Date of Award


Degree Name

MS in Biological Sciences


Biological Sciences


College of Science and Mathematics


Francis Villablanca

Advisor Department

Biological Sciences

Advisor College

College of Science and Mathematics


Migratory species are expected to demonstrate habitat selection that occurs at multiple spatial and temporal scales. Western monarch butterflies migrate seasonally to overwinter in groves at geographically predictable locations along the California coast. To date, overwintering habitat selection by western monarch butterflies has been studied assuming that habitat selection occurs where overwintering aggregations form, meaning at the spatial scale where monarchs form dense overwintering aggregations within overwintering groves. We argue that since western monarch butterflies are migratory, studies of habitat selection could have commingled selection at different scales into a single spatial scale. This likely leads to ignoring some levels of habitat selection, confounding the scale of habitat selection itself, and potentially misidentifying the habitat attributes under selection. 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 grove we collected temperature, humidity, and light data from grove edges, grove interiors, and aggregation locations over several weeks of the overwintering season. We tested the hypothesis that monarchs aggregate in locations within groves that have consistent attributes across groves. We find that locations on the outer edges of groves differed significantly in particular attributes of daily temperature and light from the interior of groves. Yet we find neither evidence supporting the hypothesis that the aggregation locations have a unique microclimate that differs significantly from other locations inside the grove nor that aggregation locations are uniform in their microclimatic attributes across overwintering groves. Rather, we find that the microclimatic attributes at the aggregation locations vary spatially with latitude. Thus, the overwintering climatic attributes that appear to be under selection varied spatially based on locations within groves and based on latitude of each particular grove. We conclude it will be necessary to consider spatial effects when studying western monarch butterfly overwintering habitat selection and that interpretations of habitat selection to date have commingled habitat selection at multiple spatial scales.