An area census method for estimating butterfly population numbers

Abstract

KNOWLEDGE ABOUT POPULATION SIZE (or density) is often desirable in ecological and other studies in butterflies. Usually population numbers are estimated from capture-recapture data. Using this method it is possible to obtain accurate estimates of population size and also of birth and death rate. However, to obtain good estimates it is often necessary to capture and mark a large number of individuals which is a time-consuming task. Moreover the individuals captured may become disturbed resulting in unnormal movements and distribution patterns. To avoid these drawbacks I proposed an area census method which is a line transect where the line covers the whole area inhabited by the butterfly population (Douwes, 1970). This method implies that all individuals observed are counted and that searching is carried out in the same manner in the whole area. In a population study of Heodes virgaureae (Lycaenidae) area censuses were compared with capture-recapture estimates and a high correlation between the two kinds of estimates was found prOVided that weather conditions were favorable when the censuses were carried out (Douwes, 1970). This promising result initiated further investigations to see if the area census method can be used for other species and by other investigators.