Monarch Population Status

Abstract
Monarchs are off to a good start and the population should increase modestly from now until the fall migration, assuming normal temperatures for the remainder of the breeding season. While an increase is likely, we still don’t have a good estimate of the size of the returning monarch population. If the population of females returning from Mexico has been low, the number of first generation monarchs heading north through May and the first half of June will also be low relative to other years and will limit population growth through the rest of the season. First sightings for March and April, as recorded by Journey North, show a population more restricted temporarily and spatially than recorded for any of the previous 10 years. The size of the overwintering population last year was 1.92 hectares (“Monarch Population Status” January 2010) and, with a modest increase this summer, the population might get back to this number. If the conditions for the rest of the summer are highly favorable, a winter population of 4 hectares is possible but that doesn’t seem likely at this point. In any case, the winter population of 2010 is certain to be below the long-term average of 7.44 hectares.

Analysis
I spend a lot of time fretting over the status of the monarch population and I’m always searching for factors or data that will help me understand the past as a way of predicting the future trends in monarch numbers. Since the status report of 2 March, I’ve been pouring over the first sightings records reported on the Journey North website. Like many of you I’ve looked at the Journey North first sightings maps from time to time and occasionally at the individual records but I’ve haven’t attempted to analyze these data.

I’ve looked at the data in some detail over the last several weeks and there are some neat patterns that I will explain in the future. My specific task at this time was to see if the first sightings data contained any clues as to the size of the returning population. I was concerned about this issue since there were no quantitative estimates of the numbers of monarchs that survived the winter storms. To answer this question I looked at the total number of first sightings reported in years with low and high numbers of returning monarchs. Unfortunately, the numbers of first sightings are not related to population size. There are several problems with working with the raw numbers. The numbers of first sightings has increased over the years as the network of monarch observers has increased making it difficult to compare one year with another. There are other problems such as trying to account for the density of observers, which is higher in and around cities compared to the surrounding countryside. In addition, it is difficult to determine if weather conditions such as cold fronts, and rainy periods in particular, affect the number of observations. And, publicity could play a role in the number of observations reported. In years when the population is low, monarchs tend to get a lot of publicity and general awareness of the low numbers might lead to an increase in the reported sightings. Further, since the number of observations does not correlate with the numbers of returning monarchs, does this mean that there is a finite number of people who will report what they see? Given all of these issues with the data, is there still a way it can be used? The short answer is yes, but caution is required. One thing we can do is look for patterns in the data and use proportions of monarchs seen in a give place or by a given date to compare one year with another.

I’m not going to dive deeply into the data but I will show you briefly how this year compares with others – and this returning population has been most unique. One way to look at the data is to ask how many of the first sightings reported in March* come from Texas and how many are reported from areas outside of TX. On average about 78% of the March first sightings are from Texas, this year an astonishing 96.8% of the first sighting were from sites within Texas. April was also distinct in this regard, since 8 of 22 (36.4%) observations from 22-30 April** were reported from Texas. In 7 of the last eleven years no Texas sightings have been reported during this period and the remaining years had 2, 2 and 1 Texas sightings. Overall, combining March and April sightings, 73% were from Texas, more than 20% higher than the long-term average. The data clearly shows that monarchs were limited to Texas this spring more than in any of the previous 10 years. What does this mean? Was the dispersal of monarchs limited this spring because of the lower than average temperatures or because the population is low or some combination of both? The answer is probably the latter; a combination of low numbers of returning monarchs and lower temperatures. Further analysis is needed.

As I pointed out in the addendum to the last status report, low numbers of returning monarchs could limit the amount of increase in the population we might expect in the breeding season even under the most favorable conditions. Assuming the returning numbers have been low, and there is no evidence to the contrary, what might we expect for the fall migration and the overwintering population in Mexico. Actually, the report based on the first sightings is not all doom and gloom. The conditions for growth in the monarch population in Texas have been exceptionally favorable the last two months. The temperatures have been moderate and due to adequate soil moisture, the milkweeds and nectar sources have been abundant. In addition, the fire ants have been scarce having not recovered from the prolonged drought of last year. So, small population or not, the monarchs should be off to a good start. Whether the population will just replace itself this year, increase above last year’s numbers or decline once again will be determined by 1) the number of first generation monarchs that come N/NE out of Texas over the next 6 weeks and, of course, 2) the weather conditions throughout the northern breeding range over the remainder of the summer. The May and June first sightings reported to Journey North do not appear to be as informative about future trends in the population as the March and April records but, we will be following them intensely just the same.

*All sightings from Florida and from west of the Rockies have been excluded from this analysis.

**April records used in this analysis were inclusive of 1-21April since an examination of the individual reports show than an increasing number of the sightings after the 21st of April clearly involve first generation monarchs.