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

Area of Science:

• Ecology
• Freshwater Biology
• Food Web Dynamics

Background:

• Forest canopy cover influences stream ecosystems through altered allochthonous inputs and primary production.
• Predatory fish can significantly impact stream food web structure.

Purpose of the Study:

• To investigate how introduced brown trout (Salmo trutta) and forest canopy cover interact to influence stream food web structure in New Zealand.
• To assess the effects of predation and habitat on invertebrate communities and their food resources.

Main Methods:

• Stable isotope analysis (¹³C/¹²C) was used to trace food resources (CPOM, FPOM, algae, invertebrates) in streams with and without trout, and in forested versus grassland sites.
• Macroinvertebrate communities were assessed for abundance, biomass, taxon richness, and species composition.

Main Results:

• Macroinvertebrate abundance and biomass were higher in grassland than forested sites, but taxon richness and composition differed significantly between trout and no-trout streams.
• Trout streams exhibited higher taxon richness, lower biomass of predatory invertebrates and shredders, and a greater proportion of shelled/cased consumers.
• Algal biomass was higher in trout streams, and the primary consumer Deleatidium's carbon isotope ratios were linked to biofilm at trout sites, but not at no-trout sites.

Conclusions:

• Both bottom-up (resource availability) and top-down (predation) processes structure stream communities.
• Predatory fish (brown trout) and forest cover exert largely independent influences on stream food web structure and function.