The impact of bird herbivory on macrophytes and the resilience of the clear-water state in shallow lakes: a model study

Abstract

Shallow lakes have the potential to switch between two alternative stable states: a clear macrophyte-dominated and a turbid phytoplankton-dominated state. Observational and experimental studies show that in some lakes herbivory by birds may severely decrease macrophyte biomass, while in other lakes, the removed biomass by herbivory is compensated by regrowth. These contradictory outcomes might arise because of interplay between top-down control by bird herbivory and bottom-up effects by nutrient loading on macrophytes. Here, we use the ecosystem model PCLake to study top-down and bottom-up control of macrophytes by coots and nutrient loading. Our model predicted that (1) herbivory by birds lowers the critical nutrient loading at which the regime shift occurs; (2) bird impact on macrophyte biomass through herbivory increases with nutrient loading; and (3) improved food quality enhances the impact of birds on macrophytes, thus decreasing the resilience of the clear-water state even further. The fact that bird herbivory can have a large impact on macrophyte biomass and can facilitate a regime shift implies that the presence of waterfowl should be taken into account in the estimation of critical nutrient loadings to be used in water quality management.

Aquatic omnivores shift their trophic position towards increased plant consumption as plant stoichiometry becomes more similar to their body stoichiometry

“…The increased consumption might be more than the re-growth of the plant, leading to enhanced top-down control on plant standing biomass. This has been demonstrated by fertilization experiments with mallard duck [ 12 ], and is also supported by modelling studies [ 13 , 41 ]. Higher consumption rates in no-choice feeding trials have also been interpreted as compensatory feeding, where the consumer needs to feed more on a poor quality resource to meet its nutrient or energy demands [ 42 , 43 ].…”

“…With this study, we demonstrate that omnivores increase their impact on aquatic plants under eutrophication by shifting their trophic position towards enhanced plant consumption. The combined stress of shading by algae and grazing pressure by omnivores and herbivores under eutrophication can lead to disappearance of submerged aquatic vegetation and a shift to a turbid state dominated by phytoplankton [ 13 , 41 ].…”

“…More specifically, it has been hypothesized that the impact of herbivores on aquatic plants increases as plant quality increases [ 12 , 13 ]. The underlying mechanism is that, when plant nutrient concentration increases with nutrient loading to a water body, these plants would be more attractive to herbivores, experience more grazing, resulting in enhanced top-down control under eutrophic conditions [ 13 ]. However, most aquatic plant-consuming animals are omnivores [ 10 , 14 ], which means that they feed on both plant and animal material.…”

Turbidity, Waterfowl Herbivory, and Propagule Banks Shape Submerged Aquatic Vegetation in Ponds

“…The potential impact of waterfowl has inspired research focusing on SAV dynamics ( Irfanullah and Moss, 2004 ; Hilt, 2006 ; Gayet et al, 2011 ; Gyimesi et al, 2011 ; van Altena et al, 2016 ). Nevertheless, birds are easily overlooked during routine assessment of macrophyte dynamics because of their mobility and migratory habits, especially outside major wetland areas or in regions not experiencing striking seasonal peaks in waterfowl numbers.…”

Habitat selection and reproductive success of coot Fulica atra on ponds under different fish size and density conditions

“…Nilsson, 1978;Paracuellos, 2006;Haas et al, 2007), its trophic impact via grazing pressure on submerged macrophytes may have ecosystem-scale consequences, i.e. it may affect macrophyte abundance and community structure and contribute to shifts between a macrophyte-dominated clear water state to a phytoplankton-dominated turbid state, analogously to fish effects (Perrow et al, 1997;Søndergaard et al, 1997;van Altena et al, 2016; but see Hansson et al, 2010;Chaichana et al, 2011;Marco-Méndez et al, 2015). Coot populations have been shown to be sensitive to fish, presumably due to negative fish impact on submerged macrophytes and aquatic insects (Houdková & Musil, 2003;MacedaVeiga et al, 2017), i.e.…”