Results
The standing biomasses at all trophic levels were strongly affected by
experimental treatments (Fig. 2). The biomass of the predator E.
modestus was significantly lower in the LP (i.e., low phosphorous
concentration) treatment compared with those in HP (i.e., high
phosphorous) and MP (i.e., medium phosphorous) treatments, and lower in
mesocosms consisting of S. dorrii and D. brachyurum than
those consisting of D. magna and D. brachyurum (Fig. 2a,
Table S2). Zooplankton biomass was influenced by the presence of
predators (F1,36 = 114.36; P < 0.001),
nutrient supply (F2,36 = 235.87; P <
0.001), zooplankton community composition (F2,36 = 40.3;P < 0.001), as well as their interactions (Fig. 2b-c
and Table S1). Specifically, zooplankton biomass was significantly
suppressed by the presence of predators, while strongly promoted by
nutrient enrichment (Fig. 2b-c). In the presence of predators,
zooplankton communities consisting of all three species had the highest
biomass across all nutrient treatments (Fig. 2b). In the absence of
predators, zooplankton communities consisting of D. magna andD. brachyurum had higher biomass than those consisting of all
three species in HP and MP treatments (Fig. 2d), due to higher biomass
of D. magna in the former case (Fig. S2e-f). In the LP treatment,
however, the inverse pattern was observed (Figs. 2c, S3a-b).Throughout the experiment, phytoplankton communities were dominated by
green algae (Fig. S4). Nutrient enrichment and the presence of predators
strongly boost phytoplankton biomass (P < 0 .001 for
both factors; Fig. 2d-e and Table S1). Our analyses also showed
significant interactive effects between predators and zooplankton
community composition on phytoplankton biomass (F2,36 =
3.9, P = 0.03) and nutrient
supply (F2,36 = 10.8, P < 0.001). At
all trophic levels, the responses of biomass production rates to
experimental treatments were similar to those of standing biomass (Fig.
S2, Table S1).
Nutrient supply not only altered the standing biomass and production
rates at different trophic levels, but also their stoichiometric
characteristics. Phytoplankton molar C:P ratios decreased with increased
ambient P concentrations (F2,36 = 253.7; P< 0.001; Fig. S4c; Table S1) and were higher in the presence
of predators (F1,36 = 5.4, P = 0.026). By
altering the stoichiometry of phytoplankton, nutrient supply also had
carry-over effects on the stoichiometry of higher trophic levels.
Specifically, an increase in ambient P concentration decreased the molar
C:P ratios of zooplankton D. magna and S. dorrii (Fig.
S4a-b), leading further to a lower body molar C:P ratios of predatorE. modestus (Fig. S4d).
The energy transfer efficiency between producers and herbivores
(hereafter herbivore efficiency) was strongly modulated by the presence
of predators (F1,36 = 16.4, P < 0.001)
and zooplankton community composition (F2,36 = 8.4,P = 0.001, Table S1). The presence of predators significantly
decreased herbivore efficiency, especially in mesocosms with zooplankton
communities of D. magna and D. brachyurum (Fig. 3a).
Similarly, the energy transfer efficiency between herbivores and
predators (hereafter predator efficiency) was higher in mesocosms with
zooplankton community D. magna and D. brachyurum , compared with
those containing other zooplankton communities (Fig. 3c). While nutrient
supply had no significant effects on herbivore efficiency and only
marginally significant effects on predator efficiency (Fig. 3b-c, Table
S1-2), it significantly affected the overall energy transfer efficiency
from producers to predators (hereafter food chain efficiency). That is,
the food chain efficiency was significantly higher in MP treatments
compared with those in HP and LP treatments (F2,18 =
7.68; P = 0.001; Table S2; Fig. 3d).
The top-down effects of predators on zooplankton biomass and their
cascading effects on phytoplankton biomass both differed significantly
among mesocosms with different zooplankton community compositions
(F2,18 = 49.51, P < 0.001 for zooplankton
biomass; F2,18 = 75, P < 0.001 for
phytoplankton biomass), but not with nutrient conditions
(F2,18 = 0.23, P = 0.796 for zooplankton biomass;
F2,18 = 0.084, P = 0.920 for phytoplankton
biomass) (Figs. 4, S6, Table S2). Mesocosms with zooplankton communities
of D. magna and D. brachyurum showed the strongest
cascading effects (on average 107.8% increase in phytoplankton biomass
in the presence of predators across three nutrient levels), followed by
those consisting of all three zooplankton species (57.3%) and those
consisting of S. dorrii and D. brachyurum (41.5%) (Fig.
4). To further disentangle the drivers of trophic cascades, we conducted
stepwise regression analyses and found that the strength of trophic
cascades was best explained by variation in predator efficiency (Fig.
5b; Table S3-4). In food chains with higher predator efficiencies, the
strengths of trophic cascade were stronger (Fig. 5b). In comparison,
neither primary productivity nor its interaction with energy transfer
efficiencies had significant impacts on the cascading effects (Fig. 5a,
Table S1).
Using food chain models with the type I functional response, we derived
analytical relationships between trophic cascade strength (STC) and
energy transfer efficiencies, i.e.,
\begin{equation}
STC=\frac{e_{C}}{e_{C}-E_{\text{pre}}}\nonumber \\
\end{equation}and between trophic cascade strength and primary productivity\(P_{\text{pro}}\):
\begin{equation}
STC=\frac{e_{1}a_{1}}{rd_{1}}P_{\text{pro}}\nonumber \\
\end{equation}These solutions suggest that the relationship between STC and\(E_{\text{pre}}\) involves only one covariate, while that betweenSTC and \(P_{\text{pro}}\) involves multiple covariates and
thus may be more variable. This expectation was confirmed by numerical
simulation in general settings. Across simulated food chains with
varying parameters, the trophic cascade strength is strongly related
with predation efficiency, but relatively weakly related with primary
production and herbivore efficiency (Figs. 5c-d, S7-12). Similar results
were found in more complex food web models with two herbivore species
(Figs. S13, S14). Overall, our mathematical models support the
mechanistic link between energy transfer efficiency and the strength of
trophic cascades observed in our mesocosm experiment.