| dc.description.abstract | The Wadden Sea is a unique ecosystem and a very significant breeding, roosting, and migratory area for birds. Moreover, it protects the Netherlands from storms and the rising ocean. Due to the dynamic nature of local islands, it is important to find a key to natural preservation and sediment accumulation. Previous research showed that bird presence affects ecosystem productivity on hard-substrate islands via allochthonous nutrient transfer (guano, dead bodies) and physical disturbance (seed dispersal). At the same time, vegetation stimulates sediment accretion. However, how soft-substrate islands are shaped through bird-affected plant growth is not well studied, while the feedback between vegetation presence and sediment dynamics exists.
To reveal the influence on vegetation and sediment around, environmental characteristics are tracked within the timeline of 2017-2022 in the Dutch Wadden Sea within and outside of 61 avian colonies in a total of 9 different species. Based on normalized difference vegetation index (NDVI) and digital elevation maps (DEMs), four parameters are comprised: mean summer NDVI as a proxy for the amount of vegetation, a linear coefficient of spring greening (greening index) for noting the slope of vegetation increase, total elevation is taken as stated in DEMs, and an absolute maximum of space slope represents steepness. Eventually, all possible combinations of these characteristics are broken down into groups and 32 clusters are composed to reflect ecotopes. To demonstrate the evolution, a linear annual coefficient of continuous data and variability of categorical data are compared between groups and per island.
Overall, birds prefer highly vegetated places which green over spring with gently or not inclined low terrains and the clusters corresponding to these attributes. These distributions correspond with species’ habitat prefer- ences. Over the years, mean NDVI and greening index increase faster within avian colonies than outside while in general there is no significant common trend for elevation or slope changes. Random forest classification also stresses the importance of vegetation parameters for bird prediction. However, the tendency is clearer if considered per island.
The stage of island development defines the magnitude and direction of the bird effect. Islands in the bio-morphological stage are susceptible to the nutrient pump as both elevation and vegetation are increasing. Whereas, on islands in the ecological stage summer mean NDVI is growing even faster but without any effect on abiotic parameters. Cluster variability also reflects these stages, where on the first kind of islands birds dwell on more dynamic territories in comparison to the outside. While both groups transition from unvegetated areas to vegetated ones, birds boost this shift. Whereas, on islands undergoing succession birds are located in more stable places in comparison to the surrounding. | |
