Water-mediated trophic cascades in a riparian forest

Background/Question/Methods

Despite the widely recognized importance of water for driving the evolution of life on earth, little research has investigated how water influences terrestrial food webs. Recent research has suggested water availability directly alters the abundance of multiple animal species along dryland rivers and influences interaction strength at multiple trophic levels. Based on previous evidence of declines in interaction strength with increased water availability, we predicted that variation in water could result in trophic cascades in these systems. We tested this prediction by manipulating large spider abundance and water availability in 20x20 m open-air plots along the San Pedro River, in SE Arizona, USA, and measuring changes in cricket abundance and leaf damage.

Results/Conclusions

We found strong interactive effects of spider removal and water addition on cricket abundance and leaf damage. As predicted, spiders reduced cricket abundance under ambient, dry conditions, but did not reduce cricket abundance where water was added.  However, contrary to predictions, spider removal increased leaf damage where water was added, but not where conditions were dry. We suggest that this pattern is related to trade-offs between cricket water stress and fear of predation by spiders, with water stress overcoming fear under dry, ambient conditions, causing high rates of leaf consumption (nearly 100%), but with fear reducing leaf consumption where water was added. Additionally, observations of leaf damage across a gradient of soil moisture, over time, allowed the prediction of the geographic distribution of likely ground arthropod water stress across the US. Our results suggest that over 60% of the US may experience conditions that lead to water stress in ground arthropods, at some point in time.  Thus, water may influence terrestrial food webs outside of drylands, in more mesic systems, whenever dry conditions occur.