Seasonal monarch populations (1991-1997), western monarch population, population genetics and theories in evolution and population genetics.

SEASONAL POPULATIONS

Winter 1996-1997
It is always difficult to obtain reliable estimates of the numbers of Monarchs at the overwintering sites in Mexico, but all returning visitors to the roosts this winter agreed that the numbers of Monarchs appeared to be much higher than previous years. One estimate placed the number of wintering Monarchs at 160 million which is much higher than the estimates of 60-120 million arrived at by various experts over the last several years. However, no two experts agree on the numbers each year and there doesn't appear to be a universally accepted way of estimating the size of these overwintering populations.

Spring 1996: Off to a Slow Start?
Although the numbers of Monarchs at the roost sites in Mexico were normal or above normal at the end of the season (early March) in spite of the snowstorm in December, unfavorable spring conditions appear to have hampered their recolonization of the breeding range. Monarchs began to leave the roosts in late February and early March but as they moved into northern Mexico and Texas, they encountered extreme drought conditions. The limited number of nectar producing flowers and the drought delayed emergence of the milkweeds seemed to reduce egg laying by Monarchs in Texas. In Fact, Bill Calvert reports that the number of adults seen, as well as the numbers of eggs and larvae found subsequently on the plants was extremely low. Reports of Monarchs to MW and Journey North from the midwest have been few a scattered. In Kansas, the first Monarchs are usually reported 6-10 April in the south central portion of the state. This year there were no reports for April and only Monarch reported so far was one seen by Chip Taylor on 16 May in Lawrence.

Drought conditions from northern Mexico combined with an unusual jet stream and spring moisture pattern may have had the effect of causing Monarchs to move in an easterly and northeasterly direction rather than northward through the dry regions of Texas, Oklahoma and Kansas. At this time (20 May), there are more reports of Monarchs from the eastern states, and even Ontario, than there are for most of the midwest. This is the third year in a row that the Monarchs appear to have moved up through the eastern states faster than they have the central midwest. Is this an artifact of the number and distribution of observers or is this pattern a consequence of weather factors that have the effect of limiting movement in the midwest while favoring flight to the northeast? Does anybody have an enterprising student who would be interested in relating patterns of spring Monarch sightings to weather conditions?

Winter 1995-1996
The numbers of monarchs reported throughout the range east of the Rockies were much lower than in the previous three seasons of the MW. High concentrations of monarchs were reported for Kansas, two locations in Oklahoma and several sites in Texas but these were the exceptions. Given the trend in most of the reports and the low tagging success throughout the northeast, our expectation was that the numbers of monarchs reaching the roosts in Mexico would be lower than normal in the winter of 1995-96. This does not appear to have been the case but it is difficult to be sure of the total numbers of monarchs roosting in Mexico. In some years, all the roosts sites are visited by monarch experts and these researchers give a rough estimate of the overall population. These estimates usually range from 60-100 million. Only two of the roosting areas were visited this year, Sierra Chincua and El Rosario. The numbers of monarchs at Chincua appeared to be normal but several monarch specialists reported greater numbers of monarchs at El Rosario than usual.

Assessment of the conditions at the roosts and the general state of the overwintering population were complicated and more than a little confused by the differing assessments of the effect of the snow storm which dumped 13-27 inches of snow throughout the Transvolcanic mountains from 29-31 Dec. Fortunately, the early estimates of 30% morality due to the storm were excessively high. Post storm assessments by several groups working independently placed the mortality rate at 5-7%.

Fall 1995
Monarch populations were extremely low in the northeastern states in the fall of 1995. Extreme drought prevailed from Ontario to Maine and through much of New York and Pennsylvania. There were many disappointed taggers throughout these states. Low numbers of Monarchs were also reported for areas to the south of this region such as Ohio, Indiana, Tennessee, and Kentucky. Along the east coast the numbers were also down, particularly at Cape May Point in New Jersey where Dick Walton has been conducting a migration survey for several years. Nevertheless, a modest number of Monarchs were successfully tagged from New Jersey through North Carolina.

The midsummer Monarch populations in the midwest seemed to be below normal and only at the end of August did it appear that populations were at the usual fall levels in the northern states. Nevertheless, the numbers of Monarchs seen moving south in Iowa and Nebraska were not remarkable and there was nothing to prepare taggers in Kansas for what happened in mid September. Monarchs arrived in large numbers on the 14th across central Kansas on a line extending from Olathe in the east to Sharon Springs in the west. Subsequent to their arrival, unseasonally cold weather delayed the migration for 4-7 days, depending on the location, and during this interval students at schools from Olathe to Hays tagged more than 6,000 Monarchs. The concentrations of Monarchs seen throughout central Kansas in this period appeared to be higher than in the previous three years of the Monarch Watch. Many observers reported seeing trees and hedgerows covered with thousands of Monarchs. In the previous season (1994), the migration stalled in Iowa for about a week and then stalled briefly again in southern Kansas before moving into Oklahoma.

A unique aspect of the migration this year was the ability to receive email messages from observers to the north. These reports allowed us to predict the arrival of the "front" or mass of the migration with some accuracy and to warn taggers to the south of the imminent arrival of the Monarchs.

Spring 1994
Rains in northern Mexico and southern Texas in early March produced an abundance of flowers upon which northward bound Monarchs could feed, and the spring migration has been progressing well. Monarchs are appearing at many locations "on schedule". Bill Calvert reported seeing his first Monarch of the year on 12 March in Austin, Texas. In the following week, many observers along the Texas coast reported seeing good numbers of Monarchs (30 per day). Mary Buford reported Monarchs in Enid, Oklahoma at the end of March, and they were first reported from Buhler and Hutchinson, Kansas on 16 April by Linda Wilbanks. We sighted our first Monarch in Lawrence, Kansas on 20 April. Normally, our first sightings in Lawrence occur between 7-21 April.

This year the northward migration is being monitored by "The World School" "Journey North" through Internet (see Internet address below). The same information can be accessed through Prodigy. The pattern of the migration this year indicates a more rapid spread into the southeastern rather than the midwestern states. This is not surprising since the weather patterns were not favorable for movement of Monarchs into states at the latitude of Kansas (37-40° N) until mid-April. In fact, at this writing (20 April), Monarchs arriving in eastern Kansas are ahead of their host plants as no new growth has appeared so far (However, 6 days later the milkweed plants were 2-6 inches in height). In general, it looks like the 1994 populations are off to a good start.

Winter 1993-1994
Alfonso Alonzo, a graduate student with Lincoln Brower at the University of Florida reports that 9 roosts were formed in the Transvolcanic mountain range west of Mexico City. In some years 11 roosts have been reported. The numbers of roosting Monarchs at the 6 sites visited by Alfonso appeared to be average or slightly above average. However, these generalizations should be viewed with some caution until Alfonso summarizes the dimensions of the roosts and apparent density of the overwintering populations. If habitat degradation is concentrating the Monarchs to fewer roosts than normal, the population could appear to be in good condition when, in fact, the numbers are actually lower. Winter conditions at the roost sites were moderate and no unusual mortality was reported. According to Alfonso, the first Monarchs began to move northward on March 6th but at Chincua, one of the main roosts, six trees still contained roosting Monarchs on 10 April.

Summer and Fall 1993
In Lawrence, Kansas Monarchs arrived on the 9th and 10th of September. These dates are within normal arrival times for the migration but were a little earlier than last year. Monarchs reached their peak population from 20-22 September. In many locations, known pathways and roost sites were not productive this year. However, normal or above-normal numbers of Monarchs were reported by Dick Walton and his tagging group at Cape May, and by Don Davis and other observers at Point Pelee National Park. In fact, one report from Point Pelee indicated that the migration was the largest seen in many years. Harriet Clark reported that the most abundant Monarch flights in Milford, Ohio occurred the weekend of September 10-11, about two weeks earlier than in '92. She also noted that in her area there was a drought this year, which she speculates could have reduced nectaring flowers and thus changed the pattern and timing of the migration through the region. In Southern Maryland, John Fales reported that Monarch populations, according to his counts, were much higher this year than in '92. In Rocky Face, Georgia, James Adams observed that wind had a definite impact on the direction of Monarch flight and noted that weather fronts strongly influenced the timing of Monarch migratory flights. As you can see, many factors including geography, weather, and food availability influence migratory patterns.

Spring 1993
In each of the last two years Monarchs started to disperse from the roosts by the end of February. This breakup of the roosts was 2-4 weeks earlier than usual. Factors such as warm winters or drifting smoke from spring fires might cause Monarchs to leave the roosts early. The effects, if any, of this early migration northward are not yet clear. Normally, a few overwintering Monarchs reach Lawrence, Kansas, by mid-April, but none were seen by ORT until May and only a few were spotted by other Kansas observers in early May. In much of the Midwest the spring was cold and wet and it seems probable that overwintering Monarchs died out before reaching the lower Midwest (Kansas, Missouri, Nebraska, southern Iowa and Illinois).

Winter 1992-1993
Observations made by Mexican authorities and other observers indicated that some Monarchs reached the roosts by the first week of November.

In February, a team of scientists and Mexican authorities visited all the known Monarch roosting areas. Members of this group estimated that 85-93 million Monarchs overwintered at these roosts in '92-93. Whether these are high or low numbers is not clear, because in previous years not all the roosts were surveyed at the same time. Estimates of populations at the best known roosts, "El Rosario" and "Chincua", seemed to indicate that these overwintering populations were of "normal" size. Even though the Monarch population was low in the eastern United States, this reduced population did not have a clear impact on the number of overwintering Monarchs.

Summer and Fall 1992
The summer of 1992 was one of record low temperatures through most of the northern states. Cloudy and rainy weather was such a memorable feature of the summer that many "ol' swimmin holes" were hardly visited, berry crops failed, and bears went hungry, and so on. Nevertheless, the Monarchs did well , and the fall migration In the Midwest, at least, was excellent. In the East, where systematic Monarch counts were made by Dick Walton and his colleagues at Cape May, the number of Monarchs was approximately 5% of what it had been the previous year. Clearly, from this report and many others, the number of Monarchs in the northeastern United States was unusually low in 1992.

Several observers reported that extremely large numbers (perhaps millions) of Monarchs reached the United States/Mexican border area near Del Rio, Texas in the last few days of September. This occurred at the same time that large numbers of Monarchs reached Austin, and southeastern Texas. Reports from Austin indicated that Monarchs were abundant from the end of September until the first week of November. Overall, the pattern of reports from Texas seemed to indicate that the fastest-moving front of the southward migration was in west Texas, and that Monarchs funnel into northeastern and eastern coastal Texas from the North and East over much longer periods.

Spring 1992
Very few Monarchs were reported along the Texas and Louisiana coasts in March and April, and the number of Monarchs reaching the eastern third of the United States was extremely low. Conditions in the Midwest were much better. Dick Walton reported seeing good numbers of Monarchs in the vicinity of Uvalde, Texas in mid-April and in May there were reports from Minnesota, Wisconsin, and Michigan of the appearance of Monarchs earlier and in larger numbers than usual. In retrospect, it appears that substantial numbers of Monarchs moved northward through Texas west of San Antonio; from there they fanned out to colonize the upper Midwest.

WESTERN POPULATION

Under the leadership of Dr. Lincoln Brower, 14 scientists have coauthored an article which appeared in the September 1995 issue of BioScience and outlines numerous reasons for not mixing eastern and western Monarch populations. Teachers may find this article useful for class discussions. The question "why should transfer between eastern and western Monarch populations be discouraged?" could be presented to the class. The article could then be used as a guideline to lead students by questioning, through many of the same biological and practical considerations presented by the scientists.

For individuals living west of the Rockies and interested in Monarch conservation, check out our page on Other Monarch Programs

POPULATION GENETICS

Another way to answer this question involves looking at the population genetics of Monarchs. Population genetics, which combines theories from evolution and genetics, studies how genes are distributed in a population. By using the tools of population genetics, biologists can evaluate the distribution of genes in Monarch populations to get a better idea of how groups of Monarchs move around and mate. Some distributions would indicate that Monarchs stick together in groups and tend to mate within their own group, while other distributions would show that Monarch populations mix either in the summer, in the winter, or during both times.

Two experiments have investigated the population genetics of Monarch butterflies, and they found some interesting and surprising results. To help you better understand the ideas behind those studies, we encourage you to go review Theories in Evolution and Population Genetics below before reading the summaries of these studies.

Genetic Structure of Summer and Migratory Monarchs

Eanes, W.F. and R.K. Koehn. 1978. An analysis of genetic structure in the Monarch butterfly, Danaus plexippus L. Evolution 32(4): 784-797.

Eanes and Koehn studied the genetics of different Monarch populations in the early 1970s. They collected 20 different sets of samples, both during the summer and during migration. Using electrophoresis to examine the same protein in different individuals, they found that Monarchs have allele frequencies that sort out into groups somewhat in the summer and become uniform again during migration. These results indicate that Monarchs divide into slightly isolated populations during the summer but mix together during migration (and, they assume, in the winter roosts although the roosts had not yet been discovered when they did this research). Migratory populations and roosts, therefore, include individuals from all over North America; all the Monarchs from a particular summer region do not necessarily overwinter in the same place, and their descendants may not return to the same region the next year. The mixing that happens during spring mating in the roosts overwhelms any genetic differentiation that occurs during summer in isolated populations.

Eanes and Koehn found another interesting pattern in allele frequencies. For three of the eleven proteins they studied, there were more heterozygotes than expected. In at least once case, males and females also differed in which allele they were likely to have (that is, males more often had one version of the protein while females more often had the other version). When alleles have different average frequencies in males and females, mating will more often produce heterozygotes. For Monarchs, there are still many unanswered questions about whether mating behavior results in different allele frequencies between sexes and what causes increased heterozygosity in the population.

DNA Variation in Monarch Butterflies

Brower, A.V.Z. and T.M. Boyce. 1991. Mitochondrial DNA variation in Monarch butterflies. Evolution 45(5): 1281-1286.

Brower and Boyce studied the mitochondrial DNA (mtDNA) of Monarch butterflies from the United States, Mexico, and the West Indies to see how similar or different their genetic material was. They were especially curious about whether the eastern and western populations of Monarchs in North America were genetically different; the eastern population overwinters in Mexico while the western one overwinters in California, and there is no evidence that these two populations ever interbreed. They looked at variation in mtDNA using restriction enzymes, a technique that identifies differences in DNA sequences. If one population, or individual, had a small change in its DNA, this technique can reveal that change. In some other insect species, studies have found that there are big differences in individuals' mtDNA between regional populations, and sometimes even within a region.

To their surprise, Brower and Boyce found almost no variation in any of the Monarch populations' mtDNA, including the ones from the West Indies. Using 13 restriction enzymes, they found only two individuals with a single difference in one site, and they attribute this difference to a single base substitution. This level of similarity in the DNA from geographically isolated populations is dramatically different from most other studied groups of animals. Vertebrates, for example, have differences at 10 times this level while other insects show differences in mtDNA even within a population.

The most plausible explanation Brower and Boyce have is that all of these Monarchs underwent a bottleneck in recent evolutionary time. Bottlenecks reduce the genetic diversity in a population (for another example, read about cheetahs) because only a small number of individuals and their DNA serve as the ancestors for the present populations. Since mtDNA is maternally inherited, it seems likely that sometime in the recent past there was a significant reduction in the number of females who reproduced. Since that bottleneck, enough time has not passed for major changes to have occurred.

THEORIES IN EVOLUTION & POPULATION GENETICS

In the beginning of the last century, scientists began to understand genetics and how offspring inherited traits from their parents. Figuring out how offspring inherited traits from their parents had stumped Darwin, and twentieth-century biologists soon recognized the importance of genetic theory to natural selection and evolution. By the 1920s, a new field of science had emerged called population genetics. These scientists expanded the scope of genetics, which had focused previously on how individuals inherited and passed on genetic information, by studying how genetic change occurs in populations. As scientists combined ideas from population genetics with Darwin's ideas about evolution, a modern synthesis theory of evolution emerged in the 1940s. This synthesis, which includes most of Darwin's ideas but focuses on populations, combines population genetics with natural selection and creates a powerful tool for examining evolution in action in the natural world.

One of the most important tools population genetics gave to the study of evolution was the principle of Hardy-Weinberg equilibrium. This principle states that there is nothing in gene replication, meiosis, fertilization, or reproduction that changes the frequency of gene alleles over time. As long as no other forces act upon the population, a gene in that today has 25% allele "A" and 75% allele "a" will still have 25% A and 75% in a million years.

The Hardy-Weinberg equilibrium is based in five assumptions, which, if they held true in nature, would create a situation where no other forces were acting upon a population and no change in gene frequencies (evolution) would occur. These assumptions are:

1. The population is infinite, isolated, and panmictic, where all individuals have an equal chance of mating with any other individual in the population.
2. There is no mutation in the genetic material of the population.
3. There is no gene flow in the population (no individuals leave or join the population).
4. There is no genetic drift.
5. There is no difference in reproductive success between members in the population (natural selection is not acting on the population).

Since these are the assumptions that must hold true for the Hardy-Weinberg equilibrium to be maintained, their opposites are the causes of evolution, or the change of allele frequencies in populations. For example, mutation can cause changes in allele frequency by creating new, altered genetic material while gene flow can change allele frequencies by introducing new alleles to the population through immigration or removing alleles through emigration. Of these five causes of evolution, only the last one involves natural selection, or directional change imposed by survival of the fittest in a harsh environment. The first four, all of which can play critical roles in evolution, involve chance events.

By studying the allele frequency in a population, and doing some math, ecologists can determine whether allele frequencies occur as predicted by the Hardy-Weinberg equilibrium principle. Hardy-Wienberg allele frequencies are likely with large populations that have lots of gene flow and random mating, and are unlikely if populations are small, isolated, or have non-random mating patterns. If a study reveals allele frequencies that do not match Hardy-Weinberg predictions, the population is probably structured in some way (e.g., is made up of more than one isolated subpopulation or has non-random mating). This is what two teams of researchers have done with monarch butterflies.