The strength of top-down (macroconsumer) control in stream ecosystems can vary across scales of observation and depends on environmental conditions. However, most experiments examining top-down control in stream ecosystems have focused on only a single, often small, spatial scale (<1 m²). With the aim of examining top-down control at multiple scales, we conducted a 32-day macroconsumer exclosure experiment (i.e., fish and decapod removal) at the Patch scale (1 m² exclosures) and Reach scale (whole-reach exclosures ~20 m length), replicated across a gradient in stream discharge in the wet/dry tropics of northern Australia. Key macroconsumer food sources periphyton (chlorophyll a biomass) and macroinvertebrate diversity and abundance were measured.
Results/Conclusions
At the Patch scale there was no evidence of top-down control, but at the reach scale strong top-down control on periphyton biomass and macroinvertebrate abundance was observed, across a gradient in stream discharge, suggestive of a trophic cascade. Our findings indicate that top-down experiments conducted at small spatial scales do not necessarily reflect larger scales (underestimation in our study) and interpolation across scales should be done with care.