Stephen Buchmann, Michael R. Kunzmann, Arthur James Donovan and Royden J. Hobbs


Gap Analysis of Pollinator (bats, bees, hummingbirds) Species Richness in Arizona:

Implications for Conservation Biology.


Arizona is the only state with a comprehensive supervised and field-checked vegetational association GIS database. The database was developed by the USGS, Biological Resources Division, Arizona Gap Analysis Project for use in assessing long-term maintenance of biodiversity. Base vegetation coverage was created from Thematic Mapper imagery (1990-1992). The output of an unsupervised classification was, along with DEM elevational data and buffered GPS-referenced airborne video sample points in a supervised mode to classify vegetational associations. Additional data from on-ground plant sampling were used to correct classification errors. With two species of nectar-feeding bats (both federally listed) and 17 species of hummingbirds, some as annual migrants from Mexico, Arizona has a rich vertebrate pollinator fauna and specialized flora which feed these animals. The vertebrate pollinator database will be especially useful to vertebrate biologists looking for species richness "hotspots" and for conservation biologists documenting the interactions of pollen and nectar host plants along "nectar corridors" for migratory pollinators in southern Arizona enroute to and from Mexico.


The Sonoran Desert of southern Arizona and northern Mexico, is one of the richest habitats for native bees and certain other pollinators. Bees make up the dominant and most efficient pollinators of flowering plants on the planet. This is largely due to their size, hairy bodies and pollen/nectar-feeding lifestyles. Some native bees are generalists and forage for pollen and nectar from a wide variety of floral hosts while others are specialists on just one family or even one genus of angiosperms. Most of the more than 25,000 species of described bee species in the world are ground-nesters leading solitary lives, while a few a social with colonies headed by a queen. The United States has about 4,000 species of native soil and wood-nesting bees. For the solitary ones, each female is totipotent since they construct their own burrows/nest cells and forage for pollen and nectar without help. Other species, such as carpenter bees in the genus Xylocopa, excavate galleries in dead wood, while others locate and utilize larval burrows abandoned by many wood-boring beetles. Surprisingly, it is the arid regions of the world (e.g. deserts, savannahs, chaparral) not the rain forests which support the greatest bee biodiversity. Around Tucson, AZ in the Sonoran Desert, one can find about 1,000 species of native bees (Buchmann, unpubl.) making this area one of the best anywhere for bee diversity.

The economic value of the pollinating activities of bees in the United States has been estimated at 8-10 billion dollars (Southwick and Southwick 1992). This value is usually attributed entirely to honey bees (Apis mellifera) when in fact, much of the credit and several billion dollars worth of crop pollination is due to native bees and other pollinators. We can literally thank pollinating bees for approximately one third of every bite of our food and much of our cotton clothing (Buchmann and Nabhan 1997).

Other pollinators are also present in southern Arizona. These include two species of migratory pollinating bats (the Lesser long-nosed bat and the Mexican long-tongued bat) and over a dozen species of resident and migrant hummingbirds. The bats are specialist feeders on the nectar, pollen and fruit of giant columnar cacti (saguaro and cardon) and century plants in the genus Agave. These are migratory pollinators which may travel thousands of kilometers annually from their roosting/maternal caves to feeding sites in Arizona and Mexico (GIF of migratory routes for bats/hummers). Both the bats and hummingbirds follow and exploit "nectar corridors" along riparian drainageways between the U.S. and Mexico (Figure 1-Migratory Routes). Hummingbirds are native to the New World, and are especially abundant and diverse in Mexico and in southern Arizona. A total of 17 species of hummingbirds can be found in SE Arizona during the year. They also follow the nectar corridors of flowering plants with tubular red flowers from the U.S. into Mexico and back. In all, approximately 2,000 vertebrate animals act as pollinators of plants worldwide, while for invertebrates, bees and other insects, this figure approaches 200,000 species.

The intent of this paper is to raise awareness about the decline of many pollinating animals and the vital roles they play in the production of food, fiber and medicines along with their often forgotten one of maintaining natural plant communities and wildlife. We focus upon certain migratory pollinators (nectar-feeding bats and hummingbirds) that cross the international border between the United States and Mexico. Data is presented on the vegetation types where most pollinators occur along with an evaluation of the ownership and stewardship of these managed and wild lands.

Land Ownership and Environmental Stewardship in Arizona:

Arizona presents us with a complex patchwork of private, state, federal and tribal lands. The state has a total area of 29,517,886 hectares. Of this, the largest percentage (27%) is in Native American Reservations. The next largest category is private (18.29%) followed by the Bureau of Land Management (14.46%), United States Forest Service (13.26%), State Trust Lands (12.93), USFS and BLM Wilderness Areas (4.16%), Military (3.82%) and the National Park Service (2.56%). Table 1 presents a complete categorization of the areas attributed to the various ownership and stewardship categories.

Table 1. Area of Arizona Stewardship Categories

Stewardship Category Area (ha) % of State

Bureau of Land Management 4,268,141 14.46

Bureau of Reclamation 1,651 0.01

Military 1,126,710 3.82

Miscellaneous 1,130 0.00

National Park Service 755,469 2.56

National Recreation Areas 268,731 0.91

National Wildlife Refuge Wilderness 583,505 1.98

Native American Tribal Lands 7,978,198 27.03

Nature Conservancy 5,963 0.02

Privately owned 5,400,255 18.29

State Parks and Recreation Areas 58,809 0.20

State Trust Lands 3,817,184 12.93

State Wildlife & Management Areas 10,166 0.03

United States Forest Service 3,912,657 13.26

USFS & BLM Wilderness Areas 1,228,036 4.16

US Fish and Wildlife Refuge 101,281 0.34

Arizona GAP Vegetation Mapping:

Arizona is one of the few states where intensive GIS and GPS studies have produced an authoritative and complete assessment (U.S. Department of Interior GAP program and vegetation map) of the natural vegetation types along with landcover designations. Further, the Arizona map and database layers have been ground-truthed by field teams and compared with GPS-synchronized airborne videography (9,000 miles of video transects), thereby not relying entirely on automated neural net vegetation and land type classification systems. A 12 month thematic accuracy assessment of these data, including the creation of a 58,169 polygon and 105 community type map, was conducted by the USGS Cooperative Park Studies Unit at the University of Arizona (Kunzmann et al., 1998).

Since the Endangered Species Act was passed in 1973, preliminary evidence in North America indicates that the numbers of plant and animal species at risk has noticeably and "dramatically" increased (National Gap Analysis Program, 1994). Increased awareness of species at risk led ecologists within the U.S. Department of Interior and other agencies to begin the development of scientifically-based methods to examine habitat integrity, habitat degradation and losses due to anthropogenic activities. They subsequently established the National Gap Analysis Program or GAP (Scott et al. 1987, Scott et al. 1993). The GAP program assumes that the costs of maintaining species in their natural habitats, as part of self-sustaining ecosystems, is less than the cost of intensive management or restoration programs to save threatened or nearly extinct species. Vegetation mapping is critically important to the GAP program in the U.S. and AZ because dominant vegetation types are proximate indicators of overall biodiversity (Franklin 1993). Further, numerous studies have demonstrated that vegetation structure and composition significantly affects species richness and the various interactions among species and individuals. Given these ecological factors, we have undertaken an approach of using GPS, GIS and GAP analysis to assess the status of migratory and other pollinators in AZ and northern Mexico.

Migratory Bats and Hummingbirds in AZ and Mexico:

Some bats have coevolved with certain flowering plants developing specialized physical, behavioral and physiological adaptations which allow them to feed on the flowers and fruits of columnar cacti and century plants in the hills of southeastern AZ and northwestern Mexico. These adaptations include a long tongue and muzzle with highly reduced dentition. Specialized hairs on the face and neck are flattened to carry pollen which the bats lick off and ingest upon returning from their nightly foraging bouts. Both species of nectar-feeding bats found in AZ (Leptonycteris curasoae- the Lesser long-nosed bat (Figure 2-A lesser long-nosed bat visiting a Cardon cactus in Mexico) and Choeronycteris mexicana, the Mexican long-tongued bat) make long-distance flights (as much as 50Km per night) from their cave roosting sites to populations of cacti and agaves where they forage actively for nectar and inadvertantly vector pollen grains from plant to plant. Later in the season, the bats disperse the seedy fruits of columnar cacti in the Sonoran desert. Bats are unique and invaluable pollinators of crop plants (the "blue Agave" for mescal and tequila and many tropical fruits including bananas and durians) and seed dispersers. Thus, they provide essential pollination services to natural desert communities maintaining columnar cacti and Agave populations which in turn provide food and shelter for myriad invertebrate and vertebrate animals in these desert regions.

These migratory pollinating bats, as well as year-round resident bats face many ever-increasing environmental threats. The most damaging threats to bat populations include the willful destruction of roosting or maternal caves which may occur unknowingly when mine entrances are closed to protect the public from hazards or wantonly as in the dynamiting of caves or burning out of bat populations as sometimes occurs in northern Mexico through irrational fear of bats and rare diseases. Other threats to migratory bats include habitat loss and destruction due to urbanization, modern large-scale agriculture and ranching practices including the planting of exotic African grasses for the cattle industry.

Hummingbirds also migrate along riparian vegetation corridors between Mexico and the "Sky Island" mountains of southern Arizona. Like their migratory bat counterparts, many of these bird species follow the bloom along what have been termed "nectar corridors." These migrations are extraordinary physiological marathons by these tiny birds. One of Arizona’s hummingbird researchers–William Calder of the EEB Department, University of AZ– stated in 1998 that "The rufous hummingbird has the distinction of flying… 49 million body lengths, the longest migration of any bird [from Alaska to Jalisco and Oaxaca, Mexico]."

Riparian areas are some of the fastest disappearing habitats in Arizona, it is no surprise that habitat fragmentation and loss is altering these fragile corridors at an alarming rate. Other threats to hummingbirds include pesticides biomagnified from ingested insects in their diet. In all, there are 17 species, which can occur in the region. All of these species depend upon small insects for their major dietary protein. These are consumed near midday. During the early morning and late afternoon hours, the birds forage for the dilute nectar of many tubular red blossoms of native plants and at sugar water feeders in urban areas.

Trap-Nesting Native Bees in Desert Habitats:

Large guilds of native bees eschew soil and nest within dead wood of trunks, branches or twigs. Some of these wood-loving bees (e.g. carpenter bees of the genera Ceratina and Xylocopa) excavate their own nest burrows and cells. Most, however, are unable to dig their own nest galleries and simply locate and move in to previously abandoned galleries left by the tunneling activities of larval buprestid and cerambycid beetles. Most of these bees belong to the leafcutter and mason bee family Megachilidae. Commonly occurring desert genera in this family include; Anthedonia, some Anthophora, Ceratina, Chalicodoma, Colletes, Megachile and Xylocopa. Many of these megachilids collect leaves, mud, pebbles, sand and resin from which they construct cell partitions and end plugs within their nests.

Habitat loss and degradation also affects these bee pollinators especially in areas of intensive agriculture and where riparian corridors are fragmented and rapidly disappearing. Some of these effects can be countered by allowing dead trees (containing beetle burrows and thus prime bee nesting sites) or creating new nest sites by drilling holes of various sizes into dead trees. Artificial wooden nesting materials can easily be placed in the field to inventory and survey the health and diversity of local wood-nesting bee populations. Trap-nest blocks consisting of pieces of boards drilled with holes of certain diamters and approximately 5-10 cm deep can be provided in the habitat as survey devices and additional nest sites allowing populations to be monitored and increase. A variety of nest hole diameters from 2-3 mm up to 10-15 mm should be used to attract a diversity of native bees (Figure 3-Researcher placing a trap nest) these trap nest blocks should be attached to shady areas of dead trees or under the eaves of wooden buildings protected from the rain. Female nesting bees quickly find and use the new holey bee real estate. Paper straws can also be inserted into the drilled wooden blocks and easily removed to inspect the contents. This can also be done non invasively by affixing the paper straws to a sticky card and using a medical X-ray unit to examine the life stages or parasites within (Figure 4-X-Ray radiograph of leafcutter and mason bee nests). In addition to land managers and restoration ecologists, such nest blocks can be used to good advantage by gardeners looking for non-stinging alternatives to defensive honey bee colonies in urban settings. Mason bees, especially the genus Osmia, is a highly effective easily managed pollinator of fruit trees including apples and cherries.

Honey Bee Colonies for BioMonitoring of Flowering Phenology, Habitat Integrity and Environmental Pollutants:

Honey bees (Apis mellifera) are superorganisms living in a social colony that may contain 50,000 individuals. Through an elaborate system of scouts and recruited forager the colony quickly evaluates the local floral community (within a typical radius of 3-5 Km) and mobilizes thousands of foragers to productive patches of floral resources. The foraging "arena" of just one colony can include from 80-100 km2 of Sonoran desert habitat. Nectar is collected and returned to the hive where it is processed into honey containing 20% water and 80% sugars. It provides the energy for flight and brood productions. Pollen is stored and inoculated with microbes serving as a protein and lipid-rich food for the developing larvae. Such a foraging strategy is termed central place foraging since all collected materials are returned to the nest. Water and plant resins, termed propolis, are also brought back to the colony. Honey is transformed biochemically by young worker bees into beeswax used to construct their double-sided hexagonal honey and brood combs. Honey bees are super collectors. Records for colonies placed on the Saguaro National Park (West Unit) from 1987-1990 revealed that managed Langstroth colonies, during a single year, collected from 10.2 to 23.7 Kg of pollen and several hundred Kg of honey (Buchmann et. al. 1991).

Foraging honey bees fly and crawl into flowers and inspect many substrates and openings. As such, they come in contact with naturally-occurring materials in the environment as well as manmade pollutants including heavy metals and pesticides. Pollen and these exotic materials stick to their hairy bodies and are carried back to the nest cavity where they often become incorporated into the beeswax, pollen and honey stores. Thus, with their wide foraging range and collecting activities, they are natural monitoring agents for investigating the ebb and flow of floral resources and toxic substances within the environment. At least one researcher has effectively used honey bees to collect pollutants including heavy metals, radionuclides and pesticides, which are concentrated within their nests and can be subsequently analyzed using modern chemical analytical instrumentation (Bromenshenk, et al. 1995). Similarly, honey can be extracted from colonies and pollen grains within identified using the methods of melissopalynology to identify the local plants in bloom. Honey bees visit from 25 — 30% of the flowering plant species within flight range of their colonies including wind-pollinated plants that offer no nectar. This makes their colonies and nests especially valuable perennial mobile biomonitors of the local environment. Special screen devices know as pollen traps can also be placed under colonies. These disrupt and collect some of the pollen loads from returning foragers. Palynological methods are then used to identify the pollen grains to family, genus and species. This data can be used to plot the blooming phenology of plants in the area and for other ecological uses.

Conservation Efforts in Arizona and Mexico for Migratory Pollinators:

In 1999, the Arizona-Sonora Desert Museum founded a consortium on Migratory Pollinators (bats, hummingbirds, white-winged doves, monarch butterflies). They have joined with organizations including Bat Conservation International, the Center for Plant Conservation, National Fish and Wildlife Foundation, Native Plant Conservation Initiative, Turner Foundation, Turner Endangered Species Fund, The Xerces Society and others in a five-year collaborative project for field research, community stewardship, education and conservation. During 1999, the ASDM sponsored a symposium and workshop on migratory pollinators in Tucson, AZ (Figure 5-Poster from the Migratory Pollinators Conference). The ASDM consortium is unique since it links field biologists, land managers and policy makers while educating and raising public awareness. Most importantly, the ambitious plan hosts workshops for Mexican biologists and trains conservation interns in local communities to serve as naturalist interpreters and protectors of areas including bat caves and feeding hot spots for bats and hummingbirds. Dr. Gary Paul Nabhan and other migratory pollinator researchers can be reached at the Arizona-Sonora Desert Museum by writing to: ASDM Migratory Pollinators, 2021 North Kinney Road, Tucson, Arizona 85743 or by visiting the Forgotten Pollinators website ( the Migratory Pollinators website ( or by sending electronic mail to:

The earliest modern effort to educate environmentalists, farmers, gardeners, land managers, policy makers and the public about the global plight of bees, bats, hummingbirds and other pollinators was the 1997 publication of the Island Press book "The Forgotten Pollinators" by Buchmann and Nabhan (, co-founders of The Forgotten Pollinators Campaign (now known as the Pollination Conservation Consortium). This book will soon appear in Spanish and Japanese language editions.

Other NGO’s, government agencies in the U.S. and Mexico, conservation biologists and land managers are now aware and making plans to protect nectar corridors and migratory pollinators, true conservation across borders. Several non-profit groups are at work involving students, bird and butterfly watchers along with other naturalists to track migrating pollinators, especially monarch butterflies. One exemplary educational effort and interactive website/database is the Journey North program at Hamlin University ( Recently, the Agricultural Research Service of the U.S. Department of Agriculture has joined with the Biological Resources Division and the U.S. Geological Survey of the U.S. Department of Interior to form an interagency task force to document pollinator populations and implement guidelines and strategies for the conservation of pollinators. Similarly, the National Science Foundation is sponsoring a taskforce and workshop in October, 1999 to gather together existing data on the status of native bees in the United States. The issues of pollinator conservation and food stability have not missed the attention of policy makers at the highest levels. United States Secretary of the Interior Bruce Babbitt remarked in a speech given on September 30, 1998 in Austin, Texas that "Pollinators: these hard-working heroes of nature are not well understood but are clearly in peril… Loss of habitat, poisonings, and fragmentation of plant life on which they depend is reducing the number of pollinators alarmingly."


Migratory nectar-feeding bats, hummingbirds and resident bee populations are demonstrably at risk due to burgeoning human populations and changing landuse patterns in the southwestern United States and northern Mexico (Nabhan and Fleming 1993). During the next 3-5 years, we will begin to map (using remote sensing, custom aerial videography and GPS ground-truthing) riparian areas and other migratory nectar corridors for nectar-feeding bats and birds in Arizona and northern Mexico. This Arc-Info database will be used by land managers and conservation biologists to examine the fragmented habitats, through which these animals must pass and stop to feed. Land managers and policy makers can also use this database and predictive model to examine the spread of invasive species, especially the exotic African Buffel grass which has been planted as cattle fodder in over one million hectares in Sonora, Mexico. We believe that the ASDM Migratory Pollinators consortium and other environmental groups will be instrumental in protecting vital nectar/pollen corridors across the U.S./Mexico international border and thereby ensuring the protection of migratory pollinators including bats, hummingbirds, white-winged doves and monarch butterflies.

Technical treatises and popular field guides with color illustrations are widely available for birds, some bats and most of the native butterflies in the United States. This is not the case for native bees where taxonomy has lagged behind largely due to the overwhelming numbers of species (4,000 native bees in the continental United States) and the dwindling number of professional taxonomists working on aculeate hymenoptera. The task is especially daunting, since many bees are small, dark in color and cannot be identified in the field without the use of voucher specimens and subsequent microscope time back at the laboratory. Funding is required to produce popular guidebooks and technical monographs on U.S. bees along with relational databases for major collections (such as the one being developed by the USDA-ARS Bee Systematics Laboratory at Utah State University in Logan, UT). Dedicated websites with photographs and keys to common bee genera would also prove invaluable to an increased public understanding of bees and their crucial role in food and fiber production. Excellent has been done in Europe (England and Germany) for bees and there is no reason we cannot have U.S. publications of the same quality, depth and regional coverage.

Further, there is a desperate need for pollinator site and regional/state lists, especially for native bees, in different areas of the United States. This has been done in many localities for butterflies and we could adopt this, as a model for research and publication needs for our native bees. There are still many undescribed bee species and much of the taxonomic literature is outdated and not readily available to conservation biologists, student and faculty researchers and land managers. To alleviate some of these problems, the American Museum of Natural History and the Smithsonian Institute will sponsor a workshop (September, 1999) on bee identification and ecology for conservation biologists and land managers at the Southwestern Research Station in Portal, Arizona. It is our hope that this will become an annual event and result in the training of competent "para taxonomists" and somewhat ease the burden for the few underpaid and often unacknowledged bee taxonomists at universities, government laboratories and private museums. Much additional long-term funding will be needed to ensure that enough taxonomists, modern generic revisions and illustrated bilingual keys are available to those needing to identify bees and other pollinators. Ambitious interagency and interorganization projects should be established immediately and new sources of private and federal funding secured to ensure that new bee taxonomists are trained and that the creation of user-friendly illustrated keys (both hardcopy and multimedia) will be available to everyone interested in a timely fashion.


We wish to thank the University of Arizona School of Renewable Natural Resources Advanced Resource Technology (ART lab) and Douglas Richardson of Baker Geo-Research (Billings, MT) for GPS and GIS support of our migratory pollinators Arc-Info-based research. Further support and ongoing collaborations came from the Arizona-Sonora Desert Museum (Tucson, AZ) and especially from Gary Paul Nabhan for his inspiration with the Migratory Pollinators efforts and the Pollinator Conservation Consortium (formerly known as The Forgotten Pollinators Campaign). We thank Theodore Fleming (University of Miami) and William Calder (University of Arizona) for their outstanding research with nectar-feeding bats and hummingbirds, respectively and for joining the ASDM in its conservation efforts with migratory pollinators. We thank our funders: Robert E. Turner III, The Turner Foundation, the Turner Endangered Species Fund, the National Fish and Wildlife Foundation, the USDA-ARS and the Arizona-Sonora Desert Museum.


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Author Information:

Stephen L. Buchmann is an Research Entomologist at the USDA-ARS Carl Hayden Bee Research Center located at 2000 E. Allen Rd., Tucson, AZ, 85719. He is also an adjunct faculty member at the University of Arizona, Tucson in the Departments of Entomology and of Ecology and Evolutionary Biology. Steve may be reached by telephone at (520) 670-6380 extension 140, or by e-mail at

Michael R. Kunzmann is an USGS-BRD Ecologist at the Cooperative Park Studies Unit located at The University of Arizona, Tucson, Arizona. Correspondence may be sent to 125 Biological Sciences East, University of Arizona, Tucson, AZ 85721. Mike may also be reached by e-mail: or by telephone at (520) 621-7282.

Arthur James Donovan is a Graduate Student in the University of Arizona’s Water Resources Department. Jim can be reached via his University e-mail at or by telephone at (520) 792-9591.

Royden J. Hobs is a Research Associate in the University of Arizona’s School of Renewable Natural Resources. Correspondence may be sent to 125 Biological Sciences East, University of Arizona, Tucson, Arizona, 85721. Roy may also be reached by telephone at (520) 621-1174 or by e-mail at