Do small-scale trophic cascades contribute to benthic heterogeneity in streams?

Background/Question/Methods

Trophic cascades can be mediated by environmental conditions and are generally examined at large (> 10 m) scales; however, where fine-grained environmental heterogeneity exists, small-scale cascades may occur and create mosaics of primary producer biomass.  “Patchy” cascades may be particularly important for benthic stream habitats in which near-bed current velocity influences both small-scale heterogeneity and species interactions.  To test this, we investigated a stonefly – mayfly – algae cascade in streamside mesocosms to replicate conditions in Copper Creek, a high gradient mountain stream by the Rocky Mountain Biological Laboratory in Gothic, CO, USA.   Two experiments examined the effects of near-bed current on 1) Megarcys stonefly capture rates of Baetis prey, and 2) Megarcys influences on Baetis grazing.  Algae was colonized on small stream cobbles placed in 24 circular channels in which near-bed current velocity averaged either 11 or 25 cm/sec.  Baetis mayflies were stocked to match mean streambed densities in Copper Creek, with one Megarcys stonefly per channel.  We employed a full factorial design with the following treatments replicated four times for each near-bed current velocity: controls (i.e., algae only), Baetis only, and Megarcys and Baetis together.  In the second experiment Megarcys could not feed because its mouthparts were glued shut.

Results/Conclusions

Results from our first experiment showed that after 12 h, Megarcys consumed an average of 3.6 more baetid prey in slow current (p < 0.05).  The second experiment found that near-bed current affected both algal accrual and the response of baetid grazers to the presence of Megarcys.  Cobbles in slow current had 53% more algae, but baetid grazers were also less effective at removing algae when Megarcys was present. Algal biomass in channels with Megarcys and slow current increased 33% and did not differ from controls.  We concluded that the presence of Megarcys affected algal removal through an interaction with near-bed current such that a top-down cascade occurred in slow but not fast current.   These results suggest that spatial variation in near-bed current may contribute to heterogeneity in algal biomass through small-scale trophic cascades.  It also suggests that the detection of trophic cascades in natural systems may be scale-dependent.