Demographics of Lestes disjunctus disjunctus (Odonata: Zygoptera) in a riverine wetland

Abstract

I investigated the demographics of Lestes disjunctus disjunctus inhabiting a riverine wetland adjacent to the St. Marys River, Michigan. The areal extent of cover types within the 4.03-ha study area was measured and the densities of all life stages of L. d. disjunctus in separate cover types were converted to population estimates. Data were then analyzed using life-table techniques. The 1982 cohort oviposited an estimated 944 300 eggs in the 4.03-ha study area. Mortality in the egg stage was estimated to be 22.6%, of which 6.0% was associated with hatching and 16.6% was attributed to habitat loss. Survival through all larval instars was 5.5%. Mean larval production was 135 mg dry mass∙m−2∙year−1. The mortality rate was greatest in final-instar larvae. Instars F-8 through F-6 dispersed from wracks to Sparganium eurycarpum beds. The female:male ratio of instars F-2 through F was 1.08:1.00 and varied from 1.91:1.00 to 1.01:1.00 in the adult stage. An estimated 20 100 adult females emerged from the 4.03-ha study area, yielding a survival rate from the egg to adult stage of 4.3%. Lower potential fecundity in 1983 (45.2 eggs/female) than in 1982 (73.5 eggs/female), combined with low survival, yielded a net reproductive rate (R0) of 0.97 and a population rate of increase (r) of −0.03.