PHENOTYPIC PLASTICITY IN REPRODUCTIVE-BEHAVIOR OF WATERSTRIDERS - TRADE-OFFS BETWEEN REPRODUCTION AND LONGEVITY DURING FOOD STRESS

This paper addresses the question of how plasticity of reproductive behaviour varies between and within waterstrider species. I focus especially on how females differ in trade-offs between longevity and reproductive investment as the food availability changes. I reared long-winged waterstriders in high and low food levels. The trade-off between reproductive investment and longevity was measured as changes in life-span and fecundity at a low food level compared to that at an abundant food level. The direction of trade-off was compared (1) between morphs that differ in flight ability, (2) between Finnish and Hungarian populations of Gerris thoracicus Schummel, and (3) among Finnish G. thoracicus, G. odontogaster (Zett.) and G. lacustris (L.) populations. Results show differences in phenotypic plasticity at all levels. When the environment was relatively permanent or as the number of generations increased, females maintained relatively high reproductive investment when food was scarce even at the cost of reduced life-span. In unpredictable environments, females reduced reproduction during food stress so that they maintained long reproductive life-spans. The cost of being able to fly differed between populations of G. thoracicus. In the Finnish population females that could fly had shorter life-spans during food stress than flightless females. In the Hungarian population flightless females had higher fecundity than flyers. Hence phenotypic plasticity in reproductive behaviour is an important life-history trait. The direction of trade-off between reproduction and longevity may vary between two females that have the same reproductive potential in optimal food conditions. Hence the two females have totally different reproductive tactics in response to variation in resources.