Literature Review: Pollinator Habitat Enhancement and Best Management Practices in Highway Rights-of-Way

Pollinator services are “central to all human beings, livestock, and wildlife” (Kevan 1999). Plant pollination by insects is one of the most well-known and important ecosystem services and is essential in both natural and agricultural landscapes. An estimated 85 percent of the world’s flowering plants depend on animals—mostly insects—for pollination (Ollerton et al. 2011). Eighty-seven of the world’s 124 most commonly cultivated crops (70 percent) are reliant on animal pollinators, and insect-pollinated forage plants such as alfalfa and clover provide feed for livestock (Klein et al. 2006). Roughly 35 percent of global crop production is dependent on pollination by animals (Klein et al. 2006). Pollinators also sustain wildland plant communities that provide food and shelter for myriad other wildlife and contribute to overall ecosystem health.

The great majority of pollinators are insects, including bees, wasps, flies, beetles, butterflies, and moths (Speight 1978; Allen-Wardell et al. 1998; Jennersten 1988; Frankie et al. 1990; Irvine and Armstrong 1990; Kevan 1999; Westerkamp and Gottsberger 2000; Kearns 2001; Larson et al. 2001), but many bird and bat species pollinate as well (Grant 1994; Valiente-Banuet et al. 2004). Bees are considered the most important group of pollinators for agricultural crops as well as for wild plants in temperate climates (Morse and Calderone 2000; Garibaldi et al. 2013).

The domesticated European honey bee (Apis mellifera L.) is the most widely managed crop pollinator in the United States. Studies indicate that honey bees are important for more than $15 billion in crop production annually (Morse and Calderone 2000; Calderone 2012).

There are also approximately 4,000 species of native bees in North America (Michener 2007), and they are also important crop pollinators (e.g., Tepedino 1981; Bosch and Kemp 2001; Javorek et al. 2002; Winfree et al. 2008; Garibaldi et al. 2013). Native bees are important in the production of an estimated $3 billion worth of crops annually to the United States economy (Losey and Vaughan 2006; Calderone 2012), and emerging research shows that this is likely an underestimate of their total value (Garibaldi et al. 2013).

There is evidence of declines in both domesticated and wild pollinators. The number of honey bee colonies has been in decline over the past half-century because of disease, parasites, lack of floral resources, insecticides, and other factors (National Research Council 2007). Since 2006, beekeepers have experienced record high annual hive losses of 29 percent or more (Bee Informed Partnership 2014), and several species of once-common bumble bees have become rare (Cameron et al. 2011).

Other pollinators are also in decline. Monarch butterfly populations have dropped by 90 percent east of the rocky mountains (Rendon-Salinas and Tavera-Alonso 2014 ) and by 50 percent west of the Rockies (Monroe et al. 2014 ), and other butterfly species have also seen significant declines. NatureServe (a primary source for species conservation data, status, and trends in the United States) has assessed all 800 butterfly species in the United States and has found that 141 (17 percent) are at risk of extinction (NatureServe 2014 ). Twenty-six species of butterflies are listed as threatened or endangered under the Federal Endangered Species Act (U.S. Fish and Wildlife Service 2014 ).

Factors leading to decline include habitat loss, pesticide use, diseases, parasites, and the spread of invasive species. Threats to pollinator communities affect not only pollinators themselves but also natural ecosystems and agricultural productivity.

Threats to pollinators specifically associated with roads include mortality due to vehicle collisions, habitat fragmentation, barriers to movement, effects of roadside vegetation management, exposure to invasive plants, drift from adjacent land, and pollution from vehicles. Despite these threats, roadsides are a conservation opportunity to increase pollinator habitat.

Roadsides can provide habitat for a diverse community of pollinators. Roadside habitat can include forage for food and breeding or nesting opportunities. Roadsides extend across a variety of landscapes and can aid dispersal of pollinators by linking fragmented habitats. By acting as refugia for pollinators in otherwise inhospitable landscapes, roadside habitat can contribute to the maintenance of healthy ecosystems and provision of ecological services such as crop pollination services.