristics of chiropterophily. Gregory
(1963, 1964) and Waser (1978) have noted that
plants with characteristics of a particular pollination syndrome may often depend on other
animals for the majority of pollination. Although
a large number of animals visiting flowers were
"thieves," pollination was predominately
achieved by diurnal insects (particularly native
bumblebees and carpenter bees) by "accidently"
making contact with receptive stigmas while foraging. The "mess and soil" pattern of ""pollination
(Faegri and van der Pijl 1979) appears to be the
primary method by which pollen is transferred to
stigmas in the populations observed, and this
90,------------
70
+---.-----.------ - --------
.day pollination
:o control
60
+-------------------
50
+-------~ - - - - - . - - - - - - - - - - -
1:
!
is=
240+--------------
:Ii
30+--------------------------·-·---
20 t----------------10
santa Rita Fruit Set %
Santa Rita Fruit Abort
Figure 4.-Percent fruit set of A. palmeri as determined by pollination
treatment
203
Data from this study indicates that pollinator
populations vary between sites and years. An intermediate or facultative pollination syndrome
may be more adaptive in species that occur in diverse habitats (A. chrysantha) or have large
geographic ranges (A. palmeri), allowing plants to
utilize a variety of pollinators that may vary both
temporally and spatially. Bats often do not arrive
to known roosts in southeastern Arizona until
mid-August (S. Schmidt, pers. comm.), however,
A. palmeri in the vicinity begins flowering in late
June-early July. The variable, seasonal and somewhat unreliable movements of Leptonycteris
(Cockrum and Petryszyn 1991, pers. comm., V.
Dalton) seem unlikely to support a tight mutualistic system. Recent studies have shown that the
long-accepted example of obligate mutualism between Yucca and the yucca moth (Tegeticula)
appears to be faculative as well (Dodd and Linhart 1994). Fruit set results of this study suggest
that a more faculative relationship exists with A.
palmeri, and the "bat-adapted" traits that A.
chrysantha and A. palmeri exhibit may be just as
advantageous to insects and other animals in hot
and arid climates where peak activity is near
dawn and dusk.
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