Population structure of pierid butterflies

Summary

The numbers, dispersal behavior, aging and residence, and Wrightian neighborhood configurations of three species of Colias butterflies have been studied in central Colorado, using mark-release-recapture techniques as major tools. All populations studied have nonoverlapping generations and mature one brood each year. A brief general review of these species' autecology is given. A system for measuring degree of physical damage to the adults is introduced. This “wear rating” varies with temporal position of any given sample in the course of a brood's flight season, the insects becoming progressively more damaged with time. The sex ratio also changes with brood aging: males eclose before females, and are in the majority early in the flight season, while females may predominate at the end of flight. Local population numbers for the montane grassland species C. alexandra may reach peak levels of 700–900 insects in favorable years, but be much lower in other years as a result of, e.g., drought. Peak densities are no more than 2/ha. The montane bog species, C. scudderi, maintains comparable low density but has much smaller local populations. The subalpine/alpine grassland species C. meadii displays peak local numbers as high as 3000, with peak density as high as 120/hectare. Dispersal varies both among and within species. Those C. alexandra who disperse show an average dispersal radius of about 1.3 km, with a radius for the whole population of about 0.6 km; maximum distance moved was 8 km. Dispersal proportions among recaptures are sharply curtailed by adverse weather, but the dispersal radius of those moving is unaffected by weather. C. scudderi's dispersal is strongly influenced by the geometry of its bog and streamside habitats. Some C. meadii populations approach isolated “island” status, but others show much dispersal. Dispersal radius of those dispersing ranges from 0.3 to 0.7 km in different populations, but the proportion of dispersals varies greatly. The longest observed movement by this species is 1.3 km, although up to 2.6 km could have been detected. Colias normally display constant loss (death plus emigration) rates with average residence expectations of 4–6 days; few insects reach their maximum physiological lifespan of approximately 1 month. Bad weather can increase the loss rate drastically. Females show shorter residence than males, appearently as a result of greater mortality. Total-numbers-per-brood estimates are given for our better studied populations. The reproductive strategy of Colias is such that Wright's models for neighborhood size apply. Neighborhood size for C. alexandra varied sixfold in numbers, and from 3 to 1.3 km in physical extent, between a favorable year and a drought year. One localized C. scudderi habitat is only 200 m in diameter, but a streamside population has a neighborhood length of 4.8 km. In C. meadii, one population of 2000–2500 insects is an 8-ha “island”, while another of similar numbers extends a single neighborhood across 1.9 km distance, 450 m altitude, and a major ecological boundary (timberline). Factors such as weather, individuals' visual cueing, and thermoregulatory behavior can influence population structure. For some Colias populations, selection may be very uniform within neighborhoods, while for others, single neighborhoods cross sharp discontinuities in selective forces. These patterns may differ for different selective forces, and may also vary with stages of the insects' life cycle. these populations will now prove a valuable resource for studying evolutionary population genetics.