The Monarch Butterfly. popular culture. A recent article by San Grewal published in the Toronto Star gives a

Transcription

1 The Monarch Butterfly The monarch butterfly (Danaus plexippus L.) symbolizes freedom and rebirth in popular culture. A recent article by San Grewal published in the Toronto Star gives a copious amount of information regarding the monarch and arms that these beautiful creatures' overwintering habitats are being threatened by anthropogenic stresses. The task at hand is to verify the scientific validity of the information which is conveyed in the article. The article first states that the monarch originates between the Rockies and the eastern part of Canada and the United States. Although no evidence was found to support or refute the mountainous origins of these insects, there is considerable evidence that the overwintering monarchs found in Mexico originate in the entire eastern half of southern Canada and the United States as stated in the article. Previously, observations along with tag and recapture methods were used to determine the origins of the monarch. However these methods never quantified natal origins nor distinctiveness due to the fact that few tagged monarchs make it back to Canada or the United States (Wassenaar et al. 1998). More recently, the keratin in the monarch's wings were analyzed for their stable-hydrogen (8D) and carbon (8"C) isotope ratios with the hope that these elements would reveal the specific natal origins of the monarch. The results of the experiments showed that the monarch's isotopic composition is highly correlated to the isotopic composition of the natal milkweed plants on which the monarch larvae fed which in turn correspond closely

2 to the geographical pattern of deuterium in the rainfall of a given area (Hobson et al. 1998). Thus, using the wings of dead butterflies in Mexico, scientists were able to accurately determine the North American origins of each particular colony (Wassenaar et al., 1998). Next, the article claims that the monarchs must ride wind currents in order to reach Mexico and are frequently blown off course. This turns out to be true since monarchs are poor fliers and must often rely on winds and thermals to reach their destination (Etheredge et ad. 1999). The article also says that it was not until 1975 that the overwintering grounds of the monarch butterfly were discovered, a fact which is confirmed by Wassenaar and his colleagues (1998). In his article, Grewal also says that it was a man named Fred Urquhart who discovered that the monarchs overwinter in a tiny volcanic forest in central Mexico. This statement is supported by Hobson and Wassenaar who also give credit that Urquhart did indeed discover that the monarchs overwinter in the Oyamel forests located between the states of Michoacan and Mexico, in the central part of Mexico (1998). According to Grewal, ever since the overwintering grounds of the monarch butterfly have been discovered, these animals have been subject to various anthropogenic stresses due primarily to the ecotourism industry as well as deforestation. Grewal states that the presence of ecotourists kicks up dust off the ground which, along with flash cameras, harms the butterflies. Grewal also reports that many makeshift villages have sprung up around that area in order to greet the ecotourists. As a consequence of this,

3 many butterflies have been forced to higher and more remote areas in the Oyamel forests. Grewal says that scientists and conservation agencies are equally worried that the monarch will be forced away from these unique grounds due to the uncontrolled ecotourism that keeps going on. Grewal then introduces his audience to a man named Jose Valdez, who makes good money (by Mexican standards) by guiding the ecotourists up to the forest to see the monarchs. According to Grewal, Valdez hopes that the money he makes off the monarchs will enable him to search for a better life in Mexico City. Unfortunately, no evidence was found that either supports or refutes the impact of ecotourists on the overwintering monarchs. The article also states that these forests are subject to deforestation which, along with the accompanying soil erosion, will alter the delicate ecosystem of that area. The destruction -of the Oyamel forests has been confirmed by Wassenaar and Hobson (1998). They state that a thinning of the Oyamel forests in the vicinity of the monarch's wintering roosts will eventually lead to an irreversible loss of these particular habitats. They also point out that the milkweed plant, which is crucial in the monarch's lifecycle, is often considered a pestilence and many of these plants are frequently sprayed by pesticides in an effort to get rid of them (Wassenaar et al. 1998). The most compelling evidence that supports Grewal's statement comes from Anderson & Brower who have studied the thinning effects of the Oyamel forest on the monarch butterflies. Even though the monarchs have moderate supercooling abilities, their results show that avoidance of wetting and a lack of exposure to the clear night sky play a pivotal role in enabling the overwintering monarchs to resist freezing. They have observed that butterflies with no

4 water on their bodies freeze at cooler sub-zero temperatures when compared to those with water on their bodies. Also, they exposed the butterflies to variable experimental exposures to the clear night sky and found that openings in the forest canopy increases body cooling and decrease the monarch's body temperatures by as much as 4 C. However, monarchs under dense forest cover had body temperatures that were the same as the surrounding air, but exposed monarchs had body temperatures below that of the ambiant air in direct proportion to the amount of open-air exposure. They also demonstrated that wetting and open-air exposure are synergistically related. Wet monarchs that are exposed to moderate levels of open forest canopy froze at -8 C whereas dry monarchs which were not exposed to the night sky froze at -0.5 C (Anderson & Brower, 1996). Another interesting study that supports the article's statement and is related to deforestation was conducted by Alonso-Meijia et al. The logging industry in Mexico was pressuring the Mexican government to grant them a logging permit which would give them access to Monarch Butterfly Special Biosphere Reserve saying that increased tree cutting would create more forest openings in which more plants could flower. They argued that an augmentation in the availability of nectar would increase the monarch's overwintering lipid reserves. In order to investigate this hypothesis, three physical characteristics were compared between monarchs collected on trees versus those collected on flowers. The researchers found that throughout the overwintering period, clustered tree-dwelling monarchs had higher water content, lean mass, larger wings and a higher lipid mass than flower dwelling monarchs. This significantly lower lipid mass in flower dwelling monarchs has serious implications for migration. Alonso-Meijia and his

5 colleagues found that flower dwelling monarchs may not have the energy reserves that are needed to migrate back in the springtime due to the high metabolic cost that is associated with migration (Alonso-Meijia et al.). It is therefore clear that the dense cover which the Oyamel forest provides plays an important role in the prevention of freezing and lipid conservation of the monarch butterflies. Lastly, and perhaps most interestingly, Grewal reports that the migrating butterflies know exactly where they are going due to their inherent genetic programming and an ability to sense the earth's magnetic field. The latter part of this statement has been experimentally confirmed. Originally, it was thought that the monarchs migrated using the sun as their compass, although this theory could not explain their migratory behavior on cloudy days. Recently however, migratory monarch butterflies were subjected to normal and reversed magnetic fields, as well as amagnetic environmental conditions in a laboratory. It was found that under a normal magnetic field, a group of monarchs would organize themselves into a typical migratory pattern and automatically orient themselves southwest. When exposed to a horizontally reversed magnetic field, the monarchs would reorient themselves in a northeastern direction, and under amagnetic conditions, these insects would lack any particular directionality (Etheredge et al. 1999). The monarch's ability to detect a magnetic field may lie in the fact that they biosynthesize a mineral termed magnetite, which endows them with this ability. It is thought that the area of magnetoreception is the thorax, which contains 65!0 of the monarch's total magnetite (Etheredge et al 1999).

6 Overall, this article accurately reports the scientific information it provides to its readers, indicating that the appropriate research was conducted prior to writing and publishing. San Grewal provides us with a wealth of background information on the monarch, along with the immediate and pressing issues concerning this species' survival. However, it is interesting to note that at the very end of the article, Grewal (via the words of professor David Gibo) questions whether anyone should bother to save the monarch because, as far as we know, its existance has no detectable impact on humans. References: - Etheredge J., Sandra P.M., Orley T.R. & Jander R. (1999) Proceedings of the National Academy of Sciences of the United States ofamerica Wassenaar L.I., Hobson K.A., (1998) Proceedings of the NationalAcademy of Sciences of the United States ofamerica Hobson K.A., Wassenaar L.I., Taylor OR (1998) 0ecologia Grewal, San "The Plight of the Butterfly: Ecotourism Threatens the Winter Homes of the Monarch_ Butterfly." The Toronto Star 26 February 2000: Accession # TST Anderson J.B., Brower L.P., (1996) Ecological Entomology Alonso-Meija A., Rendon-Salinas E., Montesinos-Patino E., Brower L.P., (1997) Ecological