Long‐term fluctuations in density of two species of caddisfly from south‐east Australia and the importance of density‐dependent mortality

Abstract

AbstractThe long‐term dynamics of freshwater insect populations have attracted little interest. The aim of this study was to examine the recruitment, larval mortality and reproductive output of two species of bivoltine caddisfly,Tasimia palpata(Tasimiidae) andAgapetus kimminsi(Glossosomatidae). In the Cumberland River in south‐western Victoria, both species displayed summer and winter generations and their grazing larvae occupied the same rock surfaces. Ten consecutive generations of these species were monitored to determine which life‐history phases were density dependent. If direct density dependence occurs, mortality during a phase of the life cycle should be positively correlated with the density of individuals entering that phase.Quantitative samples (21 each month) were taken from April 2004 to March 2009 along a 190 m reach. Mean density (R1, numbers/m2) at the beginning of a generation represented the density of larval recruits. Density when pupal density was highest represented mean density at the end of a generation (R2, numbers/m2). Densities of eggs laid (S, numbers/m2) by each generation were determined from pupal densities (P, numbers/m2), development time of pupae and egg numbers per female.The relationship between the number of recruits (R1) and egg density (S) was modelled using a Ricker stock‐recruitment curve. In this model, K1or ln (S/R1), a measure of mortality, is a linear function of S. To extend the analysis to the rest of the life cycle K2(ln [R1/R2]; larval mortality) and K3(ln [R2/P]; mortality during pupation) were calculated. K2and K3were correlated against R1and R2, respectively, to determine whether these sources of mortality were related to starting densities.The Ricker curve forT. palpatawas an excellent fit and K1was correlated with S (r = 0.96,p = 0.03). K2was unrelated to R1(r = 0.05,p = 0.78), but K3was correlated with R2(r = 0.98,p = 0.005). Thus, population regulation ofT. palpataoccurred during recruitment and pupation, i.e. the reproductive phases of its life cycle.The Ricker curve was a poor fit forA. kimminsiand K1was unrelated to S (r = 0.56,p = 0.29). However, K2was correlated with R1(r = 0.92,p = 0.05), but K3was unrelated to R2(r = 0.04,p = 0.84).A. kimminsithus showed density‐dependent mortality during its larval phase. Its larval mortality was also positively correlated with R1ofT. palpata(r = 0.88,p = 0.0008), increasing as the strength of eachT. palpatageneration increased. This suggests competition occurred between the species, most probably related to food or feeding. For both species, K values were unrelated to water temperature or discharge.The populations of the two species were not affected by strong population movements (i.e. immigration or emigration). If such movements had been substantial, they would have disrupted the K values and obscured the density‐dependent trends.