Monarch butterfly
Danaus plexippus
NatureServe conservation status
Global (G-rank): G4
State (S-rank): S2
Utah Wildlife Action Plan status
- SGCN
External links
Phenology
The eastern North American population overwinters almost entirely as reproductively inactive adults at high elevations within and area of less than 100 by 100 km in Mexico where they require a very narrow range of now rather well understood microclimatic conditions which only a few places between 2900 and 3300 meters provide. Vidal and Rendon-Salinas (2014) provide detailed information, including a map, for all 19 overwintering sites that have been documented at least once. The most recent observations indicate that only seven had any monarchs and that 88% of individuals were concentrated in two colonies, which together occupied less than a hectare (Vidal and Rendon-Salinas 2014).
Monarchs begin to arrive in the overwintering areas in about late October and continue to do so into December. Apparently once they cluster in the high altitude fir trees Monarchs usually do not feed until early spring (late February) and live off their lipid reserves. During this time they need low temperatures to reduce metabolic rate, but adult Monarchs are not highly freeze-tolerant and sometimes depend on the dense forest canopy to moderate temperature on very cold nights and to provide shelter and keep them dry during storms. The freezing point and lethal temperature of a dry Monarch is -7.7°C (18F) and for a wet one about -4.2°C (24°F), (Nail and Oberhauser, 2012). Dormant Monarchs can live in these high, cold places for up to five months. Reproductively active Monarchs live about 2-5 weeks. Overwintering Monarchs can fly, at least on sunny days, for example if disturbed. Such flights take an unknown toll on their reserves (Vidal and Rendon-Salinas 2014), which they might or might not be able to replace by feeding in early spring.
A few east coast Monarchs continue south and reach places such as southern Florida, Cuba, other Carribean Islands, and the Yucatan, where they apparently merge into resident non-migratory populations (see Zhan et al. 2014). They do not return north and die within a month. However many successfully reproduce.
Species range
North America forms the core of the monarch’s range but the overall range extends through Central America to northern South America. Monarchs also occur in Hawaii, Australia, several Pacific islands, parts of Asia, Africa, and southern Europe (Zhan et al. 2014). Several populations outside of the Americas appear to be nonnative, originating from introductions that are thought to have occurred in the 1800s (Vane-Wright 1993), but Zhan et al. (2014) suggests that introductions may have occurred much earlier. However, in some cases there are no native foodplants.
The North American populations (subspecies plexippus) are strongly migratory, resulting in vastly difference seasonal ranges. Essential overwintering areas for North American populations are limited to a few dozen places in coastal California and the mountains of Mexico. The summer range includes portions of the conterminous U.S and the southern portions of all Canadian provinces bordering the US where milkweeds occur. Populations in south Florida and the Gulf Coast are non-migratory.
Migration
The North American populations (subspecies plexippus) are strongly migratory. By September a huge majority of newly emerging eastern North American Monarchs are reproductively inactive and attempt to migrate to Mexico for the winter. Monarch migration is very well documented (see Center for Biological Diversity et al. 2014 and Flockhart et al. (2013). Brindza et al. (2008) estimates that about 1.1 million Monarchs were tagged in eastern North America from the 1992 through 2006 migration seasons. Of these, about 12,000-14,000 (over 1%) were actually recovered in Mexico, and many others in between provided data on the flight route. Stable isotope analysis confirms that Monarchs from the U.S. “corn belt” accounted for about 60% of those that reached the overwintering region in Mexico in 1996 (reviews in Butler, 2014 and Flockhart et al., 2014), but less in more recent years. Apparently almost none of the winter population originates in Florida, Mississippi, Alabama and Georgia. Monarchs breed in those states in spring and early summer but their offspring migrate north.
Those that survive the winter in the Mexican mountains mate, lay eggs, and fly north in about March, but few of these get north of Texas. Stable isotope analysis confirms that most Monarchs reaching the U.S. Midwest in late spring originate from larvae in Texas, but a few females from Mexico reach farther north or east. Monarchs begin reaching the core (corn belt) breeding range in April and May about when milkweed foliage becomes available. A few from this generation reach as far east as New Jersey. Some usually reach Maine and southern Canada by June. Northward progress each spring can be followed at the Journey North website (http://www.learner.org/jnorth/monarch/).
Habitat
Habitat is a complex issue for this species. In general, breeding areas are virtually all patches of milkweed in North America and some other regions. The critical conservation feature for North American populations is the overwintering habitats, which are certain high altitude Mexican conifer forests or coastal California conifer or Eucalyptus groves as identified in literature. It appears virtually all North American monarchs overwinter in one of these two areas. Lethal cold would preclude successful overwintering in places like the Gulf Coast and much of Florida some years and it appears these are not major wintering regions as used to be assumed. In addition certain major coastal migratory stopovers may be important conservation sites especially those in along Delaware Bay in New Jersey including Cape May where adults may holdover for several days awaiting suitable conditions for crossing the Bay. There are major, but probably less important, roosting sites farther north such as east of New Haven and probably others farther south perhaps even as far as Cuba. Coastal regions are important flyways and so nectar (wild or in gardens) is an important resource in such places. In places like Hawaii and some others the species simply breeds year round and is not really migratory.
Food habits
Larval foodplants are milkweeds (Asclepias species). Studies have shown female oviposition preference on plants that are well-watered, lower in cardenolides, and in suitable microclimates per seasonal factors.
Ecology
Can contain poison derived from their larval foodplant that helps avoid predation by birds (Scott, 1986).
Threats or limiting factors
The species as a whole is not seriously threatened and appears to be doing well or even increasing in many places, but not in its core North American range. The native North American populations are vulnerable at their overwintering grounds although the Mexican sites and some in California have some level of protection. Overwintering habitats in Mexico are primarily in a few hectares or less each and have been under pressure from logging, agricultural and urban development. See Vidal and Rendon-Salinas (2014) for maps documenting deforestation. However, recent analyses and reviews including (Miller et al. 2012, Brower et al. 2012a, Pleasants and Oberhauser 2013, Flockhart et al. 2013, 2014, Butler 2014, Center for Biological Diversity et al. 2014) conclude that the recent large-scale decline of North American monarchs is primarily the result of changes in the core breeding habitat, not the wintering habitat, apparently largely the recent loss of milkweed as a result of two changes in agricultural practices: 1) widespread adoption of genetically modified herbicide-tolerant corn and soybeans and use of the herbicide glyphosate on these crops; and 2) placing approximately 25,000,000 additional acres of mostly Roundup Ready corn since 2007. In both cases threats remain substantial. Climate change is emerging as a current threat in Mexico, California, and Texas at least. North American populations can still respond quickly to temporarily good weather conditions. Flockhart et al. (2014) project further decline for the core eastern North American monarchs. Although the species as a whole has probably declined by more than 50% in the current century, regardless of what happens in North America it is virtually at no risk of global extinction in the foreseeable future. Loss of genetic diversity, e.g. as related to migration (Zhan et al., 2014), is a real concern for both the typical subspecies and the species as a whole. Biological resource use as a threat refers mainly to concern that commercial and other rearings may result in pathogen/parasite laden individuals being released and possibly increasing incidence in the population to unnaturally high levels. This might or might not be a significant threat anywhere.