A major focus of ecology is to understand and predict ecosystem function across scales. Many ecosystem functions are only measured at local scales, while their effects occur at a landscape level. Here, we investigate how landscape-scale predictions of ecosystem function depend on intraspecific competition, a fine-scale process, by manipulating intraspecific density of shredding macroinvertebrates and examining effects on leaf litter decomposition, a key function in freshwater ecosystems. For two species, we found that per-capita leaf processing rates declined with increasing density following power functions with negative exponents, likely due to interference competition. To demonstrate consequences of this nonlinearity, we scaled up estimates of leaf litter processing from shredder abundance surveys in 10 replicated headwater streams. In accordance with Jensen’s inequality, applying density-dependent consumption rates reduced estimates of catchment-scale leaf consumption by an order of magnitude relative to density-independent rates. Density-dependent consumption estimates aligned closely with metabolic requirements in catchments with large, but not small, shredder populations. Importantly, shredder abundance was not limited by leaf litter availability and catchment-level leaf litter supply was much higher than estimated consumption. Thus leaf litter processing was not limited by resource supply. Our work highlights the need for scaling-up which accounts for intraspecific interactions.
Experimental and field survey data for amphipods and leaf litter
CODE:
The analysis for this manuscript can be performed using the R code contained in the R markdown file DEF_code_Little_et_al.Rmd.
GIS FILES:
There are 11 shapefiles in this folder which are used in the R code.
EXPERIMENTAL AND FIELD DATA:
There are eight CSV files of data which are described below. Variable names and metadata are described in the README.
GF_densitytreatments_weights.csv:
This file contains weights of Gammarus fossarum amphipods used in the experiments, in milligrams. Each row represents an individual amphipod.
DV_densitytreatments_weights.csv:
This file contains weights of Dikerogammarus villosus amphipods used in the experiments, in milligrams. Each row represents an individual amphipod.
GF_weight_and_survival_data_corrected.csv:
This file contains information on the survival of G. fossarum amphipods in the density experiment, as well as the mass loss of the leaves used in the experiment. Each row represents one experimental unit (mesocosm) used in the experiment.
DV_weight_and_survival_data.csv:
This file contains information on the survival of D. villosus amphipods in the density experiment, as well as the mass loss of the leaves used in the experiment. Each row represents one experimental unit (mesocosm) used in the experiment.
Combined_Data_amphipods.csv:
This file contains the amphipod abundance information from the April 2015 field surveys (also described in Little & Altermatt 2018 Proceedings B). Each row represents one sampling point.
Combined_Data_leaves.csv:
This file contains the benthic leaf litter and amphipod abundance information from four sampling bouts: April 2015, July 2015, November 2015, and January 2016 (also described in Little & Altermatt 2018 Proceedings B and Little & Altermatt 2018 Landscape Ecology). Each row represents one sampling point at one sampling bout.
leaf_litter_trap_cumulative.csv:
This file contains the data from leaf litter traps deployed between September and December, 2015, near Lake Constance (also presented in Little & Altermatt 2018 Landscape Ecology). Traps were first deployed on September 8, 2015, for all sites. Each row represents a sampling interval at a specific site; the traps were emptied and redeployed at each site several times over the course of the autumn, so each site has several rows of data. Rows contain information about the total amount of plant material accrued in the traps between the start of sampling and the end of a given sampling interval, that is, a cumulative total during the leaf drop period.
Land_use_in_buffers.csv:
This file contains data about the area (square meters) of three types of land cover in a 1-meter buffer on either side of the stream channels in each catchment. Land cover in the buffer zones was calculated from Swisstopo GIS layers in ArcMap as described in the methods section of the manuscript.
DRYAD_Little_et_al_DEF.zip
