Nutrient-enriched food webs have more competition and less diversity 

Background/Question/Methods

Land use affects both stream chemistry and stream biota. Changes in land use, particularly intensification of agriculture, often result in increased nutrient inputs. The availability of nutrients strongly influences the productivity of stream ecosystems. However, the consequences of nutrient enrichment on feeding relations and community structure are not well understood. We hypothesized that food web structure would shift to maximize nutrient consumption, as certain feeding groups are able to out-compete others when resources are abundant. To investigate the mechanisms which cause change in stream food web structure, we compared food webs of 24 lowland streams across a land-use gradient from natural forest to intensive agriculture. Nutrient (NO₃, NH₄, PO₄) levels, detrital standing stock (coarse and fine particulate organic matter), primary producer standing stock (chlorophyll-a), and metabolism were measured at each site to quantify basal resources and gross primary production. Macroinvertebrate and fish diversity, abundance, and biomass were measured at each site to characterize upper trophic levels. Food webs were constructed using stable isotope analysis, and verified with gut contents analysis.

Results/Conclusions

There were regional and land-use effects on water chemistry and food web structure. Increased nutrients, particularly NH₄, and gross primary production were found to affect community composition through shifts in relative abundances of functional feeding groups. Total invertebrate abundance increased with production, but diversity declined. Both predatory invertebrates and invertebrate collector-browsers declined at high levels of nutrients and primary production, whereas grazer abundance increased. These results are consistent with high nutrient levels enabling protected grazers, particularly snails, to dominate and possibly competively exclude others. Consequently, energy became concentrated in less edible/available prey, and predator populations were low despite increased abundance of prey. Overall, these findings suggest that alterations in food web structure caused by nutrient enrichment can result in trophic disconnections between predators and lower trophic levels due to competitive interactions between functional feeding groups. The land use - food web structure linkages observed here provide insight into possible mechanisms for community change, and may provide a benchmark for stream health assessments and guide future rehabilitation and restoration efforts. We suggest that to restore ecosystem function, future management needs to focus on increasing the connectedness and diversity of food webs to ensure predator populations are adequately supported. In New Zealand nutrient-enriched lowland streams this will require reducing the dominance and competitive advantage of grazers.