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Understanding a migratory species in a changing world: climatic effects and demographic declines in the western monarch revealed by four decades of intensive monitoring

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Migratory animals pose unique challenges for conservation biologists, and we have much to learn about how migratory species respond to drivers of global change. Research has cast doubt on the stability of the eastern monarch butterfly (Danaus plexippus) population in North America, but the western monarchs have not been as intensively examined. Using a Bayesian hierarchical model, sightings of western monarchs over approximately 40 years were investigated using summer flight records from ten sites along an elevational transect in Northern California. Multiple weather variables were examined, including local and regional temperature and precipitation. Population trends from the ten focal sites and a subset of western overwintering sites were compared to summer and overwintering data from the eastern migration. Records showed western overwintering grounds and western breeding grounds had negative trends over time, with declines concentrated early in the breeding season, which were potentially more severe than in the eastern population. Temporal variation in the western monarch also appears to be largely independent of (uncorrelated with) the dynamics in the east. For our focal sites, warmer temperatures had positive effects during winter and spring, and precipitation had a positive effect during spring. These climatic associations add to our understanding of biotic-abiotic interactions in a migratory butterfly, but shifting climatic conditions do not explain the overall, long-term, negative population trajectory observed in our data.
a Map of California in the Western USA, including focal sites where observations of adults during the summer flight season were recorded [Suisun Marsh (A), Gates Canyon (B), West Sacramento (C), North Sacramento (D), Rancho Cordova (E), Washington (F), Lang Crossing (G), Castle Peak (H), Donner Pass (I), and Sierra Valley (J)]. Large, open circles along the coast are overwintering locations (see main text for details) from which abundance data were collated for use in analyses; solid dots are overwintering locations from which weather data were gathered for use in a subset of climatic analyses. b Map of North America showing eastern regions represented by count data from the summer flight season, as follows: North Central (1), (2) North East (2), Mid Central (3), Mid East (4), and South (5). c Diagram of datasets analyzed, as follows: “day positives” (I; counts of days on which adult monarchs were observed per year) at ten sites across the western breeding grounds; counts of adults at five of the low-elevation western breeding sites (II); counts of overwintering adults from a subset of western overwintering sites (III; circled on map); summary data from the eastern migration including summer population indices and hectares occupied by overwintering adults in Mexico (IV); climatic data from each of the ten focal western breeding sites (V); regional climatic conditions (VI; MEI 1 and MEI 2); climatic conditions at the Pacific overwintering sites (VII, VIII; treated separately, as explained in main text). Our focal dataset (day positives) is highlighted with a gray background and connected to climatic datasets (V–VIII) by single-headed arrows to represent multiple regressions. Relationships among monarch datasets (I–IV) were explored with correlations, indicated by double-headed arrows (for simplicity, not all connections are drawn). Illustration of adult monarch butterfly, Danaus plexippus, by Anne Espeset
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Oecologia (2016) 181:819–830
DOI 10.1007/s00442-016-3600-y
POPULATION ECOLOGY – ORIGINAL RESEARCH
Understanding a migratory species in a changing world: climatic
effects and demographic declines in the western monarch revealed
by four decades of intensive monitoring
Anne E. Espeset1,6 · Joshua G. Harrison1 · Arthur M. Shapiro2 · Chris C. Nice3 ·
James H. Thorne4 · David P. Waetjen4 · James A. Fordyce5 · Matthew L. Forister1
Received: 9 September 2015 / Accepted: 29 February 2016 / Published online: 21 March 2016
© Springer-Verlag Berlin Heidelberg 2016
overwintering data from the eastern migration. Records
showed western overwintering grounds and western breed-
ing grounds had negative trends over time, with declines
concentrated early in the breeding season, which were
potentially more severe than in the eastern population.
Temporal variation in the western monarch also appears to
be largely independent of (uncorrelated with) the dynam-
ics in the east. For our focal sites, warmer temperatures had
positive effects during winter and spring, and precipitation
had a positive effect during spring. These climatic associa-
tions add to our understanding of biotic-abiotic interactions
in a migratory butterfly, but shifting climatic conditions do
not explain the overall, long-term, negative population tra-
jectory observed in our data.
Keywords Climate change · Danaus plexippus ·
Hierarchical model · Monarch butterfly · Western
population
Introduction
Issues of spatial and temporal scale have always been
among the greatest challenges that face ecologists wishing
to extrapolate beyond single species and local conditions
(McGill 2010; Chave 2013). These concerns have been
brought to the fore by recent decades of anthropogenic
influence on the environment, as the public looks to ecolo-
gists for predictions regarding changes in regional or con-
tinental floras and faunas (Morisette et al. 2008; Tyliana-
kis et al. 2008). An important advance in the process has
involved meta-analyses that allow global phenomena to be
perceived through the accumulation of smaller-scale case
studies (Parmesan 2006; Wu et al. 2011; Mantyka-Pringle
et al. 2012). A further key contribution has come from the
Abstract Migratory animals pose unique challenges
for conservation biologists, and we have much to learn
about how migratory species respond to drivers of global
change. Research has cast doubt on the stability of the east-
ern monarch butterfly (Danaus plexippus) population in
North America, but the western monarchs have not been
as intensively examined. Using a Bayesian hierarchical
model, sightings of western monarchs over approximately
40 years were investigated using summer flight records
from ten sites along an elevational transect in Northern
California. Multiple weather variables were examined,
including local and regional temperature and precipitation.
Population trends from the ten focal sites and a subset of
western overwintering sites were compared to summer and
Communicated by Klaus Fischer.
Electronic supplementary material The online version of this
article (doi:10.1007/s00442-016-3600-y) contains supplementary
material, which is available to authorized users.
* Anne E. Espeset
aespeset@nevada.unr.edu
1 Program in Ecology, Evolution, and Conservation Biology,
Department of Biology, University of Nevada, Reno, NV,
USA
2 Center for Population Biology, University of California,
Davis, CA, USA
3 Department of Biology, Texas State University, San Marcos,
TX, USA
4 Department of Environmental Science and Policy, University
of California, Davis, CA, USA
5 Ecology and Evolutionary Biology, University of Tennessee,
Knoxville, TN 37996, USA
6 1664 North Virginia Street, Reno, NV 89509, USA
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
... These results seem to be in contrast to previous continental-scale modeling efforts that did not detect a strong signal of climatic factors in historic monarch population declines in the east (Flockhart et al., 2015;Stenoien et al., 2018;Zalucki et al., 2015), though climate factors have been associated with phenology and growth of the monarch population in specific parts of the eastern range (Zipkin et al., 2012;Zylstra et al., 2021). In comparison, studies in the western range generally suggest a stronger role for climatic factors, though the relative contributions of climatic and nonclimatic factors have been difficult to separate Espeset et al., 2016;Stevens & Frey, 2010 (Nail, Batalden, et al., 2015;York & Oberhauser, 2002;Zalucki, 1982), which allowed us to infer lethal and sublethal thermal constraints. In addition, it is also possible that the timing of our study allowed us to observe the effects of direct thermal stress that have become more apparent in recent years. ...
... Our findings are consistent with previous studies suggesting seasonally specific limits on monarch recruitment (Espeset et al., 2016;Zipkin et al., 2012), with particular emphasis on the early season Espeset et al., 2016;Zylstra et al., 2021). Previous studies have suggested that warmer winter and spring conditions generally favor monarchs (Espeset et al., 2016, Zipkin et al., 2012. ...
... Our findings are consistent with previous studies suggesting seasonally specific limits on monarch recruitment (Espeset et al., 2016;Zipkin et al., 2012), with particular emphasis on the early season Espeset et al., 2016;Zylstra et al., 2021). Previous studies have suggested that warmer winter and spring conditions generally favor monarchs (Espeset et al., 2016, Zipkin et al., 2012. ...
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Seasonal windows of opportunity are intervals within a year that provide improved prospects for growth, survival, or reproduction. However, few studies have sufficient temporal resolution to examine how multiple factors combine to constrain the seasonal timing and extent of developmental opportunities. Here, we document seasonal changes in milkweed (Asclepias fascicularis)–monarch (Danaus plexippus) interactions with high resolution throughout the last three breeding seasons prior to a precipitous single‐year decline in the western monarch population. Our results show early‐ and late‐season windows of opportunity for monarch recruitment that were constrained by different combinations of factors. Early‐season windows of opportunity were characterized by high egg densities and low survival on a select subset of host plants, consistent with the hypothesis that early‐spring migrant female monarchs select earlier‐emerging plants to balance a seasonal trade‐off between increasing host plant quantity and decreasing host plant quality. Late‐season windows of opportunity were coincident with the initiation of host plant senescence, and caterpillar success was negatively correlated with heatwave exposure, consistent with the hypothesis that late‐season windows were constrained by plant defense traits and thermal stress. Throughout this study, climatic and microclimatic variations played a foundational role in the timing and success of monarch developmental windows by affecting bottom‐up, top‐down, and abiotic limitations. More exposed microclimates were associated with higher developmental success during cooler conditions, and more shaded microclimates were associated with higher developmental success during warmer conditions, suggesting that habitat heterogeneity could buffer the effects of climatic variation. Together, these findings show an important dimension of seasonal change in milkweed–monarch interactions and illustrate how different biotic and abiotic factors can limit the developmental success of monarchs across the breeding season. These results also suggest the potential for seasonal sequences of favorable or unfavorable conditions across the breeding range to strongly affect monarch population dynamics. Few studies have observed species interactions with sufficient temporal resolution to examine how multiple factors combine to constrain the seasonal timing and extent of developmental opportunities. Our results show early‐ and late‐season windows of opportunity for monarch recruitment and indicate that these windows were constrained by different combinations of factors. These findings show an important dimension of seasonal change in milkweed‐monarch interactions and suggest the potential for seasonal sequences of favorable or unfavorable conditions across the breeding range to strongly affect monarch population dynamics.
... Altogether, these perceived threats have led to the recent decision by USFWS that federal protection is warranted in the United States (USFWS, 2020). However, evidence is ambiguous whether monarchs continue to be in consistent, recent decline across the annual cycle (i.e., outside of the winter stage), with studies variously reporting steady or falling monarch numbers at different places and seasonal milestones (Brower et al., 2018;Davis & Dyer, 2015;Espeset et al., 2016;Ethier, 2020;Inamine et al., 2016;Ries et al., 2015). Uncertainty about whether breeding populations are continuing to steeply decline, or show some resiliency to overwintering losses in at least some regions or at some stages, complicates efforts to target conservation programs to points in the life-cycle where they will be most effective. ...
... Cell-specific relative abundance trends were generally the most negative in the US Northeast, parts of the Midwest, and in northwest California, and were generally the most positive in the US Southeast and Northwest (Figure 2a). Only 11 of the 334 grid cells exhibited relative abundance trends whose 95% credible intervals did not overlap zero, 10 of which were positive trends in Even though the most recent evidence indicates the monarchs west of the Rocky Mountains should not be considered a separate population (Freedman et al., 2021;Talla et al., 2020), the assessment of monarch abundance in the west has traditionally been via counts of wintering monarchs along the California coast (Espeset et al., 2016;Pelton et al., 2019). However, as we found with the larger cohort of monarchs east of the Rockies, the trend of diminishing wintering colonies in California does not appear to mirror long-term trends in breeding monarch abundance to the north or northeast, either in Oregon or Idaho (Figure 2a). ...
... which is consistent with other long-term surveys from that same region (Espeset et al., 2016), and this consistency provides confidence in the NABA data. Further, reasons for declines in wintering monarchs in California have been the subject of ongoing debate, with some speculating that western monarchs may be transitioning to a less migratory lifestyle in California, which is being fueled by homeowner plantings of non-native milkweed that thrives yearround (Davis, 2022;James et al., 2022). ...
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Many insects are in clear decline, with monarch butterflies (Danaus plexippus) drawing particular attention as a flagship species. It is well documented that, among migratory populations, numbers of overwintering monarchs have been falling across several decades, but trends among breeding monarchs are less clear. Here, we compile >135,000 monarch observations between 1993 and 2018 from the North American Butterfly Association's annual butterfly count to examine spatiotemporal patterns and potential drivers of adult monarch relative abundance trends across the entire breeding range in eastern and western North America. While the data revealed declines at some sites, particularly the US Northeast and parts of the Midwest, numbers in other areas, notably the US Southeast and Northwest, were unchanged or increasing, yielding a slightly positive overall trend across the species range. Negative impacts of agricultural glyphosate use appeared to be counterbalanced by positive effects of annual temperature, particularly in the US Midwest. Overall, our results suggest that population growth in summer is compensating for losses during the winter and that changing environmental variables have offsetting effects on mortality and/or reproduction. We suggest that density-dependent reproductive compensation when lower numbers arrive each spring is currently able to maintain relatively stable breeding monarch numbers. However, we caution against complacency since accelerating climate change may bring growing threats. In addition, increases of summer monarchs in some regions, especially in California and in the south, may reflect replacement of migratory with resident populations. Nonetheless, it is perhaps reassuring that ubiquitous downward trends in summer monarch abundance are not evident.
... Beyond simple abundance, declines have also been noted in other metrics. Western monarchs have plummeted in raw day positives-the number of days in which a monarch sighting is reported at various sites (Espeset et al. 2016)-and in population growth rate, which was significantly negative for the period 1980-2016 (Schultz et al. 2017). Monitoring of overwintering sites reveals that 41% of sites that were occupied prior to 1990 are now unoccupied (Leong et al. 2004). ...
... Furthermore, the population is at heightened risk of suffering severe consequences from Allee effects and environmental stochasticity, factors which would presumably increase extinction risks (Pelton et al. 2019). These declines seem to be located earlier in the breeding season, which suggests a correlation between declines and overwintering (Espeset et al. 2016). ...
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Western monarch butterflies (sp. Danaus plexippus) are undergoing a severe decline that rivals those occurring among insects across the globe. Despite the estimation of population abundance, growth rates, and extinction probabilities, no analyses have investigated spatiotemporal patterns of decline in the western monarch population. I performed occupancy modeling of Western Monarch Thanksgiving Count (WMTC) data. The data was constrained spatially and temporally, with sites grouped into occupancy bins by latitude and year. Occupancy probabilities (psi) were estimated for each intersection of a latitude and time bin and detection probabilities (p) were estimated for each time bin. Psi increased slightly and non-significantly from northern to southern latitude bins. However, the dataset was unable to support any models with >3 latitude bins or the intersection of latitude and time bins because the dataset contained unequal sampling distributions across both space and time and a high proportion of missing observations. These constraints are likely driven by the reliance upon citizen science for WMTC data collection, and thus those constraints may be present in other citizen science datasets. Despite inconclusion regarding my original research questions, I concluded that occupancy modeling requires robust datasets that are more complete and equally distributed across the relevant parameters than the WMTC data. As species begin to decline, datasets with these characteristics may be harder to generate, suggesting that occupancy modeling may not be suitable for western monarch butterflies or other insect populations in the future.
... While the North American butterfly counts have been used to examine population trends for single butterfly species (e.g., Espeset et al., 2016) and biodiversity patterns in particular regions (Kocher & Williams, 2000;Meehan et al., 2013), they had not before been comprehensively collated and examined as a whole for broad changes in abundance and diversity across the continent. Here, we distill 8,448,945 butterfly observations into 14,159 species × site abundance trends and ask how well abundance trends are explained by butterfly species traits, as well as climate and landscape attributes of the count sites. ...
... Figure 3a) are supported by several studies, primarily in western North America(Espeset et al., 2016;Forister et al., 2018;Wepprich et al., 2019;Westwood & Blair, 2010). ...
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Some insect populations are experiencing dramatic declines, endangering the crucial ecosystem services they provide. Yet, other populations appear robust, highlighting the need to better define patterns and underlying drivers of recent change in insect numbers. We examined abundance and biodiversity trends for North American butterflies using a unique citizen‐science dataset that has recorded observations of over 8 million butterflies across 456 species, 503 sites, nine ecoregions, and 26 years. Butterflies are a biodiverse group of pollinators, herbivores, and prey, making them useful bellwethers of environmental change. We found great heterogeneity in butterfly species’ abundance trends, aggregating near zero, but with a tendency toward decline. There was strong spatial clustering, however, into regions of increase, decrease, or relative stasis. Recent precipitation and temperature appeared to largely drive these patterns, with butterflies generally declining at increasingly dry and hot sites but increasing at relatively wet or cool sites. In contrast, landscape and butterfly trait predictors had little influence, though abundance trends were slightly more positive around urban areas. Consistent with varying responses by different species, no overall directional change in butterfly species richness or evenness was detected. Overall, a mosaic of butterfly decay and rebound hotspots appeared to largely reflect geographic variability in climate drivers. Ongoing controversy about insect declines might dissipate with a shift in focus to the causes of heterogeneous responses among taxa and sites, with climate change emerging as a key suspect when pollinator communities are broadly impacted.
... Over the last 20 yr, the western and eastern monarch populations have experienced significant population declines (Brower et al. 2012, Espeset et al. 2016, Pocius et al. 2017, Agrawal 2017, Thogmartin et al. 2017, Pelton et al. 2019, Voorhies et al. 2019, Warner et al. 2019, USFWS 2020a and, in response, concern in the cause or causes has stimulated considerable interest resulting in various hypotheses and milkweed conservation efforts (Zaya et al. 2017, Taylor et al. 2020, USFWS 2020a. In December 2020, the USFWS listed the monarch butterfly in the US Federal Register as a candidate species under the Endangered Species Act of 1973. ...
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... Although most conservation attention to date has focused on the larger eastern monarch population, the recent decline of western overwintering populations has been precipitous (Crone, Pelton, Brown, Thomas, & Schultz, 2019;Pelton, Schultz, Jepsen, Black, & Crone, 2019;Schultz, Brown, Pelton, & Crone, 2017). Declines in western overwintering monarchs have been mirrored by low summer breeding numbers (Espeset et al., 2016), culminating in a >99% reduction in counts of western overwintering monarchs since monitoring began. For three consecutive years, western monarch overwintering numbers have been below their quasi-extinction threshold, raising concerns about their long-term persistence Xerces Society, 2021; Figure 1d). ...
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We report results of a ten year study to understand tire distribution, breeding and migration of the monarch butterfly (Danaus plexippus) in Arizona. We observed breeding and migratory monarch populations throughout the state and small overwintering aggregations in some locations. Migration occurred to known migration destinations in both California and Mexico. We found that the destination is not random. Wind significantly affects whether migrating monarchs are recovered. This study provides new insights into the breeding, overwintering and migratory strategies of Arizona monarchs.
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The decline of the eastern population of the migratory monarch has become a topic of great concern, but has been based entirely on patterns observed in overwinter colony sizes. Less attention has been paid to population trends during other phases of the migratory cycle. Here, we present an analysis of trends using three monitoring programs, one focused on overwinter colony size and two focused on summer breeding grounds. We discovered an alarming steepening in the decline of winter colony size since 2008. However, population indices from two independent summer monitoring programs were characterized by high year-to-year variability and no statistically detectable trends over time. Despite the mismatch in summer and winter patterns, there is still an association between the yearly fluctuations between these key periods, suggesting a link in population dynamics throughout the year. Further, a suggestion of a downturn near the end of the summer time-series should be carefully tracked into the future. We discuss two possible reasons for this disconnect: 1) higher levels of variance or possibly biased sampling could weaken any statistical signal, and 2) losses during fall migration could potentially contribute to overwinter declines.
Article
Huge numbers of autumn-migrating monarch butterfly Danaus plexippus were noted in the US Midwest eastwards to the Atlantic coast from the mid-19th century on, a reflection of the ploughing up of the prairies and the clearing of the eastern forests, promoting growth of the host plant (milkweed Asclepias syriaca). Overwintering sites were discovered in California in 1991, but the destination of southward migrating populations remained unknown. In 1975 tagging studies established that millions of monarchs overwintered in the volcanic mountains of C Mexico. This paper describes developments in the understanding of monarch behaviour and distributions, and examining remaining questions of detail concerning migration flights, whether there are other overwintering sites, and whether there is interchange between the western and eastern North American populations. -P.J.Jarvis