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Clusters of overwintering butterflies in California. Photo by Lisa Hupp/USFWS.

Clusters of overwintering butterflies in California. Photo by Lisa Hupp/USFWS.

New tech for counting wintering monarchs

North American monarch butterflies undertake an annual migration phenomenon that results in densely clustered overwintering colonies at sites in California and Mexico. Numbers of overwintering monarchs can reach up to tens of thousands of monarchs per site in California to tens of millions of monarchs per site in Mexico. Overwintering population estimates are the primary means for monitoring the North American monarch population—information that is increasingly important given long-term population declines observed since monitoring began in the early 1990s. With thousands or millions of monarchs clustered on a few trees, precise estimates of their population can be difficult.
 
Light Detection and Ranging, known as LiDAR remote sensing technology is commonly used to map topography at broad scales. However, this high-resolution mapping technique has been used to map archaeological sites and cave structures, and recently has proven promising for assessing gray bat populations, a species that exhibits dense-clustering patterns comparable to monarchs. By mapping surface area with monarchs present, and again once monarchs have dispersed, it may be possible to produce precise estimates of abundance of overwintering colonies. If successful, this technique can be employed with partners in Mexico to provide more accurate population estimates and may also prove a useful tool for documenting the larger colonies in California.
 
In December 2016, in collaboration with Winston-Salem State University and the U.S. Geological Survey, we piloted ground-based LiDAR technology on colonies of monarchs overwintering along the central coast of California. The effort was supported by the Xerces Society, whose members conducted annual manual counts at the time. Information was collected from three sites in California - Pacific Grove, Lighthouse Field and Pismo Beach - and will be used to further develop appropriate methods and analysis techniques.
 
Thousands of people visit Pismo State Beach Monarch Butterfly Grove each year, and because LiDAR scanning occurred during open hours at the site, scientists had a unique opportunity to talk with interpreters and members of the public about the innovative research. In addition to development of survey technology, a goal of the project and partnership moving forward is to develop useful outreach and education materials that can be used by interpreters and outreach specialists like those at the Monarch Butterfly Grove.
 
By Shauna Marquardt
Missouri Ecological Services Field Office

 

Last updated: December 5, 2017