ANURAG AGRAWAL: We're trying to understand why the monarch population is declining. And so what we did was we pieced together different parts of the monarch's life cycle and ask, where are their problems? Where are their limitations that are reducing their populations?
Our key finding is that the monarch populations are declining beginning in the fall, with their fall migration, when they go about 3,000 miles from northern USA and Canada all the way down to central Mexico. And that suggests-- that's a very, very important finding, because it suggests that milkweed is probably not the limiting thing for monarch populations.
At the end of the summer, in August, when they begin that epic journey all the way down to Mexico, they no longer are feeding on milkweed. They're not nectaring on their flowers. They are simply making that journey. The only thing they eat then is water and flower nectar from other species like golden rods.
And what we found was that by the time they get to Mexico, their numbers are plummeting. But at the end of the summer, when they start that migration, their numbers are not down.
So on the one hand, that presents an important mystery. But it also presents an important finding, or that perhaps planting milkweed will not improve or increase the population as much as we thought in the past.
The conventional wisdom which is out there right now is that milkweed is what's limiting the monarch population, and that planting milkweed will improve their populations. And so from that perspective, our findings are quite different. They're challenging that notion, and suggesting that that could be a wasted effort, in a way.
On the other hand, there have been others that have been signaling that the problem is more complicated. There's researchers at the University of Georgia, researchers at Georgetown University that have suggested that the migration is very complicated, and that there may be other weak points that need to be addressed.
Our study, I think, adds a level of statistical rigor and completeness, in terms of the data analysis, that really shows that to be true. It would be a mistake to think that planting milkweed alone is going to solve this problem. It really appears that things after the milkweed is used is what the problem is.
So I hope that building on this past work that some others have done is going to open the door to more comprehensive ways to actually improve the monarch's population.
In terms of conserving the monarch butterfly, it's a very, very gnarly problem. And I say that because their annual migration occurs over 12 months. They're in a different place in different months, and it spans our entire North American continent. We have monarchs all the way down from central and southern Mexico all the way up into southern Canada.
And what that means is identifying the key weak points is absolutely critical. If we don't, we could be investing a lot of effort into conserving a portion of the migration, but it may have ultimately no impact on conserving the species overall.
With monarch conservation, our findings are important, because if milkweed is not really limiting the monarch population, planting milkweed probably isn't a bad thing to do. But it's not going to increase their populations or save them from some demise.
And so getting the science right, from my perspective, is absolutely critical, so we can identify the weak points and potentially do something positive about the problem.
This research couldn't have been done without citizen science. It's kind of an amazing story, because monarch butterflies traverse all of North America, it's a very complicated annual migration cycle. And in order to put the pieces together and understand where the problem is, we needed data.
And the Mexican government, along with the World Wildlife Fund, and some researchers have been enumerating-- counting-- the monarchs at the overwintering sites in the Mexican forests for 22 years.
But the rest of the data-- and there were literally several other data sets-- were collected by citizen scientists, who had the foresight, through various leadership 20, 25 years ago, to start collecting data on the number of monarchs. In the Midwest, in the Northeast, at funnel points where they fly south in the fall.
And we would have no clue about monarch conservation if it wasn't for this army of citizen scientists that have collected this data. And it really is a lot of people throughout the USA and Canada.
A lot of that currently is being filtered through the North American Butterfly Association. People make counts. They have data sheets, and they get organized and enumerated through the NABA-- we call it, the North American Butterfly Association.
But there are other funnel points. For example, the fall migration, there's the Cape May monitoring project in New Jersey, and the Peninsula Point monitoring project in Michigan. And those have been absolutely critical data sets. They've allowed us to put together the life cycle and ask where the weak points are.
We're not sure what's happening to monarchs on their journey south. I can say that a lot of things about the migration are changing. They seem to be migrating faster than they used to be. We don't know why.
The things that they critically need on the southern migration are water, nectar-- from plants other than milkweed-- and resting stops on trees. They only fly during the day. They rest on trees in clusters, huge clusters, at night. And landscapes that are free of insecticides, and landscapes that are unobstructed in other ways.
And so on the one hand, although it's difficult to know what exactly is the problem, we can say with quite amount of certainty that limitation of milkweed does not seem to be the problem. And these other factors, such as the water and nectar, or other things that may be obstructing their flight path could be limiting in their populations.
Yeah. One of the really amazing things about monarchs is despite the fact they breed all across the USA and southern Canada during the summer months, in the winter they go to a place about the size of New York City-- 12 mountain tops in central Mexico. And it's a very specific place, with a very specific tree-- the oyamel fir tree-- where they cluster on the branches.
And so that's always been known to be a potential weak point in their migration and in their annual cycle, because if those 12 mountain tops aren't intact, they have nowhere to go. There's historically been logging there, and the forests are degraded and they continue to be degraded.
There's various protections that have been put into place, but it's continuing to be a threat. And so the health of those forests is really critical, and that could be another issue limiting the monarch populations.
Part of our study is trying to understand, what is the role of herbicides? Or is there really not enough milkweed, and is that what's limiting the monarch population?
Our findings suggest that they are not-- monarch populations are not limited by milkweed, which in a way suggests that herbicides are not likely to be the problem. And genetically modified crops that are herbicide-resistant are not likely to be the problem for the monarch.
The current wisdom out there right now is that genetically modified crops that are herbicide tolerant are causing the monarch's decline, because in those crop fields, we're spraying herbicides, they're killing the milkweed, which is the monarch's host plant. Our research suggests milkweed is not limiting for the monarch, and that suggests that genetically modified organisms are not the problem.
Monarchs are not alone in their decline. Many other butterfly species, and especially migratory species-- including birds-- have been declining on the same time scale over one to several decades.
And that's a very sobering realization. It suggests that there are large scale problems, environmental problems, that aren't linked-- that aren't specific to particular groups, like birds, or butterflies, or particular species.
It also suggests that what we're doing to the landscape at some larger scale is causing major problems for many species, especially those that traverse large chunks of North America during their annual cycle.
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Anurag Agrawal, professor of ecology and evolutionary biology, explains the results of his recent study showing sparse autumnal nectar sources, weather and habitat fragmentation are contributing factors to the population decline of monarch butterflies.