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Abstract Human activities have dramatically altered global patterns of nitrogen (N) and phosphorus (P) availability. This pervasive nutrient pollution is changing basal resource quality in food webs, thereby affecting rates of biological productivity and the pathways of energy and material flow to higher trophic levels.Here, we investigate how the stoichiometric quality of basal resources modulates patterns of material flow through food webs by characterizing the effects of experimental N and P enrichment on the trophic basis of macroinvertebrate production and flows of dominant food resources to consumers in five detritus‐based stream food webs.After a pre‐treatment year, each stream received N and P at different concentrations for 2 years, resulting in a unique dissolved N:P ratio (target range from 128:1 to 2:1) for each stream. We combined estimates of secondary production and gut contents analysis to calculate rates of material flow from basal resources to macroinvertebrate consumers in all five streams, during all 3 years of study.Nutrient enrichment resulted in a 1.5× increase in basal resource flows to primary consumers, with the greatest increases from biofilms and wood. Flows of most basal resources were negatively related to resource C:P, indicating widespread P limitation in these detritus‐based food webs. Nutrient enrichment resulted in a greater proportion of leaf litter, the dominant resource flow‐pathway, being consumed by macroinvertebrates, with that proportion increasing with decreasing leaf litter C:P. However, the increase in efficiency with which basal resources were channelled into metazoan food webs was not propagated to macroinvertebrate predators, as flows of prey did not systematically increase following enrichment and were unrelated to basal resource flows.This study suggests that ongoing global increases in N and P supply will increase organic matter flows to metazoan food webs in detritus‐based ecosystems by reducing stoichiometric constraints at basal trophic levels. However, the extent to which those flows are propagated to the highest trophic levels likely depends on responses of individual prey taxa and their relative susceptibility to predation.
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Persistence of tri‐trophic interactions in seasonal environments
Abstract The interplay between species interactions and environmental variation is well‐understood for pairwise interactions but not for multi‐trophic interactions. Understanding how such interactions persist in a thermally variable environment is particularly important given that most biodiversity on the planet consists of ectotherms whose body temperature depends on the environmental temperature.Here we present a trait‐based mathematical framework for investigating how tri‐trophic food chains persist in seasonal environments. We report two key findings.First, the persistence of the tri‐trophic interaction is enhanced if species at upper trophic levels (e.g. top predators) are more cold‐adapted than those at lower levels (e.g. basal resources) by virtue of lower thermal optima, wider response breadths and lower mortality within the favourable temperature range. The important implication is that the assembly and persistence of multi‐trophic interactions requires that species at lower trophic levels be somewhat maladapted to their ambient thermal environment, as in the case of recent invasions.Second, differential sensitivity to thermally varying environments provides a mechanistic explanation for the conflict of interest between the intermediate consumer and top predator. The same cold‐adaptations that increase the consumer's ability to increase when rare deter the predator's ability to do so. Thus, being well‐adapted to its thermal environment makes the intermediate consumer better able to acquire resources and avoid predators.We predict that the hierarchy in cold‐adaptation should constrain the number of trophic levels that can be supported in a given thermal environment, and that ectotherm food chain lengths should increase with increasing latitude because larger‐amplitude seasonal fluctuations generate more opportunities for species to diverge in their thermal optima.
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- Award ID(s):
- 1949796
- PAR ID:
- 10453508
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Journal of Animal Ecology
- Volume:
- 90
- Issue:
- 1
- ISSN:
- 0021-8790
- Format(s):
- Medium: X Size: p. 298-310
- Size(s):
- p. 298-310
- Sponsoring Org:
- National Science Foundation
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