Habitat edges are all around us, and the way that animals behave at these interfaces could have profound effects on population dynamics by altering large scale movement patterns, like dispersal and range expansion. One commonly observed edge behavior is a preference for remaining in and around suitable habitat, i.e. staying out of the matrix. This pattern leads to an increased residence time in patches of suitable habitat. We used the monarch butterfly, a species recently undergoing declines linked to loss of their milkweed hosts (Asclepias spp.), to test for edge preferences at the interface between milkweed and grass/forb-dominated meadow matrix. We caught wild breeding monarchs in Massachusetts and released them at the edge of milkweed patches, and then followed the butterflies and marked their locations using flags and GPS, allowing us to calculate the area of milkweed along a 5-m radius around the release point. We then estimated preference, or bias in moving towards or away from the milkweed patch, by using binomial GLMs with an offset of the logit-transformed proportion of the 5-meter radius that fell within milkweed habitat, a method that accounts for the host patch size.
Results/Conclusions
Female monarchs had a significant bias towards entering milkweed patches compared with the null 0.5 expectation (estimated preference = 0.80, 95% CI = 0.61-0.92). Males also preferentially entered host patches, but the trend was not significant (estimated preference = 0.69, 95% CI = 0.48-0.84). Females, as the egg-layers, are the more relevant group for population dynamics. The preference for milkweed habitat we observed in females is stronger than previously reported preferences in monarchs (Ries and Debinski 2001) against entering fields (estimated preference = 0.58) or crossing roads (estimated preference = 0.44), and is similar to the preference for entering meadows at forest-meadow boundaries (estimated preference = 0.79). Overall, an 80% tendency in females to enter patches of host plant bears a mixed message for landscape planning: although monarchs have a strong bias in movement towards host plant patches, this preference is imperfect. The butterflies do not always move towards host plant and are not guaranteed to find or use all the available plants on the landscape. This conclusion contrasts with assumptions in some models used for conservation planning.