干旱区生态环境知识资源中心

In disturbance ecology there is a tension between ecological and evolutionary viewpoints, because while disturbances often cause mortality in populations (an ecological effect), populations may also evolve mechanisms that ameliorate mortality risk (an evolutionary effect). Flash floods cause high mortality in the juvenile aquatic stage of desert stream insects, but these ecological effects may be mitigated by the evolution of life-history strategies that allow the terrestrial adult stage to avoid floods. Life-history theory predicts that, to balance trade-offs between juvenile growth and mortality risk from floods, (1) most individuals should emerge before the peak of the flood season, (2) optimal body size at emergence should decline as flood probability increases, and (3) a second decline in body size at emergence should occur as the reproductive season ends. These predictions were tested with data on body mass at and timing of emergence of the caddisfly Phylloicus aeneus measured in three montane Chihuahuan Desert (Arizona, USA) streams over two years. P. aeneus that had not reached the adult stage were eliminated from site-years that experienced flash floods, suggesting that timing of emergence is an important fitness component. On average 86% of emergence occurred before the long-term (similar to100 yr) mean arrival date of the first seasonal flood, supporting prediction 1. The presence of two consecutive declines in body mass at emergence in most site-years was congruent with predictions 2 and 3. To test whether the two declines were associated with increasing flood probability and end of the reproductive season, respectively, maximum-likelihood methods were used to compare five body-size models: a null model that contains no parameters related to flood regime or reproductive season, a seasonal model that incorporates a reproductive time constraint, and three disturbance models that incorporate both reproductive time constraints and flood dynamics. The disturbance models outperformed the other models, suggesting that at least some of the body-mass pattern was influenced by flood dynamics. The timing of the first flood of the season was the most important determinant of observed emergence patterns. Overall, this study demonstrates that aquatic insects can compensate for flash floods by using state-dependent emergence strategies that are synchronized with long-term flood dynamics.