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Coexistence in the Gerridae Jamieson, Glen Stewart

Abstract

A comparative analysis has been made of the biologies of five species of the waterstrider Gerris, G. incurvatus, G. buenoi, G. notabi1is, G. incognitus and G. remigis, which occur together in southwestern British Columbia. The aim of this study was to investigate how coexistence is achieved, and through the construction of a computer model, to simulate the interactions arising from this coexistence. The thesis is divided into six main sections: field biology, temperature preferences on a gradient, food consumption, reactive distance and movement, gerrids as predator and prey, and the model. Field studies in 1971 and 1972 indicated that the five species of Gerris studied are not ecological homologues. Both spatial and temporal differences were detected, although considerable overlap does occur. The spatial separation results from the fact that G. remigis prefers a stream habitat, G. iscognitus prefers a littered water surface habitat, and although G. buenoi, G. notabilis and G. incurvatus all prefer an open water habitat, the distances from shore preferred by each species are respectively greater. Temporal separation of the species results from differences between the species in their response to temperature. The optimum temperature for growth in G. remiais and G. buenoi is 22°C. and in G. incurvatus, G. incognitus and G. notabilis 26°C.. The threshold temperatures below which growth ceases are 9.3°C. for G. incognitus, 10.3°C. for G. notabilis, and 12.6°C. for G. buenoi, G. remigis and G. incurvatus. Temperatures voluntarily selected by each species on a water gradient differed. G. remigis selected a temperature of about 20°C., G. incognitus temperatures between 14-30°C., and G. incurvatus, G. buenoi and G. notabilis temperatures from 25-30°C. The significance of these temperature preferences in the context of those temperatures encountered by each species in the field situation was discussed. Food consumption in a quantitative manner was described for the larvae and adults of the five species. The following parameters being determined: satiation time, the mean length of feeding and non-feeding periods in the presence of excess food, the effect of food deprivation on amount consumed, maximum gut capacity, the effect of temperature on food consumption, and the effect of developmental state on food consumption. A significant difference in digestive rate between larval and adult gerrids was found, with the time required by an adult gerrid to achieve maximum hunger being about twice that required by a larva. There were no significant differences among the larvae or among the adults. Temperatures affected food consumption, and within the temperature range tested, 5-32°C., the amount consumed increased with increasing temperature for every species except G. remigis. In this latter species, food consumption peaked at 20°C. Within any one instar, food consumption was found to peak about 40% of the way through the stadium, following a rapid rise from the previous ecdysis. After peaking, feeding declined gradually until the occurrence of the next ecdysis. The two parameters which determine the rate at which prey are encountered by the instars of the five species studied, namely the distance at which they respond to prey items and their rate of movement, were measured. These parameters allowed calculation of the swath the predator covered as it moved across the water surface. Results suggest that Gerris species prefer to attack live prey in front of them, and tend to ignore prey if the attack requires a turn of more than 100°. The state of hunger was found to affect the visual angle required to elicit an attack by G. remigis, and regardless of species, smaller gerrids required the prey to be closer before an attack was initiated. The rate of movement in Gerris was measured as a function of stride length and the number of strides made per unit time. Stride length varied according to the length of the mesothoracic leg, and the frequency of movement was observed to be species specific. G. remigis a stream species, moved 4 to 6 times as often as the four other species studied, all of which are characteristically found on non-moving water surfaces. The propensity to move in G. remigis was significantly influenced by hunger, with a maximally starved gerrid moving only one sixth as much as a satiated gerrid. Within a species, gerrid size had no significant effect on the frequency of movement, although there was a tendency for smaller gerrids to move less. The significance of the different propensities to move is discussed for the five species studied. Predation studies demonstrated that no significant differences existed among the species in the efficiency with which they killed prey. However, they did show that differences existed among the species in their propensity to attack, and that these differences resulted in significantly different numbers of prey killed by the species. G. remigis in particular, both as a larva and as an adult, killed more prey than any of the other four species. The ability of each instar to prey upon gerrids its own size or smaller was also investigated to permit assessment of the extent of cannibalism. Preference studies where the larger gerrids were presented simultaneously with all the prey size classes they could capture efficiently, demonstrated that the prey gerrids most preferred were those most efficiently handled. The effect of different "habitats" on kill success was also investigated. These studies indicated that the ease with which certain gerrids are captured approaches that with which terrestrial insects trapped on the water surface are captured, suggesting that cannibalism may be an important mechanism in population regulation and perhaps in the ability of different species to coexist. The results of this study were incorporated into a mechanistic computer model based on an experimental components analysis type of approach (Holling, 1966), which was expanded to include a number of species and to simulate an entire season under field conditions. Predictions by the model, which was based on laboratory-derived data, were compared to field observations, and were found to agree favourably. Cannibalism was predicted to be an important phenomonen in all the situations studied, and mean hunger level of the gerrids affected only slightly the predicted number of progeny produced at the end of the summer. The model when run with only single species present showed that each species completed its life cycle with the observed number of generations each year and with the temporal spacing of these generations much as observed in the field. When all five species were simulated to coexist in the same habitat, G. incognitus was found to virtually eliminate all of the ether species present. This prediction is suggested to be the reason little overlap in field distribution occurs between this species and the other pond species. The open water pond species, G. incurvatus, G. buenoi and G. notabilis, were all predicted capable of coexisting in the same habitat. The spatial separation among these species in the field may thus be concerned with long term coexistence rather than the short term problem of occupying the same habitat.

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