BioScience (2013)doi: 10.1093/biosci/bit012

In the summer of 2013, the unthinkable happened: The Eastern seaboard population of monarch butterflies—those orange and black icons, whose complex life cycle and long migration to Mexico each fall are one of the wonders of life—crashed. People saw a handful, if any, in their gardens, and scientists say that the number of monarchs that overwintered in Mexico in 2012–2013 was only 59% of those that overwintered the year before—resulting in the smallest population of monarchs ever counted in Mexico.
The crash of the monarch population is emblematic of what is happening to other pollinator species: habitat loss (what Laurie Adams of the Pollinator Partnership, a pollinators’ advocacy group, refers to as a “real estate problem”) and climate change. Add to that the effects of pesticides and herbicides, such as those that destroy milkweed, the food source for monarch larvae. Pollinators may also succumb to other poisons, infection, or poor nutrition.
The warning signs of pollinator loss were known nearly two decades ago, when Stephen Buchmann, an entomologist then with the US Department of Agriculture (USDA), coauthored The Forgotten Pollinators. Despite the warning, the declines continue, across many species. Honey bees have received the most attention, because beekeepers monitor colony health, but other pollinators are also at risk. In most cases, data about these small animals are lacking, making it a challenge to confirm that there is a problem, let alone to determine the cause.

Pollinators at Risk: Human activities threaten key species