DARE TO ENTER: Interpatch movement of the migratory monarch butterfly, Danaus plexippus

Habitat fragmented by road construction. Photo credit: Notice Nature

The roads you travel on may help get you home, but it is likely that they prevented some animals from reaching theirs. As a scientist eloquently put it: “Roads scare the hell out of ecologists” (quoted in Nijhuis, 2015) and there is reason for such fear. Human modifications to the environment through roads, development, and agriculture have been major drivers of habitat fragmentation. This form of habitat loss is one of the primary threats to wildlife in the United States. Fragmentation not only results in the loss of the total amount of suitable habitat, but also compromises the remaining habitat for animals. This process makes it difficult for migratory species to find resting sites and places to feed along their migration routes. Movement between these patches is critical for their survival and can help “rescue” these populations from local extinction and the negative effects of inbreeding depression (i.e., reduced fitness from breeding with relatives). In spite of the importance of mobility and emigration, few studies address how easily animals can move through varied environments—i.e., the rate of interpatch movement. To gain a better understanding of local extinction risks and patch occupancy, it is important to examine the willingness of an organism to leave a patch and their behavioral response to encountering a patch’s edge. Whether they choose to approach it, avoid it, or cross it, affects their dispersal success (and thus, their ability to survive) across varying landscapes. 

Such decisions are crucial for animals like the migratory monarch butterfly (Danaus plexippus). As individuals journey to overwintering sites in Mexico, they encounter miles of landscape heterogeneity. How do monarchs respond to fragmented environments? Previous research by Ries and Debinski (2001) helped to address this question. Their findings piqued our interest in studying edge effects on our campus monarchs.

Specifically, Ries and Debinski (2001) wanted to compare the responses of two butterfly species to edges in fragmented prairies and whether their responses were affected by variables like wind, flower abundance, and time of year. In this study, the researchers focused on two species, Speyeria idalia and D. plexippus: a non-migratory habitat specialist and a habitat generalist, respectively. Using plots in eleven prairies with different boundary types (crop, treeline, roads, fields), Ries and Debinski tracked the behavior of the individuals to quantify their edge response (i.e., proportion that exited patches and proportion that returned to the plot interior; turning frequency to avoid crossing edges). They found that the frequency that D. plexippus would cross edges was strongly affected by wind direction and time of year; individuals were more likely to leave prairie plots later in the summer and when the wind was blowing towards the boundaries of the study sites. Both species were less likely to cross over treeline edges- but the non-migratory S. idalia was a lot more sensitive to subtle changes in vegetation and avoided crossing edges adjacent to row crops. These findings demonstrate that habitat edges can be barriers to movement. The interspecies variability in leaving or remaining in patches shows that environmental factors can influence and modify edge response, regardless of edge type—further emphasizing the importance of local conditions in analyzing species movement.

What does this mean for our local population of Danaus plexippus at Grinnell College? Those individuals that visit the reconstructed prairie patches near the athletic fields encounter various types of edges (concrete paths, turf, railroads). 

Campus study site: Bowers Prairie patches near Springer Field. The patches border a concrete walkway, turf-grass lawns, and a railroad. Scale: Soccer field measures 120 x 75 yards.  Aerial image from Mapquest.

In our campus project, we are attempting to investigate the variation in interpatch movement among monarchs and how local variables like flower abundance, area size, and distance between neighboring patches might affect their flight patterns and visitation frequency in certain plots. We hope to use the results discussed in Ries and Debinski (2001) to inform our project and create a specific framework of how the landscape (near the Bear Recreational Center, at least) will affect small-scale movements in these monarchs. Our study may lend support for a plan to make campus landscaping more butterfly-friendly. But we are also optimistic about informing larger-scale applications such as roadside plantings. Is it possible to combat the harmful effect of roads by providing better feeding and resting sites for pollinators like D. plexippus? We invite you to think about that on your next trip down I-80.

REFERENCES:

Nijhuis, M. (2015, March 20). What Roads Have Wrought. The New Yorker, Retrieved from http://www.newyorker.com/tech/elements/roads-habitat-fragmentation. 

Ries, L., & Debinski, D. M. (2001). Butterfly responses to habitat edges in the highly fragmented prairies of Central Iowa. Journal of Animal Ecology, 70(5): 840-852.