Tracking multi-generational colonization of the breeding grounds by monarch butterflies in eastern North America

Abstract

Insect migration may involve movements over multiple breeding generations at continental scales, resulting in formidable challenges to their conservation and management. Using distribution models generated from citizen scientist occurrence data and stable-carbon and -hydrogen isotope measurements, we tracked multi-generational colonization of the breeding grounds of monarch butterflies (Danaus plexippus) in eastern North America. We found that monarch breeding occurrence was best modelled with geographical and climatic variables resulting in an annual breeding distribution of greater than 12 million km(2) that encompassed 99% occurrence probability. Combining occurrence models with stable isotope measurements to estimate natal origin, we show that butterflies which overwintered in Mexico came from a wide breeding distribution, including southern portions of the range. There was a clear northward progression of monarchs over successive generations from May until August when reproductive butterflies began to change direction and moved south. Fifth-generation individuals breeding in Texas in the late summer/autumn tended to originate from northern breeding areas rather than regions further south. Although the Midwest was the most productive area during the breeding season, monarchs that re-colonized the Midwest were produced largely in Texas, suggesting that conserving breeding habitat in the Midwest alone is insufficient to ensure long-term persistence of the monarch butterfly population in eastern North America.

Keywords: Danaus plexippus; carbon-13; deuterium; insect migration; migratory connectivity; stable isotopes.

Figure 1.

The probability of occurrence of monarch butterflies in eastern North America throughout the breeding season. This predictive map takes the maximum probability of occurrence of each cell in the landscape from all monthly distribution maps. The 0.5, 0.25, 0.1 and 0.05 isoclines are indicated. (Online version in colour.)

Figure 2.

Assigned natal origins based on model for using citizen science observations and stable-hydrogen (δ²H) and stable-carbon (δ¹³C) isotope composition of monarchs captured in (a) April and May with high wing wear scores, and therefore overwintered in Mexico (n = 115), (b) April and May with low wing wear scores and therefore first-generation offspring (n = 78), (c) June and therefore second generation (n = 140), (d) July and therefore third generation (n = 259), (e) August and therefore fourth generation (n = 217), and (f) September and October and therefore fifth generation (n = 30). The cell value as indicated by the scale is the sum of the binary maps of all individuals using a 2 : 1 odds ratio for each generation (see text). For example, of the overwintered butterflies in (a), the value 49 represents that cell being consistent with representing an origin of 49 of the 115 butterflies within the sample. Capture locations of butterflies are indicated with circles. (Online version in colour.)

Figure 3.

Combined assigned natal location surface of all monarch butterflies (n = 839) based on stable-hydrogen (δ²H) and stable-carbon (δ¹³C) isotopes. The cell value as indicated by the scale is the sum of the binary maps of all individuals using a 2 : 1 odds ratio for each generation (see text). For example, the value 363 represents that cell being consistent with representing an origin of 363 of the 839 butterflies considered in the study. Capture locations of butterflies are indicated with circles. (Online version in colour.)