Top-down and bottom-up processes in grassland and forested streams

Abstract

The influence of predatory fish on the structure of stream food webs may be altered by the presence of forest canopy cover, and consequent differences in allochthonous inputs and primary production. Eight sites containing introduced brown trout ( Salmo trutta) and eight sites that did not were sampled in the Cass region, South Island, New Zealand. For each predator category, half the sites were located in southern beech (Nothofagus) forest patches (range of canopy cover, 65-90%) and the other half were in tussock grassland. Food resources used by two dominant herbivores-detritivores were assessed using stable isotopes. (13)C/(12)C ratios were obtained for coarse particulate organic matter (CPOM), fine particulate organic matter (FPOM), algal dominated biofilm from rocks, and larvae of Deleatidium (Ephemeroptera) and Olinga (Trichoptera). Total abundance and biomass of macroinvertebrates did not differ between streams with and without trout, but were significantly higher at grassland sites than forested sites. However, taxon richness and species composition differed substantially between trout and no-trout sites, irrespective of whether streams were located in forest or not. Trout streams typically contained more taxa, had low biomass of predatory invertebrates and large shredders, but a high proportion of consumers with cases or shells. The standing stock of CPOM was higher at forested sites, but there was less FPOM and more algae at sites with trout, regardless of the presence or absence of forest cover. The stable carbon isotope range for biofilm on rocks was broad and encompassed the narrow CPOM and FPOM ranges. At trout sites, carbon isotope ratios of Deleatidium, the most abundant invertebrate primary consumer, were closely related to biofilm values, but no relationship was found at no-trout sites where algal biomass was much lower. These results support a role for both bottom-up and top-down processes in controlling the structure of the stream communities studied, but indicate that predatory fish and forest cover had largely independent effects.

The Stream Biome Gradient Concept: factors controlling lotic systems across broad biogeographic scales

“…Studies in grassland systems suggest that both top-down and bottom-up processes regulate foodweb structure (Huryn 1998, Stagliano and Whiles 2002, Nyström et al 2003. Where dense canopy cover is lacking, reliance on autotrophic production should be greater (Whiting et al 2011).…”

Environmental variability and population dynamics of juvenile brown trout (Salmo trutta) in an upstream and downstream reach of a small New Zealand river

“…The average biomass of benfhic invertebrates during summer 2003-04 in the upstream and downstream reach was 2589 mg dry weight (DW)/m 2 and 2960 mg DW/m 2 , respectively. This biomass is generally higher than has been found in other New Zealand rivers (Glova & Sagar 1994;Harding & Winterbourn 1995;Nystrom et al 2003). The average biomass of drifting invertebrates during summer was 0.24 mg DW/m 3 in the upstream reach and 1.06 mg DW/m 3 in the downstream one.…”

Nutrient limitation controls the strength of behavioral trophic cascades in high elevation streams

“…In some cases trophic cascades are strong when system productivity is low but are weakened when productivity is increased (Chase 2003, Riley et al 2004). Other studies report no change in predator effects on primary producer biomass over a productivity gradient (Nyströ m et al 2003). In still other situations, increasing nutrients has been shown to strengthen top-down interactions (Pace et al 1999, Letourneau et al 2004, Menge 2004.…”