Precipitation and guano deposition: their combined impact on vegetation dynamics

Abstract

Coastal ecosystems provide vital ecosystem services, including habitat provision, nutrient cycling, and shoreline protection. Vegetation helps to sustain these functions by acting as ecosystem engineers through soil stabilisation, nutrient flows regulation and food web support. Its growth depends on water mainly from precipitation, and nutrients that can be supplied by sources like bird guano. This research studies the influence of precipitation on vegetation productivity, focusing on the role of guano-derived nitrogen and plant community composition. It was hypothesised that precipitation positively affects vegetation productivity, moderated by guano input and plant communities, and that precipitation also impacts nitrogen uptake and species shifts.

To test these hypotheses, a study was conducted on five uninhabited islands in the Dutch Wadden Sea: Griend, Richel, Rottumeroog, Rottumerplaat, and Zuiderduintjes. Normalized Difference Vegetation Index (NDVI) values from 2017 to 2023 were compared to precipitation of the same time frame to assess temporal dynamics. Of both factors, anomalies were used to account for seasonality. Field- and laboratory work was carried out for vegetation surveys in 2022 and 2023 together with nitrogen isotope analysis.

NDVI anomalies were significantly influenced by precipitation anomalies, with vegetation responses varying in timing and duration across islands. Moreover, two distinct NDVI peaks were observed in relation to precipitation. Among the researched factors possibly influencing the NDVI-precipitation relationship, island identity showed the strongest effect, followed by guano input and plant community type. While precipitation appeared to have limited direct impact on guano-derived nitrogen uptake, shifts in species composition were observed, favouring nitrophilous, less salt-tolerant species with shallower root systems which potentially increases erosion risk. These findings highlight the importance of islandspecific monitoring and management to support long-term ecosystem resilience in the face of changing precipitation regimes.