INFLUENCE OF GUANO DEPOSITION ON PLANT COMMUNITY IN THE DUTCH WADDEN SEA

Abstract

The Dutch Wadden Sea islands play an invaluable role in the coastal protection of the mainland from flooding during storm surges. The islands can be divided into two categories, the barrier islands that separate the North Sea from the Wadden Sea, and the islands located behind those islands, the back-barrier islands (Govers & Reijers, 2021). At present, these islands are threatened by human disturbances, predator introduction, and anthropogenic greenhouse emissions causing accelerated sea level rise and increased storm surge frequencies (Trilateral Working Group on Coastal Protection and Sea Level Rise (CPSL), 2001; Veerman & Deltacommissie., 2008; Wang et al., 2012). These threats are reason for concern, as they endanger the survival of the islands and corresponding species. The Dutch Wadden Sea islands play a central role in the global flyway networks for migratory shore- and seabirds (Boere & Piersma, 2012; Piersma et al., 2016). They offer feeding, breeding and roosting opportunities. In return, these birds increase ecosystem productivity through the provision of a marine derived nutrient influx, guano deposition (Anderson & Polis, 1999; Benkwitt et al., 2021; Buelow et al., 2018; Lundberg & Moberg, 2003). Guano is the accumulated excrement of migratory shore- or seabirds and contains high levels of nutrients such as nitrogen and phosphate phosphorus (Anderson & Polis, 1999; Ellis, 2005; García et al., 2002; Ryan & Watkins, 1989; Wait et al., 2005). The size and resilience of the islands are dependent on vegetation growth (Durán & Moore, 2013). Yet, their sandy soil is often nutrient poor, constraining the growth of vegetation(V. C. Reijers et al., 2020). This leads to the question if the island’s size and resilience are dependent on guano deposition by migratory shore- and seabirds. To answer this question, the effect of guano deposition on island vegetation must be elucidated first. For example, it has been shown that excessive inputs of phosphorus and nitrogen can shift the community composition, resulting in a decrease in species diversity (Avolio et al., 2014; de Schrijver et al., 2011; Payne et al., 2013; Suding et al., 2005). However, research on this topic is not always in agreement. On a community level, migratory shore- and seabirds have been shown to decrease the abundance of native species (Hogg & Morton, 1983; Vidal, Médail, Tatoni, Roche, et al., 1998; Vidal, Médail, Tatoni, Vidal, et al., 1998). However, they can also contribute to the persistence of rare native species (Dean et al., 1994; Norton et al., 1997). Moreover, conflicting results have been published on the effect of migratory shore- and sea birds on species richness. Several studies reported that species richness was higher in areas that were unaffected by birds in comparison to colony areas (Ellis, 2005; Gillham, 1960; Ishida, 1996; Vidal et al., 2003), while other studies reported that the species richness was higher at intermediate levels of bird disturbance (Hogg & Morton, 1983; Vidal et al., 2003). Some reported that the effect of migratory shore- and sea birds on species richness was dependent on the island size (Hogg et al., 1989; Vidal et al., 2000; Vidal, Médail, Tatoni, Roche, et al., 1998) However, these results were also contradicting. To close this knowledge gap, this thesis aims to investigate the effect of guano deposition on the plant community at the Dutch Wadden islands. It is proposed that the island plant community is dependent on guano deposition. The distance to colony will affect the species diversity, species richness, plant height, root depth and biomass. However, the magnitude of the effects will be dependent on the ecotope that the vegetation originates from. The ecotopes reviewed here are sandy barriers and dunes and salt marshes. Sandy barriers and dunes are constantly changing due to wind and water action. In this