RESULTS
We found evidence of a decline in the species richness of the
whole-community over time (Fig. 2a and Table S2). Likewise,
top-carnivore, mesocarnivore, and detritivore species richness decreased
over time (Fig. 2a and Table S2). We also found temporal declining
trends in the abundance of whole-community and all trophic guilds (Fig.
2b and Table S2). Similarly, temporal declining trends in the energy
flux were observed in whole-community and single trophic guilds (Fig. 2c
and Table S2). Particularly, the energy flux for whole-community,
top-carnivores, mesocarnivores, omnivores, and detritivores was reduced
by at last 71%, 72%, 67%, 70%, and 78% from the first to the last
year, respectively (Fig. 2c). The
proportion of energy flux between trophic compartments has also changed
systematically over time, becoming highly concentrated in the omnivore
and detritivore compartments, with the top- and meso-carnivore
compartments losing energy flux (Fig. 2d and Table S2).
At all three river sites, we found positive associations between
rarefied species richness and energy flux for the whole community and
for single trophic compartments. Increased whole-community species
richness was associated with greater energy flux (Fig. 3a and Table S3)
(linear mixed-effects model; log(whole-community energy flux): effect
size for log(whole-community richness) 2.60 ± 0.47 (mean ± s.e.m.)).
Increased top-carnivore species richness was related to greater
top-carnivore energy flux (Fig. 3b and Table S3); (the effect size for
log(top-carnivore richness) was 0.50 ± 0.09 (mean ± s.e.m.)). Increased
mesocarnivore species richness was related to increased mesocarnivore
energy flux (Fig. 3c and Table S3); (the effect size for
log(mesocarnivore richness) was 0.26 ± 0.05 (mean ± s.e.m.)). Increased
omnivore species richness was strongly related to increased omnivore
energy flux (Fig. 3d and Table S3); (the effect size for log(omnivore
richness) was 1.87 ± 0.50 (mean ± s.e.m.)). Lastly, increased
detritivore species richness was also strongly related to increased
detritivore energy flux (Fig. 3e and Table S3); (the effect size for
log(detritivore richness) was 1.13 ± 0.32 (mean ± s.e.m.)).
Structural equation modeling (SEM) revealed direct and species
richness-mediated indirect negative effects of human footprint on energy
flux (Fig. 4 and Table S4). The negative effects of human footprint on
species richness and energy flux were maintained after accounting for
key drivers of diversity and ecosystem functioning, such as
precipitation, N:P ratio, and river properties (i.e., water discharge
and turbidity). Specifically, human footprint indirectly decreased
top-carnivore energy flux by decreasing top-carnivore species richness
(Fig. 4a-d; –0.32). Similarly, the human footprint indirectly decreased
mesocarnivore energy flux by decreasing mesocarnivore species richness
(Fig. 4d-f; –0.27). Furthermore, the human footprint reduced the energy
flux of omnivores (β = –0.55) and detritivores (β = –0.14) only
directly (no species richness-mediated indirect effects; Fig. 4). There
was also a strong positive effect of time on human footprint, which
indirectly decreased both diversity and energy flux (Fig. 4). There were
direct and diversity-mediated indirect positive effects of precipitation
on the energy flux of top-carnivores and detritivores (Fig. 4b,k).