Compensatory growth in an aquatic plant mediates exploitative competition between seasonally tied herbivores

Abstract

Abstract. The degree to which vertebrate herbivores exploitatively compete for the same food plant may depend on the level of compensatory plant growth. Such compensation is higher when there is reduced density-dependent competition in plants after herbivore damage. Whether there is relief from competition may largely be determined by the life-history stage of plants under herbivory. Such stage-specific compensation may apply to seasonal herbivory on the clonal aquatic plant sago pondweed (Potamogeton pectinatus L.). It winters in sediments of shallow lakes as tubers that are foraged upon by Bewick's Swans (Cygnus columbianus bewickii Yarrell), whereas aboveground biomass in summer is mostly consumed by ducks, coots, and Mute Swans. Here, tuber predation may be compensated due to diminished negative density dependence in the next growth season. However, we expected lower compensation to summer herbivory by waterfowl and fish as density of aboveground biomass in summer is closely related to photosynthetic carbon fixation. In a factorial exclosure study we simultaneously investigated (1) the effect of summer herbivory on aboveground biomass and autumn tuber biomass and (2) the effect of tuber predation in autumn on aboveground biomass and tuber biomass a year later. Summer herbivory strongly influenced belowground tuber biomass in autumn, limiting food availability to Bewick's Swans. In contrast, tuber predation in autumn by Bewick's Swans had a limited and variable effect on P. pectinatus biomass in the following growth season. Whereas relief from negative density dependence largely eliminates effects of belowground herbivory by swans, aboveground herbivory in summer limits both above-and belowground plant biomass. Hence, there was an asymmetry in exploitative competition, with herbivores in summer reducing food availability for belowground herbivores in autumn, but not the other way around.

Aquatic herbivores facilitate the emission of methane from wetlands

“…Considering the well-documented presence and effects of herbivorous waterfowl on aquatic macrophytes (e.g., Van Eerden et al 1997, Van den Wyngaert et al 2003, Gauthier et al 2005, Hidding et al 2009) in wetlands systems globally, it is highly unlikely that our observations are of incidental nature. Nevertheless, methane emission has to our knowledge never been assessed in any plantherbivore interaction study.…”

“…Nevertheless, methane emission has to our knowledge never been assessed in any plantherbivore interaction study. Waterfowl can reduce stands of wetland plants by grubbing for tubers and rhizomes (Hidding et al 2009), which may result in an altered C balance of ecosystems (van der Wal et al 2007). Grubbing activities of waterfowl feeding on belowground plant organs have been shown to affect the underlying microbial processes of methane emission, methane production, and oxidation (Bodelier et al 2006).…”

Underuse of stopover site by migratory swans

“…Tuber loss due to other causes than swan predation is small over the short exploitation period (3 % per month in winter exclosures; Hidding et al 2009). Hence, the difference between the initial and final tuber density should roughly equal the consumption C by the swans, and the observed patterns of depletion and stopover duration should be reproducible from the functional response of swans feeding on tubers (Nolet et al 2006b).…”

Net Energy Intake Rate as a Common Currency to Explain Swan Spatial Distribution in a Shallow Lake

“…At the Lauwersmeer, food biomass, foraging costs, and accessibility of the food items had a great deal of spatial and temporal variability ). Moreover, wind fetch and grazing by waterfowl in summer might also affect the development of aboveground biomass differently per inlet and per year (Scheffer et al 1992;Wersal et al 2006;Hidding et al 2009;Gyimesi et al 2011). Therefore, interannual changes in food density do not have the same direction in all the inlets.…”

“…First, grazing on aboveground vegetation late in the summer (after assessing aboveground biomass) affects tuber production (Hidding et al 2009;Gyimesi et al 2011). Second, other tuber-eating birds, like diving ducks and coots, may also consume a part of the tuber biomass, thus leave less behind for Bewick's swans (Gyimesi, van Lith and Nolet, unpubl.…”