• Andy Davis

The incredible resiliency of the monarch (that means they keep bouncing back)

This week we're going to talk about something that you rarely see in the headlines about monarchs. Most of the time, the news stories only talk about the highly-touted declines in the overwintering sites (probably because that's what gets people's attention). But where are the headlines about all the monarchs people are seeing this summer? By all accounts, monarchs have made a huge comeback this year - people are reporting hundreds of eggs, larvae, etc, in their backyards or neighborhoods, and the monarch summer breeding range is absolutely huge this year. Take a look at the Journey North map for summer sightings (July and august) of adults this year:

Incidentally, the folks at Journey North have only recently begun compiling sightings of adults in the summer, so maps like this one are a fairly new thing. But this map does show that there are monarchs absolutely everywhere this summer in the east. Wait, but how can that be, if there were so few at the overwintering site last year? The reason is the amazing reproductive potential of the monarch species (that means they lay lots and lots of eggs). Female monarchs are capable of laying 500-700 eggs in their lifetime, so if the conditions are right in the spring and summer, the entire population can grow from a small number to a large one in a heartbeat (or at least a summer).

This resiliency is probably the main reason why the counts of breeding monarchs (from multiple citizen-science programs) do not show the same long-term decline as the overwintering numbers do. Several papers showed this in the recent special collection of monarch papers in the Annals of the Entomological Society of America (see previous blog post on this). So even though the numbers in Mexico keep going down, the adults that do manage to return to breed seem to be compensating by laying lots of eggs (or at least enough to keep the breeding numbers from declining in the long-term).

So think about this - the numbers in Mexico keep going down each year, but yet the numbers of breeding monarchs does not (and never has this been more apparent than this year). This raises a very interesting question - how much does the overwintering colony size influence the next phases, especially the spring migration? Well this question was recently asked and answered in a paper that appeared in the special monarch collection in the Annals of the Entomological Society of America.

This was a paper I myself worked on with the director of Journey North, Elizabeth Howard. She and I set out to uncover how the spring migration is affected by the changes in the overwintering colony size over time. To do this we had to come up with a way to assess how well the monarchs' spring migration progressed each year. The spring migration is essentially where the monarchs recolonize their summer breeding range in a series of generations that march northward. Journey North has been tracking this recolonization for years (18 when we wrote the paper). In the Journey North program, people report when they see their first adult monarch, and then these sightings are posted on maps like the one above. So we came up with a way to track how 'big' the spring migration was each year. We divided the country up into squares or 'blocks' that were 50,000 square km each, then we counted how many of these blocks contained monarch sightings. This gave us a single number each year that indicated the overall size of the migration. The reason we didn't simply count the number of sightings is because there has been a steady increase in sightings ever since the program started in 1997 - this is because of increasing participation.

What we found was that the number of blocks in the early part of the migration (i.e. up to April) has been slowly declining over time - this reflects the declining number of overwintering monarchs. The decline was on the order of 9% (not 90%) in the last 18 years. BUT, when we looked at the size of the final range size, that is, the number of blocks by the end of June, we found this number has been actually increasing over time. Part of this increase is likely because there are more people looking for monarchs now compared to the late 90s, but in any case, you can see the general trend in the graph below.

The orange line in this graph shows the size of the spring colonization range each year, in terms of the amount of land area it encompasses by the end of the spring and the start of the summer. The blue bars show the size of the overwintering colony each year, for the years of overlap in the two data sets. This graph was modified from the one in our paper (Fig. 3), in which we plotted the long-term trend in the number of blocks up to April. This one here is for the entire migration up to June.

Again, it's hard to know how much of the long-term increase in the orange line is due to more people watching for monarchs these days, or if it's because the summer range of the monarch is actually increasing over time. In any case, this graph could help to explain why we can have summers like this one (monarchs everywhere), despite the dramatic reductions in overwintering colony size - the monarchs are able to compensate with high reproduction in the spring (when conditions are good).

So the bottom line for this post is that the number of monarchs in Mexico doesn't seem to have a huge impact on how many monarchs we get in the US and Canada in the summer, and this year is a shining example! Let's hope they have good success in the fall migration.

The full reference for the paper is here:

Howard, E. and A.K. Davis*. 2015. Investigating long-term changes in the spring migration of monarch butterflies using 18 years of data from Journey North, a citizen science program. Annals of the Entomological Society of America.

The science of monarch butterflies

A blog about monarchs, written by a monarch scientist, for people who love monarchs