[Pollinator] Monarch Crash of 2013

Thu Jun 17 10:08:04 PDT 2021

*This article from the Monarch Watch blog is relevant to the conditions for
Western monarch populations in 2021.*

The Monarch Crash of 2013
Chip Taylor, Ph.D.

What contributed to the monarch population crash in 2013?

The text below was written at the time the FWS was evaluating evidence of
all aspects of the monarch population prior to determining whether the
monarch should be listed as a threatened species. As you may remember, in
December 2020, monarchs were listed as threatened but precluded based on
the priorities given to other species which were more clearly threatened
and endangered. A final decision on the status of monarchs is scheduled for
2024. The below was neither published nor posted to the Blog. It should
have been since the analysis is relevant to what happened between 2016-2018
in California that led to the collapse of the Western monarch population.
I’ve reorganized and added to the original text to provide more background
and clarity.

Introduction

There are numerous ways to associate trends in the growth or declines in
populations with physical (weather related) and biological factors.
Ideally, there are real numbers to work with but that is seldom the case,
and lacking hard counts, we often have to link outcomes tentatively with
causative factors until we see the patterns appear over and over again in
generation to generation or year to year records. That’s the case with
monarchs. Numbers are generally lacking with cause-and-effect relationships
first appearing as speculation or inference. Those insights, if confirmed
again and again, can morph into hypotheses, and in the best cases,
correlations, which, in themselves, do not guarantee causation. The next
step is prediction. If the outcomes can be predicted based on the observed
relationships, we can be reasonably sure we are on the right track.

My approach to trying to understand the trends in monarch numbers has been
to use what is called a stage specific model. I break the annual cycle into
6 stages based on the dominant activity within a stage. Each stage is
treated as a discrete period that demographically represents either an
increase or decrease in numbers depending on conditions during the period
but not excluding the condition of the butterflies entering a stage. For
example, drought starved monarchs could arrive at the overwintering sites
with shorter wings, smaller mass and reduced fat bodies that could affect
both mortality during the stage and realized fecundity of the survivors.
Further, each stage sets the initial numbers for the next stage, and, in
effect, provides limits on what can happen in the next stage irrespective
of the conditions during that stage. In other words, the outcome of the
next stage depends both on the starting numbers and fitness (mass, fat
body, wing area) of the starting population and the conditions during that
stage. Good starting numbers may not result in an expected increase if
conditions are poor, and conversely, poor starting numbers can effectively
cap the outcome even under the most favorable conditions. It also appears
to be extremely difficult for a population to recover from a poor start in
a breeding season that only involves 3-4 generations.

This approach evolved from attempts to understand the variation in
year-to-year measurements of the size of the overwintering monarch
population in Mexico. My quest started with a simple question: what was the
explanation for the decline from 18.19 hectares in the winter of 1996-1997
to the following winter of 1997-1998 with a population of 5.77 hectares?
The answer appears to be an April frost in Texas that killed large numbers
monarch eggs and larvae. That quest led to an analysis of the factors
possibly associated with population growth for all years from 1994 forward
as well as searches through the weather records back to 1895. The point I’m
making is that, although this text focusses of what happened from
2011-2015, the data base I’ve used to interpret these records is extensive.

Methods

The metrics used in this analysis are large scale, or reflect measures of
large-scale, that is multi-regional, events. Regional weather and local and
regional impacts due to predators, parasites and pesticides all have a role
in determining population numbers but are difficult to measure and hard to
integrate into a stage specific model. With this in mind, I have selected
only a few measures that appear to be correlated with outcomes in a way
that give them predictive value. Among these measures are first sightings
of adult monarchs in the spring, March temperatures in Texas, May
temperatures north of 40N, summer temperatures and NDVI, a measure of
greenness of the vegetation on a landscape (Table1). Success or failure of
population growth is measured by the increase or decrease in the areas
occupied by monarchs at the overwintering sites from one year to the next.

Results

The population declined from 2011 to 2012 and again from 2012 to 2013 and
increased in both 2014 and 2015 (Table 1). The declines occurred as the
result of high March temperatures in TX (2011, 2012), summer temperatures
>1.9F above the long-term means (2011, 2012), drought conditions (low NDVI)
in the South Region (2011, 2012) and a low number of first sightings
(2013). The increases in 2014 (0.67 to 1.13) and 2015 (1.13 to 4.02)
followed favorable mean temperatures for population growth in March, May
and June-August. The NDVI index, which can be taken as a proxy for the
availability of nectar sources, was high for each year as well

The associations of high mean temperatures and drought conditions with
population decreases is seen throughout the record from 1994 through 2019.
Similarly, population growth from one year to the next is associated with
mean temperatures that are close to or below the long-term average for
March in TX. The longer record shows that populations increase when summer
means are up to 1.9F above long-term means but decrease when mean
temperatures substantially exceed +1.9F. Populations also decline when mean
temperatures are >-1.5F below average (2004, 2009).

The temperatures for September and October represent the conditions for the
first and second halves of the fall migration (Table 1). Although there is
no clear relationship between these means and the size of the overwintering
population in these data, this may be changing. The extreme high
temperatures for September 2019 delayed the migration by up to two weeks.
Those temperatures and a drought in Texas and northern Mexico resulted in
an unusually low recovery rate (N=392) for tagged monarchs suggesting that
attrition due to these factors limited migratory success. Similarly, above
average September temperatures in the Northeast in recent years are
associated with lower recovery rates of tagged butterflies from this
region.

Discussion

The low number of monarchs reported at the overwintering sites in Mexico in
the winter of 2013-2014 appears to have been the result of a series of
negative weather events that began in the summer of 2011. Excessive
temperatures and droughts in 2011 and 2012 followed by low initial
colonizing numbers in the spring of 2013 account for the decline. More
favorable conditions for population growth allowed the population to
increase in 2014 and 2015.

Temperatures

Summer temperatures can have a significant impact on the size of the fall
migratory population. A review of all the records suggests that population
growth is enabled when temperatures range from a little over -1F to almost
+1.9F. Lower temperatures of >-1.5F, as in the summers of 2004 and 2009,
both due to a southward dip in the jet stream, resulted population
declines. These reductions may have been due a reduction in egg development
which is temperature dependent, egg laying rate or simply a shorter growing
season or all three. Average temperatures in the Upper Midwest greater than
+1.9F were also associated with declines. Such temperatures are likely to
negatively affect realized fecundity by reducing the reproductive activity
and longevity of adults. In addition, such temperatures affect resources
available to adult monarchs by shortening flowering intervals, and
hastening senescence of host plants.

Droughts

A metric known as normalized difference vegetation index (NDVI), a measure
of greenness derived from satellite imagery, is frequently used to measure
the severity of droughts. Low values indicate severe drought conditions and
the NDVI values for Texas were low in both 2011 and 2012. These low values
are associated with low recoveries of tagged monarchs and lower than
expected overwintering numbers. Drought conditions also occurred in some
parts of the Upper Midwest in 2012. Those conditions may have further
reduced the migration and the size and robustness of the butterflies
originating from that region. Reports of “small” monarchs occur with some
frequency during droughts in the Midwest.

First sightings

While first sightings posted to Journey North tend to be urban and suburban
centric and can be limited by the number of people willing to report
sightings, they appear to tell us two things that are relevant here. First,
low numbers, well below the recent average numbers of sightings, as in
2013, tell us that population growth can be limited by the number of
returning monarchs. This record also tells us that there is no obvious
association between the size of the overwintering population and the number
of first sightings. For example, while the overwintering population of 2012
(1.19 hectares) led to 322 first sightings in 2013, the lower population of
2013 (0.67 hectares) accounted for 521 first sightings in 2014. Similarly,
the larger population of 2014 (1.13 hectares) produced only 547 first
sightings. Sorting out the human factor from the dynamics that produce the
numbers of monarchs moving north in March and later in May will be a
challenge. In the meantime, the low number returning in 2013 gives rise to
several questions about the fitness of the cohort of monarchs that arrived
at the overwintering sites in the fall of 2012. We know that most monarchs
originate from the Upper Midwest and that the region experienced a
semi-drought that summer along with the highest temperatures in the record
for that region from 1994 to present. The tagging record for 2012 indicated
that numbers tagged that fall were among the lowest we’ve recorded for the
Upper Midwest. Therefore, it’s possible that the last generation monarchs
were less fit due to lower fat reserves, or size, to survive the migration,
the winter period and the migration northward the following spring. High
levels of mortality during the migration northward from the colony sites to
Texas could also account for the low number of first sightings in the
spring of 2013.

Additional considerations

There are a few more things to say about 2012 and 2013 that are not
apparent from the above data. The first sightings data show that the return
migration for 2012 was the earliest in the record with large numbers of
overwintered monarchs advancing beyond 40N in late April and the first 10
days of May. In contrast, due to late arrivals and cool conditions in May
of 2013, the movement of first-generation monarchs beyond 40N was
predominantly at the end of May. These two back-to-back years demonstrated
that overwintering and first-generation monarchs can advance both too early
and too late into the summer breeding area for optimal population
development. If too early, females lay eggs at more northerly latitudes
with cooler, sometimes freezing, temperatures that delay age to first
reproduction. Such delays have the effect of reducing the reproductive
success of the returning cohort that overwintered in Mexico. Late
recolonization also negatively affects population growth in that it
shortens the breeding season.

This analysis suggests that it is extremely difficult, maybe impossible,
for a population to recover from a poor start in a 6.5-month breeding
season that involves 3, and more rarely 4, generations. Of course, this
depends on the definition of a poor start, and in this case, I’m thinking
of the poor starts in 2004, 2012, 2013. There may have been carry-over
effects from the droughts of 2002 and 2012 that had an impact on the
populations the following years. The population that developed in 2012,
following the drought of 2011 also declined, but that decline may have been
due as much to conditions in March and August as to the condition of the
butterflies surviving from the previous winter.

This example shows that the decline and recovery of a population can be
explained IF measures can be identified that have a broad geographic impact
on a population at a particular stage. Negative events can have a ripple
effect from one stage to another and can carry over from one year to the
next. Further, a series of negative events can cause a population to spiral
downward rapidly. Is it possible that the downward trends in the Western
monarch population from 2016-2020 were the result of a similar series of
negative events? Yes, that’s possible.

The crash in the population numbers in 2013, together with what appeared to
be a long-term decline that included other years with alarmingly low
numbers (2009-2010 - 1.92 and 2012-2013 - 1.19) led to the petition filed
in August 2014 to declare the monarch a threatened species. This petition
was preceded by an invitation from the White House to a stake-holders
meeting at the Eisenhower Office Building in April 2014. Attendees
represented a broad array of organizations interested in monarch and
pollinator conservation. A bit later, on the 20th of June 2014, President
Obama issued a memorandum requesting that all Federal Agencies with some
control of landscapes become engaged with monarch and pollinator
conservation. Given the prospect that a small overwintering population
would be extremely vulnerable to catastrophic overwintering mortality that
could reduce the numbers to levels that would threaten the very existence
of the monarch migration, these responses were not unreasonable at the
time. The US Geological Survey, the lead research agency of US government,
took the lead in convening meetings to examine the monarch situation. These
meetings were populated with monarch biologists and appropriate expert
personnel from federal agencies. A number of publications resulted from
these meetings, the most cited of which is “The all hands-on-deck” paper (
Thogmartin, et al, 2017). The analysis therein indicates that an
overwintering population of 6 hectares is required for monarchs to be able
to rebound from known adverse weather or other events. The analysis also
points out that to achieve a population of this size will require the
restoration of 1.8 billion milkweed stems. While there are many restoration
efforts under way, it’s not clear whether these efforts are sufficient to
offset the 2 million or more acres of potential monarch habitat that are
converted to croplands and lost to development each year.

The low overwintering numbers of 2013-2014 led me to ask whether monarch
numbers had been this low or lower in the past. Yes, that is likely, but
there is no data on this subject. What we can do is look at past climates.
Those records show that monarchs have experienced far greater extremes in
the past than anything seen since 1994. We can only speculate about the
impact of these events on monarch numbers, but it seems likely monarchs
declined to extremely low levels several times in the last 125 years. And
before that was the “little-ice-age”, a much colder period with erratic
shifts in climate from 1300 to 1850. While monarchs clearly survived
numerous extremes through the centuries, they now face new threats posed by
climate change such as severe winter storms, high temperatures and droughts
in all portions of the breeding season and even the migration. Those
impacts can already be seen in California.

Table 1. Temperature means, first sightings and NDVI records for 2011-2015.

Year

2011

2012

2013

2014

2015

Overwinter hectares/prev season

4.02

2.89

1.19

0.67

1.13

First sightings 1 March–30 April

241

255

129

201

203

March mean temp TX

+5.4

+6.8

+0.9

-1.6

-0.2

May mean temp 40N UM

-0.2

+4.8

+0.2

+0.5

+1.3

First sightings >40 N

1 May–9 Jun

362

408

193

320

344

June–August mean temp

Upper Midwest

+2.0

+3.0

+0.3

-0.5

-0.3

September mean temp

Upper Midwest

-0.5

-0.1

+3.3

+0.5

+6.4

October mean temp

South Region

0.0

-1.3

+0.1

+2.9

+2.6

Total first sightings

603

663

322

521

547

NDVI

0.462

0.494

0.536

0.542

0.533

References

Wayne E Thogmartin, Laura López-Hoffman, Jason Rohweder, Jay Diffendorfer, Ryan
Drum, Darius Semmens, Scott Black, Iris Caldwell, Donita Cotter, Pauline
Drobney, Laura L Jackson, Michael Gale, Doug Helmers, Steve Hilburger,
Elizabeth
Howard, Karen Oberhauser, John Pleasants, Brice Semmens, Orley Taylor, Patrick
Ward, Jake F Weltzin and Ruscena Wiederholt. (2017). Restoring monarch
butterfly habitat in the Midwestern US: 'all hands on deck'. Environmental
Research Letters <https://iopscience.iop.org/journal/1748-9326>, Volume 12
<https://iopscience.iop.org/volume/1748-9326/12>, Number 7
<https://iopscience.iop.org/issue/1748-9326/12/7> 074005

Laurie Davies Adams

Pollinator Partnership

475 Sansome Street, 17th Floor

San Francisco, CA 94111

e:  lda at pollinator.org

w:  www.pollinator.org

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