Fragile Beauty: The Life History And Range Of Monarch Butterflies

Abstract

Monarch butterflies in North and Central America are unique in the extensive migrations they undertake, which can extend up to 2500 miles from their overwintering colonies in central Mexico to their summer range in the northern United States and southern Canada. This extensive migration takes four generations of Monarchs to complete. Monarch butterflies have a four-stage life cycle, which is intimately tied to their annual migration. This annual migration is dependent upon the presence of milkweed, a plant with which Monarchs are co-evolved and dependent upon for reproduction. Human activities such as agriculture, housing development and road mowing as well as anthropogenic climate change have interfered with the ability of Monarchs to successfully migrate, leading to a decline in Monarch butterfly populations.

Introduction

Known for their striking colors and delicate beauty, Monarch butterflies are one of the most recognizable butterfly species. What may be lesser known, however, is the remarkable life cycle of these insects, and the extensive migration they undertake. Their journey is one that spans up to 2500 miles and four generations, from the mountainous forests of central Mexico to the grasslands and forests of Southern Canada and the Northern United States. The journey is perilous, energy consuming, and entirely dependent upon the presence of one plant: the milkweed. As the climate changes, and human activities alter natural landscapes, the range of Monarch butterflies has likewise been affected. In this paper I will discuss 1. The life history of Monarch butterflies, 2. The migration and range of Monarch butterflies, and 3. Limitations on Monarch migration, and the effect of habitat changes on Monarch distribution.

Life History of Monarch butterflies

The life of a Monarch butterfly (Danaus plexippus), is one of dramatic changes. The life cycle of the Monarch takes place in four distinct stages. A Monarch begins life as an egg laid on the leaves of a milkweed pant, which is the first stage. Female monarchs lay their eggs individually, securing each egg to the leaf of the milkweed plant. Females often lay “300 to 500 eggs over two to five weeks of egg laying” (University of Minnesota, 2018). While each egg is quite small, averaging 1/1000th of the mass of the adult female, females frequently will lay more than their own mass in eggs over their lifetime. The egg stage usually lasts between three and five days. The length of time spent in the egg stage of development can vary depending upon variations in temperature, with eggs laid in warmer environments developing more rapidly (Oberhauser, 2015).

The next stage is the larva, or caterpillar stage. The biological goal of this stage is simple and all-consuming: growth. Once born, these insects are ravenously hungry. They begin by eating the shell of the egg they emerged from, before moving on to consuming the leaves of the milkweed plant onto which they were born. They eat determinedly, taking few breaks to rest. This frenzy of eating and growth lasts between 9 and 14 days. As the caterpillar grows, it will molt several times. The stages between larval molts are referred to as instars. Over the duration of these instars, the caterpillar will increase its body mass about 2,000 times (Oberhauser, 2015). This rapid growth is necessary for the Monarch to undergo its next stage in development: the pupa stage.

The third stage in the Monarch butterfly life history is the pupa stage. This is the chrysalis stage, from which the insect begins its second dramatic transformation. At this time, the caterpillar will leave its host milkweed plant for the first time in search of a safe place to undergo this change. Once it has determined its pupation location (usually on a nearby plant), the caterpillar will create a silk mat, from which it will attach itself and dangle upside-down for a period of 9 to 15 days (Oberhauser, 2015). In this position, the caterpillar will shed its skin for a final time, forming a chrysalis around itself. This chrysalis resembles a leaf and serves effectively as camouflage for the transitioning insect. In the final days of the pupation stage, the chrysalis will become more translucent, and the vibrant colors associated with the Monarch butterfly will begin to be visible within. The final stage of the Monarch life history is the adult stage. The butterfly emerges from the chrysalis with the primary goal of reproduction, although those adults born during the late summer or early fall will have an additional goal of migration to the overwintering colonies.

Adults born in the summer generations have a life expectancy of 2 to 5 weeks, while adults of migrating generations can live up to 9 months (Oberhausen, 2015). The large disparity in life expectancy between adults in summer generations and migrating generations is due to the significant energy demands of reproduction. Overwintering adults do not reproduce until spring, and therefore have more energy to expend on survival, with the additional aid of cooler temperatures which slow metabolism. Adults of summer generations have the sole goal of reproduction, which has an increased energy requirement. They will begin mating as soon as 3 to 8 days old, with females beginning to lay eggs immediately following their first mating (Oberhausen, 2015). Both sexes will mate several times within their lifetime. The ability of both summer and overwintering generations to survive and reproduce depends on several factors, such as the ability to evade predators, survive environmental extremes, and locate their co-evolved host plant, the milkweed.

Migration and Range of Monarch Butterflies

Milkweed is the common name used for plants in the Asclepiadaceae family. Monarch butterflies are stenophagous (predators limited to a specific prey), and co-evolved with milkweed plants such that the northern range limit of the milkweed plant coincide exactly with the northern summer range for Monarch butterflies (Macdonald, 2003). Migration is common among butterflies, however, the migration of the monarchs is unique in “the dramatic differences between the sizes of the breeding range (over 390,000 square miles) and the wintering area (a few acres)” (Jepsen, 2015). In winter, the eastern Monarch populations cluster in the oyamel fir forests found in the mountains of central Mexico and the western Monarch populations gather on the central coast of California. The eastern Monarch population far outnumbers the western Monarchs. The Monarchs make this journey to avoid the freezing winter temperatures experienced in their summer range of the northern United States and southern Canada.

The scope of this migration is even more astonishing because the journey spans four generations. The Monarchs that leave their summer range in North America are headed to locations last seen by their great-grandparents. Scientists have determined that “migrating monarchs use a time-compensated Sun compass to navigate” (Froy, 2003) in conjunction with an internal circadian clock, although research is on-going to determine the exact mechanisms behind this remarkable ability. In February or March, adult Monarchs that have overwintered begin the journey northward, stopping to reproduce in areas where their host plant, milkweed, can be found. It will take three generations for the Monarch population to arrive in their summer range.

Changes in Habitat and Distribution

As a result of anthropogenic changes to native habitats, as well as changing global climate patterns, Monarch populations have been in decline. There are several different factors influencing this trend: removal of milkweed plants and/or pesticide usage near milkweed plants, logging near overwintering sites, and global climate change (Jepsen, 2015). North America has 27 native species of milkweed used by Monarchs for reproduction, which are usually found in “rangelands, agricultural areas, riparian habitats, wetlands, deserts, prairies, meadows, open forests, woodlands, and roadsides (Jepsen, 2015). Expansion of urban areas, increase in suburban sprawl, and conversion of former grassland and forests for agricultural purposes have all affected the abundance of milkweed in North America.

In other areas, agricultural use of pesticides contributes to Monarch decline. Researchers found an 81% decline in milkweed abundance near Midwestern agricultural lands from 2000-2010, which coincided with an increase in the use of pesticides on genetically modified corn and soybean crops (Oberhausen, 2015). Road mowing and use of insecticides for mosquito control may also play a role in reduction of milkweed abundance as well as decline in Monarch populations. Because of their exclusive reliance on milkweed in reproduction, a reduction in the abundance and distribution of milkweed will result in a reduction in the abundance and range of Monarch butterflies.

Overwintering colonies are subject to threat from human activities as well. Monarchs require a thick tree canopy for protection at the overwintering sites, and increased logging in these areas of central Mexico may be contributing to declining numbers of Monarch populations.

Similarly, clearance of trees for housing developments in California may be having the same effect on western Monarch populations. Global climate change may play a role in altering the range of Monarchs as well. This issue is ongoing and not yet fully understood, so it is not yet clear how Monarch populations will respond. Monarchs are very sensitive to climatic conditions and depend on environmental cues to successfully complete their life cycles and migration. Temperature is often a trigger for Monarch life changes and to begin and end their migrations (Jepsen, 2015). Rapid climate changes may affect these temperature triggers, resulting in changes to their range and abundance.

Conclusion

While the beauty of Monarch butterflies is striking, it is their unique and impressive migration that truly makes this species unique. Their highly specialized evolution requires specific temperature ranges and the presence of milkweed, their co-evolved host plant. It is this very specialization, however, that may become a liability for Monarch butterfly populations as humans continue to alter the natural landscape and contribute to climate change. The ability of Monarch populations to adapt in the face of rising temperatures and changing habitats will determine their continued biological success. The outlook is not entirely bleak, however. Efforts are underway to bolster Monarch butterfly populations through restoration of native habitats by strategic planting of milkweed and stricter regulations regarding use of herbicides. Conservation groups are making legal efforts in Mexico to protect Monarch overwintering sites from illegal logging activities. With efforts to reduce climate change and restore native habitats, humans have the potential to correct the harm caused to migratory Monarch butterfly populations so that future generations can continue to marvel at these unique and beautiful creatures.

Bibliography

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