Invertebrate grazing during the regenerative phase affects the ultimate structure of macrophyte communities

Abstract

1. Although the biomass of freshwater macrophytes consumed by invertebrate herbivores (excluding crayfish) is usually low, there is growing evidence that invertebrates do exert a structuring effect on macrophyte communities. To explain this, we postulated that the effect of invertebrates may be concentrated on macrophytes during their regenerative phase.2. We tested this hypothesis by means of a mesocosm-based experiment, in which we investigated the effects of different densities of pond snails [Lymnaea stagnalis (L.)] on macrophytes regenerating from the natural sediment propagule bank.3. After 2 months, a diverse macrophyte community had established in the absence of snails, mainly from sexual propagules. Under moderate snail grazing (4 individuals m-2), the ultimate biomass of macrophytes was similar, but its species composition differed dramatically. Only a few unpalatable taxa, such as Ceratophyllum demersum and Nymphaeaceae, persisted. Moreover, the relative success of macrophytes regenerating from vegetative rather than sexual propagules improved. Under higher snail grazing (20 m-2), all macrophytes disappeared before the end of the experiment.4. These results confirm that snails at natural densities can have a strong effect on the ultimate structure of macrophyte communities by selectively consuming some species at a juvenile stage. Therefore, the regenerative phase can be seen as a window of opportunity for invertebrate grazers, which can have a qualitative effect on communities that is disproportionate to the biomass consumed

Biotic resistance by snails and fish to an exotic invasive aquatic plant

“…These findings also parallel the role of other snails (e.g. the lymnaeid snail Lymnaea stagnalis L.) that can have major impacts on the macrophyte communities due to herbivory during early colonisation (Elger, Willby, & Cabello-Martinez, 2009). In contrast, the fish M. forestii does not represent a potential control mechanism of the short-term growth and development of H. verticillata, but it can potentially decrease the long-term performance of H. verticillata by reducing its total amount of photosynthetic tissue due to leaf consumption of rooted plants.…”

Response of Submerged Macrophyte Communities to External and Internal Restoration Measures in North Temperate Shallow Lakes

“…Fish removal may indirectly (due to more periphyton grazing invertebrates) reduce periphyton shading in summer, a major mechanism preventing macrophyte survival after nutrient load reduction (see Response of Macrophytes to Nutrient Load Reductions). The relevance of this process for macrophyte recovery after fish removal has not yet been directly tested, although mesocosm trials in which the density of periphyton grazers are manipulated produce predictable outcomes in terms of periphyton biomass and macrophyte composition (Elger et al, 2009 ). Excretion of allelopathic substances, which has been detected for many of the typical species that colonize after biomanipulation (Table 5 ), may also contribute to lower periphyton densities.…”

The relative importance of local conditions and regional processes in structuring aquatic plant communities

“…Predation, herbivory and competition are much less likely than dispersal limitation to be responsible for the regional pattern. Aquatic plants are grazed by many vertebrate and invertebrate animals, which can dramatically reduce plant abundance and distribution within a community (Lodge, 1991), but there is little evidence that grazing can be so extensive as to remove a species from natural communities (this can occur in mesocosms; Elger, Willby & Caballo‐Martinez, 2009). Therefore, herbivory is less likely to affect regional analyses of species presence, such as ours, than those involving abundance.…”