Abstract
Each year, millions of monarch butterflies (Danaus plexippus) migrate up to 3000 km from their overwintering grounds in central Mexico to breed in eastern North America. Malcolm et al. (1993) articulated two non-mutually exclusive hypotheses to explain how Monarchs re-colonize North America each spring. The ‘successive brood’ hypothesis proposes that monarchs migrate from Mexico to the Gulf Coast, lay eggs and die, leaving northern re-colonization of the breeding range to subsequent generations. The ‘single sweep’ hypothesis proposes that overwintering monarchs continue to migrate northward after arriving on the Gulf coast and may reach the northern portion of the breeding range, laying eggs along the way. To examine these hypotheses, we sampled monarchs throughout the northern breeding range and combined stable-hydrogen isotopes (δD) to estimate natal origin with wing wear scores to differentiate between individuals born in the current vs. previous year. Similar to Malcolm et al. (1993), we found that the majority of the northern breeding range was re-colonized by the first generation of monarchs (90%). We also estimated that a small number of individuals (10%) originated directly from Mexico and, therefore adopted a sweep strategy. Contrary to Malcolm et al. (1993), we found that 62% of monarchs sampled in the Great Lakes originated from the Central U.S., suggesting that this region is important for sustaining production in the northern breeding areas. Our results provide new evidence of re-colonization patterns in monarchs and contribute important information towards identifying productive breeding regions of this unique migratory insect.
Highlights
Despite the massive scale of some insect migrations, remarkably little is known about how populations are spatially connected between different periods of the annual cycle [1,2]
We estimated that the majority (,90%) of monarchs re-colonizing the Great Lakes region during spring are 1st generation individuals born in the Gulf Coast and Central regions of the U.S.A. but that a small proportion of individuals (,10%) originate directly from Mexican over wintering sites
The confidence in assignments to specific breeding regions based on stable isotope values was high, it is possible that some monarchs were mis-assigned to have been born in the previous year or sampling year due to some uncertainty in the relationship between wing wear and age
Summary
Despite the massive scale of some insect migrations, remarkably little is known about how populations are spatially connected between different periods of the annual cycle (i.e. migratory connectivity) [1,2]. The entire eastern North American population of monarch butterflies (Danaus plexippus) migrate thousands of kilometres southward to discrete overwintering sites located between 2400–3600 m.a.s.l. in the Oyamel forests of central Mexico [6,7]. The ‘successive brood’ hypothesis (hereafter termed ‘‘SB strategy’’) proposes that overwintering monarchs migrate north to the U.S Gulf Coast, lay eggs, and die, leaving the re-colonization of the Great Lakes region to the 1st spring generation. The ‘single sweep’ hypothesis (hereafter termed ‘‘SS strategy’’) proposes that re-colonization is accomplished by the overwintering generation arriving at the Gulf Coast in early spring to lay eggs, but continuing northward towards the Great Lakes, effectively recolonizing the entire eastern breeding range in a ‘single sweep’
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