Effects of snail herbivory
Shading and snail herbivory interactively affected the growth ofP. crispus . Under full light conditions, low biomass of snails promoted the growth of P. crispus , while during high biomass of snails the macrophyte growth was inhibited. This may be a consequence of preferential feeding of snails on periphyton over macrophytes (Bronmark, 1989; Guo et al., 2021; Koleszár et al., 2021; Zhi et al., 2020). During low biomass of snails there is plenty of periphyton available to graze, and herbivory in this case releases macrophytes from competitive pressure for light with periphyton, promoting the growth of macrophytes (Yang et al., 2020). In contrast, during high biomass of snails, periphyton food is depleted and snails will graze on macrophytes, thus inhibiting their growth (Zhi et al., 2020). However, during shaded conditions, the preferential food availability to snails is limited at a high grazing pressure, forcing snails to graze on macrophytes.