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Population study of waterstriders (Gerridae: Hemiptera) in Marion Lake, B.C. Maynard, Kathleen Jennifer

Abstract

This study consisted in part of general observations of the biology of four species of waterstrider (Gerris buenoi, incurvatus, notabilis, and remiges), in Marion Lake, B.C, and in part of an experimental manipulation of density and food supply in replicated populations of penned G. notabilis. Gerrids have a generation time of one year; adults overwinter, lay their eggs in the spring, and then die; nymphs become adults in about two months. In the natural population storms and food shortage probably caused the greatest mortality. In the penned populations survival of nymphs was inversely proportional to spring density of adults, and directly proportional to food supply. In low density pens fewer nymphs hatched but relatively more survived; in high density pens more nymphs hatched but relatively fewer survived; thus fall numbers were much the same (within food treatments) regardless of initial density. Increased density also lowered survival of adults in both food treatments, and, in turn, low adult survival enhanced the expectation of survival of nymphs, in the unfed pens only. However these effects were unimportant compared with the direct effects of density and food on nymph survival. Most of the nymphs died during the first stadium, probably owing to cannibalism by older nymphs and parents in the fed pens, and to both starvation and cannibalism in the unfed pens. In view of the fluctuating food supply to which gerrids are subject, their opportunistic method of feeding and concomitant cannibalistic behaviour is probably of selective advantage to the individual, as well as being a potential population regulating mechanism.

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