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.


A. 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.81Despite 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