Detrital nutrient content determines growth rate and elemental composition of bromeliad-dwelling insects
- In many aquatic ecosystems, detritus is the main basal resource and is able to support highly productive food webs. However, detritus is a relatively poor-quality resource compared with living plants. Despite substantial research on how the nutrient content of living plants limits herbivores, there is little information on whether the nitrogen (N) and/or phosphorus (P) content of plant detritus similarly limits detritivore performance, especially under natural conditions.
- We examined this question by rearing larval chironomids and caddisflies within detrital-based tank-bromeliad ecosystems on different food qualities. We manipulated detrital nutrient content by increasing N and P simultaneously or separately.
- Chironomids and caddisflies showed faster growth rates when they fed on enriched detritus. Both N and P contents of the detritus limited the growth of chironomids, but combined effects were antagonistic (i.e. growth rates were less than expected according to single additions of N or P). By contrast, the growth of caddisflies was greater when fed on N-rich detritus, but was unaffected by P enrichment. These results suggest that under natural conditions, chironomids and caddisflies show reduced growth rates because they face strong nutrient limitation.
- We also found that increased growth rates were correlated with an increase in the body P content of chironomids and caddisflies, which is consistent with the growth rate hypothesis. This hypothesis predicts that high body P content permits the rapid ribosomal RNA production needed for high growth rates. This study therefore provides some of the first direct evidence for the growth rate hypothesis in detrital systems. Although detritus quality was an important factor determining the growth of detritivores, it had no effect on the survival of these insects.
- Our study highlights the importance of N and P additions as well as their interaction in constraining the growth of different consumers. Our findings point to the need for future studies integrating organism life history traits and ecological stoichiometry to better understand how food quality affects the functioning of detritus-based ecosystems.