Pollination: A Vital – and Threatened – Ecosystem Service

 What are Ecosystem Services?

Ecosystems are composed of organisms and the physical environment that they inhabit. The continuous interactions between them create the conditions and processes through which ecosystems function. It is these conditions and processes that help to sustain life and enrich the planet’s biological diversity. They also provide critical services, such as water purification, air cleansing and recycling, flood control, natural pest control, medicinal remedies, and pollination, upon which human sustainability depends. Without these and a whole host of other ecosystem services, human society could not be supported.[1]

 

Natural ecosystems seem to provide ecosystem services for free. As a result, it is easy and not uncommon to take these services for granted–at least until ecosystems stop providing them. When a large area of forest is clear-cut, for example, it may no longer be able to absorb sufficient amounts of water to prevent flooding and mudslides during periods of heavy rain. Because ecosystem services are often poorly understood, they also tend to be undervalued. Pollination is an example of an ecosystem service that is often overlooked—and greatly undervalued.

 What is Pollination?

Pollination is central to successful reproduction in most plants. Simply stated, it is the transfer of pollen grains from the anthers of one flower to the stigmas of the same or another flower.[2] Some plants are able to pollinate themselves or are wind-pollinated, but most depend on insects, birds, bats, and other organisms, collectively referred to as pollinators, to transport the pollen for them. The coevolution of pollinators and the pollination process is one of nature’s unique solutions to the dilemma of sexual reproduction among stationary plant organisms.

 Modes of Pollination

The transfer of pollen is a vital process for reproduction in the majority of plant species—flowers that are not pollinated are simply not able to produce fruits—but moving pollen is not a simple task for plants, which, quite literally, are rooted to the spot. Movement of pollen between flowers on separate plants is called cross-pollination. Movement of pollen within a flower or between flowers on the same plant is called self-pollination. Self-pollination will produce seeds and fruit but the mixing of genes that result from cross-pollination is necessary to maintain a healthy plant community. In some cases self-pollination can lead to a condition called inbreeding depression in which breeding between close relatives may result in a reduction in genetic diversity, and the possible expression of negative traits in the population and a loss of ability to evolve in response to a changing environment.

 

A significant number of plants, particularly grasses, conifers, and oaks, rely upon the wind to spread pollen. Some of the world’s most important food crops, such as corn, wheat, and rice, are wind pollinated. A few even use water, such as horned pondweed, hornwort, and some species of water starwort. However, the great majority of plants, more than 70 percent of species, depend on insects, birds, bats, and other animals to transport the pollen for them.

 

The coevolution of plants and their pollinators and the resulting pollination process ensures precise transfer of vital genetic material between flowers of the same species. In contrast, wind pollinated plants release vast quantities of dust-like pollen grains of which only a few reach their target.

 Plants and Their Pollinators

The relationships between flowering plants and their pollinators have evolved since the early Cretaceous period, some 140 million years ago.[3] [4]These relationships are usually mutually beneficial to both parties: flowering plants produce nectar, a highly nutritious sugar-based substance that has no direct benefit for the plant but is a critical source of energy for pollinators, and in return, pollinators assist in the reproduction of plants by transporting pollen. In some cases, however, this association is not mutually favorable. For example, many beetles eat the pollen instead of transferring it to another flower. In addition, some pollinators have become adept at “nectar robbing,” taking nectar from flowers without passing the anthers of the flower where pollen is located.[5] [6]

 

There are also differences in how effective pollinators are at transferring pollen. Bees, for example, are generally recognized as the most important single group of pollinators. There are three reasons for this. First, bees are the primary flower-visiting insects that actively collect and transport pollen (the exception is a small group of wasps, the family Masaridae, that also collects pollen). Other insects forage on flowers for nectar or pollen or eat parts of the flower and make contact with pollen only by chance. Second, bees are central-place foragers, consistently foraging in the same area around their nest, so plants within that area get many visits. Finally, bees visit a single species of flower during a foraging trip (a habit known as flower constancy), ensuring that pollen is efficiently and accurately moved.

 

By contrast, all but a few butterflies gather only nectar as they forage across the landscape, visiting any suitable flower. An interesting exception are some tropical Heliconine butterflies that collect balls of pollen from which they extract amino acids by regurgitating nectar onto them.[7] Flies and beetles generally only take nectar, although some will also eat the pollen (a good source of protein and nutrients) or even parts of the flowers themselves. They also visit flowers to search for mates or to sunbathe for warmth and in doing so may make contact with pollen. The abundance of flies and beetles and their high levels of flower activity mean that although pollen transport is by chance and may only be a few grains at a time, they are valuable pollinators. This is particularly true for flies in colder (alpine or arctic) regions, where they may be more important than bees.[8]

 

Although mutually beneficial, plant-pollinator interactions differ greatly in the degree of dependency that exists between the plant and pollinator species. The vast majority of flowering plants depend on multiple pollinators to fulfill their reproductive needs. This is often reflected in the shape of the bloom or the accessibility of the nectar.[9] Plants in the families Asteraceae (aster) and Apiaceae (carrot) are examples of flowers that have accessible nectar and thus many pollinators. Flowering plants benefit by attracting many different species of pollinators because the number of pollinator visits is high, and one pollinator species can take the place of another should populations fluctuate or decline. There are, however, drawbacks to this type of relationship. For example, some pollinators are more efficient than others, and by attracting generalist pollinators, flowering plants risk getting visits from less effective pollinators. In addition, plants that use generalist pollinators have to compete with one another for these pollinators.[10] [11]

 

Some flowers, such as lupines (genus Lupinus) and larkspur (genus Delphinium) rely on a few, specialized pollinators. These plants have complex blooms with the nectar or pollen hidden away and accessible only to, for example, a hummingbird or bumble bee.[12] African water lilies (family Nymphaeaceae) are mainly pollinated by rhinoceros beetles (Scarabaeidae).[13]

 

Bat pollinated flowers tend to have several distinctive features. They are usually large, firm, and wide mouthed, shaped for a bat to push its head past the pollen bearing anthers and reach its long tongue down to the nectaries. Bats are thought to be color blind, and bat pollinated flowers are generally dull colored and nocturnal blooming, with strong fruity odors. Saguaro (Carnegiea gigantea) and organpipe cactus (Stenocereus thurberi) flowers are good examples of these features.

 

A much smaller group of flowering plants is wholly dependent on a single species for pollination services. An example of this specialized (obligate) relationship exists between the yucca plant (family Agavaceae) and their yucca moth (family Prodoxidae) pollinators. Highly specialized plant-pollinator relationships are ideal for both parties because pollination efficiency is high and competition for food resources among pollinators is low. Yet this type of relationship makes both the plant and the pollinator vulnerable to fluctuations in their counterpart’s populations.[14] Figs are another group of plants for which the wasp pollinators are typically highly specific.

 Generalist and Specialist Pollinators

The pollinators themselves can also be categorized as generalist or specialist according to their foraging habits. Most pollinators are generalists, happy to take nectar or pollen from wherever they can reach it. Most bees are generalists, as are the majority of flies, beetles, and butterflies.

 

Examples of specialist pollinators can be found in both flies and bees. Specialized flies tend to be adapted to drink nectar from particular flowers. The tangle-veined fly Moegistorhynchus longirostris, found in southern Africa, has a proboscis three times the length of its body—a world record for fly tongue length. This extreme adaptation allows it to drink from Lapeirousia anceps, a plant in the lily family that has a long thin corolla.[15] Specialist bees, in contrast, are specific about the flowers from which they collect pollen, and not where they drink nectar. An example is the aptly named squash bees (genus Peponapis), which collect pollen from flowers in the squash family. More choosy again are those bees that collect pollen from only a single species of plant. In the Virgin Basin of southwestern Utah, the Mojave poppy bee (Perdita meconis) gathers pollen only from the bearclaw poppy (Arctomecon humilis).[16]

 

Whether specialists or generalists, pollinators are essential to the survival of over 70 percent of the 250,000 flowering plants species on the planet today.[17] The availability of pollinators is as important as moisture, sunlight, and soil fertility to the reproductive success of nearly half the world’s flowering plants.[18]

 Pollinators and Human Society

Pollination is also vital to the well being of humans. The most obvious example of our link to pollination is through agriculture. Pollination services by managed honey bees and native pollinators are a key component of the seed, fruit, and fiber yields of the crops that we eat and wear. Almost all fruit and grain crops require successful pollination in order to produce the harvested crop. While it is true that some very important agricultural crops, such as canola, corn, and wheat, are self- or wind-pollinated, the majority require the services of pollinators. In fact, pollinators are important for more than 150 food crops produced in the U.S. including apples, alfalfa, almonds, blueberries, cranberries, kiwis, melons, pears, plums, and squashes.[19] [20]

 

Unfortunately, many people are simply not aware that pollination services are critically important for maintaining both healthy, diverse ecosystems and human sustenance. A survey showed that three-quarters of the visitors to an exhibit about pollination related pollen to allergies, but did not recognize its role in plant reproduction.[21]

  Who are the Pollinators?

A. Native Pollinators. Pollinators comprise a significant portion of the total diversity of species on this planet. In fact, between 200,000 – 300,000 invertebrate species—such as butterflies, beetles, moths, flies, and bees—are estimated to serve globally as pollinators.[22] [23]A good estimate for vertebrates species is around 2,000, including birds, mammals, and reptiles.[24] A few examples of pollinators around the world include:[25] [26] [27]

• The monarch butterfly and the bumble bees are common pollinator species enjoyed in gardens across the U.S..
• Male mosquitoes are effective pollinators of many plants worldwide, including rare orchids in the peat bogs of America’s upper Midwest.
• The bumble bee hummingbird is slightly larger than a bumblebee and can hover in front of a flower by beating its wings up to 100 times a second while removing nectar.
• The white-winged dove and lesser long-nosed bat both serve as pollinators to the saguaro cacti, the massive cacti synonymous with images of the southwest U.S..
• The hawkmoth from Madagascar, with its 12-inch proboscis, can reach into the deep floral tubes of some orchids, where others cannot, to obtain nectar.
• The flying fox, a giant bat from Southeast Asia with wingspans up to five feet, is one of over 300 bat species that serve as pollinators wor

 

Native pollinators maintain the health of ecosystems and habitats on which many other creatures rely. They also are important, but often overlooked, crop pollinators. Native bees, butterflies, moths, and flies are at least equally and in many cases even more adept than honey bees at pollinating the 100–150 major U.S. crops requiring pollinators.[28] Recent research conducted in California’s Central Valley has demonstrated that when adequate habitat remains, native bees can meet the needs of even those crops with a heavy pollination demand such as watermelon. This pollination is not provided by a single species of bee but by a suite of over 30 species that give year-on-year security despite natural population fluctuations.[29]

 

On a global scale, at least twenty genera of animals other than honey bees provide pollination services to many of the world’s 100 most important crops. Collectively, these species pollinate at least as many crop species as do managed honey bee colonies.[30] For example:

• In the U.S., wild-living native bees pollinate many crops that are ineffectively pollinated by honey bees including blueberries, cranberries, eggplants, kiwi fruits, and tomatoes.[31] [32]
• On oceanic islands, bats pollinate many economically important plants such as wild bananas, agave, durians, and several species of eucalyptus and palms [33], as well as most of the rainforest canopy trees.
• Biting midges are the primary pollinators of cocoa.[34]
• Most of the world’s 750 fig species rely on wasps for Figs are a critical resource for both people and animals living in many tropical forest communities. In some areas, figs may constitute as much as seventy percent of the diet of vertebrate species.[35]

B. Managed Bees. Honey bees (Apis mellifera) are often the first things that come to mind when someone mentions pollinators. This common association is a likely consequence of the honey bee’s prevalence in agriculture, and its regular appearances on nature programs, even in children’s books. Honey bees are not native to North America; they were brought here by European settlers in the seventeenth century to provide honey and wax, two important products much valued by households of the time.[36] The honey bee’s ability to adapt to new environments means that feral (naturalized) colonies can be found pollinating flowers and scouring for food in most backyards, gardens, and wild areas throughout the continent. The rise in importance of managed honey bees as crop pollinators occurred during the twentieth century for two reasons, 1) improvements in the hive and its management that occurred in the nineteenth century [37], and 2) the decline in native bees due to habitat loss and pesticide use .[38] Pollination is now big business: since 1992, over one million honey bee colonies have been rented from commercial beekeepers yearly for pollination of agricultural crops in the U.S..[39]

 

There has been recent concern among the agricultural industry as researchers have accumulated evidence showing that honey bee colonies are seriously threatened in the U.S.. Managed and wild colonies declined by twenty-five percent during the 1990s and by over fifty percent since the 1940s. Available data show that in 1947 there were 5.9 million managed colonies in the U.S.; in 2005, there were 2.4 million.[40] [41] Information is less clear for feral honey bees, although it is believed that their numbers may have fallen by 75 percent or more in the last thirty years.[42]

 

Many factors are believed to be responsible for what is being called “Colony Collapse Disorder”.  Four broad categories of potential causes currently being studied are: pathogens; parasites; environmental stresses, which include pesticides and extreme weather conditions; and management stresses, including nutrition problems, mainly from nectar or pollen scarcity.[43] Many experts suggest that these declines illustrate the danger of our heavy reliance on a single species for most of our pollination needs.

 

An increasing number of other bees are being managed for crop pollination in North America. This is partly in response to the troubles facing honey bees and partly because honey bees are not the most efficient pollinator for all crops. The blue orchard bee (Osmia lignaria), for example, is widely used to pollinate apple and cherry orchards. Pollination of an acre of apples may require as few as 250 of these solitary-nesting bees, a job that would require up to 2.5 honey bee hives (c. 50,000 bees).[44] Other bees are important pollinators of particular crops. The alfalfa leafcutter bee (Megachile rotundata) and the alkali bee (Nomia melanderi) pollinate alfalfa in western North America. Bumble bees (genus Bombus) are used in glasshouses for tomatoes and other Solanaceae crops that were previously hand pollinated. Of course, these managed bees are not without their own problems. The movement of captive reared bumble bee colonies appears to have spread the Nosema pathogen to native Bombus occidentalis in the western U.S. leading to a steep decline range wide[45] [46] and the possible extinction of two other species (NRC report ⁴¹).

 

 

Valuation of Pollination Services

Putting a dollar figure on the value of pollination services may be important, if, as many researchers argue, financing conservation by determining economic value is an effective method for protecting ecosystem services. Yet, as a result of our incomplete understanding of the entire complex process, the exercise has proven to be exceptionally challenging—how does one put an accurate price on a service that is a cornerstone for life on earth? As a result, attempts to create a mechanism for identifying an economic value for pollination services have been scarce in the U.S.[47] This section lays out those existing economic arguments for valuing pollination services and then presents some additional indirect value considerations.

 

A. Economic Considerations.  Worldwide, at least thirty percent of 1500 crop plant species depend on pollination by bees and other insects.[48] [49] Historically, the U.S. agricultural industry has depended heavily on the honey bee for its pollination needs. Consequently, most of the few existing studies that evaluate the economic importance of pollination services focus on agriculture and the honey bee. For example, Morse and Calderone estimate the value of agricultural crop production due to honey bee pollination was $14.6 billion in 2000.[50]  Morse and Calderone recognized that native pollinators made a significant contribution to crop values, but did not attempt to estimate this figure. A more recent study by Losey and Vaughan put a value of just over $3 billion on the pollination of U.S. fruits and vegetables by native insects.[51]

 

A decline in pollinator activity is, in fact, not a hypothetical scenario. Decreases in services have already caused problems for some crops. For example, in early 2004, California almond growers realized there were looming honey bee shortages and scrambled to get sufficient hives for their crop. Despite preparing for the 2005 bloom, serious bee shortages forced almond growers to pay rental fees of upwards of $100/hive and even to import honey bee colonies from Australia in order to save their $2.5 billion crop.[52]

 

The significance of pollinators to the agricultural industry is not the only aspect of pollination services that has economic value. In fact, pollination services can be linked to many other parts of present day economies. For example, most flowers use size and color to attract pollinators and humans have placed a high value on their uniqueness, beauty, and aroma. As a result, the production of cut flowers and potted plants for the florist trade, and use of plants for perfumes, shampoo and other cosmetics have developed into multinational industries that rely to some degree on the services of pollinators. The pharmaceutical industry, cattle grazers, and people throughout the U.S. with small gardens in their backyards are also dependent upon and realize economic benefits from pollinators.[53] [54] In the Western U.S., federal agencies are using wildflowers for restoration and rehabilitation of degraded lands. Wildflower seeds are traditionally harvested in the wild, an expensive and time consuming process. The USDA Agricultural Research Service laboratory at Logan, Utah has been developing management techniques for commercial production of wildflowers using native bees. Production has already begun in several states.[55]

 

 

1. Noneconomic Considerations. Identifying economic value is not the only means for conveying the significance of pollination services and the relationships between pollinators and plants. With well over 200,000 flowering plant species dependent on pollination from over 100,000 pollinator species, pollination interactions have been a catalyst in developing, and are important to maintaining, the vast wealth of biodiversity on the planet. Consequently, pollination is a keystone process in both human-managed and natural terrestrial ecosystems. Without this service, many interconnected species inhabiting, and processes functioning within, an ecosystem would collapse.[56] Flowers also produce the seeds and fruits that constitute the diets of many animal species. Pollinator declines can limit seed and fruit production and disrupt food supplies in natural communities. Pollinator-dependent plant communities help to bind the soil, reducing erosion that fouls creeks and impacts habitat for a wealth of aquatic life from salmon to mussels. Finally, pollinators have only recently been acknowledged for their contribution as consumers and distributors of energy-rich floral biomass. One study found that harvesting of plant primary production through collection of pollen, nectar, and resin by stingless social bees in Panama is greater than that of leafcutter ants, game animals, frugivores, vertebrate folivores, insect defoliators (excluding ants), and flower-feeding birds and bats.[57]

 

 

 Pollinators in Decline – Causes

Examples of localized pollinator declines or disrupted pollination systems have been reported on every continent except Antarctica.[58] Hundreds of pollinator species, primarily vertebrates, are on the verge of extinction. In the U.S. alone, there are fifteen vertebrate pollinator species listed as endangered. The Fish and Wildlife Services maintains a list of species protected under the U.S. Endangered Species Act (http://endangered.fws.gov/wildlife.html.)

 

The conservation status of insect pollinators is less well known. The Xerces Society for Invertebrate Conservation reviewed the status of butterflies and bees of North America.[59] The Red List of Pollinator Insects of North America includes dozens of butterflies and bees that are facing significant threats and population declines that are not listed under the U.S. Endangered Species Act or the Canadian Species at Risk Act. In Britain, the United Kingdom Biodiversity Action Plan has identified over three dozen pollinator insects in need of urgent conservation efforts, including five of the nation’s twenty-five bumble bee species (http://www.ukbap.org.uk/default.aspx).

 

Despite these advances in our knowledge, the comparative lack of information on insects, compared to birds or mammals, that is accessible to decision makers concerns many scientists, because insects are by far the largest category of pollinators. Yet, due to their small size and inconspicuous nature, declines in insect species can go unnoticed until they approach local extinction. While species of native pollinators that visit agricultural crops are well documented, researchers are continually surprised as studies of native plants reveal new insects as pollinators. Unfortunately, these plant and pollinator species appear to be declining at a far greater pace than scientists are identifying their relationships.

 

The growing evidence of localized declines of pollinators is a cause for concern. The National Academy of Sciences noted that declines in many pollinator groups are associated with habitat loss, fragmentation, and deterioration; diseases and pathogens; and pesticides (Status of Pollinators in North America, NRC 2006).

The resulting impact on pollinator-dependent flowering plants could be devastating. In fact, the World Conservation Union predicts that 20,000 flowering plant species will disappear in the next few decades. While pollinator declines are not the sole cause of these plant extinctions, and few plant-pollinator systems are absolutely obligate between two species, large-scale losses of either flowering plants or pollinators are likely to result in cascading declines within both groups. (For an example of cascading declines, see case study, “Fig Trees and Fig Wasps in Tropical Forest Communities”.) Animal species dependent upon fruit and seeds for forage may be negatively impacted as well.[60] [61]

 

At present, there is not enough information available to predict the severity of the ongoing disruption to pollinator activity, yet the potential for significant and irreplaceable losses of biodiversity through cascading extinction is very real. And although the notion of a global disruption in pollination systems is not currently supported by empirical evidence, it is suspected that the well-documented localized declines are symptomatic results of the more wide-scale losses in biological diversity. It should come as no surprise that significant causes of both declines are often very similar: habitat loss, fragmentation and modification; agricultural and grazing practices; pesticide use; and the introduction of nonnative species.[62] [63] The following sections look at how each of these issues affects pollinators.

 

1. Habitat Loss, Fragmentation and Modification. As within the larger context of global biodiversity, habitat loss and fragmentation are the biggest problems for pollinators. Although research in this area is limited, experts increasingly recognize the dependence of wild pollinator populations on appropriate habitat. A review of data on pollinators and fragmentation concluded that as habitat area decreases, abundance and diversity of insect pollinators also decrease.[64] The ongoing loss of suitable habitat for pollinators in the U.S. due to sprawl and related land use changes intensifies this problem. Recent evidence suggests that global climate change could have a serious detrimental effect on flowering plant species and their pollinators.[65] [66] [67]

Habitat loss and fragmentation affect pollinators in two ways. First, pollinators have basic food requirements. The availability of a variety of native plants is important because not all pollinators can gain access to the nectar found in introduced flowers. Pollinators also depend on the availability of various flowering plants throughout a season.[68] Habitat loss can negatively affect the timing and amount of food availability, thereby increasing competition for those limited resources.

 

Loss of habitat can also disrupt the nesting or egg-laying requirements of pollinators. For example, some caterpillars are like the endangered Karner Blue, Lycaeides melissa samuelis, which feeds only on wild lupine (Lupinus perennis).[69] Most bees also have specific conditions for nesting, such as bare soil or beetle-riddled snags.[70] [71] Development pressures from human activity and land management methods decrease the availability of caterpillar host plants, remove suitable bee nesting habitats, and modify the remaining habitats in other ways across the landscape.

 

Changing landscapes may also introduce positive features for pollinators. The compacted soils of roadsides can be favored by ground-nesting bees and wasps, wooden buildings and fences provide nest sites for other bees, and gardens and parks can offer foraging or butterfly egg-laying sites, [72] although these benefits likely do not outweigh the losses of natural habitat from other human activities especially with rare or specialist pollinators.

 

Whereas habitat loss can seriously impact all pollinator organisms, increased fragmentation of habitats is particularly troublesome for those pollinators that travel great distances. Migratory pollinators, such as the monarch butterfly, the rufous hummingbird, and the lesser long-nosed bat, travel hundreds or thousands of miles each year as the seasons change. These trips require high levels of energy, and it is critical for the migrants to have consistent food resources all along the way. Fragmentation of habitat increases the distance between suitable food and shelter sites along migratory routes, thereby disrupting the journey. Some scientists believe that if fragmentation continues at its current rate, many migratory corridors will soon be closed.[73]

 

B. Agricultural and Grazing Practices. In addition to development pressures that result in habitat loss and fragmentation, modern agricultural practices have increasingly made farms a poor habitat for wild pollinators. Throughout the U.S. monoculture plantings, the removal of fencerows and buffer strips to maximize growing areas, and the use of hybrid seeds are common practices on farms. Monoculture farming and the removal of buffer strips reduce suitable habitat for wild pollinators. A study of the margins of agricultural fields pointed out that small areas with native flowering plants, such as fencerows, could be effective in attracting and maintaining stable pollinator populations. Cumulatively, today’s agricultural practices not only disrupt wild pollinator activity, but they also increase our dependence on costly managed honey bee colonies.[74] [75]

Grazing is also a threat to pollinators. A study of grazing practices in California found evidence of sheep removing pollinator food resources, destroying underground nests and potential nesting sites, and direct trampling of bees.[76] This evidence of pollinator disruption is exacerbated by the notion that sheep, cattle, and other grazing animals depend on insect-pollinated legumes, such as alfalfa and clover, for forage.

 

C. Pesticides. Heavy reliance on a broad spectrum of pesticides by both the agriculture industry and individual homeowners poses yet another major threat to pollinators. Insecticides affect pollinators directly through unintentional poisonings, and herbicides affect them indirectly through a loss of insect forage and other wildflowers important in maintaining some insect populations.[77] [78] While a significant hazard to all pollinators, the increased dependence on pesticides is particularly problematic for managed honey bees whose exposure is greater due to their use as crop pollinators. Despite efforts to raise awareness among farmers, beekeepers continue to report many pesticide and herbicide poisonings of honey bees each year. The physiological impacts of pesticides on native and honey bees are fairly well known (see case study) but the effect on agricultural production is less well-known. One study found that loss of pollinators following application of the organophosphorous pesticide Fenitrothion resulted in blueberry crop yields in New Brunswick, Canada significantly below those of neighboring Nova Scotia and Maine (see case study).[79]

 

Even when applied as regulated, pesticides undeniably create significant hazards for pollinators. Unfortunately though, it is too often the case that pesticides are overused and applied carelessly, reaching unintended areas and exacerbating their impact. For example, in the case of aerial applicators, factors such as wind and human carelessness can greatly influence the actual coverage area of an applied pesticide, jeopardizing pollinators inhabiting areas within and adjacent to agricultural fields.[80] This problem emphasizes the importance of buffer strips in agricultural areas, not only as a critical habitat for pollinators, but also as protection from pesticide oversprays.

 

D. Introduced Species. For hundreds of years nonnative species, including plants, mammals, insects, and pollinators, have been introduced both intentionally and inadvertently to new habitats. In some cases the effects are beneficial or benign, but introduced species can also have serious effects on their new ecological systems. The most prevalent example of an introduced pollinator is the European honey bee, which has been imported to virtually every corner of the world.[81] Despite its well-documented benefits to commercial agriculture, there is evidence that the honey bee has disrupted native pollination systems.[82] Through competition for floral resources, honey bees reduce the abundance of native pollinators. Native species, which have often co-evolved with local plant species, are in many cases more effective pollinators of crops and native wildflowers than the exotic honey bee.[83] [84] Introduced pollinators can also disrupt the reproduction of native plant species and facilitate the spread of invasive plants. For example, the fig wasp was introduced into California at the beginning of the twentieth century. Its introduction caused some existing nonnative fig trees to produce fruit and spread as pests throughout the region.[85]

 

Pollination activity is also disrupted by other introduced insects and mammals. In Hawaii, native bees, moths, and the majestic but highly endangered silversword plant are at risk of extinction from the introduced Argentine ant. The spread of wild pigs onto the Hawaiian Islands has also destroyed critical habitat for endangered flowering plants and their pollinators, including the crested honeycreeper.[86]

 

Introduced pathogens and parasites cause significant declines in both managed and native bee populations in North America. Honey bee colonies, both managed and feral, are being devastated by the external parasitic mite Varroa destructor that was introduced to the continent.[87] Similarly, the protozoan pathogen Nosema bombi caused great problems for reared colonies of the bumble bee Bombus occidentalis and has apparently lead to the wide scale declines of native B. occidentalis across the West Coast and also to declines in other bumble bees in the subgenus Bombus, particularly the eastern species B. affinis.[88] [89]

 

 

Pollinators in Decline – What can be done?

Threats to pollinators are pervasive. Researchers have presented evidence that pollination systems have been disrupted and some pollinator populations are declining. Clearly, measures must be taken to document the actual extent of pollinator declines, especially among the poorly studied native insect pollinators. Concurrent steps should be taken to avert a potential pollination crisis.

 

A. Policy and Management. While no national strategy currently exists to deal with the pollinator declines, steps can be taken to strengthen and maintain efficient pollination systems. Some of the more notable approaches include:[90]

• Improving agricultural practices and regulations that encourage, for example, targeted rather than broad-spectrum pesticides and the use of buffer strips.
• Restoring habitat and species through effective land use planning policies and adaptation of existing farm support programs.
• Reintroducing native plants and pollinators coupled with the removal of alien pollinator
• Valuing native diversity and promoting native gardens.

 

B. Research Needs. Scientific understanding of pollination dynamics and the consequences of diminishing pollinator levels is at best incomplete. Further research is needed to fill gaps in a wide array of pollination issues:

• The relationship between pollinators and plant populations.
• The effects of pesticides, grazing, logging, and suburban sprawl on native and feral pollinator
• The importance of declining pollinator populations and the potential for cascading extinction.
• Identifying pollinators on the World Conservation Union’s endangered species list.
• Competition among native and nonnative pollinator
• Migratory dynamics of pollinator
• Pollinator

 

Research to identify economically important plant-pollinator relationships is also vitally important.[91] [92] [93]

 

 

Conclusions

Pollination is an essential ecosystem service, vital to humans and to the stability of natural environments. This is not an esoteric issue of interest to researchers; pollinator conservation is an issue that has direct links with our everyday lives. The potential loss of pollinators and pollination services poses serious economic and social consequences for humans. Yet the risk remains largely unrecognized. Due to its far-reaching impacts, dealing with a decline in pollination services will require efforts at the local, regional, national, and international levels.

 

We should not underestimate the significance of pollination services or delay steps to ensure their sustainability. As two notable pollination researchers, Carol Kearns and David Inouye, put it: “Predicting the effects of the loss of a particular pollinator is extremely difficult, but it is important to remember that no species exists in isolation. Each is part of an ecological web, and as we lose more and more pieces of that web, the remaining structure must eventually collapse”.[94] Within this context, and given the current global biodiversity crisis in which localized pollinator declines persist, now is the time to raise awareness, stimulate actions, and expedite additional research.

 

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* For a detailed list of cultivated crop plants serviced by pollinators, visit http://gears.tucson.ars.ag.gov/book/index.html

 

Revised June 20, 2008

 

Reference as: Ecological Society of America. 2008. Communicating Ecosystem Services Pollination Toolkit: Science Summary. Updated June 20, 2008. Online at www.esa.org/ecoservices