Search

Leaf breakdown is an important process in forested headwater streams. A common method used to quantify the role of macroinvertebrate and microbial communities in leaf litter breakdown involves using paired mesh bags that either allow or exclude macroinvertebrate access to leaves. We examined common assumptions of the paired litterbag method to test (1) whether mesh size alters microbial respiration and (2) whether the effects of abrasive flows (e.g., from water and sediment) differ between coarse‐ and fine‐mesh litterbags. We measured rates of microbial respiration on Acer rubrum and Rhododendron maximum leaves incubated in coarse‐ and fine‐mesh litterbags. We also measured rates of abrasion using aerated concrete blocks in pairs of coarse‐ and fine‐mesh bags in ten streams across a gradient of discharge. We found that rates of microbial respiration on Acer rubrum leaves conditioned in fine‐mesh bags were 65% greater than the rates of respiration in paired coarse‐mesh bags, but respiration rates on Rhododendron maximum were similar in coarse‐ and fine‐mesh bags. Abrasion was, on average, 56% greater in coarse‐mesh than paired fine‐mesh bags, and these effects were greater in streams with higher discharge. These results suggest that more caution is required when attributing the difference in leaf breakdown between coarse‐ and fine‐mesh bags to macroinvertebrates. Because the effect of mesh size on microbial respiration of Acer leaves and abrasion are opposite in direction, the effect that dominates and creates bias likely depends on both environmental context and experimental design. [ABSTRACT FROM AUTHOR]

Functional traits of organisms, especially feeding traits, influence how organisms mediate ecosystem processes. As climate change, landscape modification and industrial waste heat release continue to increase water temperatures, shifts in the composition of feeding traits within aquatic macroinvertebrate communities may alter ecosystem processes. However, it is unclear whether thermal traits of macroinvertebrates vary systematically across functional feeding groups (FFGs; i.e., categories based on feeding ecology such as herbivores, shredders, predators, etc.) or phylogeny.We used previously published datasets on hundreds of macroinvertebrate taxa to evaluate how thermal traits differed across FFGs. We also examined the strength of phylogenetic signal in both FFG and thermal traits, using a new phylogeny of insect taxa. Then, we tested whether phylogenetic patterns offered a plausible explanation for differences in thermal traits among FFGs by comparing phylogenetic and non‐phylogenetic regressions.Shredders tended to have lower temperature preferences, optima and maxima (three of five of the thermal traits evaluated) than other FFGs. Patterns for other FFGs differed by thermal trait, but predators, collector‐gatherers and filterers had some of the highest thermal trait values. FFG explained 40% of the variation in critical thermal maximum, but <12% of the variation in the four other thermal traits.Phylogeny explained 26%–88% of the variation in thermal and feeding traits. For the subset of taxa and trait data that were available, phylogeny explained more than double the variation in thermal traits relative to FFG, but comparison of phylogenetic and non‐phylogenetic regressions highlighted that FFG explained variation in thermal traits that was independent of phylogeny.Our results highlight phylogeny and FFG as predictors of thermal traits in aquatic macroinvertebrates. Our results suggest that warmer water temperatures could favour predators, filterers and collector‐gatherers over shredders. Furthermore, our results confirm that certain orders of macroinvertebrates, such as Diptera, may be better suited to warmer temperatures than other orders, such as Plecoptera. [ABSTRACT FROM AUTHOR]

The majority of terrestrial net primary production decomposes, fueling detrital food webs and converting dead plant carbon to atmospheric CO2. There is considerable interest in determining the sensitivity of this process to climate warming. A common approach has been to use spatial gradients in temperature (i.e., latitude or elevation) to estimate temperature sensitivity. However, these studies typically relate decomposition rates to average temperatures at each site along such gradients, ignoring within-site temperature variation. To evaluate the potential effects of temperature variation on estimates of temperature sensitivity, we simulated plant litter decomposition using both randomly generated and real time series of temperature. This simulation approach illustrated how temperature variation leads to higher decomposition rates at a given mean temperature than is predicted from simulations in which temperature is held constant. Increases in decomposition rate were most evident at cooler sites, where temporal variation in temperature tends to be greater than at warmer sites. This unbalanced effect of temperature variation shifted the slope of the relationships between average temperature and decomposition rate, resulting in lower estimated temperature sensitivities than were used to simulate decomposition. For example, estimates of activation energy (Ea) were as much as 0.15 eV lower than the true Ea when decomposition was simulated with the true Ea set to the canonical respiration value of 0.65 eV. We found that the estimated Ea was lower than the true Ea for surface, soil, and air temperatures, but not for stream temperatures, for which there was only a weak relationship between temperature variation and mean temperature. Our results suggest that commonly used methods may underestimate the temperature dependence of litter decomposition, particularly in terrestrial environments. We encourage publication of temperature data that include variation estimates and suggest an alternative method for calculating temperature sensitivity that accounts for variation in temperature. [ABSTRACT FROM AUTHOR]

Ecological stoichiometry uses information on a few key biological elements (C, N and P) to explain complex ecological patterns. Although factors driving variation in these elements are well established, expanding stoichiometric principles to explore dynamics of the many other essential elements comprising biological tissues (i.e. the ionome) is needed to determine their metabolic relationships and better understand biological control of elemental flows through ecosystems.In this paper, we report observations of ionomic variation in two species of salamander (Ambystoma opacum and A. talpoideum) across ontogenic stages using specimens from biological collections of two wetlands sampled over a 30‐year period. This unique data set allowed us to explore the extent of ionomic variation between species, among ontogenic stages, between sites and through time.We found species‐ and, to a lesser extent, site‐specific differences in C, N and P along with 13 other elements forming salamander ionomes but saw no evidence of temporal changes. Salamander ionomic composition was most strongly related to ontogeny with relatively higher concentrations of many elements in adult males (i.e. Ca, P, S, Mg, Zn and Cu) compared to metamorphic juveniles, which had greater amounts of C, Fe and Mn.In addition to patterns of individual elements, covariance among elements was used to construct multi‐elemental nutrient balances, which revealed differences in salamander elemental composition between species and sites and changes in elemental proportions across ontogenic development. These multi‐elemental balances distinguished among species‐site‐ontogenic stage groups better than using only C, N and P.Overall, this study highlights the responsiveness of consumer ionomes to life‐history and environmental variation while reflecting underlying relationships among elements tied to biological function. As such, ionomic studies can provide important insights into factors shaping consumer elemental composition and for predicting how these changes might affect higher‐order ecological processes. [ABSTRACT FROM AUTHOR]