Overwintering Microclimate Selection by Western Monarch Butterflies

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Western monarch butterflies spend each winter in hundreds of groves primarily along the California and Baja California coast (Tuskes and Brower 1978). These groves occur in protected areas (State, City and Regional Parks, Monarch Sanctuaries, and Military Installations), in public areas (U.S. Forest Service lands), and in private holdings. Each overwintering grove is thought to contain areas that are either suitable microhabitat or unsuitable microhabitat (Leong 1990, Leong 2004, Leong 2016), given that only a portion of each grove are used by overwintering monarchs. Therefore, all monarch overwintering groves need some level of habitat management in order to maintain these pockets of suitable overwintering habitat (Griffith and Villablanca 2015). Active management is required because as groves mature, age, and trees eventually senesce, the microhabitat provided by those trees is modified (Griffith and Villablanca 2015, Pelton et al. 2016). If unmanaged, groves eventually lose the microhabitat attributes that allow overwintering by western monarchs (Griffith and Villablanca 2015, Pelton et al. 2016). In addition, ⿿management⿝ of groves in urban forests and State, City and Regional Parks, can result in unintended modifications to monarch overwintering microhabitat through tree trimming, thinning, and removal of ⿿hazard⿝ limbs and trees. Also, coastal development on private lands has resulted in the loss and degradation of overwintering habitat. For example, in the last decade approximately five overwintering sites per year have either been destroyed or made unsuitable for monarchs due to development (Pelton et al. 2016). Grove management is a primary focus for monarch conservation in the western U.S. (USFWS, Xerces Society, Monarch Joint Venture).

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