ASU Research E-Magazine: Desert Plant Polygamy

For many days over three summers, Liz Slauson put up with storms, sleepiness, and sweltering temperatures. She did it to document exactly which creatures visit and pollinate two closely related agave plants, and to finally settle a bet on a bat.

If morals guided the plant kingdom, some plants would get a green thumbs-up for being chaste. The yucca, for example, accepts only the yucca moth as its pollinator. The moth, in turn, pollinates only the yucca, and its larvae feed on the developing fruits that result. The relationship between the two is called obligate mutualism, and could be considered the plant version of monogamy.

Other plants, like the slender, pretty agaves, aren’t so selective about their callers. These polygamous plants even adorn themselves with bright, colorful flowers and release tantalizing odors and nectar to woo suitors of any kind.

“The flowers aren’t specialized, so if anyone wants to come visit, they can,” says Liz Slauson, Desert Botanical Garden curator. “Their policy is, the more animals and insects that visit, the more opportunity there is that someone’s going to touch the stigmas and pollinate the flowers.”

Slauson recently completed her doctoral degree in botany at Arizona State University. She spent seven years as a nurse before switching to the nursery.

“It’s always been a dream of mine to study plants,” she says. “As a child, I loved to go into the woods and I loved to help my grandmother with her garden. She would always want me to come work with the vegetables, but I always wandered toward the flowers.”

Slauson studied the pollination ecology of agaves, or “century plants,” which are rosette-shaped, perennial leaf succulents native to Southwestern deserts. She developed her research interests while working as an intern at the Desert Botanical Garden in Phoenix, which she says has one of the world’s largest agave collections.

For several days over three summers, Slauson fought storms, sleepiness and sweltering temperatures to document who visits and pollinates two closely related plants, Agave chrysantha and Agave palmeri. She wanted to test the accuracy of a 1980s study that claimed, like the yucca, A. chrysantha needs a specific pollinator—the lesser long-nosed bat—to survive. Pollinators for A. chrysantha were unknown.

The 1980s study, conducted by an Arizona mammologist, suggested that other animals do not make contact with A. palmeri’s stigma, the female part where pollen is deposited for fertilization. The study also suggested the drop in bat numbers was causing a decrease in A. palmeri’s fruit and seed sets. The lesser long-nosed bat is listed as endangered, though recent studies show their numbers may be higher than originally estimated.

“This (study) has been looked at from the bat’s point of view,” Slauson says. “I looked at it from the plant’s point of view.”

The lesser long-nosed bat is a tiny, virtually toothless creature, covered with a fur coat more luxurious than mink. Each year, thousands of the bats leave central Mexico, traveling up the western coast and arriving in southeastern Arizona anywhere from the last week in July to the third week in September. In the northern part of their range, the bats feast on cacti, then turn to agave when those resources run out.

“They’re kind of erratic, they’re somewhat unreliable, you don’t know when or if they’ll arrive at a certain roost year after year,” Slauson says. “Their movement and migration patterns are not well understood, though we’re beginning to better understand them now.”

To Slauson, the mammologist’s hypothesis didn’t figure, because some populations of A. chrysantha finish flowering before the bats even arrive. A. palmeri blooms asynchronously, so some plants can be done before others start. The plant’s peak bloom time is late July to early August, though the early bloomers start in late June.

Slauson also found problems with some of the methods used in the 1980s study. For example, while fruit and seed sets were reported to be high where bats were present, no data documenting visitors or visitation rates was recorded, she says.

Also, the hypothesis was based on a decline in fruit and seed set of herbarium specimens over a 30-year period. But herbarium specimens are usually sampled to illustrate differences in morphometric, or physical, characteristics, Slauson says. They don’t accurately represent reproductive success.

In 1993, Slauson set out to see if the bats were as vital to the agave as originally claimed. She included A. chrysantha in her study because like A. palmeri, it exhibits traits that suggest bat pollination.

Both produce nectar and present pollen at night, though at different times. Both plants have a “batty smell,” though A. chrysantha’s is sweeter, like ripening coconut. A. palmeri smells more like rotting or fermenting fruit.

Flower color also differs between the plants. A. chrysantha’s flowers are bright yellow-orange, unlike A. palmeri’s drab, whitish or creamy colored ones, which suggest bat pollination.

Despite A. chrysantha’s “bat characteristics,” most of the plants are not within the range of the lesser long-nosed bat, Slauson says. While A. palmeri grows in southern Arizona, New Mexico, and Mexico, A. chrysantha grows only as far south as the Tucson area, then extends north. The two plants come in contact around Tucson.

In lower elevations, A. chrysantha can also start blooming as early as May, several weeks before the bats arrive. Slauson wanted to know several things: Who pollinates A. chrysantha? What kind of fruit set does it produce, since it’s out of the bats’ range? Is it in danger of going extinct, or has it adapted to other pollinators?

Slauson hoped her study of the two related plants would also shed light on taxonomic questions. Some botanists say A. chrysantha is a subspecies of A. palmeri; others say it’s a separate species. Few botanists have collected specimens because agaves have prickly teeth, spines, and harsh juices, and they are hard to process. They also grow in rugged, almost inaccessible mountainous areas.

During August of 1993, 1994, and 1995, Slauson and a team of volunteers traveled to five sites in Arizona and New Mexico to study the plants. They looked at A. chrysantha in the northern edge of its range at Parker Mesa in the Sierra Ancha Mountains, above Parker Creek, and near its southern edge in the Santa Catalina Mountains above Peppersauce Canyon.

Sites for A. chrysantha were located in the foothills of the Santa Rita Mountains, the Mustang Mountains, and in the Coronado National Monument. Slauson’s site at Coronado National Monument was less than two miles from a known roost of the lesser long-nosed bat.

At the sites, Slauson and her team recorded visitors to the plants, noting each creature’s identity and behavior, the time, and environmental conditions. To see the after-dark callers, she wore night-vision goggles. She watched to see which animals transferred pollen from one plant to the receptive stigma of another.

Agaves cannot pollinate themselves. Slauson looked for the animals who “robbed” the umbels, or flower clusters, taking resources but not returning the favor. Knowing who visited and when provided clues to pollinator importance.

Visitors included honeybees, bumblebees, carpenter bees, hummingbirds, orioles, hawkmoths, butterflies, and wasps. Slauson observed the lesser long-nosed bat only in 1995, at Coronado National Monument, though that does not mean the bats never visited the other A. chrysantha sites when she was not there.

She counted only 32 bat visits in more than 15 hours of observation. Slauson says few bats came to her site that year because they likely had plenty of agave resources near their cave and did not need to forage far for food. Also, a bat researcher she worked with has suggested that the bats prefer to visit plants covered by trees or bushes. Slauson’s site was in the open.

Peak visitation at both plants occurred at dawn, with smaller bursts of activity at dusk. Day visitors came most often at all sites. The honeybee was the predominant visitor at all sites except at Coronado National Monument, where the bumblebee made the most appearances.

Pollination was generally haphazard. Many visitors, because of their small size, were able to rob nectar by nimbly avoiding the flower’s extended, receptive stigma. Pollination mostly occurred when larger, clumsier insects such as bumblebees and carpenter bees accidentally bumped into the stigma—a pattern called “mess and soil.”

The honeybee was most guilty of thieving agave resources. At Peppersauce Canyon, for example, honeybees paid an average of almost one and a half visits per minute to A. chrysantha, out of 690 minutes observed. Yet only one stigma contact resulted.

The bats, however, earned their keep. At Coronado National Monument, they went 32 for 32—one stigma hit for every visit.

Still, day visitors proved to be the most important pollinators for A. chrysantha and A. palmeri. Fruit sets varied widely among plants, sites, and years. But in all cases, the sets from the day-pollinated umbels were higher than the night-pollinated flowers.

Slauson calculated the mean fruit set of the umbels by dividing the total number of possible fruits (fruits produced + aborted ones) by the number of actual fruits.

Even at sites the bats did not visit, the fruit set for the control umbel averaged about 20 percent, which falls in the 10 to 30 percent range that’s normal for outcrossing plants. That means other animals and insects are adequately pollinating A. palmeri and A. chrysantha, Slauson says.

Slauson concluded the bats need the agaves more than the agaves need the bats.

“From the bat’s point of view, (agaves) are pretty important because there’s not much else for them to forage on during late summer,” she says. “But from the plant’s point of view, bats aren’t vital because there are a lot of other animals that can get the job done, though maybe not as effectively.”

Slauson’s research into the reproductive biology and physical traits of the two plants also led her to conclude A. chrysantha and A. palmeri are different species.

“As A. chrysantha moved north and the bats became unreliable visitors, shifts in the plant’s floral color, scent, pollen distribution, and peak nectar production may have evolved to attract day visitors,” she says. She believes A. chrysantha’s bat pollination traits may be remnants from the past, when the plant or its ancestors may have depended more on the bats.

Slauson hopes her study of A. chrysantha and A. palmeri will highlight the importance of preserving the culturally and ecologically important agave. The plants have a long history. Since pre-Columbian times, people have used them to make food, beverages, and fiber. They also provide food and shelter to hundreds of insects and animals.

“They’re like a big grocery store, and they’re ethnically diverse,” Slauson says. “You should see how many animals live in and use them. They’re a whole little ecosystem in and of themselves.” —Amanda Kingsbury