When Danielle Bronson was a little girl, Pismo Beach Monarch Butterfly Grove was home to about 250,000 monarchs. Now the state park interpreter at Pismo Beach, Bronson’s grove only welcomed 500 butterflies this year.
Western monarch butterflies experienced their second-lowest population count in history this season, according to Bronson. The all-time low occurred in 2021.
The Hollywood species, a term coined by Bronson due to their immense popularity, are famous for their annual migration that spans North America. Once northern conditions become too cold, they retreat south towards locations with warmer climates, including much of the California coast.
The population’s downward trend can be attributed to many factors. Despite being one of the most-studied vertebrates, it is difficult for researchers to pinpoint an exact cause for this year’s population dip, Bronson said.
Record-high late summer temperatures along with drought in the Western United States are among the potential factors in this year’s monarch butterfly population decrease, said Kyle Nessen, a biological sciences master’s student.
Droughts heavily affect the monarch butterflies overwintering sites, as they create unfavorable environments for the sites’ eucalyptus trees.
However, California is composed mostly of grassland and shrubs, so the density of eucalyptus trees heavily appealed to the monarch butterflies, according to Nessen.
“The drought, particularly in 2016, terminally hurt these eucalyptus trees,” Nessen said. “Almost all of our groves are in really poor condition.”
Pismo Beach Monarch Grove is working to combat the drought effects by planting new trees and implementing irrigation. These efforts help boost tree growth and contribute to the health of eucalyptus trees, Bronson said.
“Pismo is actually doing quite well compared to other groves I’ve seen,” Nessen said. “But, it still has a number of trees that were all planted around the same time a little too close, and they’re vulnerable to being knocked down in these strong wind events.”
Along with drought, low numbers are likely caused by fertilizers, pesticides or lack of native milkweed, Bronson explained.
“The more positive messages that we get out, the more people will truly understand the state that [monarch butterflies] are in and be more inclined to find ways that they themselves can help,” Bronson said.
Monarch butterflies were recently proposed as threatened under the Endangered Species Act, which would help protect the species, Nessen said. Destroying a monarch butterfly habitat would become a felony, in contrast to smaller fines.
Although monarch numbers are low this year, their sensitivity grants them the ability to fluctuate greatly based on the quality of their conditions. They can lay up to a few hundred eggs depending on the quality of their environment.
“You can have these huge [population] swings because of this ability of the butterflies to reproduce very fast,” Nessen said. “Every time they reproduce, they can potentially have hundreds of viable offspring that can then go on to make hundreds more.”
Nessen encouraged everyone to visit the Pismo Beach Monarch Butterfly Grove if possible.
“We live in a very special place to see this,” Nessen said. “The monarch migration is one of the most famous examples of animal migration that we have.”
