3 years ago



By multiplying the

By multiplying the number of eggs per milkweed in the growing season by the density of milkweeds in the landscape and the proportion of the landscape in crop fields versus other land uses, Pleasants and Oberhauser (2012), as updated in Pleasants (in press), estimated the total productivity of different habitats for monarchs and found that a significant proportion of the monarchs from the Midwest once originated in cropland. Based on milkweed densities in various habitats in Iowa in 1999 (Hartzler and Buhler 2000), they estimated that corn and soybean fields produced 78 percent of the state’s monarchs, with another 16 percent from land enrolled in the Conservation Reserve Program (based on data supplied by John Pleasants). Milkweeds vary in nutritional quality based on species and age. Southern milkweeds generally have higher cardenolide concentrations than northern milkweeds, which may help protect monarchs from bird predation during much of their breeding cycle and which may thus also influence migration strategy (Malcolm and Brower 1986, Malcolm et al. 1993, Lynch and Martin 1993, Rasmann and Agrawal 2011). Monarchs need milkweeds that are young, nutritious, and that supply the appropriate amount of protective cardenolides. Common milkweed leaves in shaded habitats tend to be larger, less tough, and have lower cardenolide content and lower induced latex production which possibly increases their quality for monarch larvae (Oyeyele and Zalucki 1990, Agrawal et al. 2012). Egg densities on milkweeds with young or re-sprouted leaves tend to be higher than on older leaves (Zalucki and Kitching 1982). The re-sprouting that follows non-glyphosate herbicide application may contribute to higher egg densities on milkweeds in agricultural fields (Oberhauser et al. 2001), though application of any herbicide causes defoliation that prevents development into larvae of monarch eggs laid prior to treatment (Pleasants in press). Some butterflies have been shown to be more likely to oviposit on leaves with higher nitrogen content, though this is not conclusive in monarch studies (Oyeyele and Zalucki 1990). Monarchs can compensate for lower nitrogen content in leaves by consuming more leaves (Lavoie and Oberhauser 2004). In addition to milkweed, monarch habitat requirements during the breeding and migrating season include trees for roosting. During migration, monarchs have to make frequent stops to rest, to feed on nectar to maintain fat reserves, and during bad weather (Davis and Garland 2004, Brower et al. 2006, McCord and Davis 2010, Davis et al. 2012, Brower et al. in press). Monarchs form communal roosts at some of these stopover sites, particularly during the fall. Based on an analysis of four years of roost data collected by citizen scientists during fall migration for Journey North, a student wildlife monitoring program, monarchs can use trees with different branching patterns and leaf characteristics for roosting (Davis et al. 2012). Monarchs in northern states primarily roost in conifers and maples, while monarchs in the south commonly roost in pecan and oak trees. No particular land cover type is correlated with roosts, however, monarch roost sites are associated with large bodies of water, such as rivers and lakes, although reasons for this are unknown. In the southern part of the flyway, monarchs are found more often in grassland than would be expected by chance. Monarchs do not appear to consistently roost in the same locations within the flyways each year, suggesting that roost site selection is somewhat random (Davis et al. 2012). The ephemeral nature of monarch roost site selection increases the importance of protecting nectar resources in the flyways, because nectar sources can be more easily predicted by land Monarch ESA Petition 30

managers than roost sites (Brower et al. 2006, Howard and Davis 2008, Davis et al. 2012). Though monarch caterpillars are entirely dependent on milkweed, numerous species of flowering plants can provide suitable nectaring habitat for adult monarchs (Tooker et al. 2002). Climate, including weather patterns and temperature, also plays a significant role in defining monarch habitat seasonally because suitable temperature regimes are required for monarch survival and reproductive success (Zalucki and Rochester 2004, Taylor and Lentz 2005, Stevens and Frey 2010). Although basic overwintering habitat requirements are common to the subspecies, some details differ for D. p. plexippus east and west of the Rocky Mountains. The western monarchs roost in coastal areas of California in the winter, whereas the much larger numbers of monarchs east of the Rockies roost in a small area of Mexico, and these roosting locations have distinctive flora and microclimates. Overwintering monarchs have very specific microclimatic habitat requirements, such as protection from wind and storms, absence of freezing temperatures, exposure to dappled sunlight, and presence of high humidity (Chaplin and Wells 1982, Calvert et al. 1983, Anderson and Brower 1996, Leong 1999). Fall or winter blooming flowers that provide monarchs with nectar may be important to maintain lipid reserves required for winter survival and the spring migration (Tuskes and Brower 1978). In inland Mexico, monarchs gather on oyamel (sacred) fir (Abies religiosa) trees on the border between Michoacán and Mexico State in the mountains of the Trans-Mexican Volcanic Belt. The high altitude forests provide the microclimatic conditions that monarchs must have to survive the winter. Colonies are ecologically and geographically constrained to densely forested sites that are at high elevations (~2,900–3,300 m [9,500–10,800 ft]) and they are usually restricted to arroyos near streams on southwest-facing slopes that are moderately steep (Slayback et al. 2007, p. 28). The cool temperature and moisture inside the oyamel forests maintain the butterflies in a state of reproductive diapause and allow them to conserve lipid reserves that fuel the wintering period and the spring remigration north (Brower et al. 2011, p. 28). The benefits of the dense canopy and mature trees have been likened to an umbrella, a blanket, and a hot-water bottle, protecting the butterflies from rain and keeping them warm enough not to freeze but cool enough that diapause is not broken (Ibid.). The monarch’s overwintering in habitat in Mexico is threatened by logging, forest disease, forest senescence, climate change, and severe weather events. Site fidelity and extreme localization of colonies within such a small area of available habitat heightens monarch vulnerability and highlights the urgent need for protecting the butterflies’ habitat (Slayback et al. 2007, p. 38). In coastal California, most overwintering sites are dominated by exotic blue gum (Eucalyptus. globulus) or red river gum E. camaldulensis), although many sites also contain native trees such as Monterey pine (Pinus radiata), Monterey cypress (Cupressus macrocarpa), western sycamore (Platanus racemosa) and other species (Xerces Society 2013). Recent research shows that monarchs do not prefer Eucalyptus over native tree species (Griffiths and Villablanca 2013), especially later in the season as storms become more severe. Historically, the composition of vegetation on the California coast differed from the contemporary composition, and groves of Monarch ESA Petition 31

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