Tuesday, November 17, 2015

Monarch Census--The Winter Survey

Guest Blog by Mary Nemecek, Missouri Master Naturalist Osage Trails Chapter.

On November 3rd, around mid-day, monarchs began arriving in Angangueo, Mexico.  Angangueo sits just below the two largest wintering grounds formonarch butterflies.  This signals the end of their fall migration and the beginning of their winter spent on the Oyamel trees in the Monarch Butterfly Biosphere Reserve.   

Here the monarchs will overwinter, hanging in heavy clusters, earning  them the honor of being the 2nd most dense concentration of animals in the world (krill being #1).  In March, they will become active, begin to breed and move back north.

During the winter scientists will survey the amount of land covered by monarchs on the biosphere and estimate population numbers.  The number of hectares (ha) covered by monarchs is usually reported in late January and an estimate of the monarch population is released.  Monarchs are counted at 50 million monarchs per hectare.  One hectare equals 2.47 acres.
These monarchs are roosting in Kansas
on their way to Mexico. Photo by Mary Nemecek

The history of the number of monarchs estimated per hectare is interesting and heartbreaking.  Originally, after years of study through different methodology, monarchs were estimated at 10 million/ha.  That was until a winter storm hit the overwintering grounds in 2002 and millions of butterflies were killed.  It's estimated 75% of the population was lost during the storm.  However this gave scientists the opportunity to count butterflies. One square meter held  2,241 dead monarchs.  This alone would have brought the number per hectare to 22.4 million.  But not all the monarchs perished and there were still many more in the trees.  In other areas the numbers were even higher.  Eventually scientists came to the conclusion that each hectare held a population of 50 million monarchs.
These are monarchs roosting at their Mexican
overwintering site in Mexico.  Photo from Learner.org.

An all time low of .67 ha was reached during the winter of 2013/2014.  Last winter the number bumped up to 1.13 ha but nowhere near what scientists would like to see for a sustainable population.  Dr. Chip Taylor with Monarch Watch would like to see the number at 4 ha and US Fish and Wildlife Services have set a goal for 6 ha.

Earlier this month, when Secretary of the Interior Sally Jewel visited the reserves with Mexican Environment Secretary Rafael Pacchiano , news sources around the world reported predictions of a large increase in overwintering populations to 3 or 4 times over  last winter.  If true, this could get the monarchs to Dr. Taylor's minimum threshold of 4 ha.  However, Dr. Taylor is more conservative in his predictions.  "I've been predicting at least a doubling of the population and that seems justified based on the success of the tagging program," wrote Dr. Taylor in a recent post. 
   
The number of data sheets returned for tagged monarchs shows almost double the number of monarchs tagged this year compared to last year.  Dr. Taylor said roughly 80,000 plus monarchs were tagged in 2015.  Additionally, Dr. Taylor points out, the conditions this year were similar to 2011 when the overwintering population came in at 2.89 ha.


As  the world holds their breath for good weather on the wintering grounds and a big count, there is still more work to do to ensure a continued, sustainable, migratory monarch population.  USFWS estimates it takes 29 stems of milkweed to produce one overwintering monarch.   That leaves an additional 1.5 billion milkweed stems needed in the mid-west to achieve a 6 ha overwintering population.  Lots of planning and planting left to do.

No comments:

Post a Comment