Jeffrey P. Crolla
6-366 Bronson Avenue,
Ottawa, Ontario K1R 6J4
J. Donald Lafontaine
Research Branch, Agriculture Canada,
Submitted to the Canadian Wildlife Service, Water and Habitat Conservation Branch, Habitat Conservation Division, March 29, 1996.
Table of Contents
The Monarch, Danaus plexippus (Linnaeus), is a large showy butterfly that is famous for its spectacular migration between breeding areas in the United States and Canada and overwintering sites in Mexico and California. The small size and fragile nature of the overwintering sites has brought the species into public attention, and made the Monarch a symbol of natural history conservation. Recent disasters and human pressures at the overwintering sites have made the Monarch susceptible to a catastrophic decline in numbers that could potentially push the North American populations of the Monarch to the brink of extinction. Within the breeding areas in the United States and Canada, the species has greatly declined in its former range in the Great Plains, while expanding into the eastern United States and southeastern Canada with the clearing of the forest s and creation of open habitat suitable for Monarch breeding. Because of the vulnerability of the species, it has become increasingly necessary to preserve remaining Monarch breeding habitat, especially in the eastern United States and southeastern Canada. This is complicated by the fact that milkweed is considered a noxious weed in most areas. Recommendations for the recognition and preservation of remaining habitat, and the removal of milkweed from noxious weed acts are discussed.
North American Distribution
The Monarch is widely distributed across North America from Central America northwards to southern Canada, and from the Atlantic Coast westwards to the Pacific Coast. Within this broad area of occurrence, there are three geographically distinct populations of the Monarch (the eastern, western and Central American populations) which collectively make up the total North American range of the species. The appearance of the adults of these three populations is so similar that no-one has proposed treating them as distinct subspecies; however, these populations obviously differ genetically in that their migratory patterns are distinct and preclude little if any gene exchange through interbreeding.
The Central American population of the Monarch occurs in Guatemala, El Salvador, Honduras, British Honduras, Nicaragua, Costa Rica, Panama and southern Mexico. Unlike the eastern and western populations, the Central American Monarch population is relatively sedentary, although short distance (10 to 100 km) seasonal migrations in Costa Rica between highland and lowland regions, in an east to west direction, have recently been documented (Haber 1993). In contrast to the other North American populations, Central American Monarchs are reproductively active throughout the year, and the migration of the species in Costa Rica is dictated by dry and wet seasonal conditions and their favourability for breeding. This is the only North American population that has a relatively fixed year-round distribution, and it does not contribute to the numbers of Monarchs found in Canada and the United States each year.
The eastern population of the Monarch is the largest of the three North American populations, and includes all Monarchs that occur each year east of the Rocky Mountains in the United States and Canada. The present annual breeding range of the eastern population extends from the Gulf Coast States (Texas, Louisiana, Mississippi, Alabama, Georgia and Florida) northwards to southern Canada (Alberta to New Brunswick and Nova Scotia), and from the Great Plains States and Prairie Provinces eastwards to the Atlantic Coast and the Maritime Provinces. The entire eastern population annually migrates to a small number of overwintering sites in central Mexico where they congregate in vast numbers.
The western population includes all Monarchs found west of the Rocky Mountains in the United States and Canada. The present annual breeding range of western Monarchs extends from the southwestern United States (Arizona and New Mexico) northwards to southern Canada (British Columbia) and from the Rocky Mountains westwards to the Pacific Coast. The entire western North American population overwinters each year at numerous sites along the coast of California.
In the last 150 years there has been a major shift in the North American distribution of the eastern population of the Monarch (see Brower 1995). Until the 1880's, the prairie region of central North America appears to have been the main breeding area of the eastern Monarch population. The native prairie flora includes about 22 species of habitat-specific milkweeds (Asclepias), many of which can serve as larval hostplants, and an abundance of flowering plants that provide a diverse array of nectar resources for adult Monarchs.
In the latter half of the nineteenth century, plowing destroyed 433 million acres of the midwestern prairie, and by 1910 most of the native prairie had been converted to crop land (Brower 1995). Concurrent to this widespread destruction of the prairie flora, the deciduous forests of eastern North America were being cleared on a vast scale. Most of the deciduous forest in the eastern United States had been cleared by 1860, and from 1860 to 1890 an additional 50 million acres of forest in the Great Lakes region was cut (Brower 1995). One result of the opening of the eastern deciduous forest was a rapid and widespread proliferation of the weedy Common Milkweed (Asclepias syriaca) in cleared lands across the northeast. The destruction of the native prairie flora of the midwest (with its associated milkweeds), and the concurrent rapid spread and increase in abundance of Common Milkweed in the northeast, appears to have resulted in a major shift in the main breeding range of the eastern population of the Monarch, from the Great Plains to northeastern North America This expansion into the northeast was characterized by a shift from utilization of a variety of prairie milkweeds as larval hostplants, to the utilization of Common Milkweed as the primary larval host of the eastern population of the Monarch in North America.
The historically cleared eastern deciduous forest region corresponds to the principal area of concentrated breeding of the eastern population of the Monarch in the present day. This consists of a large area of the eastern United States and southeastern Canada, extending from 32 o north latitude northwards to 48o north latitude, and from 95 o west longitude eastwards to the Atlantic Coast (Urquhart 1960).
Distribution in Canada
The distribution of the Monarch in Canada is shown in the distribution map below. The map incorporates both historic and current collections and observations of the Monarch, based on information in the database of the Canadian National Collection (CNC) of Insects at Agriculture Canada in Ottawa. Each dot represents a location in Canada where the Monarch has been recorded, and an individual dot may be based on one or many records of occurrences at a given location. The majority of records shown are adult butterflies, but, since the Monarch is easily recognized as both a caterpillar and a pupa, records of Monarchs encountered in all stages of its life cycle are indicated.
In Canada, the Monarch has been recorded in all ten provinces and in the Northwest Territories. Its breeding range occupies the area lying mainly south of 50 o latitude, although in the Prairie Provinces the range extends north to about 54 o latitude. This northern range limit in Canada represents the northern limit of the Monarch's breeding range in North America, which corresponds with the northern range limit of milkweeds (Asclepias). The western population of the Monarch occurs in Canada only in southern British Columbia. Western Monarchs generally reach British Columbia only in summers with protracted periods of warm, sunny weather in the Pacific Northwest, when conditions are most favorable for northward bound migrants. When they do occur, because they are strong, high fliers, flights of Monarchs spreading out over the province will usually reach available patches of milkweed. Breeding occurs in scattered locations in the province, particularly in the Okanagan Valley and along the Fraser River.
The eastern population of the Monarch occurs in Canada from Alberta eastwards to Newfoundland, and accounts for over 90% of the Canadian distributions of the species. Two occurrences from northern British Columbia and one from Great Slave Lake in the Northwest Territories are records of adult Monarchs from the eastern population that wandered well north of the normal breeding range.
Eastern Monarchs reach Canada with regularity each year, although numbers can very dramatically from year to year depending on the success of overwintering in Mexico, and the size of spring generations produced annually in the Gulf Coast States during the spring migration. Distribution east of the Rocky Mountains is varied, but can be divided into three principal regions where occurrence is similar: the Prairie Provinces (Alberta, Saskatchewan and Manitoba), southern Ontario and southern Quebec, and the Maritime Provinces (New Brunswick, Nova Scotia, Prince Edward Island and Newfoundland).
In the Prairie Provinces extensive breeding occurs only in the southern portion, and the abundance of Monarchs decreases northwards and westward from Manitoba to Alberta. Southern Ontario and southern Quebec represents the most extensive area of breeding in Canada. Abandoned farmland, the prime habitat for Common Milkweed, is widespread in southern Ontario and Quebec, and this region supports the main breeding population of Monarchs in Canada each year. Monarchs reach the Maritime Provinces with less regularity, and breeding occurs only at scattered locations due to the limited distribution o f milkweeds. In New Brunswick, breeding occurs mainly along the banks of the Saint John River, and in Nova Scotia breeding is confined mainly to the Annapolis Valley. The Monarch reaches Newfoundland as a migrant, sometimes in considerable numbers, but does not breed there since milkweeds do not occur in the wild and do not fare well when planted. In Prince Edward Island, Swamp Milkweed occurs in the wild, and Common Milkweed has been introduced, but both have a very limited distribution in the province, and no records of Monarch breeding could be found.