Tropical
and subtropical ecosystems rest on a foundation of floral pollinators. Without
pollinators, such as bees, wasps, moths, flies, beetles, birds, bats, and
butterflies, and the multitude of special relationships between these
pollinators and plants, modern plant communities could not exist. Rather, they
probably would resemble those simple prehistoric communities present many
millions of years ago (Buchmann and Nabhan 1996). Plant communities and
pollinators have evolved over the years to the point that many species of plants
are entirely dependent on pollinators; and virtually all pollinators need
flowers for survival. Any major disturbance of either the plant community or the
pollinators can result in a decline, perhaps catastrophically, of the other
group, with a possible cascading effect reaching to the animals dependent upon
the plants directly for food, or indirectly by supporting prey.Pollinators can be classified into two broad general categories: specialists and generalists. Specialists are adapted in structure, behavior, and periods of activity to exploit efficiently one, or a few, species of flowering plants. Often a plant is also highly dependent on its specialist pollinators to be fertilized and set seed. Generalist pollinators visit a wide variety of flowers and usually are not dependent on any one species for their success. Generalist pollinators, like specialists, typically pollinate the flowers they visit, but they usually are not as efficient as specialists, and sometimes simply rob the floral nectar without pollinating the flower.
Honey bees (Apis mellifera) are the ultimate of generalist pollinators. They efficiently exploit much of the floral resources, sometimes to the serious detriment of other pollinators, and can even affect the density and diversity of plant species in the environment Matheson et al. 1996; Sugden et al. 1996). Honeybees are native to Africa and Western Eurasia. Historically, Organ Pipe Cactus National Monument was a complex of subtropical thorn-scrub and other communities that was pollinated primarily by specialist pollinators. Honey bees first arrived in the environment a few hundred years ago with colonizing Europeans, who brought with them their honey bees. These bees escaped and became feral throughout most of the New World, including southern Arizona The effect of these feral European honey bees on the monument is unknown because they arrived before any floral surveys were conducted. There is also uncertainty about their impact because European honey bees do not seem to develop populous and abundant colonies in the harsh climate of the monument. In 1956, a new population of vigorous tropically adapted honey bees, called Africanized honey bees, were brought into Brazil, and began moving northward (Goncalves 1974). Africanized bees arrived in Texas in 1990 (Sugden and Williams 1990; Rubink et al. 1996) and were expected shortly thereafter in Arizona, including Organ Pipe Cactus National Monument. These bees are much more vigorous competitors in tropical climates than their feral European cousins and might adapt better to the climate of Organ Pipe Cactus. If such a postulation is correct, arriving Africanized bees could potentially damage the plant and animal communities of the monument by outcompeting native specialist pollinators and causing their serious decline or extinction (Sugden et al. 1996). This might induce a cascade with the loss of specialist plants in the monument.
The purpose of this long-range investigation was to determine the density and reproductive biology of feral European honey bees in the monument over a variety of climatic conditions ranging from very wet to very dry years and to compare the biology of the feral honey bees in the monument with that of Africanized bees after their arrival. A further goal of this study was to provide a window to view the phenomenon of population replacement as Africanized bees moved into the area and possibly replaced the existing feral European population. The final goal was to determine if the densities of Africanized bees would increase sufficiently to potentially disrupt the current plant-pollinator relationships and adversely affect some species in the monument.
Feral bee populations were surveyed using swarm traps baited with synthetic Nasonov pheromone (Schmidt et al. 1989). The swarm traps were cavities constructed of wood pulp and designed to simulate hollow trees. Nasonov pheromone is used by honey bees during reproductive or absconding swaaning and, in the case of swarm traps, served to attract swarms in the area into the trap (Schmidt 1993). From 1988 through 1994, fifty swarm traps, each placed in the partial shade of a tree at a height of 1-3 m above ground, were positioned in locations along the Puerto Blanco drive, including near Quitobaquito Springs, the Senita Basin road, the Ajo Mountain Drive, and around the residential and sewage treatment pond areas. Thirty traps were placed in xeric areas that contained no permanent or temporary water, 12 were placed in riparian areas that were near water and contained more lush vegetation, and 8 were placed around residential areas that were xeric in terms of plant community, but contained man-made sources of water and nest sites. The traps were checked several times a year, typically in winter before the normal swarming season commenced, and later in the summer after the main swarming season. During each survey, traps were checked for the presence or absence of bees as determined by opening the swarm trap and for the presence of honey comb, which was scored as evidence of a swarm that had inhabited the trap but had died or absconded. Fresh pheromone lures were also placed in the traps during the surveys. Starting in 1993, samples of bees from each attracted swarm were collected in alcohol and later analyzed morphmetrically according to the methods of Daly and Balling (1978) to determine if they were Africanized or European.
Some traps were lost or destroyed between surveys. The final number of recovered traps were 50 in 1988, 1990, and 1994; 49 in 1991 and 1993; 47 in 1989; and 46 in 1992. These small reductions in numbers had little overall effect on the results, other than to change some percentage values in the data. Only European bees existed in the monument until 1994, when Africanized bees arrived. Overall, the number of swarms generated was highly dependent upon the yearly rainfall pattern. Years were categorized as severe drought (<40% normal rainfall, 1990); dry (<80% normal rainfall, 1969); normal (8~ 120% normal rainfall, 1988, 1994); wet (>120% normal rainfall, 1993); very wet (>140% normal rainfall, 1991); and extremely wet (>160% normal rainfall, 1992). During the normal-to-dry years of 1988-90, insufficient floral resources were available in most areas for good reproduction by European bees. Only 19 swarms were attracted during those three years for an average trap attractancy rate of 13%. However, within the different habitats dramatic differences were observed: 16 swarms (44% occupancy) were in traps in riparian areas, whereas only 2 were in traps in xeric areas (2% occupancy) and 1 was in a residential area (4% occupancy). These results indicate that during normal and dry years feral European bees are under such stress that only those colonies in riparian areas are able to obtain sufficient floral resources to grow and reproduce by swarming. Thus, the riparian areas are somewhat shielded from the effects of rainfall and act as refugia for the bees.
During the wet years of 1991-93, the feral European bee population exhibited a dramatically different swarming pattern. During these years, a total of 87 swarms were attracted, including 26 in riparian areas (72% occupancy), 51 in xeric areas (56% occupancy), and 10 in residential areas (42% occupancy). The occupancy rates for the different areas were not significantly different (chi-squared test). Evidently, during these years, enough floral resources were available in all areas for resident feral colonies to expand in population and to reproduce. The advantage of being in a riparian area all but disappeared. Comparisons between dry-normal years and wet years revealed several interesting differences: