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The Forgotten Pollinators
By Stephen L. Buchmann, Gary Paul Nabhan, Paul Mirocha
ISLAND PRESSCopyright © 1996 Stephen L. Buchmann and Gary Paul Nabhan
All rights reserved.
Silent Springs and Fruitless Falls
The Impending Pollination Crisis
The Virgin River basin of southwestern Utah is a place of rarities, from towering crimson canyon walls and Joshua trees to a host of low-lying, lesser-known living treasures. It was my first time to the Virgin, but Gary had made pilgrimages here before, collecting rare wild sunflowers to be used in breeding disease resistance into commercial hybrid sunflower crops. This trip, however, we came to the Virgin not for its sunflowers but to stalk a rare diminutive poppy. That poppy grew on gypsum-laced bluffs and hummocks too bleak for much of anything else to grow, but it had one local animal associated with it that we hoped to meet—a bee as rare as the plant itself.
We had flown into the virtual-neon-baking-hot reality of Las Vegas one morning in late spring. There we rented a vehicle to take us two hours to the northwest, into the higher reaches of the Virgin watershed where the crimson cliffs of the Chinle Formation enclosed sandy valleys. Below us, in one valley, a patchwork of greener-than-green alfalfa fields juxtaposed themselves against the badlands of pink dunes, cocoa-colored mudstones, and pale gray, bleached-out gypsum hummocks. From the very base of the nearby cliffs, low-growing familiar shrubs of pungent creosote, snakeweed, and saltbush were sparsely dotted across the flats, their roots competing for the meager rainfall of that semi-arid land. It seemed like an unlikely spot for us to look for one of the most endangered pairs of plant and animal in all of North America, but our hunt was on.
The poppy in question has a lovely, ivory-colored blossom, but a name that few people have heard: Arctomecon humilis. One animal associate of the "bearclaw" poppy is recognized by even fewer people, for it is a bee recently described as a new species: Perdita meconis. The story of their barely surviving together in a not-so-virgin habitat is a reminder of how little we know about the rarest-of-the-rare, even in a nation that spends more on environmental monitoring and protection than any other country in the world.
It could be that the bearclaw poppy was never widespread nor naturally abundant. Nevertheless, it has become exceedingly rare over the last century of habitat degradation, and the last decade of frequent drought. It was listed as a federally endangered species in 1979. Soon after that, during a five-year dry spell, nearly every seed-producing poppy plant died off, and hardly any new seedlings were recruited to replace them. Most of these perennials live a scant five years at the most, but far less than that when stressed. By the late 1980s, bearclaw poppy numbers had become so thinned that the conservationists who knew it best could only predict that it would be extinct by the year 2000.
We arrived at the end of a wet spring, so the prognosis did not look so bleak to us. Still, Gary and I wandered up and down the mudstone and gypsum hummocks of the Moenkopi Formation for well over an hour before I stumbled across the first flowering poppy. Excitedly I yelled for Gary to come over, for I doubted that he could see it from any distance away: it was barely 6 inches tall, with waxy blue leaves. The flowers bore velvety white petals and a central mass of bright orange-yellow stamens, looking like a miniature fried egg, sunny-side up.
We looked around, and realized that we were in a broad arc-like swath of a hundred poppies or more spread over 50 yards or so. Most of the plants had finished flowering, but the bulk of them had bloomed later than usual, so we were lucky to see quite a few blossoms. In fact, we were lucky to see any poppies at all. Cattle hoofprints and off-road vehicle tracks wandered right through the largest patch of poppies, despite a plethora of formidable signs posted nearby warning that the area was closed to traffic.
The abundant rains had encouraged high fruit production for the coming season, but when I knelt to examine the fruit, I realized that good weather was not enough to ensure that the fruits would contain a full complement of seeds. Each partially opened fruit looked like a miniature Easter basket, complete with handle, holding the seeds until they could germinate with the summer rains. A fully pollinated fruit might have over 30 thick, shiny black seeds in it. Yet many of the fruits that I scanned were already shrunken from lack of pollination, or perhaps from abortion resulting from the poor nutritional condition of the mother plant.
We took no seeds from the site, but we could easily count their numbers in the earliest-maturing fruits. They had just begun to dehisce, drying and splitting in fissures. A few fruits had 25 to 30 seeds in them, but most had far fewer. I rattled off my counts to Gary: 23, 20, 12, 14, 7, 18, 1, 4, and 11 seeds per fruit. Either many of the plants lacked the nutritional reserves to mature all the ovules into viable seeds in their fruits, or their flowers had not been pollinated to begin with: the average fruit held less than half the maximum seed complement it could produce under optimal conditions.
The only potential pollinators we saw during our dawn and dusk visits to the poppies were introduced honeybees, likely from a nearby apiary. They were busy on evening primrose and buckwheat blossoms, and for the most part, they left the bearclaw blossoms alone. Native pollinators may have been more active on this site earlier in the season, but they were nowhere to be seen while we were present over a two-day period.
Fortunately, in 1988, a group of entomologists from Utah State University had better luck at the site than we did. In May of that year, Vince Tepedino, a USDA-ARS research entomologist stationed in Logan, Utah, encouraged his student assistant Bonnie Snow to collect at the site. Bonnie swept up a species of solitary bee hitherto undescribed by entomologists, one that had evaded notice for more than a century after the poppies themselves had been described by biologists.
In fact, the bee had been collected just once before, on another kind of poppy, in the Kelso Dunes of eastern California well over a hundred miles west of the Virgin River. Terry Griswold was the collector of the Kelso Dunes bee, and when he saw that Bonnie Snow brought the same kind into his lab, he realized that these specimens were unlike any other from the region. Griswold named the new species Perdita meconis. His colleague Vince Tepedino soon went back out into the field to learn more about the relationship between the poppy-loving solitary bee and the endangered poppy.
Tepedino found that in the Virgin watershed, the poppy-loving bee has an unswerving allegiance to the bearclaw poppy despite the local availability of a wide variety of other plants blooming within its flight range. The bee has not even been found on other kinds of poppies closer than the dunes at Kelso. Over the entire time it has been observed, this solitary bee has maintained its fidelity to just these two poppy species, which have yet to be found growing together.
In short, Tepedino, Griswold, and colleagues have documented what theorists have assumed to be a highly improbable occurrence in the natural world: a pollinator that, on at least one site, specializes exclusively in visiting a rare plant, one with an extremely restricted distribution. A bee that visits only one kind of flower is called a monolege; the poppy-loving bee, however, is more precisely a highly restricted oligolege, a specialist on a small set of closely related flowers, one at each site. There are only a handful of well-documented cases in North America of truly monolectic bees, ones that associate with just a single source of pollen. Among them are Trachusa larreae, which is exclusively dependent on creosote bush in the desert Southwest; Cemolobus ipomoaeae, which is hosted exclusively by a single morning glory; and Hesperapis oraria, which visits but one dune-dwelling sunflower inhabiting a 200-mile stretch of Gulf Coast plains between New Orleans and the Florida Panhandle.
Such extreme specialization is clearly not the norm in the natural world, but it is the kind of exception that proves the rule: the fates of many plants and their associated animals are, at one level or another, intertwined. This is sometimes called the "Dodo Principle of Linked Extinctions," for when the dodo was extirpated on the island of Mauritius 300 years ago, a tree dependent on the Dodo began to decline in numbers. Why? Some scientists argue that its seeds require passage through the digestive tract of the extinct bird—or one with an equally impressive ability to crack open hard seedcoats—in order to germinate. In the case of monolectic bees and their host plants, the plant's decline would be expected to trigger a marked response in its associated animal population.
Theorists have long argued that such restricted relationships hardly make evolutionary sense: why would a pollinator confine its foraging to a single resource, particularly if that resource was naturally scarce to begin with? In the case of the bearclaw poppy, it may have never had a wide range, but it has undergone a dramatic local decline in numbers. Apparently the poppy-loving bee has not read current theories of optimal foraging, or it would have abandoned the poppy and switched to other floral resources.
Instead, as Tepedino has learned, the poppy requires a bee to serve as a vector for pollination, since cross-pollination produces significantly more seeds. This bee visits multiple flowers of the poppy during a single foraging bout, regularly contacting the stigma of each blossom, and has been observed repeatedly moving between the plants, resulting in cross-pollination. In short, the poppy-loving bee is not a casual or promiscuous floral visitor. Its behavior and form have evolved in such a manner that it has become a consistently effective pollinator of this rare plant.
Sitting in his office in Logan, Utah, one day, Tepedino proposed to us that "this bee should perhaps be listed as an endangered species just as the poppy is, since it is known from so few places." If we duly acknowledge how human pressures have already exacerbated problems posed by their natural rarity, this would become the first case in the continental United States where both a plant and one of its key pollinators would both be federally protected. This is not to say that the poppy itself would face extinction if the bee were extirpated from the Virgin watershed, for two other bees also visit the bearclaw poppy. But the three bees visit sequentially, in waves, over the poppy's flowering season. The first to appear is a fairly common native bee known by the melodious name of Synhalonia quadricincta; next comes the rare poppy-loving bee, Perdita meconis; last, and "certainly least" says Tepedino, comes the European honeybee, an outsider.
We had come to the Land of the Bearclaw Poppy so late in its flowering season that it was not even being visited by many of the managed or feral honeybees in the area. A few miles to the north of us, where poppies formerly grew, housing developments had taken their place. We can only guess at the former range of the bees themselves. Years ago, Stan Welsh, the dean of Utah's botanists, wrote that the bearclaw poppies "should be regarded as national prizes, as jewels of great price, and protected for future generations." What he did not know is that future generations of the poppy-loving bee will not survive unless these floral prizes remain alive and well within their native habitat.
Consider what might occur if, by happenstance, a global or local warming trend extends the duration of drought past seven years in length—the presumed maximum lifespan of a cohort of poppies. The amount of pollen and nectar available to feed the bees will be too small to nourish their population. Should moist conditions return, dormant poppy seeds may later germinate out of the Moenkopi Formation's gypsum soils, but they may not find even one bee left to pollinate their flowers.
It is conceivable that introduced honeybees or other (insect) generalists might provide some pollination services to the poppy. Nevertheless, a millennia-old relationship between poppy-loving bee and bearclaw poppy will have ceased to function. And almost certainly, the poppy's survival will be at greater risk, for its backup pollinator—the once-abundant honeybee—is now suffering dramatic population declines throughout North America.
There is good news and bad news when it comes to placing this parable in some sort of global context. The good news is that monolectic and oligolectic bees comprise a rather small percentage of all known pollinators. As we hinted earlier, few plants dance with just one partner. The bad news is that linked extinctions should not be the only topic of concern for conservationists who care about endangered plants or their rare pollinators. A formerly diverse natural community may become impoverished even when one or both partners in pollination decline, but do not become globally extinct.
Auburn University botanist Robert Boyd has recently written: "The most prudent approach to conservation of [a locally rare plant] would be to preserve and manage not solely [the plant] but as many components of the ecosystem as possible. This would likely include the pollinators, seed dispersers, and other organisms that perform important roles in the plant's life cycle." But such an ecosystem approach might be costly, for it assumes that we know a lot about the dynamics in interactions between plants and animals. In fact, we are only beginning to understand even the most obvious relationships between flowers and their pollinators.
In some cases, it is not the plant population that first begins to dwindle, but the animal with which it may have coevolved as they reciprocally adapt to one another over the centuries and millennia. I have witnessed such a situation several times over the last few years, a thousand miles away from the Mohave Desert in the heart of Mexico City. I cannot imagine a habitat more distinct from that of the poppy's as I found in the Pedregal, an overgrown lava field located on the city-like campus of UNAM, among the largest universities in the world. There, within easy walking distance of some of Mexico's finest laboratories, ecologist Luis Eguiarte has taken me to a pollinator monitoring site that he established with our mutual friend Alberto Burquez in 1982.
On many nights between 1982 and 1994, Luis and Alberto have sat out among some of their favorite flowering plants from sunset through early evening. They have tallied the numbers of nectar-foraging bouts made by the lesser long-nosed bat, a species presumed to be declining in central Mexico. The flower that Luis and Alberto favor is a distant relative to mescal-producing century plants. It is a small, rosette-forming succulent with no common English name, known only as Manfreda brachystachya. I have nicknamed the plant "Señora Manfreda," although I am sure Luis and Alberto do not condone this anthropomorphizing.
Señora Manfreda, like many of her mescal-producing kin, emits a musky scent and abundant nectar within her pale tubular flowers. The flowers themselves are stacked laterally along a 4 to 6-foot-tall stalk and arranged for easy access by nectar-feeding bats. Although other kinds of bats as well as hawkmoths and hummingbirds visit Señora Manfreda, the lesser long-nosed bat remains her most common visitor. This bat is not just another nectar-sucking pretty face; it is a highly effective pollinator.
And pollinate they do. Luis has shown me how he tags each flower stalk that has been visited, so he can later correlate bat visitation frequencies with the number of fruits set by each plant. I can recall the surprise in his voice when he recounted the discovery of a 75 percent reduction in fruit set between 1982 and 1985: "At first we thought we were witnessing a permanent decline in fruit set. Why? Then we realized that our first season's visitation by bats had been much higher than during our second season in 1985." Luis and Alberto went on to demonstrate that declines in fruit set directly reflected declines in bat visitation rates to flowers, even though other floral visitors—hawkmoths and hummingbirds—were still present. They also noted that in a botanical garden not far from their site, century plants that had received 100 bat visits a night in 1982 were down to 20 visits a night in 1985.
I probed Luis for more detail. "After a few years' worth of more data," he said, "I realized that we were witnessing fluctuations—but not necessarily unidirectional declines—in both the local abundance of bats and the fruiting of plants." Luis sat down and showed me chart after chart documenting a tight correlation between two phenomena: visits by pollinators and fruit set. When bats visited the plants frequently, fruit set was high; when they were nearly absent, fruit set was low. Evict the bats from the Pedregal, the data predicted, and rates of fruit set would take a corresponding dip.
Excerpted from The Forgotten Pollinators by Stephen L. Buchmann, Gary Paul Nabhan, Paul Mirocha. Copyright © 1996 Stephen L. Buchmann and Gary Paul Nabhan. Excerpted by permission of ISLAND PRESS.
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Table of Contents
INTRODUCTION - Remembering the Pollinators,
CHAPTER 1 - Silent Springs and Fruitless Falls,
CHAPTER 2 - Flowers,
CHAPTER 3 - Pollinators,
CHAPTER 4 - The Perils of Matchmaking,
CHAPTER 5 - Bees in the Bestiary, Bats in the Belfry,
CHAPTER 6 - Fractured Fairy Tales,
CHAPTER 7 - Need Nectar, Will Travel,
CHAPTER 8 - Holding the Globe in Our Hands,
CHAPTER 9 - Keepers of the Flame,
CHAPTER 10 - New Bee on the Block,
CHAPTER 11 - The Little Lives Keeping Crops Fruitful,
CHAPTER 12 - Cultivating Lasting Relationships,
APPENDIX I - A Call For a National Policy on Pollination,
APPENDIX 2 - Pollinators of the Major Crop Plants,
APPENDIX 3 - Conservation and Research Organizations,
APPENDIX 4 - Sources,
APPENDIX 6 - Common Agricultural Pesticides,