This iconic butterfly is classified in the family Nymphalidae, the brush-footed butterflies. There are two populations of Monarchs in North America, an eastern and a western group, with the Rocky Mountains forming the geographic separation between the two populations. Each population also has its own migration pattern and different wintering sites. The two North American populations are genetically similar, and have not acquired differences in spite of reproductive isolation. The smaller population found west of the Rocky Mountains normally migrates during winter to coastal southern California, but may also overwinter in Mexico. The eastern population is the largest, with the concentration of migrating Monarchs traveling along a so-called Monarch Highway, a corridor roughly marked by I-35 as it traverses the states of Texas, Oklahoma, Arkansas, Iowa and Minnesota. The western edge of this large eastern population travels through New Mexico, primarily east of the Rio Grande corridor, where Monarchs may be seen during spring and fall migration. Most photos here were taken in early September at the Oak Flats recreation area of the Cibola National Forest, south of Tijeras.

Monarchs breed in the northern United States and females lay eggs only on the milkweed plant. The life cycle is termed complete metamorphosis, and goes from the egg, through caterpillar and pupa stages, to adult in about 25 days in warm weather, but may require 6-7 weeks in cooler conditions. Females lay single eggs on the underside of milkweed leaves, producing 300-500 eggs over several weeks. After about a week, caterpillars emerge and feed on the host milkweed plants. The caterpillar goes through 5 instar stages of development over about two weeks and then pupates, creating a green chrysalis in which the adult will take shape after about two more weeks. The caterpillars build up concentrations of steroid chemicals called cardenolides in their bodies as they feed on the milkweed, which are retained in the adults. These chemicals are toxic, affecting cardiac function in predators that consume them, and the bright coloration of the adult monarchs serves as a protective warning to potential avian predators to avoid these butterflies.

The adult reaches sexually maturity after less than a week. Males are somewhat larger than females and have a black spot on an inner vein on each hind wing. The females can also be distinguished from males by their thicker black wing veins. The migration is termed multigenerational because several reproductive life cycles will occur along the spring migratory route. The amazing navigational feat accomplished by several generations in the spring migration necessitates an innate knowledge of the route and eventual destination of the migration, as none of the individuals arriving at the breeding sites have never been there before. The fall migration is completed by a single generation.

The Monarch is also the only butterfly to make a spring and fall migration, like that seen in many bird species. Because Monarchs only travel during the day, they seek roosting sites each night, and these favored sites are habitually used each year. Thousands of butterflies may congregate in masses to preserve heat. They prefer pine, fir, and cedar roosting trees.

The Xerces Society is dedicated to the conservation of all invertebrate species, emphasizing their vital contribution to healthy ecosystems through pollination, recycling of organic matter, providing nutrition to many other species, and other often overlooked ecological services. A branch of the Xerces Society promotes Monarch education and conservation. They note that the large eastern population has declined by some 80% in recent years due to a variety of causes, such as the introduction of genetically modified crops, use of chemical herbicides and pesticides, and conversion of native grasslands to agriculture and urban development. These habitat alterations have also led to a reduction in the availability of the milkweed plant, essential to successful Monarch reproduction, thus impacting Monarch populations nationwide.

The primary winter refuge site of the eastern population is in Mexican Oyamel fir forests (Abies religiosa) in the mountains west of Mexico city at elevations from 9,500-10,800 feet. A Monarch Butterfly Biosphere Reserve is protected in this forest but illegal logging activities and habitat losses due to drought have been responsible for a reduction in the wintering areas available to Monarchs, and butterfly abundance has been declining there in recent years.

The Xerces Society reports that the western population, which overwinters along the coast of California and the northern Baja peninsula, is under a serious extinction threat, having undergone a decline of over 95% from many millions in the 1980s to several hundred thousand by 2015. In the years 2018-2020 western populations numbers were estimated at under 30,000 individuals. As with the eastern population, habitat loss through conversion to agriculture and urbanization, chemical application to croplands, and environmental impacts of global warming, are primary causes of the dramatic decline of this smaller western population of Monarchs. The Society has instituted a special Monarch Call to Action, identifying specific rapid-response steps to help conserve and restore the western population in California. The Society reports that by 2021 Monarch numbers estimated in the annual Thanksgiving count in California had rebounded to a few hundred thousand again, confirming that restoration efforts are proving effective.

Butterflies add joy to any outdoor activity. But butterflies and caterpillars also play important roles in the ecosystems where they live. They also act as food sources for a variety of other species, including birds, rodents and other mammals, reptiles, and other insects and arthropods. Caterpillars are voracious consumers of vegetation and may cause harm to crops and trees. However, their recycling of plant nutrients into components that can be used as nutrients by other organisms is a valuable ecological service. Some caterpillars, like those of the gossamer-wing butterflies, are carnivorous, feeding on aphids and other insects, and thus removing parasites that would otherwise cause damage to plants.

Many of the nectar feeding butterflies collect pollen from the flowers they visit and distribute that pollen to other plants, thus completing the sexual reproduction cycle of the host plants in the process. These flowering plants thus depend upon this mutualistic relationship with butterflies for their survival.

Due to the vulnerability of butterflies to environmental contaminants and stresses, they provide bell weathers of deleterious habitat alterations that may be recognized by monitoring the health of local butterfly species. Identifying factors affecting a decline in butterfly populations could permit mitigation efforts before more widespread ecological damage occurs. Provision and protection of linked habitats favorable to butterfly species, such as woodlands, meadows, grasslands and wetlands can ensure that butterfly species continue to thrive. Cultivation or protection of wildflower species in suburban environments also provides needed resources for resident butterflies as well as traveling migrants.