Nectar Trails of Migratory Pollinators
By Gary Paul Nabhan
Not far from the U.S./Mexico border, Mark Larkin grows a dozen vegetable crops as well as fruits and pasture grasses. One season he noticed that honeybees were nearly absent from his fields and orchards along the Rio Santa Cruz floodplain. Larkin is what you might call a stripped-down farmer—a short crew cut, sleeveless T-shirt, a doorless pickup, minimal chemical use, a small tractor or an occasional draft horse—but no farmer wants his land stripped of its pollinators.
News of declining birds and bees does not make many farmers happy, let alone one who grows a diversity of crops that benefit from—and in some cases require—cross-pollination for abundant harvests. Parasitic mites unknown to North America’s beekeepers prior to 1985 had rapidly spread across the country, devastating hive after hive of both managed and feral honeybees (1). In fact, Mark and his neighbors began to witness a paucity of wild and managed honeybees in their fields about five years ago. At the same time, biologists documented that such declines were not restricted to mite-infested bees, but that they also affected other pollinators formerly found in abundance in the binational corridor of the Rio Santa Cruz and in other riparian habitats spanning the U.S./Mexico border. Since 1995, U.S. and Mexican farms and ranches have suffered what the USDA has called the worst “pollination crisis” in American history (2). In 1998, the Society for Conservation Biology approved its first commissioned paper, which addressed the potential causes and consequences of pollinator declines on biodiversity and food yields, finding evidence that such declines were caused by inappropriate pesticide use, genetically-engineered crops, habitat conversion, and fragmentation (1).
Most farmers like Mark recognized that pollination is one of nature’s services essential to forage and crop production. Yet, few were in such a unique position to verify what relatively inexpensive land use changes could do to restore and enhance pollinator diversity and abundance. Mark had already been collaborating with ecologists to determine the effects of treated sewage effluent and rest-rotation grazing on riparian restoration, and he allowed us to initiate pollinator population monitoring just as the managed bee colonies on the farm were removed, and the last feral colony went into demise. Within three years, Mark knew that he never had to rent managed colonies of honeybees again to guarantee sufficient fruit yields; between his riparian restoration efforts, his maintenance of hedgerows and woodlots, and his minimal use of toxins in integrated pest management, native pollinators were provided with enough nectar and nesting habitat to ensure that his fruits and vegetables—as well as many wild plants—had adequate pollination services provided to them. At the same time, Larkin could rightfully claim that his farm was part of a restored migratory corridor that benefited many forms of wildlife, not just pollinators.
Such success stories on private lands often get overlooked, especially in contrast to more glamorous conservation efforts that focus on establishing new protected areas. Yet, as important as reserves are, without good stewardship of the private lands that lie between them, they are inadequate. Private lands (including many farms) provide the “stepping stones” or stopovers for the 300-some migratory species that move between the reserves of tropical Mexico and those of the temperate and boreal U.S. In addition, pollinator habitat restoration can result in direct benefits to farmers over and above the presence of “watchable wildlife.” If we are to promote participation of Mexican and U.S. farmers in protecting and restoring wildlife corridors, the economic benefits of pollinating services provided by these corridors are not a mute point. Here is a land management action that can bridge rather than divide the conservation sector and food production sector of our societies.
The Ecological Roles of Pollinator Corridors
Conservation corridors typically conjure up images of continuous linear habitats or greenways that provide for the movements of large predators and other wide-ranging species. In contrast, migratory corridors for winged pollinators might be more aptly described as a mosaic of stepping stones within a larger matrix, with each stone a stopover that migrants use for “refueling” while in transit along 2000-6000 km flyways. The “glue” providing the connectivity in this mosaic is the shared presence of certain flowering plant genera and the mobile pollinators that visit them.
For example, lesser long-nosed bats (Leptonycteris curasoae) use dense stands of columnar cacti, agaves, and morning glory trees near cave roosts as stepping-stones on their northward flight from Jalisco to southern Arizona. Many of these nectar-producing plants visited by long-nosed bats are patchily distributed succulents that favor hot, rocky hillsides and cliffs. The frequency of these patches maybe just as naturally limiting to nectarivorous bats as the availability of roosts in caves and rock shelters. If this hypothesis is true, it suggests that migratory pollinators such as long-nosed bats moved considerable distances to find one stepping-stone after another, even before the intervening matrix was degraded.
Donna Howell, a bat ecologist, may have been the first to explicitly suggest that at each stepping stone along a migration route, “It is not uncommon to find several bat-pollinated species in association [with one another at the same site] exhibiting similar phenologies.” From winter through late spring, these clusters of bat-pollinated plants bloom sequentially from south to north, creating the effect of a blooming wave cresting northward (3). Near simultaneous blooming of several nocturnally flowering species at the same site has the effect of presenting a concentrated energy source to nectar-feeding migrants, which keeps them at a particular stopover roost until the nectar resources there begin to decline. Then the pollinator population is “forced” northward to seek the next emerging bloom in the northward-reaching wave.
A nectar trail is the entire circulation pattern that pollinators follow as they migrate from one sequentially blooming plant population to the next (3). The loosely co-evolved relationships between migratory pollinators and plant populations contributing to the blooming wave may be thought of as sequential mutualisms. Should one or more of the plant mutualists be eliminated from the sequence by habitat destruction, bad weather, competition, pests, or diseases, pollinator nutrition and movements may be disrupted to the extent that they cannot visit other mutualists.
A sequential mutualism implies that an animal may be linked in space and time with several flowering plant populations, and in the case of lesser long-nosed bats, may move pollen and seeds between them. Obviously, because the plants are sessile but the pollinators are not, nectarivorous bats, hummingbirds, doves, butterflies, and moths serve as mobile links among plant populations in different landscapes, facilitating pollen and gene flow over considerable distances. Similarly, lesser long-nosed bats and white-winged doves (Zenaida asiatica) also facilitate seed dispersal and spatial mixing of genotypes from geographically isolated populations, thereby serving as mobile links between cactus populations during two different phases in the plants’ life cycle.
While migratory pollinators ensure landscape-level linkages among many different plant populations, many non-migratory pollinators visit these same flowers and secondarily benefit from genetic mixing stimulated by the migrants. Should the landscapes that are linked by pollinators fall within officially designated wildlife reserves or protected areas, these migrants may be among the few mobile links that visit most or all units in a regional reserve network.
Vulnerability of Pollinator Corridors
In arid and dry subtropical landscapes, farmlands found between protected areas can serve either as oasis-like stopovers for these migrants (4) or as barren, chemical-ridden sites that further stress pollinators during the most energy-intensive phase of their annual cycle (4, 5). Over the last half century, millions of hectares of desert and thornscrub vegetation in western Mexico and the Southwest U.S. have been converted to chemically intensive agriculture or to pastures of exotic grasses, creating 100-200 km stretches of flyways devoid of suitable forage and roost sites for nectarivores. We are only beginning to fathom the long-term effects on migratory bats, doves, hummingbirds, and butterflies of having fewer nectar plants for forage and fewer safe roost sites available as stopovers.
More than 70 percent of all birds, bats, and butterflies that migrate between the U.S. and Mexico travel routes bounded by the Continental Divide in the Sierra Madre Occidental and Rockies to the east and by the Colorado River/Sea of Cortes to the west (6). Despite earlier concerns that these migrants were being threatened by deforestation in the tropical and temperate forests (7), more thorough analyses suggest that both vertebrate and invertebrate migrants may undergo severe stress while in transit across arid areas of low productivity and high climatic variability (8, 5).
Consider, for example, the western populations of the rufous hummingbird (Selasphorus rufus), which move through coastal Sonora, Arizona, and Southern California. These tiny birds have been found in transit at such low body weights that they cannot continue their migration (8). Their weights were marginal for a number of reasons: loss of nectar resources due to competition from invasive plants, global warming and drought, and roosting/foraging habitat loss.
Other migratory pollinators showing local or regional declines include the Sonoran Desert populations of white-wing doves, coastal populations of lesser long-nosed bats, and those monarch butterflies (Danaus plexippus) that migrate to Michoacan, Mexico, from west of the Rio Grande watershed (2, 5).
Whether generated by climatic variability, herbicides, pesticides, or land conversion, en route stresses on migratory pollinators can have devastating effects. Pollinators require a tight synchrony between their migration and the peak nectar availability of flowering plants along their “nectar corridor” (3).
Restoring Ecological Connectivity
The best way to ensure adequate connectivity in regional reserve networks is to better manage intervening private lands in a manner consistent with the needs of migratory wildlife. Yet, in their current state, many private lands are the weak links in the migratory chain. Restoring the ecological connectivity of these lands will require stronger stewardship collaborations among public agencies, private land owners, and rural ejido collectives.
Dr. Exequiel Ezcurra (formerly the lead scientist for Mexico’s Instituto Nacional de Ecologia) echoed this point in a keynote address remembered for its political wisdom as well as its excellent science. In May 1998, at the international conference on the Conservation of Migratory Pollinators and Their Corridors held at the Arizona-Sonora Desert Museum, he pointed to the increasing political difficulties of establishing additional large protected areas in Mexico and the U.S. He predicted that few new reserves are likely to be established in northwest Mexico. As such, restoring ecological connectivity through private lands between federally protected areas will be critical to binational regional conservation efforts.
One success story of public-private collaboration is the remarkable recovery of riparian corridors using treated sewage effluent along binational riverbeds in the Arizona-Sonora borderlands. Because of its southeast-northwest alignment contiguous to north- south running rivers in Sonora, the Rio Santa Cruz is part of a 400km corridor of intermittent streams and associated riparian vegetation stretching across some of the driest portions of arid North America. This corridor has unprecedented importance to binational wildlife movements, given that only 10 percent of the historic riparian vegetation remains along the rivers and streams of southern Arizona (6).
In 1980, the Nogales International Waste Treatment Plant began to augment historically diminishing instream flow with treated effluent. The plant now provides continuous flow and replenishment of the shallow aquifer below the flood plain for 40 km north of Nogales, Sonora. By 1992, along a stretch of floodplain that had formerly lost most of its gallery forests, newly established stands of cottonwoods, willows, and mesquites covered more than 45 percent of the Upper Rio Santa Cruz flood plain (6). Additional restoration efforts using treated sewage effluent along the Rio Santa Cruz are currently being implemented by Pima County as part of its Sonoran Desert Protection Plan. This is an ambitious multi-species Habitat Conservation Plan, which has strict guidelines for targeting and managing these waters to regenerate floodplain habitats for several species of conservation concern, including migratory pollinators.
Mark Larkin began his restoration efforts where the Nogales Treatment Plant left off. He seasonally reduced or increased grazing in different patches to create healthy stands capable of long-term growth on the available water budget of treated effluent. With his consent, we began attempts at active restoration of pollinator habitat in 1997. These efforts included wildflower plantings, artificial nest placements, and other pollinator population enhancement techniques described in detail elsewhere (2, 6).
In addition to twenty-five species of migratory pollinators benefiting from these passive and active restoration efforts, we have documented some 322 species of invertebrate pollinators now in residence on Tubac Farms. There were potential seasonal increases in other species as well. Within the last decade, ornithologists have recorded nearly two hundred birds in the watershed’s headwaters. While it was not possible to assess population changes for so many species, certain neotropical migrants show clear signs of recovery.
Within the Upper Rio Santa Cruz corridor, we can now point to the success of a decade of efforts on Tubac Farms. This private land experiment demonstrates the utility of promoting pollinators’ “nectar trails” as a means to maintain healthy corridors across private lands between protected areas. These efforts not only benefit the pollinators themselves but also provide habitat for numerous other species, including habitat-modifying keystone plants and animals, frugivores, and perhaps even carnivores. The ecological restoration and subsequent management shifts at Tubac Farms convince us of the value of collaborating with a range of private land owners to enhance the ecological functionality of an entire corridor.
In addition to documenting their ecological role in linking landscapes, plants, and animal populations, our emphasis on pollinators as a means to define corridors has had several practical political advantages in promoting land stewardship. Efforts to define, promote, and restore corridors to ensure pollination services, for example, will likely meet with far more acceptance among farmers and ranchers than advocating for corridors to increase the movements of carnivores. Moreover, government initiatives such as the USDA’s Wildlife Habitat Incentives Program (WHIP) and the Sustainable Agriculture Research and Education (SARE) Program can subsidize pollinator habitat restoration as a means to benefit both crop yield stability and wildlife in general. Rather than emphasizing carnivores in the already contentious debates among farmers, ranchers, biologists, and conservation activists, we might do well to find common ground in our shared interest in maintaining pollinator services and then evaluate the extent to which this strategy indirectly provides more corridor habitat for carnivores.
Even if stepping-stone corridors suited to migratory pollinators do not function for all carnivores, it is hard to imagine how existing data on carnivore movements will be sufficient—in and of themselves—to empirically confirm where natural corridors still function and where they need to be restored. In contrast, there are thousands of migratory bird, bat, and butterfly observations and flowering plant records available to empirically define nectar trails. DNA and isotope tracking techniques can determine empirically which faunal samples taken at different stopovers are from the same breeding populations. Observations made by volunteer naturalists (compiled on the Arizona-Sonora Desert Museum web site: www.desertmuseum.org) may help locate other corridor segments that are in need of protection and/or restoration. Tracking the precise binational movements of carnivores—and establishing where corridors function for them—will take much longer. Nevertheless, the Migratory Pollinators Project, the Wildlands Project, the Sky Island Alliance, and Northern Arizona University are now beginning to compare the efficacy of using carnivores versus pollinators in the design of corridors to capture maximum levels of biodiversity possible in the U.S./Mexico borderlands.
Gary Paul Nabhan is Director of the Center for Sustainable Environments at Northern Arizona University.
1. Nabhan G.P. et al. 1998. The potential consequences of pollinator declines on the conservation of biodiversity and stability of food crop yields. Conservation Biology 12(1):8-17.
2. Buchmann, S. and G.P. Nabhan. 1997. The Forgotten Pollinators. Island Press, Washington D.C.
3. Fleming, T.W. 2000. Pollination of columnar cacti in the Sonoran Desert. American Scientist 88(5):432-439.
4. Lavee, D. and U. N. Safriel. 1989. The dilemma of cross-desert migrants: stopover or skip a small oasis? Journal of Arid Environments 17:69-81
5. Pyle, R.M. 1998. Chasing the Monarchs. Houghton-Mifflin, New York.
6. Nabhan, G. P. and J. Donovan. 2000. Nectar trails for pollinators: Designing corridors for conservation. Arizona-Sonora Desert Museum Technical Monograph 4, Tucson Arizona.
7. Terborgh, J.W. 1989. Where Have All The Songbirds Gone? Princeton University Press, Oxford.
8. Calder, W.A.1997. Hummingbirds in Rocky Mountain meadows. In K. Able, ed. A Gathering of Angels: The Ecology and Conservation of Migratory Birds. Cornell University Press, Ithaca. p. 149-168.
We thank the many researchers who contributed to the Migratory Pollinators Project and its precursor, the Forgotten Pollinators Campaign, especially staff at the Arizona-Sonora Desert Museum, the Programa para la Conservacion de los Murciélagos Migratorios founded by UNAM and Bat Conservation International, and the University of Arizona. We particularly thank the following team members for sharing their data: Jim Donovan, Stephen Buchmann, Ty Fitzmorris, Karen Krebbs, Pete Siminski, Steve Hopp, Ginny Dalton, Keith Labnow, and Laurian Escalanti. Mark Larkin hosted us at Tubac Farms, and Steve Walker facilitated collaboration in the region. Christine DeCarlo assisted with manuscript preparation. Support was provided by the Turner Foundation, the Wallace Global Fund, the Turner Endangered Species Fund, the Wallace Research Foundation, the C.S. Fund, the W. Alton Jones Foundation, the Roy Chapman Andrews Fund, and Border 21.
Buchmann, S. and G.P. Nabhan. 1997. The Forgotten Pollinators. Island Press, Washington D.C.
Soulé, M.E. & J. Terborgh (eds.). 1999. Continental Conservation: Scientific Foundations of Regional Reserve Networks. Island Press, Washington D.C.