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Microhabitat selection and regional coexistence in water-striders (Heteropetra: Gerridae) Spence, John Richard


This study considers the natural history and ecology of water-strider species occurring on the Fraser Plateau of south-central British Columbia. The overall aim was to assess the effects of spatial heterogeneity on factors controlling the distribution and relative abundance of gerrid species. The relationships among temperature, population dynamics and habitat use were investigated. From a regional perspective, spatial heterogeneity allows species population dynamics to converge in time while keeping them separate in space. , Laboratory rearing studies were used to calculate physiological time-scales for developmental processes. Patterns of mating behaviour, fecundity and fertility are described for Serris comatus and G. pinqreensis in the laboratory and G- incoqnitus in the field. Egg laying and juvenile growth are shown to be strongly temperature dependent in all species studied. Temperature thresholds for development differ, both among species, and often among stages of a particular species. Low thresholds recorded for G. pinqreensis can lead to significant growth advantages for this species during early spring. Instar differences seem to be adapted to seasonal temperature regimes experienced by gerrids. Gerris species and instars showed distinct optimum temperatures for survival. These optima vary with developmental thresholds. It is suggested that species may be best adapted for growth under different temperature regimes. A method was developed for estimating absolute densities in the field from relative abundance measures using linear regression techniques. Gerrid size and presence or absence of vegetation markedly affect capture rates. No effect of species or type of emergent cover was demonstrated. Availability for capture varies with leg-length in G. buenoi and G. pinqreensis. This relationship is used to estimate availability constants for other water-strider species. Field surveys between 1975 and 1977 established that G. buenoi, G. comatus and G. pinqreensis were the most abundant water-strider species in- the study area. Each of these was strongly associated with a single type of vegetation in the field; G. buenoi with grass/sedge habitats, G. comatus with floating vegetation and G. pinqreensis with bulrush habitat. Limnoporus dissortis and L. notabilis were commonly encountered on small, temporary ponds, G. incognitus was first taken during 1976 in the study area and small populations are confined to brushy, well-shaded habitats. G. buenoi, G. comatus and G. pinqreensis are all potentially bivoltine in the study area; Limnoporus spp. are univoltine. Generation timing varies tremendously among lakes and periods of maximum abundance for each species are not separated in time. Strong between-lake habitat associations in the field result proximately from habitat fidelity at the time of spring colonization. The tendency of gerrids to overwinter near the mother pond and trial and error habitat selection during spring dispersal enforce habitat fidelity during colonization. Species distributions within lakes are affected by habitat availability. Habitat preference experiments demonstrate that G. pinqreensis and G. comatus have active preferences for emergent cover and open habitats respectively. G. buenoi is a habitat generalist but its distribution can be affected by a tendency to avoid other species. Smaller stages of each species are found close to shore and often in areas of dense emergent vegetation. Enclosure experiments demonstrated that G. pinqreensis can exclude G. buenoi and G. comatus from bulrush habitats, which are most favorable for the growth and development of all species. Fifth instar G. buenoi and G. comatus showed greatest weight specific differences in foraging efficiency among late instars may help produce the habitat associations observed for these two species. Fifth instar G. pinqreensis showed poor survival when enclosed in freshwater habitats, suggesting the hypothesis that its distribution is restricted by the presence of surface-feeding predators other than water-striders. It is suggested that competition for space, predation, density-independent mortality and colonization dynamics all interact on the template of spatial heterogeneity to produce regional patterns of distribution and abundance.

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