Stonefly life-history differences between runoff-dominated and spring-fed streams: A reciprocal transplant experiment

Background/Question/Methods Understanding whether life-history traits are constrained or plastic is important when forecasting resilience following drastic environmental changes, such as anthropogenic alterations to flow regime.  To explore this, we conducted a reciprocal transplant experiment between a runoff-dominated and a spring-fed tributary to the McKenzie River, OR.  We expected insects in the runoff-dominated stream to emerge prior to summer low flows to avoid degraded in-stream conditions.  In contrast, we expected insects inhabiting the spring-fed stream, with little seasonal variation in flow, to continue emerging throughout the summer.  Yoraperla nigrisoma (Peltoperlidae) larvae were reciprocally transplanted in replicated enclosures in both stream types.  Results/Conclusions Of the insects that emerged, more were of spring-fed than runoff-dominated origin (p<0.005), indicating a constrained natal stream effect.  Of the insects originating from the spring-fed stream, those transplanted to the runoff-dominated stream had larger head widths than those placed in the natal spring-fed stream (p<0.05).  This difference was due to a greater proportion of (69%) females, which are larger than males (p<0.05), emerging from the runoff-dominated stream compared to the proportion of females emerging from the spring-fed stream (18%).  Life-history differences between natal streams and between transplanted and native treatments indicate some degree of both genetic constraint and phenotypic plasticity.