Impacts of detritivore diversity loss on instream decomposition are greatest in the tropics

Abstract

The relationship between detritivore diversity and decomposition can provide information on how biogeochemical cycles are affected by ongoing rates of extinction, but such evidence has come mostly from local studies and microcosm experiments. We conducted a globally distributed experiment (38 streams across 23 countries in 6 continents) using standardised methods to test the hypothesis that detritivore diversity enhances litter decomposition in streams, to establish the role of other characteristics of detritivore assemblages (abundance, biomass and body size), and to determine how patterns vary across realms, biomes and climates. We observed a positive relationship between diversity and decomposition, strongest in tropical areas, and a key role of abundance and biomass at higher latitudes. Our results suggest that litter decomposition might be altered by detritivore extinctions, particularly in tropical areas, where detritivore diversity is already relatively low and some environmental stressors particularly prevalent.

Fig. 1. Generalised additive models exploring the influence of detritivore diversity, abundance and biomass on decomposition in different latitudinal zones (tropical: ≤23°; temperate: 24–60°; and boreal: >60°).

Variation in total and detritivore-mediated decomposition (measured as the proportion of litter mass loss per degree day, dd; mean ± SE) with a detritivore diversity (number of families per litterbag), b log-transformed abundance (number of individuals per litterbag) and c log-transformed biomass (mg per litterbag), in different latitudinal zones. Lines represent the smoothers and shading the 95% confidence intervals from generalised additive models for significant relationships (p-value < 0.05); whole-model results are given in Supplementary Table 3.

Fig. 2. Global distribution of study sites in different biogeographic realms (Pa, Palearctic; Na, Nearctic; Au, Australasian; Nt, Neotropical; At, Afrotropical; Im, Indomalayan); n = 38.

Box plots show the median, interquartile range and minimum-maximum range of litter-consuming detritivore diversity (number of families per litterbag), abundance (number of individuals per litterbag), biomass (mg per litterbag) and mean body size (mm) in each realm (ordered from highest to lowest diversity); different letters indicate significant differences. The NMDS ordination of litter-consuming detritivores with realms is represented by polygons of different colours as in maps and box plots. Significant differences in assemblage structure were: Pa vs. Na, At, Au, Im; Na vs. Nt, Au; Nt vs. Au.

Fig. 3. Global distribution of study sites in different biomes (Tu, tundra; TeBF, temperate broadleaf forest; TeCF, temperate coniferous forest; MeF, Mediterranean forest; XeS, xeric shrubland; TrWF, tropical wet forest; TrS, tropical savanna); n = 38.

Box plots show the median, interquartile range and minimum-maximum range of litter-consuming detritivore diversity (number of families per litterbag), abundance (number of individuals per litterbag), biomass (mg per litterbag) and mean body size (mm) in each biome (ordered from highest to lowest diversity); different letters indicate significant differences. The NMDS ordination of litter-consuming detritivores with biomes is represented by polygons of different colours as in maps and box plots. Significant differences in assemblage structure were: TrWF vs. TeBF, TeCF, MeF.

Fig. 4. Global distribution of study sites in different climates [A, equatorial (Af, fully humid; Am, monsoon; As, with dry summer; Aw, with dry winter); C, warm temperate (Cfa, fully humid with hot summer; Cfb, fully humid with warm summer; Csa, with dry and hot summer; Csb, with dry and warm summer); D, snow (Dfb, fully humid with warm summer; Dfc, fully humid with cold summer)]; n = 38.

Box plots show the median, interquartile range and minimum-maximum range of litter-consuming detritivore diversity (number of families per litterbag), abundance (number of individuals per litterbag), biomass (mg per litterbag) and mean body size (mm) in each climate (ordered from highest to lowest diversity); different letters indicate significant differences. The NMDS ordination of litter-consuming detritivores with biomes is represented by polygons of different colours as in maps and box plots. Significant differences in assemblage structure were: Aw vs. Cfb, Cfa, Dfb; Af vs. Cfa, Cfb, Dfb.

Fig. 5. Distribution of detritivore families in our study, which was predominantly Laurasian (blue) or Gondwanan (green); insert indicates origins of those two regions (≈200 Ma).

Photographs represent a subset of families (ordered left to right from the most to the least abundant in our study) and asterisks denote families that were globally distributed but more abundant in one of the two areas. A complete list of families is provided in Supplementary Table 1. Photograph credits: L. Boyero, A. Cornejo, R. Figueroa, N. López-Rojo, F. Masese, J. Pérez, J. Rubio-Ríos, J. Vieira and C. M. Yule.