🦇 Pollinators Under Threat 🐝

By Joan McIntyre, Extension Master Gardener

Phyciodes tharos (pearl crescent) butterfly on Melampodium. Photo © Mary Free

Pollinators provide an essential ecological service by moving pollen around so that plants can reproduce and incidentally provide seeds and fruit that feed people and many other animals. They pollinate 75 percent of human food crops, which constitute about a third of the plants we consume and most of the fruits and vegetables we need for a nutritious diet.  Our native wildflowers depend on their services to propagate.  Pollinators and other insects are also a crucial component in the food web with many birds, reptiles, fish, and mammals relying directly or indirectly on insects for food.  Without pollinators and other insects, life as we know it would cease to exist.

Pollinators include honey bees and other bees, moths, butterflies, beetles, ants, bats, and hummingbirds.  Bees are the most efficient pollinators, with pollen sticking to their bodies and then moved from flower to flower.  Bees, particularly native bees, are generally nonaggressive and many do not sting at all.  In some cases, plants have evolved a symbiotic relationship with their pollinators.  Wild ginger (Asarum canadense), with its brownish flower near the ground, evolved to attract small pollinating flies looking for decomposing carcasses when emerging in the spring[1].  Bats pollinate cocoa, mango, banana, and agave[2].


Pollinators and insects in general are under threat  

Based on a review of more than 70 reports documenting insect decline, researchers in a 2019 study estimated that roughly 41 percent of insect species are in decline, twice as high as for vertebrates.  They estimated that around a third of all insects are threatened with extinction in the countries included in the study.  The Illinois Natural History Survey, for example, showed that half of 16 bumblebee species (Hymenoptera) had declined between 1900 and 2007, and 4 species had gone extinct[3].

Research also indicates a relationship between insect declines and declines of other species.  In a Puerto Rico rain forest, researchers reported that steep declines in the biomass of arthropods (insects, spiders, and crustaceans) were paralleled by sharp declines in frogs, lizards, and birds over 36 years, leading them to conclude that arthropod declines were  “indirectly precipitating a bottom-up trophic cascade and consequent collapse of the forest food web.”[4]  Similarly, a 2019 article in Science reported that bird populations fell by almost 3 billion, or about 30 percent, since 1970—birds rely on insects, particularly caterpillars, to feed their young[5].

Researchers are attributing the declines to habitat change, pollution, biological factors, and climate change.  Habitat change resulting from development for housing and transportation, urbanization, and other land use changes have shrunk and fragmented natural habitats.  The shift in agricultural practices in the 20th century toward industrial scale production, monocultures, and widespread use of synthetic fertilizers and pesticides appears to be another key factor.


Landscape practices help create a hostile environment

Hedera helix (English ivy) smothering a tree. Photo © Elaine Mills

Conventional landscape practices consisting primarily of extensive lawns with a few, mostly non-native foundation plants offer little in the way of food and habitat for pollinators.  The domination of non-native plants in our landscapes deprives pollinators of the host plants that they require to reproduce such as milkweed for monarch butterfly caterpillars. Turf grass (also non-native), in particular, has become a monoculture in our landscape, offering none of the nectar or pollen needed to support our pollinators. Many non-native, commonly used foundation plants such as Japanese holly (Ilex crenata), heavenly bamboo (Nadina domestica) Japanese barberry (Berberis thunbergii), and privet (Ligustrum spp.) as well as ground covers such as English ivy (Hedera helix) and creeping liriope (Liriope spicata) have escaped into our natural areas, pushing out native plants needed for pollinators to survive.

Other landscape practices, such as those listed below, can make it downright dangerous for pollinators and undermine the environmental health of our local ecosystems.

  • Use of synthetic fertilizers and pesticides to maintain a green, weed-free landscape or to control annoying pests such as mosquitoes and ticks is harmful to pollinators and also degrades soil fertility and water quality.
  • Heavy use of mulch can smother insects and plants.
  • Cutting back stalks of flowers and grasses and cleaning out leaf litter from garden beds deprives insects of necessary habitat to survive the winter.
  • Impervious surfaces and compacted soil around trees interrupt the natural life cycle of caterpillars that need to drop from trees and burrow into the soil to finish their transformation to butterflies or moths and also contributes to soil erosion and harmful stormwater runoff.
  • Lights used to illuminate areas outside of our homes at night are harming insects, particularly moths – one third of insects you see circling a light bulb will die by morning[6].

Homeowners have an opportunity to restore native habitat for pollinators, birds, and other wildlife.  If only half the 40 million acres now devoted to lawn were converted into more natural landscaping, an area larger than the combined acreage of most of our major national parks could be transformed, creating what Doug Tallamy is calling Homegrown National Park[7].  National Pollinator Week (June 22–28, 2020) is a good time to think about adding even just a few flowering plants that will make your yard more inviting to pollinators.  See Creating an Oasis for Pollinators for tips on what you can do to make your yard an oasis for pollinators.


Related Articles and Publications


Footnotes

[1] Larry Stritch, “Plant of the Week: Wild Ginger (Asarum Canadense L),” U.S. Forest Service.

[2] Michael Douglass, “Animals you might not know pollinate flowers,” BBC,  May 14, 2015.

[3] Sánchez-Bayo, F., & Kris A. G. Wyckhuys. 2019. “Worldwide decline of the entomofauna: A review of its drivers.” Biological Conservation.
https://doi.org/10.1016/j.biocon.2019.01.020

[4] Lister, B.C., & A. Garcia,  2018.  “Climate-driven declines in arthropod abundance restructure a rainforest food web.” Proceedings of the National Academy of Sciences. https://doi.org/10.1073/pnas.1722477115  

[5] Rosenberg, Kenneth V., Adriaan M. Dokter, Peter J.  Blancher, John R. Sauer, Adam C. Smith.  2019. “Decline of the North America avifauna” Science.
https://doi.org/10.1126/science.aaw1313

[6] Carrington, Damian,” Light pollution is key ‘bringer of insect apocalypse’. The Guardian. November 22, 2019 https://www.theguardian.com/environment/2019/nov/22/light-pollution-insect-apocalypse

[7] Tallamy, Douglas, Nature’s Best Hope: A New Approach to Conservation that Starts in Your Yard, Timber Press, 2020

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