Fig. 1 Conceptual illustration of two hypotheses underlying
trophic cascade strengths: the productivity hypothesis (a) and
the energy transfer hypothesis (b) . R (horizontal green bars),
H (horizontal brown bars), and C (horizontal red bars) represent the
biomass of plants, herbivores, and predators, respectively. The upper
arrows between adjacent trophic levels represent energy transfer, where
Eher (green arrows) andEpre (brown bars) represent herbivore
and predator efficiency, respectively, with the thickness of arrows
representing the relative magnitude. The bottom dark-green upper arrows
(P ) refer to the primary productivity. The dashed arrows indicate
the indirect effects of the apex predator on plant biomass (i.e.,
trophic cascades) with their thickness representing the relative
strength. The productivity hypothesis posits that a higher productivity
can increase the biomass of apex predators and thereby enhance trophic
cascades (Fig. 1a, Hypothesis I). The energy transfer hypothesis posits
that a higher efficiency of energy transfer promotes the biomass
accumulation of apex predators and subsequently strengthens trophic
cascades (Fig. 1b, Hypothesis II). The dashed line with a smaller slope
in Fig 1a (resp. Fig. 1b) indicates weakened production effects (resp.
weakened effects of energy transfer) under a lower energy transfer
efficiency (resp. lower production).
Fig. 2 Responses of standing biomass at different trophic
levels to experimental manipulations: (a) predator E.modestus , (b, d) zooplankton herbivores, (c, e) phytoplankton.
Different capital characters indicate significant differences (P< 0.05) among treatments varying in zooplankton community
composition, and different lowercases indicate significant differences
among treatments varying in nutrient supply. The bars and error bars
show the averages and standard errors across three replicates,
respectively.
Fig. 3 Responses of energy transfer efficiencies across trophic
levels to experimental manipulations: (a-b) Herbivore efficiency, (b)
predator efficiency, (d) FCE. The
red arrows in the illustration schemes represent the focal energy
transfer processes. Different capital characters indicate significant
differences (P < 0.05) among treatments varying in
zooplankton community composition, and different lowercases indicate
significant differences among treatments varying in nutrient supply. The
bars and error bars show the averages and standard errors across three
replicates, respectively.
Fig. 4Responses
of trophic cascade strength to experimental manipulations. Different
capital characters indicate significant differences (P< 0.05) among treatments varying in zooplankton community
composition, and different lowercases indicate significant differences
among treatments varying in nutrient supply. The bars and error bars
show the averages and standard errors across three replicates,
respectively. The horizontal grey lines correspond to no cascading
effect, i.e., no change in phytoplankton biomass following the removal
of the top predator.
Fig. 5 Relationships between trophic cascade strength with
primary production (a, c ) and predator efficiency (b,
d ), derived from experimental data (a, b ) and theoretical
models (c, d ). In a and b , each symbol
represents one mesocosm, where circles (SD) represent zooplankton
communities consisting of S. dorrii and D. brachyurum,triangles (DD) represent zooplankton communities consisting of D.
magna and D. brachyurum ; and squares (SDD) represent zooplankton
communities containing all the three species. The dashed black line in
(a ) represents a non-significant relationship (P> 0.05), whereas the solid black line in (b )
indicates a significant relationship (P < 0.05). Shaded
areas are the error bands and denote 95% confidence intervals. Inc and d , each point represents one simulated food webs
with randomized assimilation efficiencies, i.e.,\(e_{H}\sim U\left[0.3,0.6\right]\) and\(e_{C}\sim U\left[0.5,0.9\right]\), where trophic
interactions follow the type II functional response. Other parameters
are specified in Methods. The horizontal grey lines correspond to no
cascading effect.