Population Fluctuations of the Monarch (Danaus plexippus) in the 4th of July Butterfly Count 1977-1994

Abstract

-Monarchs reported in North America N of Mexico in the 4th ofJuly Butterfly Count (4JC) for 1977-1994 were analyzed for population fluctuations. From 1977 to 1986 the mean number of monarchs per hour in eastern North America varied significantly in only one of nine pairs of consecutive years. From 1986 to 1994, eastern monarchs varied significantly in five of eight year-pairs, with a nearly equal number of increases and decreases. However, the amount of variation within each year-pair was similar for 1977-1986 and 19861994 (median difference ca. a factor of 2). Although the Atlantic and midwestern subregions of the eastern population covaried in four of eight year-pairs from 1986-1994, differences between the two subregions sometimes damped fluctuations in the eastern population overall. Data from independent transect surveys in Wisconsin for 1990-1994 agreed with the direction of fluctuations in Wisconsin 4JCs. In 4JCs from 1987-1994 in the Pacific coastal states, monarchs varied less than the eastern population, with a significant increase in 1989 and significant decrease in 1988. However, a nonsignificant decline of an order of magnitude occurred in 1993, and a considerable but smaller increase occurred in 1994. Fluctuations in monarch abundance in eastern North America during 1977-1994 often, but not always, coincided with years affected by major widespread perturbations of typical weather such as the El Ninlo-Southern Oscillation, major volcanic eruptions, droughts and floods.