Summer is over for monarch butterflies and this year’s super-generation is flying south. Morro Bay has welcomed the first arrivals from the western subpopulation who winter in California, and butterflies are still leaving Washington. The eastern subpop usually begins migrating to Mexico in August, although in some areas migration was delayed by hot temperatures. Consequences of climate change and habitat loss have reduced North American monarchs to the lowest population levels ever recorded. Compared to 1995, there are at least 90 percent fewer monarchs now.
Monarchs are fussy and don’t migrate when it’s too hot. Decreasing day length, cooler temperatures, and dying back of milkweed (the larval host plant) tell the butterflies when to move south. Monday was the first day of migration through Lawrence Kansas, almost a month later than usual. A group of monarchs were seen on the University of Kansas campus midday and by late afternoon all were gone. Lack of a breeze at ground level suggested ideal conditions for gliding and soaring up higher. The butterflies probably caught a thermal and headed southwest towards Mexico says Dr. Chip Taylor, Ecology Professor at the university and Director of Monarch Watch.
Overall monarchs are delaying their exit from the north this year and larvae are still hatching in the southern areas. If temperatures remain mild, migration season will be longer than usual but the butterflies will arrive at their overwintering habitats. These butterflies migrating right now are the same individuals who will fly north in spring, so next summer’s population depends on successful migration.
Citizen scientists and specialists reported seeing very few monarchs this summer. For example, this year there were 4 larvae in an area where 15 were seen last year. The Monarch Larva Monitoring project collects sighting information from volunteer observers. They estimate the eastern subpopulation this year to be half of last year’s, which was the second lowest recorded in a decade. Look at the first graph below and see the bar for last year, second from right.
As part of the process to determine if monarchs should be listed under the Endangered Species Act (ESA), scientists are looking at population trends. They say this year’s population may be lower than previous estimates, in part due to bad weather last winter in Mexico and Texas. Some study models indicate anywhere from a 10 to 56 percent chance our monarch population will be extinct in 20 years. All the monarchs in North America are one population divided into two subpopulations by the Rocky Mountains. Those who summer east of the Rockies overwinter in Mexico while those summering west of the Rockies winter in California.
The scarcity of monarchs this summer is disappointing because numbers from Mexico’s overwintering sites last season were encouraging. In February, I reported that monarchs occupied three and a half times the area of the previous year.
Overwintering monarch populations in Mexico from 1994-95 (left) through 2015-16. Population data exists for years prior to 1994, but sampling methods were different so the data aren’t comparable.
Only a few weeks after the good news was reported, an unusual winter storm in the Monarch Biosphere brought snow at the time monarchs were emerging from winter dormancy and beginning to move north. The butterflies are most vulnerable to cold wet weather then because they are flying around, nectaring, and mating in preparation for migration. They are not hanging in tight clusters sheltered in the trees with their metabolic rate reduced, a state similar to hibernation. Estimates at that time suggested that mortalities were low (perhaps 7%), but the numbers were unreliable as no one knew how many monarchs had left the Biosphere already and if the storm hit them. Migrating butterflies also were caught by a bad storm in Texas.
In Mexico, the storm damaged trees in the high elevation forest that provide roosting sites and shelter. Acres of overwintering habitat were degraded, although how much wasn’t known right away. Examination of the Biosphere this summer showed a much larger amount of habitat damage than was estimated right after the storm.
Last season’s data from the California overwintering habitat showed a larger population than 2014-15. More sites were occupied by a total 271,924 monarchs. However, this is only a fraction of the 1.2 million recorded in the late 1990’s and is 39 percent lower than the longterm average population size.
Overwintering populations in California 1997-2015
Population data from The Xerces Foundation’s annual census of California’s overwintering monarchs from 1997 (left) to the 2015 season (right). The green shows total monarchs reported among all sites, and the beige is the average number of monarchs per site.
This summer, population data for both the western and eastern subpopulations has suggested serious declines. Because the monarchs in their summer ranges are mobile and move across the landscape, the best time to evaluate population size is in their overwintering habitat. During the winter, the monarchs congregate in the Biosphere Reserve in Mexico and along California’s Central Coast. Population sizes are determined by estimating numbers of individuals in sample areas and multiplying this by the number and size of monarch sites. Thus, we won’t know the extent of damage from last winter’s snow storm in Mexico and how that population fared through the summer until this winter’s population is evaluated in late 2016 and early 2017.
I’ll be writing soon about the following monarch news:
- extent of the past winter’s storm damage in Mexico and other problems in the Biosphere;
- status of the overwintering sites in California and the western subpopulation;
- habitat enhancement projects to help monarchs in their summer breeding range;
- proposed listing under the ESA and the reasons for a recent legal suit against the USFWS by conservation groups; and
- scientists’ claims that citizens are loving monarchs to death through captive-rearing and release.
How to Count Monarchs When you watch migrating monarchs, find ways to quantify (count or measure) what you're seeing. Please tell us how many monarchs you see and how many minutes or hours you're watching. Also, compare today's observation to what you saw yesterday, last week, or last year.
More about North american monarchs and their habitats
Monarch Magic at El Rosario Sanctuary describes the largest public-access overwintering sites in Mexico’s Monarch Butterfly Biosphere Reserve and preliminary census data for 2015-16.
Monarchs Help Solve Their Own Mystery (but local people always knew the answers) is the story of how scientists discovered where the eastern subpopulation of monarchs spent the winter.
Sugar up, mate, and fly away — California’s overwintering monarchs leave, but Mexico’s wait for March explores the California habitat and announces 2015-16 population data.
Mexico’s monarchs mostly survive unusual cold storm so far gives the official population estimates for last season and reports on damage from the snowstorm.
