Dynamic Life-History Strategy of the Waterstrider Gerris thoracicus as an Adaptation to Food and Habitat Variation

Abstract

This study addresses the problem of how insect females make the trade-off between reproduction and longevity when the availability of food varies greatly and when there is poor correlation between the quality of the adult and juvenile environments. The effect of food stress was studied in the waterstrider Gerris thoracicus Schumm. living in Finland mainly in rock pools and brackish water bays. In the laboratory experiments when food was abundant G. thoracicus females had a high reproductive output and a short lifetime. When food was scarce females decreased reproduction and lived longer. When the food ration was varied reproduction followed pulses of food with a time lag of a few days. The food ration used in the experiment had no effect on the longevity of males. The pattern studied in G. thoracicus is likely be an adaptation to unpredictable environments since Gerris lacustris L., an inhabitant of more permanent habitats, responded to food scarcity both with decreased egg production and a shorter lifetime. The conclusion is that G. thoracicus has a more dynamic life-history strategy. It shifts the trade-off in allocation between reproduction and longevity when the food ration changes: During food abundance it has a high reproductive rate at the expense of a reduced life span. During food scarcity it switches off reproduction and is thus able to live longer.