Species-specific traits predict whole-assemblage detritus processing by pond invertebrates.

Functional trait diversity determines if ecosystem processes are sensitive to shifts in species abundances or composition. For example, trait variation suggests detritivores process detritus at different rates and make different contributions to whole-assemblage processing, which could be sensitive to compositional shifts. Here, we used a series of microcosm experiments to quantify species-specific coarse and fine particulate organic matter (CPOM and FPOM) processing for ten larval caddisfly species and three non-caddisfly species in high-elevation wetlands. We then compared trait-based models including life history, dietary, and extrinsic traits to determine which traits explained interspecific variation in detritus processing. Finally, we compared processing by mixed caddisfly assemblages in microcosms and natural ponds to additive predictions based on species-specific processing to determine if single-species effects are additive in multi-species assemblages. We found considerable interspecific variation in biomass-specific CPOM (13-fold differences) and FPOM (8-fold differences) processing. Furthermore, on a mass-specific basis, amphipods, chironomids, and caddisflies processed similar amounts of detritus, suggesting non-shredder taxa could process more than previously recognized. Trait models including dietary percent detritus, development rate, body size, and wetland hydroperiod explained 81 and 57% of interspecific variation in CPOM and FPOM processing, respectively. Finally, species-specific additive predictions were strikingly similar to mixed-assemblage processing in microcosms and natural ponds, with the largest difference being a 15% overestimate. Thus, additivity of species-specific processing suggests single-species rates may be useful for understanding functional consequences of shifting assemblages, and a trait-based approach to predicting species-specific processing could support generating additive predictions of whole-assemblage processing.

Decomposing decomposition: isolating direct effects of temperature from other drivers of detrital processing.

Understanding the observed temperature dependence of decomposition (i.e., its "apparent" activation energy) requires separation of direct effects of temperature on consumer metabolism (i.e., the "inherent" activation energy) from those driven by indirect seasonal patterns in phenology and biomass, and by longer-term, climate-driven shifts in acclimation, adaptation, and community assembly. Such parsing is important because studies that relate temperature to decomposition usually involve multi-season data and/or spatial proxies for long-term shifts, and so incorporate these indirect factors. The various effects of such factors can obscure the inherent temperature dependence of detrital processing. Separating the inherent temperature dependence of decomposition from other drivers is important for accurate prediction of the contribution of detritus-sourced greenhouse gases to climate warming and requires novel approaches to data collection and analysis. Here, we present breakdown rates of red maple litter incubated in coarse- and fine-mesh litterbags (the latter excluding macroinvertebrates) for serial approximately one-month increments over one year in nine streams along a natural temperature gradient (mean annual: 12.8°-16.4°C) from north Georgia to central Alabama, USA. We analyzed these data using distance-based redundancy analysis and generalized additive mixed models to parse the dependence of decomposition rates on temperature, seasonality, and shredding macroinvertebrate biomass. Microbial decomposition in fine-mesh bags was significantly influenced by both temperature and seasonality. Accounting for seasonality corrected the temperature dependence of decomposition rate from 0.25 to 0.08 eV. Shredder assemblage structure in coarse-mesh bags was related to temperature across both sites and seasons, shifting from "cold" stonefly-dominated communities to "warm" communities dominated by snails or crayfish. Shredder biomass was not a significant predictor of either coarse-mesh or macroinvertebrate-mediated (i.e., coarse- minus fine-mesh) breakdown rates, which were also jointly influenced by temperature and seasonality. Unlike fine-mesh bags, however, temperature dependence of litter breakdown did not differ between models with and without seasonality for either coarse-mesh (0.36 eV) or macroinvertebrate-mediated (0.13 eV) rates. We conclude that indirect (non-thermal) seasonal and site-level effects play a variable and potentially strong role in shaping the apparent temperature dependence of detrital breakdown. Such effects should be incorporated into studies designed to estimate inherent temperature dependence of slow ecological processes.