Bees and other insect pollinators are beset by the same environmental challenges as other species, including habitat loss, degradation, and fragmentation; non-native species and diseases; pollution, including pesticides; and climate change.
Habitat Loss, Degradation, and Fragmentation
Much pollinator habitat has been lost to agriculture, resource extraction, and urban and suburban development. Although these land uses can provide floral resources and benefit some pollinators, many bees and butterflies are habitat-specific, and the loss of habitat that provides sites for overwintering, foraging for pollen and nectar, or nesting can be detrimental to these species.
Habitat degradation, the decline in habitat quality, is another serious concern. For example, the loose, friable soil required by ground-nesting bees may be trampled by heavy foot traffic or the use of off-road vehicles. In cities, ground-nesting species may be particularly limited due to the large amount of landscape that has been covered with concrete or other impervious surface.
Many pollinators are adversely affected when large, intact tracts of habitat are broken up into smaller, isolated patches by road construction, development, or agriculture. These habitat fragments may not be large enough to meet all pollinator needs by themselves. Establishing and maintaining connectivity—safe passage among patches—is key to pollinator persistence in these areas.
Non-native Species and Diseases
Plants or animals brought here from other places can decrease the quality of pollinator habitat. When non-native shrubs such as autumn olive and multiflora rose take over open fields, they crowd out the wildflowers needed by certain butterfly and bee species for pollen, nectar, or larval food. For example, Japanese barberry shades out native spring ephemerals like Dutchman’s breeches, which provide food for early spring bumble bees. Some non-native plants also attract pollinators away from native species that are superior food sources. West Virginia white butterflies sometimes lay their eggs on non-native garlic mustard instead of native toothwort, for example, and the young caterpillars fail to thrive. In other instances, non-native species can compete with native plants or animals for resources—in fact, in some habitats European honey bees have been shown to compete with native bees for pollen and nectar.
Introduced parasites and diseases are still another threat to pollinators. Thus far, the effects of these parasites have been species-specific, including the mite and virus species that have severely compromised honey bee colonies.
Thus far, 16 species of non-native bees have been documented in New York City, including the European Honey Bee. It is unclear if many of them are having a negative effect on native bees. Surveys indicate that populations of these non-native bees currently are not very large in most locations (although there are some exceptions), suggesting that right now they may not be having a major impact on native bees. However, given the history of introductions that subsequently cause major ecological damage, it is important to carefully monitor populations of introduced bee species.
Although they were intended to be beneficial at the time, some non-native organisms introduced as biocontrol agents have caused problems for native species. For example, non-native lady beetles introduced to this country for aphid control have eliminated New York’s state insect, the nine-spotted lady beetle. These introduced lady beetles have even been found to feed on Monarch butterfly caterpillars. Since the 1950s, the tachinid fly Compsilura concinnata, introduced in the early 1900s to control gypsy moths, has likely contributed to the regional decline of the large sphinx moths that pollinate flowers such evening primrose and certain orchids.
It’s important to note that overabundant native species can also be a problem for plants and animals in the metropolitan region. White-tailed deer numbers have increased over the years, due in part to the fragmentation of forest habitat into the open areas they prefer. Deer browse removes understory vegetation that caterpillar larvae and other pollinators depend upon. For example, deer often preferentially feed on native shrubs such as spicebush, the host plant of the spicebush swallowtail caterpillar, while avoiding thorny non-native bushes like Japanese barberry.
Pollution, Including Pesticides
Air pollution is a very real problem for bees and other pollinators that rely on scent trails to find flowers. Light pollution can harm moth pollinators by increasing their susceptibility to predation by bats or birds when they are attracted to artificial lights at night.
Pesticide misuse and drift from aerial spraying are a major threat to insect pollinators, especially spraying with so-called persistent chemicals that remain in the environment for a long time before degrading. Systemic insecticides applied to seeds can contaminate the pollen grains that are an essential source of food for bees and their young. Pesticides often kill directly, but sub-lethal amounts can also be detrimental to bees and other pollinators by impeding their ability to navigate or forage. (See this 2012 report, Are Neonicotinoids Killing Bees for the latest information about this class of pesticides.) The use of herbicides that eradicate important forage plants for bees and other pollinators is an additional problem.
Studies predict that climate change will alter the close relationship between insect pollinators and the plants that depend upon them for reproduction. Flowering plants migrating north or to cooler, higher elevation habitat in response to warming temperatures or other changes may not move in sync with their pollinators. The composition of pollinator communities is expected to change. According to the Xerces Society, anecdotal observations have found that bumble bees adapted to cooler temperatures are in decline, while bumble bees adapted to warmer temperatures are expanding their ranges northward. What effect this will have on local plants is unknown.