Winter aggregations of monarch butterflies in California may be viewed as a metacolony made up of many winter sites distributed along a narrow one to four mile wide coastal strip from northern Mendocino County to as far south as Baja California, Mexico.  Their distribution and abundance resembles a bell-shaped curve with the greatest concentration of winter sites and overwintering butterflies located within central California, from Santa Cruz to Santa Barbara, where conditions are most suitable for winter aggregations.  The number of winter sites and overwintering butterflies decreases significantly as their distribution range extends north or south of central California.  The possible reasons for lower numbers of winter sites and population numbers may be attributed to colder temperatures and higher rainfalls associated with their northern distribution and warmer temperatures and lower relative humidity and morning dew associated with their southern distribution.  In San Luis Obispo County, there are many winter sites that include Pismo North Beach, Morro Bay Golf Course, San Simeon, and the nineteen acre Monarch Dunes Butterfly Habitat in Nipomo.

The overwintering monarch butterflies carry the “seeds” of future generations, and their winter survival depends upon finding suitable sites.  Their mass winter aggregations in California and in Mexico were classified in 1983 as an endangered phenomenon by the International Union for the Conservation of Nature and Natural Resources (IUCN) in the IUCN Invertebrate Red Data Book due to the dwindling numbers of suitable winter sites.

In California, winter sites are limited to discrete forested areas designated as sanctuaries for overwintering monarch butterflies.  These sites are fragile, dynamic and capable of maintaining conditions for winter aggregations for only a few decades.  Suitable conditions for winter aggregations will eventually fail, because of changing grove conditions due to the loss of limbs and trees to winter storm winds, diseases or just the normal maturation of grove trees.

Winter groves should and must be managed for the butterflies to ensure their winter survival and to preserve their mass winter aggregations in California.  Management involves maintenance or restoration of conditions suitable for winter aggregations through purposeful enhancement activities.  Grove improvement should be based on: (1) the seasonal wind profile of the winter grove; and (2) the butterflies’ access to filtered morning and afternoon sunlight.

Grove’s wind profile:  The seasonal wind profile provides a graphic display of the course of disruptive winds (≥2 meters/second or 4.5 miles/hour) within the grove and whether these winds are able to invade the cluster arena.  Decisions as to where the planting of trees can best buffer these winds can be logically made when analyzing the wind profile of the grove.

Filtered sunlight:  Access to filtered morning and afternoon sunlight is essential for the overwintering butterflies.  Morning sunlight is needed to warm their body temperatures so they can leave their clusters to forage for water or nectar in order to metabolize their fat reserve for survival, sun or to find mates.  Afternoon sunlight is necessary to re-form their nightly aggregations.  Evaluation of the butterflies’ access to filtered sunlight during critical morning and afternoon hours would determine where selective trimming of limbs and trees could enhance optimum exposure to filtered sunlight.

Monarch Dunes Butterfly Habitat in Nipomo is the first winter site in California that is managed by volunteers in concert with their community’s land developers.  Their joint goal is to maintain or re-establish suitable conditions for winter aggregations through monitoring of grove conditions and making appropriate changes where indicated.

Monarch Dunes Butterfly Habitat

The western monarch butterfly prefers overwintering in the fragrant eucalyptus groves that are abundant along the central coast region of California.  These trees provide the climate the monarchs need:  proper humidity, filtered light, shade, temperature, and some protection from the wind.  Blue gum eucalyptus trees are not well suited for sheltering monarch butterfly clusters, but they do provide the butterflies with a convenient nectar source, as these trees bloom in the winter.  Western monarchs will also cluster nightly in other trees, including pine and Monterey cypress.

The Monarch Dunes Butterfly Habitat in Nipomo is a nineteen acre rectangular shaped grove of Tasmanian blue gum eucalyptus trees that encompasses the historical aggregation trees and two wind buffer zones on the western and southern borders of the habitat.  Dense growing Monterey cypress trees were added to the standing eucalyptus to create these buffer zones.  Some of the original inner grove trees were removed to increase the amount of solar radiation entering the cluster arena.  The resultant central clearing was seeded with a hydro seed grass and flower mix and an irrigation system was added to provide water for the seeded plants, the cypress trees and as a water source for the overwintering butterflies.

Five Monterey cypress saplings were planted in the main cluster area to create an additional layer of wind protection and to provide future trees on which the butterflies may form their winter aggregations.  In the open area between the row of southern buffer trees and the grove’s southern border, there is a paved trail leading from the parking lot on the southeastern corner to the southwestern portion of the grove’s border, and then curving back to an observational area within the grove.  The diagonal entry of the trail into the grove (west to east) was purposefully designed to prevent the funneling of southern storm winds into the aggregation area.

An irrigated grassy area was established along the grove’s southern perimeter to serve as an important source of moisture for overwintering butterflies during the winter months.  The Nipomo butterfly habitat is located approximately five miles inland from the Pacific Ocean, while most of the other California overwintering sites are within a mile of the coast and may thus record higher humidity levels.  The irrigation system in Nipomo provides additional water to the naturally occurring moisture sources of winter rain, coastal fog and morning dew.

Residents and visitors often picnic at the tables provided and enjoy the solitude of a wooded area within the Trilogy at Monarch Dunes community.


Last updated 24 May 2013.