Detrital nutrient content determines growth rate and elemental composition of bromeliad‐dwelling insects

Abstract

Summary 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.

Caught in the web: Spider web architecture affects prey specialization and spider–prey stoichiometric relationships

“…Whereas the N requirements of the spiders in our study were based on body elemental content, it is important to recall that silk production requires the allocation of large amounts of N (Craig, 2003), and therefore, web chemical content and web investment (i.e., silk amount) are a very likely factor influencing the N requirements of web‐building spiders (Savory, 1960; Vollrath, 2005) and other silk‐producing organisms, such as caddisflies (González, Romero, & Srivastava, 2014). Web‐building spiders show significant differences in the N content of silk, with tangle having 13.2%N, orbs 15.8%N, and sheet‐tangle 15.5%N in their webs (ALG, unpublished data).…”

Assessing the role of canopy cover on the colonization of phytotelmata by aquatic invertebrates: an experiment with the tank-bromeliad Aechmea lingulata

“…However, the volume of water and the amount of FPOM, which are important variables for the colonization process (Kitching, 2000;Armbruster et al, 2002;Dézerald et al, 2015b), did not differ between treatments and contributed to the high similarity of the communities sampled throughout the experiment. These results differ from those found in studies developed in tropical forest ecosystems (Montero et al, 2010;González et al, 2014) and are probably related to the characteristics of restinga (i.e., less dense vegetation composed of species adapted to low water availability and high temperatures and solar radiation) (Zaluar and Scarano, 2000;Scarano, 2002). Some studies have suggested the volume of water is the main factor that influences the colonization of phytotelm ecosystems because it determines habitat size and susceptibility to colonization (Srivastava et al, 2008;Dézerald et al, 2014;Gossner et al, 2016).…”

“…Although CPOM represents an important energy source in many aquatic ecosystems (Vannote et al, 1980), typical invertebrate shredders were not found in the communities sampled. Organisms assigned to this functional feeding group have been observed in bromeliads of forest ecosystems (see Montero et al, 2010;Marino et al, 2012;González et al, 2014). Shredders are important to aquatic food webs because they provide food to other consumers (Graça, 2001).…”

“…Shredders are important to aquatic food webs because they provide food to other consumers (Graça, 2001). González et al (2014) observed that the growth of the insect shredders Polypedilum sp. Kieffer, 1913 (Diptera, Chironomidae) and Phylloicus bromeliarum Müller, 1880 (Trichoptera, Calamoceratidae) was higher when exposed to leaf detritus enriched with nutrients (N and P) and suggested that leaf quality is also important to the development of shredders in bromeliad tanks.…”

Bottom-up effects of streambed drying on consumer performance through changes in resource quality

“…Although growth on a DRY leaf diet was reduced, it was not significantly lower than on the PERM diet. This result was unexpected given the lower ingestion rates and N concentration in leaves, which has been related to dampened growth rates in caddisflies (Iversen 1974;González et al 2014;Frainer et al 2016). Because leaves rapidly lose large quantities of fatty acids (including polyunsaturated and essential fatty acids) during decomposition in streams (Torres-Ruiz and Wehr 2010), more labile C, such as essential fatty acids and carbohydrates, might have , and dry flow (DRY, only for leaves)] on consumers' (Stenophylax and Physella, respectively) relative consumption rate (RCR; a n = 9-10 and c n = 6-9) and relative growth rate (RGR; b n = 10 and d n = 8-10).…”