Long-term nutrient enrichment decouples predator and prey production

Abstract

Increased nutrient mobilization by human activities represents one of the greatest threats to global ecosystems, but its effects on ecosystem productivity can differ depending on food web structure. When this structure facilitates efficient energy transfers to higher trophic levels, evidence from previous large-scale enrichments suggests that nutrients can stimulate the production of multiple trophic levels. Here we report results from a 5-year continuous nutrient enrichment of a forested stream that increased primary consumer production, but not predator production. Because of strong positive correlations between predator and prey production (evidence of highly efficient trophic transfers) under reference conditions, we originally predicted that nutrient enrichment would stimulate energy flow to higher trophic levels. However, enrichment decoupled this strong positive correlation and produced a nonlinear relationship between predator and prey production. By increasing the dominance of large-bodied predator-resistant prey, nutrient enrichment truncated energy flow to predators and reduced food web efficiency. This unexpected decline in food web efficiency indicates that nutrient enrichment, a ubiquitous threat to aquatic ecosystems, may have unforeseen and unpredictable effects on ecosystem structure and productivity.

Fig. 1.

Average annual biomass (mean ± SE) and secondary production of primary consumers (A and B) and predators (C and D) during the 7-year experiment. The arrow indicates the beginning of nutrient enrichment. Each year represents an average of 12 monthly samples with 4 samples per stream. Note difference in scales between primary consumers and predators. AFDM is ash-free dry mass.

Fig. 2.

Relationship between primary consumer and predator secondary production for the reference stream (gray circles), the treatment stream (black circles), and previously published data (open circles). The arrows represent the temporal trajectory of the treatment stream starting with the 2 years of pretreatment (P1 and P2) and ending with the fifth year of enrichment (E5). The data labels correspond to the sampling year for the reference and treatment streams. The previously published data include 5 years of production data from the reference stream (C53) and a similar Coweeta stream (C55) that had experimentally reduced terrestrial leaf inputs during 4 of those years (21). It also includes previously published data from an unmanipulated year that compared our current reference (C53) and treatment (C54) streams (22). AFDM is ash-free dry mass.

Fig. 3.

Size-specific secondary production of the 10 dominant primary consumers in the reference and treatment streams. For any given year, the displayed secondary production represented 70–90% of total primary consumer production. Each individual within these 10 taxa was classified as either small-bodied individuals (body length ≤ 10 mm; circles) or large-bodied individuals (body length > 10 mm; triangles), and their production was subsequently summed. Large-bodied individuals were relatively predator-resistant compared to small-bodied primary consumers. The arrow indicates the beginning of nutrient enrichment. AFDM is ash-free dry mass.