Recently populations of monarch butterflies have been in decline, and their overwintering sites in California are considered vulnerable areas by CA Dept of Fish and Game. Eucalyptus trees were first introduced to CA in the 1850's, and since native trees have declined over the years due to poor land management and disease, these non-native trees serve as ideal habitat for the butterflies. Below is a good little summary from the Ventana Wildlife Society of how conservation of this butterfly might entail protecting some of these non-native tree groves.
Historically, butterflies likely utilized native conifer stands of Monterey pine (Pinus radiata), Monterey cypress (Cupressus maculatum) and Coast redwood (Sequoia sempevirens), but in the last century monarchs have been observed roosting in introduced eucalyptus. Eucalyptus have strong vertical layering, grow quickly and densely, and are not susceptible to disease while Monterey pine is susceptible to pitch canker and is not densely constructed. Further, extensive land development, logging, and poor land management have reduced the number of native tree stands that support over-wintering monarchs. Use of isolated stands of non-native eucalyptus trees makes monarchs vulnerable to land management plans that mandate the removal of non-native tree species. Today, protection and management of monarch overwintering sites usually entails balancing planting eucalyptus coupled with its eradication and removal of diseased pine.
If you missed seeing the butterflies this past winter, check them out this December in Ellwood.
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