F. van Rees1,2,3*, V. Reijers2, P.Guseva2, C. Tuijnman2, M. Zwarts2, K. Camphuijsen1, G. Ruessink2, L.Govers1,4
1 Royal Netherlands Institute for Sea Research; 2 Utrecht University; 3 Deltares; 4 Rijksuniversiteit Groningen
Bird faeces contains high concentrations of nitrogen, phosphate and potassium. This natural fertilizer fosters vegetation growth at hard-substrate islands (Buelow et al., 2018). Yet, for sandy islands it is unknown if similar links between bird presence and vegetation development exist. Whereas, especially on sandy islands, vegetation acts as an autogenic ecosystem engineer driving island morphodynamics through vegetation-sedimentation feedback processes (Bonte et al., 2021). We therefore hypothesise that birds interact with island morphology through fertilizing the sandy islands they breed on. Insight into relationships between coastal bird presence, vegetation growth dynamics and subsequently landscape development is advantageous in integrated coastal zone management in which policy serves both coastal safety and biodiversity.
We assessed to what extent the plant community composition and its trait expressions depend on bird presence by conducting a field survey on 11 different sandy islands in the Waddensea. To verify whether plants grew on guano-subsidized soils, we analysed stabile isotopes of nitrogen within- and outside of bird colonies. Subsequently, we elucidated whether bird-meditated reciprocal interactions between plants and sediment can be linked to ecotope shifts on these islands. We classified ecotopes using a supervised classification on satellite imagery and elevation data between 2017 and 2022.
Pre-liminary results show that community composition, dry biomass, rooting depth are a function of distance to bird colony. Stabile isotope 15N/14N ratios revealed vegetation was nourished by 15N-rich guano. Moreover, ecotopes classified as bare areas develop faster into vegetated- and elevated landscapes when guano was present compared with bare areas under no guano influence.
Figure 1: Transition from sandbar to vegetated island under bird influence.
Bonte, D., Batsleer, F., Provoost, S., Reijers, V., Vandegehuchte, M. L., Van De Walle, R., Dan, S., Matheve, H., Rauwoens, P., Strypsteen, G., Suzuki, T., Verwaest, T., & Hillaert, J. (2021). Biomorphogenic Feedbacks and the Spatial Organization of a Dominant Grass Steer Dune Development. Frontiers in Ecology and Evolution, 9(October), 1–12. https://doi.org/10.3389/fevo.2021.761336
Buelow, C. A., Baker, R., Reside, A. E., & Sheaves, M. (2018). Nutrient subsidy indicators predict the presence of an avian mobile-link species. Ecological Indicators, 89(June 2017), 507–515. https://doi.org/10.1016/j.ecolind.2018.02.029