Return of the Monarchs

by Holly Sletteland

It's been a good year for monarchs at the preserve. We've seen dozens of them fluttering around the tops of the eucalyptus trees near the ponds in the morning sun. We've even had a couple of small clusters.  Ryan Slack, who is conducting a formal survey of monarch (Danaus plexippus) activity on both the central and eastern sections of the preserve, discovered a cluster of about 35 monarchs on one tree and another with about 50 butterflies. Ryan works with the Monarch Alert Project, a citizen-based research project conducted by graduate students and faculty from Cal Poly.  The project monitors overwintering monarch populations in San Luis Obispo and Monterey counties by conducting weekly surveys from the first week in November through the last week in March. 

Apparently we're not alone in seeing higher numbers this year, although unfortunately it's not consistent across the state.  Many sites have welcomed more monarchs than they were expecting, whereas others have seen fewer this year. Official counts won't be available until later this spring, but it appears that overall populations are still trending downward. There has been a 90 percent drop in the Western monarch population over the past 15 years. (See Monarch Butterflies Return in Surprising Numbers) And, of course, our numbers at Sweet Springs – while encouraging – are still a far cry from what they were in the past.  I never had the good fortune to see the monarchs at their peak on the preserve, but Dennis Sheridan, a local photographer and entomologist, recorded as many as 7,000 butterflies in the late eighties and early nineties.  He also chronicled their regrettable disappearance just a few years later. 

Some of the decline at the preserve is undoubtedly associated with the decline of the population as a whole.  No one knows precisely why the numbers have fallen throughout North America, but theories include the loss of overwintering groves to development, degradation of monarch habitat, higher temperatures and drought created by climate change and the reduction of milkweed (Asclepias sp.) due to increased herbicide applications and land conversion. The availability of milkweed appears to be the most significant factor contributing to the monarch's demise. Female monarchs will only lay their eggs on or near milkweed plants and the young caterpillars will only eat milkweed.  Unfortunately, milkweed is far less common than it used to be.  Our sprawling developments have consumed many of the acres where it used to grow. And we've sprayed much of the rest alongside our roads and in our agricultural fields with herbicides.

The decline in the overall population still doesn't explain why monarchs have stopped overwintering at Sweet Springs entirely. They continue to seek out other groves nearby.  I suspect something must have changed in the microclimate at the preserve. Monarchs are known to be very picky about having just the right amount of humidity, light, shade, temperature, and protection from the wind. Although the area where they historically congregated at the preserve has been relatively free of any major disturbances,  Dennis Sheridan did note that the trees have all grown considerably taller over the past 20 years. The trees to the south of the protected bowl they use to use are blocking more sun than before.  In fact, the monarch clusters we've seen this year aren't using the historic site.  They are clustering just to the west of it, where there is more sun.  The understory has also changed over the years and this may be another factor.  Monarchs seem to prefer a tiered understory, with plants of varying heights below the trees.  The trees have lost much of their foliage close to the ground as they have grown taller and there has been a significant decrease in the number of shrubs and herbaceous plants growing below them due to the accumulation of leaf litter. This is something we certainly have some control over, so I hope we can reestablish more vegetation in this area – ideally plants that can provide some nectar to our winter travelers.

The migration of any animal species – whether it be an Arctic Tern, an African wildebeest, or a gray whale – is a pretty amazing phenomenon.  But there is something particularly awe inspiring about the monarch's annual migration over 2000 miles.  It is hard to imagine how a creature so small, so fragile, and so very delicate can travel so many miles.  And, in fact, one monarch doesn't travel the full distance. It actually takes four generations of monarchs to make the trip. It is the great-great-grandchildren of the monarchs we're seeing now that will return next year.

It's still a mystery how they manage to return to the same spot as their ancestors - sometimes even to the very same tree. We do know that journey begins in October of every year when the shorter, cooler days of fall signal the coming of winter.   As if on cue, monarchs east of the continental divide start traveling to forests high in the mountains of Mexico to spend the winter, whereas those west of the Rockies head for small groves of trees along the coast of California.

Although genetically identical, the eastern population is the larger of the two, with millions of butterflies settling in for the winter in the fir trees of Michoacán, Mexico. The western monarchs cluster primarily in the protective branches of native Monterey pine, Monterey cypress, Sycamores, and non-native eucalyptus trees.  The longer days and warming temperatures in late January stimulate mating activity among the butterflies and the females are the first to depart shortly thereafter. They search for milkweed upon which to deposit their eggs and then die. After hatching, the second, third, and fourth generations of western monarchs fan out across the West reaching as far north as Canada and as far east as Idaho until the cycle starts all over again the next fall.

Although the decline in monarch populations is certainly disconcerting, there are a lot of constructive actions that each of us can take to help reverse the trend.

First, we can support organizations that are conducting research on monarchs and actively trying to restore their populations, such as the Xerces Society, Monarch Alert and Monarch Watch.  We can plant milkweed in our gardens, parks, and open space. We can buy organic food so we don't contribute to the loss of milkweed to pesticides.  We can support compact development and urban infill to slow the pace of land conversion.  And finally, we can work to conserve and enhance monarch overwintering habitat. 

I'm hopeful that we may yet again see large numbers of overwintering monarchs at Sweet Springs, if we continue to try to restore the place they used to call home.  Everyone is very welcome to join us in that effort on the 2nd Saturday morning of every month.  More help is always needed!

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Top photo of Monarch Butterfly by A. Gunther

Western Pond Turtle Image on Banner by Dave Johnson