The jet stream as a driver of continental phenology and its impact on monarch migration

Abstract:

Every year, monarch butterflies set off on a multi-generational, continental migration as they follow the wave of milkweed blooms from Mexico to Canada and back again. This annual migration matches the seasonal redistribution of solar energy and the corresponding latitudinal movement of the northern hemisphere jet stream (NHJ), responsible for regional patterns of climate at the surface. Surface climate, and especially temperature thresholds, directly correspond to insect physiology and fitness, and (sometimes combined with precipitation delivery) can also influence host and nectar plant phenology and availability along the migration path.

Here, we investigate how much variability in monarch migration is linked to jet stream-influenced climate at critical periods in the migration. We leverage and integrate multiple datasets, including: previously published seasonal and geographically-referenced monarch observations; monthly latitudinal positions of the NHJ resolved at 2.5 degree longitudinal windows; monthly climate fields; satellite NDVI-derived products of the start, peak, end and length of season; and the bloom time of 53 milkweed host plant and nectar plant species from the US National Phenology Network.

We find significant correlations between monthly NHJ anomalies and monarch estimates throughout the year that coincide with changes in climate over breeding grounds and plant phenology along the migration path. In particular, northern anomalies from the climatological mean NHJ position reflect warmer August temperatures in northeastern US breeding grounds and lead to higher overwintering acreage in Mexico that year. In late September to early October, southern NHJ anomalies over the Great Plains corresponded with more precipitation, cooler temperatures, higher NDVI (an indicator of nectar availability) and changes to the growing season- all contributing to more favorable resource availability and conditions for returning monarchs passing through in October and November. By characterizing the mechanisms behind jet stream influence on monarch migration, we highlight the importance of regional climate connectivity to a long distance migrating organism in contextualizing the effectiveness of conservation efforts focused on host plant maintenance in the summer breeding grounds.