Abstract. Alternative successional trajectories (AST) may result in multiple climax states within an ecosystem when disturbances affect colonization history. In the boreal forest, ungulates have been proposed to drive AST because, under herbivore pressure, preferred species may go extinct and apparent competition may benefit browsing-resistant species. Over a 15-year period following logging, we tested whether deer herbivory altered plant species composition and whether the competitive advantage of resistant species was maintained following herbivore removal. We compared exclosures built immediately after logging with delayed exclosures built eight years later on Anticosti Island, Quebec, Canada. Although the palatable tree Betula papyrifera (paper birch) and some palatable herbs recovered in delayed exclosures, we observed legacies in both tree and herb cover. Woody regeneration in delayed exclosures was dominated by Picea glauca (white spruce), and Poaceae (grasses) were abundant in the field layer. Given that only early-successional species recovered, whereas latesuccessional broadleaf species and Abies balsamea (balsam fir) remained rare, succession may follow an AST after a limited browsing period during early succession.
Macrophytes play a keystone role in shallow aquatic ecosystems. In lakes, macrophytes stabilize clear‐water conditions with high biodiversity and their decline can cause a shift to a turbid state with lower biodiversity. Various mechanisms have been suggested as triggers of macrophyte collapse. Herbivory by waterfowl and fish seems to be one of the obvious factors, but the response of macrophytes to herbivory is ambiguous. We hypothesized that herbivory alone does not typically cause macrophyte collapse, but that shading from periphyton can enhance the effect of herbivores. Shading of macrophytes is supposed to increase with eutrophication due to changes in the top–down control cascading from fish via macroinvertebrates to periphyton. We elaborated on this idea by fitting a macrophyte growth model with different herbivore grazing and periphyton shading scenarios. In addition, we performed a meta‐analysis on existing experimental herbivore exclosure studies with respect to periphyton growth. The model supported our proposed hypothesis and the reviewed field studies appeared to point in the same direction. We suggest that a significant herbivore impact may indicate a reduced resilience of vegetation to eutrophication, making it an early warning signal for an imminent macrophyte collapse leading to a sudden shift of the system to turbid conditions.
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