San Francisco State University
Geography 316: Biogeography
The Biogeography of the Monarch Butterfly (Dannaus plexippus)
by David Munro, student in Geography 316, Fall 1999
Species: Dannaus plexippus
|(Source: Urquhart 1987)|
Description of Species:
The monarch butterfly is undoubtedly one of the most recognized animal species in the world. This is partially due to its wide distribution, but probably more due to its beautiful, striking coloration.
The monarch is a medium sized butterfly, measuring about 3 inches from wingtip to wingtip. Its body is about one inch long. Its four wings are generally a field of yellow, orange or gold, with veins of black running through them. A band of black, thickest at the front, rings the wings, and the body is black as well. This black band is usually speckled with white spots, larger at the front and smaller at the back.
The Monarch Butterfly (Dannaus plexippus) has a rich natural history that has been studied extensively by entomologists and biologists. Despite this scrutiny, new discoveries are still emerging about this beautiful insect. It has an extensive home range, but specific habitat needs. Its mating habits are in some ways the opposite of what one would expect, and its complex adaptations continue to cause argument amongst researchers to this day.
The ecology and the home range of the monarch butterfly are closely intertwined, as with most species. Put simply, it is dependent upon milkweed plants, belonging to the family Asclepiadaceae, of which about 2,400 species exist (Urquhart, 1987). The distribution of the monarch is controlled by the distribution of milkweed, it regulates their density in a given area, and it is for this plant that the monarchs migrate for long distances every year. So dependent upon milkweed is the monarch that where one finds the monarch, one will also find milkweed.
Milkweed is the host plant for most of the monarchs life cycle. Eggs are deposited and hatch on the underside of leaves of the milkweed plant. Upon hatching, the larva will feed upon the fine hairs on the leaves of this plant, and stay on this same plant throughout its five molting stages. After molting, the larva will leave the milkweed and construct its chrysalis somewhere else. However, once an adult monarch emerges from the chrysalis, it will soon head back to a milkweed plant for foraging and shelter (Urquhart,1987).
The monarchs will spend their summer either in the New England-Great Lakes area, or else in the canyons of the eastern Rocky Mountains. The Great Lakes population will migrate southwest in the fall, and spend the winter in the Sierra Madre mountains of central Mexico. Despite having been studied for so long, it was not until 1976 that their overwintering ground was discovered (Urquhart,1987). That population which spends its summers in the Rocky Mountains will migrate to California and spend the winter in a variety of roosting sites as far north as Monterey, CA. Although their overwintering grounds do not necessarily depend upon the presence of milkweed, they will always return to areas rich in this plant to deposit their eggs.
|(Source: Urquhart 1987)|
The monarch butterfly is a fascinating animal that has earned its nickname "The International Traveler" (Urquahart 1987). Although it is one of the most successful species of animal on the planet, the population in its original range, North America , is threatened by deforestation of its overwintering areas in the Sierra Nevada of Mexico. It is further threatened by the fragmentation of its summering habitat in the northern United States. Due to its wide range and somewhat predictable nature, it is a prime indicator species for the ecological health of a very large area.
The monarch butterfly has been classified and reclassified several times as a result of new data coming to light. As with most species, theories of its evolution are based on its systematic similarities to other closely related species, as well as genetic similarities to those species.
To understand the evolution of the monarch butterfly (Danaus plexxipus), it is important to understand first where it evolved. On this point, there has long been disagreement, although the most recent evidence may put an end to the dispute. Some researchers believe that it evolved in South America, then split into different subspecies (Ackery and Vane-Wright 1984). Others believe that it is an "Old World" species which subsequently evolved in the "New World", after being transported here by one means or another (Poulton 1909).
Puolton's theory surmised that D. plexippus is a subgenus which diverged from genus Salatura about 250,000 years ago (Poulton 1909). He placed the location for this event somewhere in Asia, and theorized that after the divergence occurred, D. plexippus invaded North America by way of the Aleutian Islands. The fact that the northern areas would have been icebound at that time casts doubt on that theory (Sutcliffe 1985). Kitching et al. suggested that if D. plexippus diverged from a different species about 1.75 million years ago, this theory might be valid, as there seemed to be a period of warming at about that time (Sutcliffe 1985). Poulton based his theory on evidence that there was a lack of Batesian mimics in the Americas, relative to other continents (Poulton 1909).
The most recent reclassification of the genus Danainae, also known as the milkweed butterflies, was in 1984 (Ackery and Vane-Wright). This reclassification divided the genus into two "tribes", the Euploeini and the Danaini. Both of these tribes were further subdivided into two genera each. Both the tribes and their subdivisions are South American species, but are very closely related to the North American genera, Danaus Danaus plexippus (Kitching et al.) This is the species which migrates annually through North America into its wintering grounds in Mexico (Urquhart 1988).
The reclassifications are based on several factors. Different genera were termed "sister-species" based on phenotypic similarities observed in the pupae (Kitching 1985). Examples of these observed factors are spots on the abdomen of certain genera which corresponded with other species, as well as smoothness of the skin of the abdominal segments. Genotypic relationships were also established by analysis of allozymes (Eanes and Koehn 1979).
According to Ackery and Vane-Wright, these observations indicate that D. plexippus is closely related to D. erippus, a southern South American species. Both of these as a subgenus share a sister relationship with D. cleophile, a species endemic to the Caribbean. This evidence supports the theory that the genus Danainae evolved in Central America and the northern part of South America, and is endemic to those areas (Ackery and Vane-Wright 1984). The recession of the ice sheets in the last 12,000 years would have allowed D. plexippus to migrate northward and D.erippus to migrate to its present range in South America (Sutcliffe 1985).
Based on genetic and phenotypic evidence, the theory that D. plexippus is a descendant of a divergent genera which migrated to the Americas 250,000 years ago suffers. The evidence instead suggests strongly that D. plexippus evolved as a species endemic to the Americas.
The monarch has adapted or evolved several interesting traits which have helped it become so successful. One of these is a well-documented ability to make itself unpalatable to birds by synthesizing the nectar or the milk of the milkweed into a bitter substance (Bates,1862). This is an example of Batesian mimicry, a process whereby a species will adopt certain qualities of its food source. This tendency in the monarch has served as the model for a long-held belief among scientists that other species of butterfly do the same thing. This idea is coming into question, and some researchers are beginning to believe that certain species may be intrinsically unpalatable to birds (Ackery,1993). It is now believed that the monarch is one of the only species of butterfly to exhibit the ability to make itself unpalatable, while not being intrinsically so. It is also a well-established theory that the Viceroy butterfly has adapted itself as a species to appear similar to the monarch to take advantage of birds aversion to the bitterness of the monarch, in another example of Batesian mimicry (Brower 1958).
The Monarch Butterfly has a wide distribution, much wider now than in the past and widening all the time. The reasons for this continual increase in distribution are numerous, but the main reason is that since its main host plant, Milkweed, is widely dispersed, the Monarch can successfully colonize a wide area. Although endemic to North and Central America, its existence is threatened in its original habitat for a number of reasons. Furthermore, being migratory, its distribution changes significantly over the course of a year.
The home range of the monarch has exploded from North America and Central America to most of the world between 45 degrees north and south (Urquhart,1987). The North American population exhibits a yearly migration for which it is well known, but is the only population known to do so. The Columbus Hypothesis offers one explanation for these phenomena. According to this theory, the North American population has not always migrated, but began doing so after the arrival of European colonists. The Europeans destroyed much of the forest that covered North America, and was replaced by milkweed, an opportunistic species that flourished in the deforested areas. The spread of this weed enabled the monarch population to spread as well, resulting in an overspill which eventually colonized a large part of the temperate regions of the world. Furthermore, there is no evidence that monarchs performed such long migrations before about 1865. The fact that populations found in all other regions do not migrate so extensively (in Australia, for example) suggests that this migratory behavior evolved in the North American population after other populations had become established elsewhere (Vane-Wright,1193). However, this theory may need some refinement in the face of findings that monarchs guide themselves on their migrations by use of an internal magnetoclinic compass (Schmidt-Koenig,1993). It seems doubtful that such a trait could have evolved in so short a time, but another explanation could be that other populations have simply lost this guidance mechanism.
The breeding grounds of the Monarch are found in New England, the Great Lakes region, and the northern Rocky Mountains. These are the areas where they will spend the months from May through late August to mid-September (Urquhart 1987, 115). Those which breed in New England or the Great Lakes will migrate southwest to their wintering grounds in the Sierra Madre mountain range of Mexico. Those breeding in the Rocky Mountains will migrate southwest as well, but will end up in California. Their migratory route is well established and is a strong case study for co-evolution, in this case the co-evolution of the Monarch and Milkweed.
Migration routes of the North American population
of the monarch butterfly (Urquhart 1987).
Other interesting issues:
Recently, D. plexippus was at the center of a controversy concerning pollen from bioengineered corn. Palevitz (1990 reported that a study by Losey et al. (1999) found that when pollen from corn which had been bioengineered to be toxic to certain types of pest insects was ingested by monarch larvae, only 56% of the larvae survived. This level of mortality is much higher than that which is found under normal circumstances. Furthermore, surviving larvae were on average less than half the weight of control larvae.
Several issues make this study controversial. First, researchers Laura Hansen and John
Obrycki (Hansen and Obrycki 1999) reported that levels of pollen decline only 3 meters
from a Bt corn field. When they fed monarch larvae leaves with pollen from the corn fields
edge, 19 percent died. Furthermore, even ecologists and butterfly researchers admit that
habitat loss, particularly in overwintering grounds in Mexico, is a greater threat to
monarchs than Bt corn (Palevitz 1999). Nonetheless, this incident does bring up questions
about the effect of bioengineered products on different aspects of our environment, and
points to the monarch as a useful indicator species for environmental problems.
Ackery, Phillip R. Host-plant Exploitation by Aposematic Nymphalid Butterflies. Pre-adaptation vs. Stepwise Sequestration. In, Stephen B. Malcom and Zalucki, Myron P., Eds. 1993. Biology and Conservation of the Monarch Butterfly. From, the 2nd International Conference on the Monarch Butterfly. Natural History Museum of Los Angeles County. Los Angeles, CA. Pp.116-126.
Ackery, P.R. and R.I. Vane-Wright. 1984. Milkweed butterflies: Their cladistics and biology. London: British Museum of Natural History; and Ithaca, N.Y.: Cornell University Press.
Bates, H.W. 1862. Contributions to an insect fauna of the Amazon Valley. Transactions of the Linnean Society of London. 23:495-566.
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Masters, Alan R. Temperature and Thermoregulation in the Monarch Butterfly.In, Stephen B. Malcom and Zalucki, Myron P., Eds. 1993. Biology and Conservation of the Monarch Butterfly. From, the 2nd International Conference on the Monarch Butterfly. Natural History Museum of Los Angeles County. Los Angeles, CA. Pp.145-154.
Poulton, E.B. 1909. Charles Darwin and the Origin of Species. Addresses, etc., in America and England in the year of the two anniversaries. London: Longmans, Green and Co. Pp.302.
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Vane-Wright, Richard I. The Columbus Hypothesis: An Explanation for Dramatic 19th Century Range Expansion of the Monarch Butterfly. Stephen B. Malcom and Zalucki, Myron P., Eds. 1993. Biology and Conservation of the Monarch Butterfly. From, the 2nd International Conference on the Monarch Butterfly. Natural History Museum of Los Angeles County. Los Angeles, CA.Pp.183-185.
Van Hook, Tonya. Non-random Matings in Monarch Butterflies Overwintering in
Mexico. Stephen B. Malcom and Zalucki, Myron P., Eds. 1993. Biology and Conservation
of the Monarch Butterfly. From, the 2nd International Conference on the Monarch
Butterfly. Natural History Museum of Los Angeles County. Los Angeles, CA.Pp.16-18.
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