3 years ago



native trees presumably

native trees presumably hosted dense monarch aggregations (Lane 1984, 1993). Monarch overwintering habitat in California is directly threatened by logging and other forest degradation for commercial and municipal development. Habitat alterations, such as tree trimming or tree removal, or natural factors such as fire, severe storms, or disease or senescence of trees, can alter the structure and microclimate of an overwintering site and reduce its suitability for monarchs (Sakai and Calvert 1991, Commission for Environmental Cooperation 2008). All of the California sites are at low elevations (

POPULATION DISTRIBUTION AND STATUS The historic distribution and abundance of monarchs is not known with certainty, but would have been broadly defined by the distribution of milkweed. Historically D. p. plexippus populations east of the Rockies would have bred mainly in the grasslands and prairies of the Great Plains that were populated by a mix of native milkweed species (Brower 1995) and copious nectar sources (Figure 9). Monarchs likely also inhabited meadows, Native American agricultural fields, and other open areas throughout North America wherever milkweeds occurred and weather conditions permitted. The butterflies would have been rare in heavily forested regions, mountainous areas, and arid zones. Monarchs were almost certainly confined to continental North America from pre-history until the mid- to late-1800s. Figure 9. Historic monarch distribution east of the Rockies likely coincided with pre-European prairie extent. Figure 2 from USGS 2013 Prairie Past and Present, caption included: It is likely that prairie milkweeds were abundant and supported high monarch populations, though abundance and distribution of particular milkweed species before widespread plowing of the prairies is unknown. Milkweed species and abundance have been measured in some current prairie remnant habitats in Iowa and extrapolated to provide an estimate of pre-agricultural milkweed occurrence. One measure of milkweed abundance is percent coverage of the landscape by milkweeds in relation to all other plant species in an area – how much space they take up. Pleasants (in press) estimates that statewide, the milkweed species in former prairies contributed 0.65 percent of the vegetation coverage in Iowa, which would have provided habitat to support highly abundant monarch populations. As of 1999, common milkweed comprised only 0.194 percent of coverage in Iowa, and that percentage has decreased nearly three-fold, to 0.068 percent by 2012, as the widespread planting of glyphosate-resistant Roundup Ready crops has Monarch ESA Petition 33

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