Reproductive Tactics of the Waterstrider Gerris Thoracicus in Unpredictable Environments: A Simulation Study

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Abstract
The waterstrider Gerris thoracicus Schumm. (Heteroptera) is met with in Finland only in the archipelago and coastal areas of the Baltic, mainly in rock pools and brackish-water bays, in which living conditions change unpredictably for waterstriders. We modelled reproductive success in each habitat. In the rock pools the critical environmental variable is the drying-up of pools, which depends on the length of rainless periods. We simulated different hypothetical population persistence by changing the timing of reproduction and batch size using precipitation data over 20 yr. The tactics were either big-bang or spread-out ones. All big-bang tactics suffered from high or fairly high extinction probabilities. Only those tactics that spread oviposition over July or over June and July could survive. In the sea the critical parameter is the length of the time when the water is warm enough for reproduction and development. There were at least three years out of the 17 studied when the sea surface temperature was so low throughout the summer that offspring production was highly improbable in the sea. Severe space and food limitations in rock pools suggest that these must substantially increase local extinction probabilities. Due to the negative correlation between crunch years in the rock pool system and the sea, a tactic mixing reproduction in these two habitats might maintain the population. The result is compatible with field data.