Each summer, female monarch butter-flies flutter around their breeding grounds in northern North America in search of nectar, a mate, and a milkweed plant on which to lay eggs. And they have an audience. Thousands of volunteers periodically survey the charismatic black and orange insects, helping scientists track population trends. Others count monarchs as they migrate south each fall to warmer climes. Now, one of the most systematic analyses of these tallies and other data has raised questions about how well researchers understand why monarchs have seen a dramatic decline on their major wintering grounds in Mexico.
The confusing picture emerges from seven monarch studies published this week in the Annals of the Entomological Society of America. One paper suggests that, even as wintering populations of monarchs have plummeted over the past 2 decades, there’s been no similarly steep decline in a key summer breeding area that stretches across the midwestern United States and southern Canada. Others find that some fall migration counts also show no major downward trend. At the same time, the butterflies may be laying fewer eggs overall, concludes one study.
Such findings “present a puzzle,” says ecologist Leslie Ries, now of Georgetown University in Washington, D.C., and an author of one of the studies. They “make me wonder if we really have the strongest evidence to be able to say we know what’s causing the decline that we see in Mexico.” The uncertainty could pose problems for conservationists trying to protect the butterflies.
The ambiguity is fueled, in part, by the monarch’s complex population structure and life cycle, which includes a lengthy migration completed, relay-style, by several generations of insects (see graphic, below). Each spring, many monarchs head north from Mexico to the southeastern United States, where they produce the first of up to three generations that can swell the total population. The offspring then spend the summer feeding and breeding on milkweed throughout the northern United States and southern Canada. In the fall, a final generation makes the trek back to Mexico. (Other regional monarch populations have different migration and breeding patterns.)
The cycle can make it hard to get a handle on summer and fall monarch numbers, but researchers agree on one thing: Over the past 20 years, the number of monarchs returning to the largest known wintering ground in central Mexico’s highland forests has plunged by more than 90% (Science, 7 February 2014, p. 583). Many have blamed the decline primarily on the expansion of herbicide-resistant crops in the summer breeding grounds, which has led to the wide use of chemicals that kill milkweed. But few studies have systematically examined monarch population trends in these areas.
The new papers help fill that gap. One draws on 18 years of monarch sightings collected by citizen scientists. Between 1997 and 2014, at least some monarchs were spotted in all of the butterfly’s historical eastern breeding range (some breed in the far west), suggesting the summer population is hanging on. And a second study, relying on hourly monarch counts made by volunteers at hundreds of summer breeding sites between 1993 and 2014, found no statistically significant population trend, up or down.
There’s “a disconnect” between those results and the wintering data, Ries says. That’s because if milkweed loss is driving the winter decline, then summer populations seemingly should be shrinking, too.
One possible answer, Ries and others say, is that the volunteers may have under-sampled agricultural areas hit hard by herbicides, instead favoring sites popular with butterfly-watchers, such as parks and protected areas. Another view is that the last 7 or 8 years of the summer numbers do contain evidence—albeit statistically weak—of the monarch’s decline. “If they had 9 or 10 years [of data], it probably would be [statistically] significant,” contends entomologist Lincoln Brower of Sweet Briar College in Virginia, who was not involved with the studies.
Other researchers say the egg-laying study suggests summer monarchs are struggling. It found that monarch egg densities on milkweed plants have declined since 2006. “It’s pretty scary,” says biologist and co-author Karen Oberhauser of the University of Minnesota, Twin Cities. “It probably means there aren’t enough monarchs out there to find the remaining patches” of breeding habitat.
The results have also put a spotlight on the need to better understand what is happening to monarchs during their fall migration south, researchers say. Two of the studies examine counts of migrating monarchs made from the mid-1990s to 2014 at stations in northern Michigan and Ontario, Canada. The Ontario tallies show a moderate decline, whereas the Michigan numbers show no clear trend. That suggests monarch deaths occurring farther south during the migration could be responsible for the sharp winter decline, says ecologist Andrew Davis of the University of Georgia, Athens, a co-author of two of the papers. Migrating monarchs run “a gauntlet of dangers,” including predators, parasites, and even speeding cars, he says. “There is a tremendous amount of mortality, and we don’t know how much.”
The biggest knowledge gap is for Texas, Brower says, which is in the middle of both the main fall and spring flyways. Researchers “desperately” need monitoring data from that throughway, he says.
Such studies could help ensure that government agencies and environmental groups focus their time and money on the right solutions. Currently, the U.S. Fish and Wildlife Service and others have focused on conserving or restoring milkweed breeding habitat. “But if the problem is that the monarchs are dying during the migration,” Davis says, “I’m not sure just trying to produce more at the start of the [fall] migration is the answer.” Other steps, such as protecting migratory pathways, may also be needed. The concerns over migration are real, says ecologist Ernest Williams of Hamilton College in Clinton, New York, but “they should be added to—but not replace—the other issues we know to be affecting monarchs.”
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Write a Scientific Paper
On a separate page, write your title, and the names and locations of people doing the study. Your title should summarize the question you studied. The title page should include the information in the following example:
Effects of different host plant species on growth rates and larval survival in Danaus plexippus
Ayan Abdinur1, De Cansler2, Janelle Firl2,
Julia Goldburg2, Mary Watson1 and Mai Phia Yang1
1 Century High School, Rochester MN 55901
2 Willow Creek Middle School, Rochester MN 55904
Your abstract should be a concise summary of your question, methods and results. Many people find it easiest to write the abstract last. It should contain no more than 200 words. A useful format is to recall what you did, explain the purpose, state the results, and finally summarize the implications of these results. Here is a 99-word example from one of Karen Oberhauser’s papers:
We studied the relationship between the timing of mating and oogenesis in monarch butterflies to determine 1) the potential for male nutrient input into eggs and 2) whether mating stimulates egg development. Most females mated soon after they started maturing eggs. (this sentence tells what we did and why we did it) One and two days after mating, females contained the same number of mature oocytes as virgin females of the same age, while three days after mating they contained more mature oocytes than did virgins. (this sentence summarizes the results of the study) These results confirm the potential for male-derived nutrients to augment oocyte production, but indicate that mating is not required for oocyte maturation to occur. (this sentence tells the implications of the study)
Your introduction should include a fairly detailed summary of the question you addressed, with some background on this problem. You want to convince people that this is an important and interesting question. You may want to do some research to learn about related studies, and discuss them in this section. For example, one of Sonia Altizer’s papers includes the following introduction:
I explored geographic variation in host resistance and parasite virulence among populations of monarch butterflies infected with the neogregarine protozoan parasite, Ophryocystis elektroscirrha. (this sentence tells what she did) Monarchs and this parasite are distributed worldwide, and parasite prevalence is highly variable among populations (Ackery and Vane Wright 1984, Leong et al 1997, Altizer et al. 1998). One possible cause of this variation in prevalence is that populations have genetically diverged with respect to host susceptibility or parasite infectivity. (these sentences give some background information, with references, and introduce her approach to studying the cause of existing variation)
After introducing the topic, briefly describe your research procedure, and then list your hypotheses (it is OK to have more than one hypothesis). Here is an example of the end of Sonia’s introduction to this paper:
To test the potential for genetic differences in hosts and parasites among populations, I conducted cross-inoculation experiments with hosts and parasites from three North American populations. Because virulence is often associated with the degree of parasite replication within hosts, I measured both host survival and the parasite loads of inoculated monarchs. (this sentence explains what was done and why) I predicted several effects of host and parasite origin, including 1) higher replication of parasite strains on native hosts, indicating that parasites are locally adapted, 2) lower replication of parasites strains on native hosts, indicating that hosts are resistant to local parasites, 3) higher host resistance among the longest-distance migrants, resulting from an increased cost of infection, 4) higher parasite virulence in non-migratory populations due to increased horizontal and vertical transmission opportunities, or 5) no effect of either host of parasite origin on host survival or parasite replication. (numbers 1-4 are several alternative hypotheses, some of which are mutually exclusive (1 and 2), and 5 is the null hypothesis)
This section should be a brief, concise summary of what you did. It should be detailed enough that someone else could repeat your study, but should not go into long, boring detail (e.g. We obtained milkweed plants from an unmowed area behind our school is better than During third period we walked to an unmowed area 0.5 km from our school. We picked milkweed plants from this area, then carried them back to our classroom). You should include locations, dates, and sample sizes in this section.
This section will summarize the answers to your question. Before you start writing the Results section, list everything that you learned, and decide what is most important and how to organize your results to make the important points. The section should include tables, charts and graphs to illustrate these points. As a general rule, graphs or charts (called figures) do a better job of making a point than tables, but it is not always possible to get all of the information you need into a figure. Tables and figures should both have captions. Whenever you use a table or figure, be sure to refer to it by number in the text of the results section. Never include a table or figure that you don’t don't discuss in the text. Look at other reports on this site for examples.
In this part of the report, summarize your findings, and discuss their implications with respect to your hypotheses. If relevant, compare your results to those found in similar studies in the past. In addition, you could suggest future directions for research. If you feel that there were methodological problems with your work, mention them here, and state how they might have affected your results. If you think that your results were inconclusive, state what you might want to do differently in the future.
In this section, acknowledge people who helped you with this work. For example, you could say "I would like to thank my mother, Sylvia Plexippus, who checked my cages while I was at school." Here is an example from one of Karen’s papers:
I thank De Cansler, Ann Feitl, Rachel Hampton, Brenda Jenson and Christine Jessup for help counting and weighing eggs. Don Alstad, Carol Boggs and Christer Wiklund provided helpful comments on the manuscript. Research was supported by the National Science Foundation (DEB-9220829).
List all of the published sources you used to get information for your report. These should include the author’s name, year of publication, title of article or book, publisher or journal, and issue and pages for journal articles. Below are examples of citations for a book and article:
Choe, J. C. & B. J. Crespi. 1997. Mating systems in insects and arachnids. Cambridge University Press.
Oberhauser, K.S. and R. Hampton. 1995. Relationship between mating and oogenesis in monarch butterflies. J. Ins. Behav. 8:701-713.
If you use websites, give the organization and the address of the site. (e.g. Monarch Joint Venture website: www.monarchjointventure.org)