Multiscale seasonal factors drive the size of winter monarch colonies

Abstract

Monarch butterflies in eastern North America have declined by 84% on Mexican wintering grounds since the observed peak in 1996. However, coarse-scale population indices from northern US breeding grounds do not show a consistent downward trend. This discrepancy has led to speculation that autumn migration may be a critical limiting period. We address this hypothesis by examining the role of multiscale processes impacting monarchs during autumn, assessed using arrival abundances at all known winter colony sites over a 12-y period (2004–2015). We quantified effects of continental-scale (climate, landscape greenness, and disease) and local-scale (colony habitat quality) drivers of spatiotemporal trends in winter colony sizes. We also included effects of peak summer and migratory population indices. Our results demonstrate that higher summer abundance on northern breeding grounds led to larger winter colonies as did greener autumns, a proxy for increased nectar availability in southern US floral corridors. Colony sizes were also positively correlated with the amount of local dense forest cover and whether they were located within the Monarch Butterfly Biosphere Reserve, but were not influenced by disease rates. Although we demonstrate a demographic link between summer and fine-scale winter population sizes, we also reveal that conditions experienced during, and at the culmination of, autumn migration impact annual dynamics. Monarchs face a growing threat if floral resources and winter habitat availability diminish under climate change. Our study tackles a long-standing gap in the monarch’s annual cycle and highlights the importance of evaluating migratory conditions to understand mechanisms governing long-term population trends.

Design Implications for Surveys to Monitor Monarch Butterfly Population Trends

“…Pleasants et al (2016Pleasants et al ( , 2017 challenged these studies, indicating that population trend estimates based on survey sites that are not representative of all possible survey sites can differ from estimates based on randomly-located and representative survey sites. Similarly, Saunders et al (2019) highlights the inability to determine whether the lack of trends on the breeding grounds occurred due to a true absence of trend or a result of bias in data collection. Our study corroborates Pleasants et al (2017) claim with empirically-derived density estimates, showing that monarch density on the breeding grounds was lower on average and by year on random site types compared to non-random site types.…”

Configuration and Location of Small Urban Gardens Affect Colonization by Monarch Butterflies

“…Although monarchs foraging locally may approach and leave milkweed patches from all directions (Zalucki and Kitching, 1982b), unimpeded north/south access to gardens may be particularly important for them to be encountered and used when adults are flying predominantly southward during their fall migration or northward during spring migration. North-south access may also be important because availability of nectar sources, particularly during autumn migration, may be critical to monarchs' migration success (Saunders et al, 2019). Interestingly, neither overall percentage of hardscape within a 100 m radius of the gardens, nor the percentage of total (360 • ) access blocked by buildings, was a significant determinant of monarch use.…”

“…Given that monarch larvae feed exclusively on milkweed (family Apocynaceae, subfamily Asclepiadoideae), and that adults migrate to locate host plants across diverse landscapes, two primary concerns facing monarch populations are shortages of milkweed, and floral nectar to fuel migration (Pleasants and Oberhauser, 2013;Oberhauser et al, 2017;Malcolm, 2018;Saunders et al, 2019). Conserving and restoring monarch habitat, especially planting of milkweeds and nectar resources on public and private lands, has emerged as the central conservation strategy to meet monarch population goals set by the USFWS and adopted by Mexico, Canada, and the United States 1,2 .…”

Reply to Wepprich and Ries et al.: Alternative methods do not provide support for the contribution of GM crops to monarch declines

“…Ries et al (2) further note that strong geographic biases exist in monarch museum records, and they propose limiting the monarch dataset to those specimens collected in particular regions and times of year that correlate well with monarch abundance at their overwintering grounds in recent years (9). We believe that this is an approach that may also prove useful in other species when their demography is well understood.…”