CES - Pollination Tool Kit: Pollinators in Decline

Pollinators in Decline — Causes

Examples of localized pollinator declines or disrupted pollination systems have been reported on every continent except Antarctica (14). Hundreds of pollinator species, primarily vertebrates, are on the verge of extinction. In the United States alone, there are fifteen vertebrate pollinator species listed as endangered including the Mexican long-nosed bat from the southwest United States. (The US Endangered Species List can be viewed at endangered.fws.gov/wildlife.html

The conservation status of insect pollinators is less well known. This unknown status concern many scientists, as insects are by far the largest category of pollinators. Yet, due to their small size and inconspicuous nature, declines in insect species can go undocumented 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 are 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 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 (8,27).

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 and fragmentation, agricultural and grazing practices, pesticide use, and the introduction of non-native species (3,14). The following section looks at how each of these issues impacts pollinators.

Habitat Loss and Fragmentation
As within the larger context of global biodiversity, habitat loss and fragmentation are the biggest problem 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 decreases (20). The ongoing loss of suitable habitat for pollinators in the United States due to sprawl and related land use changes intensifies this problem.

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 (13). 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 requirements of certain pollinators. For example, many species of bees nest in underground burrows and in hollowed-out logs. Development pressures from human activity can decrease the availability of suitable nesting habitats.

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 and the rufous hummingbird, travel 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 (29).

Agricultural and Grazing Practices
In addition to development pressures that result in habitat loss and fragmentation, modern agricultural practices have made farms an increasingly poor habitat for wild pollinators. Throughout the United States today, 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 impact wild pollinators by reducing suitable habitat. 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. It is also thought that the use of hybrid crop seeds requires increased pollination services. Cumulatively, today's agricultural practices not only disrupt wild pollinator activity, but they also increase our dependence on costly managed honeybee colonies (13,14).

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 (26). 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.

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 bees and other insect 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 (12,15). While a significant hazard to all pollinators, the increased dependence on pesticides is particularly problematic for honeybees whose exposure is greater due to their use in agriculture. Despite efforts to raise awareness among farmers, beekeepers continue to report many pesticide and herbicide poisonings of honeybees each year. The impact of pesticides on feral honeybees and other wild pollinators is less well-known. However, considering that pesticide use in the United States has doubled in the last forty years, the disruption is likely to be significant as well (16).

Even when applied as regulated, pesticide use undeniably creates a significant hazard for pollinators. Unfortunately, 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 error can greatly influence the actual coverage area of an applied pesticide, jeopardizing pollinators inhabiting areas within, and adjacent to, agricultural fields (4). 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.

Introduced Species
For hundreds of years non-native species, including plants, mammals, insects and pollinators, have been introduced both intentionally and inadvertently to new habitats. In some cases the effects are beneficial, but introduced species can also have serious effects on their new ecological system. The most prevalent example of an introduced pollinator is the European honeybee, which has been imported to virtually every corner of the world (3). Despite its well-documented benefits to commercial agriculture, there is evidence that the honeybee has disrupted native pollination systems (9). Through competition for floral resources, honeybees 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 honeybee (5,13). 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 century. Their introduction caused some existing non-native fig trees to produce fruit and spread as pests throughout the region (14).

Pollination activity is also disrupted by the introduction of other, non-pollinator species. 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 caused the destruction of critical habitat for endangered flowering plants and their pollinators, including the crested honeycreeper (19).