Monarch Population Update

In Conservatory by dfiik


monarch arrival journey south

Photo from Journey North Monarch Blog – Homero Gomez, Nov 2016.


The Monarchs arrived in Mexico! Almost like clockwork, they were reported as arriving in sanctuary areas on Nov 1 & 2 2016, just in time for the local Day of the Dead celebrations.

There have been speculations on how this year’s overwintering population will compare to the last. Every summer MonarchWatch issues a prediction, based on the numbers from the previous year, the weather patterns, and how the northward spring migration fared.

Unfortunately the predictions weren’t great for this fall’s overwintering population, and it seems that they won’t be far off the mark. A slow-start spring combined with a hot, dry summer has hit the monarchs hard. The overwintering population is expected to be similar to the numbers from 2014, which was the lowest recorded year.

monarch population chart 2016

A lot of it has to do with the low numbers of 1st generation monarchs that moved northward in the spring. If the summer population doesn’t get a strong chance to get the numbers up before the fall, then they are less likely to have a good start for the winter population. Of course the decline in breeding habitat (milkweed) doesn’t help either.

In Ontario, we are at the farthest north of their range, and each year we seem to be seeing less monarchs. With climate change affecting our seasons and growth times, it may be the monarchs won’t bother (or be able to support) coming as far north as they once used to.

Because of citizen science projects like the tagging program through MonarchWatch, we are able to have information and gather data on the monarch population. Tagging is the best way for scientists to get a good population estimate. They’ve found that there is a positive association between the number of monarchs tagged and the size of the overwintering population. (If there’s more monarchs out there to tag, then citizen scientists can tag more.

Based on the numbers coming in so far of monarchs that were tagged this fall season, it does seem that the population is starting on the low side. After an optimistic spike in the overwintering population in 2015, this is doubly disappointing that the monarchs will be taking a serious hit again in 2016.

However, it goes to remind us of the importance of tagging monarchs! If it weren’t for hundreds of people volunteering their time to tag butterflies, we wouldn’t have all this data and knowledge. Here are a few interesting facts about tagging:

  • There are estimated to be 10-40,000 untagged butterflies for every 1 monarch that is tagged.
  • It can take volunteers and paid staff 2-4 hours to find just one tagged monarch amongst all the untagged individuals in Mexico.
  • Of the 1.2 million monarchs tagged between 1998-2015, only about 14,000 were recovered, some which were reported years after the date they were tagged. This is recovery rate of less than 2{445e89025a4c50fcfeeac19430422221da6859948fb46c166dae93841e3a783b}!!
  • The El Rosario sanctuary in the Michoacan area of Mexico is the largest overwintering population of monarchs. The reserve covers about 56,000 hectares and the overwintering monarchs tend to take up less than 5 hectares of this sanctuary.