Field Dispersal and Population Sizes of Native Drosophila from North Carolina

Abstract

Native Drosophila were periodically captured, dusted with micronized fluorescent dust and released during the summer of 1977 at two locations near Raleigh, North Carolina. Two or three days after each release, flies were collected in baited traps placed at regular intervals along a transect (800 m or 1,200 m long) centered on the point of release. Recaptured data provided estimates of the shape of the distribution of release flies as well as the dispersal rate of Drosophila, in particular D. melanogaster and D. simulans. Simultaneously collected unmarked flies provided estimates of natural population densities and effective sizes during the summer and early fall. Movement of marked flies averaged 150 m per day with a variance of around 20,000 m2. The observed distribution of a large sample of marked flies appeared to fit a Brownian motion model, in spite of a tendency for Drosophila to aggregate in moist, shady spots. The density of flies varied over the 4 mo tested, ranging from near zero to roughly 2-3 flies per 100 m2 for both D. melanogaster and D. simulans at the two experimental sites. Wright's formulas give estimates of effective population size in the range of 1,000-10,000 individuals during the peak summer populations, while the neighborhood (breeding) radius varied between 200-400 m. Dispersion and population size may be underestimated since evidence suggested dispersion of marked flies beyond the experimental area.