Abstract
Reed (Phragmites australis (Cav.) Trin. ex Steud.) beds are important habitat for marsh birds, but are declining throughout Europe. Increasing numbers of the native marsh bird, the Greylag goose (Anser anser L.), are hypothesized to cause reed bed decline and inhibit restoration of reed beds, but data are largely lacking. In this study, we experimentally tested the effect of grazing by Greylag geese on the growth and expansion of reed growing in belts along lake shorelines. After 5 years of protecting reed from grazing with exclosures, reed stems were over 4-fold denser and taller than in the grazed plots. Grazing pressure was intense with 50–100% of the stems being grazed among years in the control plots open to grazing. After 5 years of protection we opened half of the exclosures and the geese immediately grazed almost 100% of the reed stems. Whereas this did not affect the reed stem density, the stem height was strongly reduced and similar to permanently grazed reed. The next year geese were actively chased away by management from mid-March to mid-June, which changed the maximum amount of geese from over 2300 to less than 50. As a result, reed stem density and height increased and the reed belt had recovered over the full 6 m length of the experimental plots. Lastly, we introduced reed plants in an adjacent lake where no reed was growing and geese did visit this area. After two years, the density of the planted reed was six to nine-fold higher and significantly taller in exclosures compared to control plots where geese had access to the reed plants. We conclude that there is a conservation dilemma regarding how to preserve and restore reed belts in the presence of high densities of Greylag geese as conservation of both reed belts and high goose numbers seems infeasible. We suggest that there are three possible solutions for this dilemma: (1) effects of the geese can be mediated by goose population management, (2) the robustness of the reed marshes can be increased, and (3) at the landscape level, spatial planning can be used to configure landscapes with large reed bed reserves surrounded by unmown, unfertilized meadows.
Highlights
Riparian zones located at the interface between aquatic and terrestrial ecosystems, are generally rich in biodiversity (Nilsson and Svedmark, 2002; Brauns et al, 2007; Valkama et al, 2008)
Reed stem density was significantly higher in exclosures compared to control treatments where geese had access; after 5 years of protection from grazing, reed stems were over four-fold denser than in the grazed plots [Figure 2A; F(1, 45) = 230.97; P < 0.001]
The proportion of grazed stems was close to zero inside exclosures and varied between 50 and 100% of the stems being grazed in control plots, where geese had access [Figure 2C; F(1, 45) = 1811.08; P < 0.001]
Summary
Riparian zones located at the interface between aquatic and terrestrial ecosystems, are generally rich in biodiversity (Nilsson and Svedmark, 2002; Brauns et al, 2007; Valkama et al, 2008). Reed beds are important habitat for marsh birds, of which many species are a conservation concern (Graveland, 1998; Vermaat et al, 2008; Beemster et al, 2010; Voslamber and Vulink, 2010). It is of utmost importance that existing reed beds are being protected and deteriorated reed beds are being restored. A new conservation dilemma emerged with regard to reed bed protection and restoration. The recent increase in the number of Greylag geese (Anser anser L.) causes concern about their impact on reed vegetation during summer (Vermaat et al, 2016; Buij et al, 2017)
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