Applying Remote Sensing to Assess Habitat Viability for the Western Monarch Butterfly (Danaus plexippus plexippus) in California

Abstract

Among the Lepidoptera, butterflies play a significant pollination role (Rader et al., 2016). Additionally, as widely regarded species that structure ecological communities, conservation biologists consider them “charismatic flagship” and “umbrella” species (Guiney & Oberhauser, 2008; New, 1997). The migratory monarch butterflies (Danaus plexippus plexippus) in North America have garnered much interest in the arena of education and ecology. Despite their significance, the abundance of western monarchs is susceptible to ecological and anthropogenic stressors, such as the loss or degradation of overwintering and of breeding sites, and climate change (Jepsen et al., 2015; Malcolm, 2018; Pelton et al., 2019). In addition, western monarch abundance has been in a continual decline for the last two decades, abruptly dropping in 2018, with low monarch abundance persisting the following year (Xerces Society Western Monarch Thanksgiving Count, 2022). Understanding what factors may be inducing this massive population decline is critical to conservation prospects for its reversal. Therefore, the main objective of my thesis research was to explore how the extent and role of large-scale habitat variation relates to overwintering monarch abundance.

To accomplish this objective, four hypotheses were tested: (1) habitat quality of coastal tree groves showed a downward trend during the study period, (2) the coastal groves showed an increase of negative anomalies in the recent decade as compared to the previous decade, (3) land cover changed in ways that are detrimental to monarch abundance, and (4) the overwintering habitat suitability in the recent decade has decreased compared to the previous decade. Therefore, my specific aims were to conduct a two-decade spatiotemporal trend analysis of the groves along the California coast where western monarchs overwinter, as well as to evaluate whether the suitability in the recent decade has changed when compared to the previous decade. Satellite remote sensing datasets from MODIS and GPM, as well as western monarch Thanksgiving count datasets were acquired for this study. Software tools such as QGIS, Maxent, and Excel were used to process and conduct the data analysis.

The results indicated that not only the quality of the overwintering coastal groves has been deteriorating over time, but also the negative anomalies were more prominent during the recent decade (2010-2019). In addition, the dominant land cover types have been changing, and specifically: a decrease of overall canopy covered area of forest trees that provide roosting and protection to monarchs (adj R2 = 83.85%; p-value = 2.61E-06); a compensatory increase in the proportion of woody savannas cover (adj R2 = 82.13%; p-value = 2.71E-07); a decrease of shrublands (adj R2 = 22.30%; p-value = .0271) that provide food sources to monarchs; and a steady increase of urbanization (adj R2 = 97.04%; p-value = .0142). When comparing the suitability maps in 2003 with 2018, the overall extent of suitable habitat identified decreased by 20%, particularly along the south coast of California. For comparison of 2003 vs. 2018, the overwintering monarch distributions indicate a potential range shift. It is possible that this shift has occurred in response to precipitation & land surface temperature varying among study areas and, presumably throughout the monarch’s tolerated range of habitats. Focusing study on California’s entire coastal swath, as undertaken here, may facilitate stewardship planning by conservation planners and policymakers, with application to other pollinators as well.