Evaluating the Migration Mortality Hypothesis Using Monarch Tagging Data

Abstract

The decline in the eastern North American population of the monarch butterfly population since the late 1990s has been attributed to the loss of milkweed during the summer breeding season and the consequent reduction in the size of the summer population that migrates to central Mexico to overwinter (milkweed limitation hypothesis). However, in some studies the size of the summer population was not found to decline and was not correlated with the size of the overwintering population. The authors of these studies concluded that milkweed limitation could not explain the overwintering population decline. They hypothesized that increased mortality during fall migration was responsible (migration mortality hypothesis). We used data from the long-term monarch tagging program, managed by Monarch Watch, to examine three predictions of the migration mortality hypothesis: (1) that the summer population size is not correlated with the overwintering population size, (2) that migration success is the main determinant of overwintering population size, and (3) that migration success has declined over the last two decades. As an index of the summer population size, we used the number of wildcaught migrating individuals tagged in the U.S. Midwest from 1998 to 2015. As an index of migration success we used the recovery rate of Midwest tagged individuals in Mexico. With regard to the three predictions: (1) the number of tagged individuals in the Midwest, explained 74% of the variation in the size of the overwintering population. Other measures of summer population size were also correlated with overwintering population size. Thus, there is no disconnection between late summer and winter population sizes. (2) Migration success was not significantly correlated with overwintering population size, and (3) migration success did not decrease during this period. Migration success was correlated with the level of greenness of the area in the southern U.S. used for nectar by migrating butterflies. Thus, the main determinant of yearly variation in overwintering population size is summer population size with migration success being a minor determinant. Consequently, increasing milkweed habitat, which has the potential of increasing the summer monarch population, is the conservation measure that will have the greatest impact.

Further Insights on the Migration Biology of Monarch Butterflies, Danaus plexippus (Lepidoptera: Nymphalidae) from the Pacific Northwest

“…These recovery rates are similar to those reported for 2012–2016 [ 13 ]. They are also within the range reported for Arizona [ 11 ] and eastern US monarchs [ 33 ]. However, it is likely that the recovery rate for eastern US monarchs is an underestimate because it only uses individuals found dead at the Mexico overwintering sites [ 33 ].…”

“…They are also within the range reported for Arizona [ 11 ] and eastern US monarchs [ 33 ]. However, it is likely that the recovery rate for eastern US monarchs is an underestimate because it only uses individuals found dead at the Mexico overwintering sites [ 33 ]. If live tagged butterflies were counted in Mexico, the recovery rate would likely be much higher.…”

“…A large number (21)(22)(23)(24)(25)(26)(27)(28)(29)(30)(31)(32)(33)(34)(35)(36)(37)(38)(39) of individual citizen scientists in Oregon participated in this program annually, each rearing a varying number (3-600) of monarch butterflies in protected home gardens (~10% of rearers) or indoors during August-October. All captivereared monarchs were locally sourced from eggs or larvae found within Oregon, often in the back yard or property of the citizen, and were mostly reared in muslin cages on narrow-leaved milkweed (A. fascicularis Dcne.)…”

Commentary: Evaluating the Migration Mortality Hypothesis Using Monarch Tagging Data

“…Thus, the possibility was raised that the fall migration to Mexico and overwintering period might be a weak link in the migratory life cycle, which would, in turn, suggest conservation attention should not be focused too narrowly on summer milkweed resources. Taylor et al (2020) present new tagging data, concluding that none of the expectations of the migration mortality hypothesis are supported. They conclude the following: (1) the apparent disconnect between late summer and overwintering numbers reflects poor summer data used in previous analyses, (2) the recovery rate (equated with migration success) of tagged monarchs is not correlated with overwintering numbers, and (3) recovery rate has not decreased over time.…”

“…Thus, details of the monarch decline matter: unlike many insects, it is a species for which we might hope to identify causes of decline. It is in this spirit that we offer a reconsideration of the data presented by Taylor et al (2020) who tested hypotheses pertaining to eastern monarch population decline.…”