每日英語跟讀 Ep.844: Why Is the Monarch Butterfly Population Declining?
Western monarch butterflies spend their winters in Pismo Beach and other sites on the central California coast. A few months later, they breed in the Central Valley and as far north and east as Idaho.
But where they go in between remains an open question.
Now, a group of researchers wants the public’s help to solve that mystery.
They would like anyone who spots a monarch north of Santa Barbara this spring to snap a quick picture. The researchers — from Washington State University, Tufts University, the nonprofit Xerces Society for Invertebrate Conservation, and the University of California, Santa Cruz — need photographic evidence, a date and a location to confirm where the monarchs might be living. (Photos and information can be emailed to [email protected] or uploaded on the iNaturalist app.)
“Something’s going on in early spring,” said Cheryl Schultz, a professor at Washington State University, in Vancouver.
Researchers know that winter survival isn’t the issue in the short term, she said. But they don’t know whether the monarchs are not making it to breeding sites, not finding plants to nourish them along the way, or not able to find mates.
The Western monarch population, which lives west of the Rocky Mountains, stood in the millions in the 1980s. In 2017, an annual count found 200,000 butterflies. In 2018, the tally fell to about 30,000 — a figure that held steady last year, said Elizabeth Crone, a biology professor at Tufts University, in Medford, Massachusetts.
The monarch’s decline is part of a larger trend among dozens of butterfly species in the West,said Matt Forister, an insect ecologist at the University of Nevada.
Research pins the loss of Western butterflies on a variety of factors, including development, climate change, farming practices, and the widespread use of pesticides by farmers and on home and business lawns, Forister said.
For example, Schultz said farms used to have rough, weedy borders that were great breeding grounds for the types of plants that monarchs love. Newer farming practices have pushed crops to the edge of the fields, leaving no room for these weedy margins, she said.
Another factor, she said: Some homeowners, eager to attract monarchs, have planted tropical milkweed. Although the butterflies will feed on them, these plants tend to spread disease because they don’t drop their leaves, Schultz said, which may be contributing to the declining monarch population. Native milkweed supports the population without this risk, she said.
Source article: https://paper.udn.com/udnpaper/POH0067/352162/web/