Climate change may alter breeding ground distributions of eastern migratory monarchs (Danaus plexippus) via range expansion of Asclepias host plants.

Nathan P. Lemoine

doi:10.1371/journal.pone.0118614

Abstract

Climate change can profoundly alter species’ distributions due to changes in temperature, precipitation, or seasonality. Migratory monarch butterflies (Danaus plexippus) may be particularly susceptible to climate-driven changes in host plant abundance or reduced overwintering habitat. For example, climate change may significantly reduce the availability of overwintering habitat by restricting the amount of area with suitable microclimate conditions. However, potential effects of climate change on monarch northward migrations remain largely unknown, particularly with respect to their milkweed (Asclepias spp.) host plants. Given that monarchs largely depend on the genus Asclepias as larval host plants, the effects of climate change on monarch northward migrations will most likely be mediated by climate change effects on Asclepias. Here, I used MaxEnt species distribution modeling to assess potential changes in Asclepias and monarch distributions under moderate and severe climate change scenarios. First, Asclepias distributions were projected to extend northward throughout much of Canada despite considerable variability in the environmental drivers of each individual species. Second, Asclepias distributions were an important predictor of current monarch distributions, indicating that monarchs may be constrained as much by the availability of Asclepias host plants as environmental variables per se. Accordingly, modeling future distributions of monarchs, and indeed any tightly coupled plant-insect system, should incorporate the effects of climate change on host plant distributions. Finally, MaxEnt predictions of Asclepias and monarch distributions were remarkably consistent among general circulation models. Nearly all models predicted that the current monarch summer breeding range will become slightly less suitable for Asclepias and monarchs in the future. Asclepias, and consequently monarchs, should therefore undergo expanded northern range limits in summer months while encountering reduced habitat suitability throughout the northern migration.

Highlights

Over the past century, climate change has altered range distributions of many species [1,2,3]
The overall Asclepias distribution was best explained by a combination of all environmental and geographic variables (Table 2)
Mean annual temperature explained over half of the variance, as Asclepias only occurred in areas where mean temperatures were above 0° but below 30° C

Summary

Introduction

Climate change has altered range distributions of many species [1,2,3]. Lepidopterans (i.e. butterflies) have exhibited significant poleward distributional shifts due to climate warming [1, 3, 6,7]. Host switching allows more rapid northward range expansion by generalist lepidopterans than by more specialized species [8]. If specialist lepidopterans cannot switch hosts, the rate at which they expand or shift their range will depend on range expansion of their host plant species. In cases where species are unable to shift their distributions northward or upslope due to lack of suitable habitat, i.e. a lack of host plant availability, climate change can impose severe bottlenecks or even cause extinctions [9]. Given the likely increases in greenhouse gas emissions and concomitant changes in climate, there is considerable interest in forecasting species distributions into the future to enable adequate conservation measures [10]

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Conclusion