Climatic Niche Model for Overwintering Monarch Butterflies in a Topographically Complex Region of California

Monarch Butterflies in Western North America: A Holistic Review of Population Trends, Ecology, Stressors, Resilience and Adaptation

Monarch butterfly populations in western North America suffered a substantial decline, from millions of butterflies overwintering in California in the 1980s to less than 400,000 at the beginning of the 21st century. The introduction of neonicotinoid insecticides in the mid-1990s and their subsequent widespread use appears to be the most likely major factor behind this sudden decline. Habitat loss and unfavorable climates (high temperatures, aridity, and winter storms) have also played important and ongoing roles. These factors kept overwintering populations stable but below 300,000 during 2001-2017. Late winter storm mortality and consequent poor spring reproduction drove winter populations to less than 30,000 butterflies during 2018-2019. Record high temperatures in California during the fall of 2020 appeared to prematurely terminate monarch migration, resulting in the lowest overwintering population (1899) ever recorded. Many migrants formed winter-breeding populations in urban areas. Normal seasonal temperatures in the autumns of 2021 and 2022 enabled overwintering populations to return to around the 300,000 level, characteristic of the previous two decades. Natural enemies (predators, parasitoids, parasites, and pathogens) may be important regional or local drivers at times but they are a consistent and fundamental part of monarch ecology. Human interference (capture, rearing) likely has the least impact on monarch populations. The rearing of monarch caterpillars, particularly by children, is an important human link to nature that has positive ramifications for insect conservation beyond monarch butterflies and should be encouraged.

Complexity, humility, and action: a current perspective on monarchs in Western North America

Recent studies have continued to shed light on the ecology of monarch butterflies (Danaus plexippus) in western North America. These studies have documented a declining overwintering population over several decades, punctuated by unexpected variability in recent years. Understanding this variability will require grappling with the spatial and temporal heterogeneity of resources and risks presented to western monarchs throughout their annual life cycle. Recent changes in the western monarch population further illustrate how interacting global change drivers can create complex causes and consequences in this system. The complexity of this system should inspire humility. However, even recognizing the limits of our current understanding, there is enough scientific common ground to take some conservation actions now.

Hierarchy and Scale Influence the Western Monarch Butterfly Overwintering Microclimate

Migratory species are expected to demonstrate habitat selection that occurs at multiple spatial and temporal scales. Western monarch butterflies migrate seasonally to overwintering groves at geographically predictable locations along the coast of California. To date, overwintering habitat selection by western monarch butterflies has primarily been studied assuming the microclimate hypothesis. Specifically, that microclimate habitat selection occurs when monarchs form dense overwintering aggregations in overwintering groves. However, western monarch butterflies are migratory; thus, previous habitat selection studies could have commingled selection at different scales into a single local scale in the site of aggregation. Therefore, we explore monarch overwintering habitat selection to determine whether an explicit spatial framework is necessary. We studied nine groves on the coast of California, and at each we collected temperature, humidity, and light data from grove edges, grove interiors, and aggregation locations for several weeks during the overwintering season. We tested the hypothesis that monarchs aggregate in locations in groves that have a unique microclimate that is consistently selected across groves (the microclimate hypothesis). We find no evidence supporting the hypothesis that aggregation locations have a unique microclimate that differs significantly from that of other locations inside the grove or that aggregation locations are uniform in their microclimatic attributes across overwintering groves. Rather, we find that microclimatic attributes in aggregation locations vary spatially with latitude, and that aggregation conditions exist in a large portion of each grove. We conclude that it will be necessary to consider spatial effects when studying or managing western monarch butterfly overwintering habitats, and that interpretations of habitat selection to date likely commingle habitat selection on the local and geographical scales.

The Role of Community Science in Entomology

Community (or citizen) science, the involvement of volunteers in scientific endeavors, has a long history. Over the past few centuries, the contributions of volunteers to our understanding of patterns and processes in entomology have been inspiring. From the collation of large-scale and long-term data sets, which have been instrumental in underpinning our knowledge of the status and trends of many insect groups, to action, including species management, whether for conservation or control, community scientists have played pivotal roles. Contributions, such as pest monitoring by farmers and species discoveries by amateur naturalists, set foundations for the research engaging entomologists today. The next decades will undoubtedly bring new approaches, tools, and technologies to underpin community science. The potential to increase inclusion within community science is providing exciting opportunities within entomology. An increase in the diversity of community scientists, alongside an increasing taxonomic and geographic breadth of initiatives, will bring enormous benefits globally for people and nature.