Seabird droppings are important for coastal landscapes

Dutch scientists have analyzed how seabird droppings (guano) affect vegetation and geomorphological processes on sandy islands in the Wadden Sea. The study fits into the concept of so-called biogeomorphological systems, in which organisms and physical processes drive each other. Vegetation stabilizes sediments and promotes sand accumulation, while changes in relief affect plant growth.

How the effect of guano on the process of dune formation was studied

The study, the results of which were published in the journal Biogeosciences, was conducted on five uninhabited, originally sandy islands located in the Wadden Sea. All of them are nature reserves populated by seabird colonies, and the biogeomorphological processes occurring there depend on feedbacks between vegetation development and aeolian processes and sedimentation. The number of breeding pairs and bird species varies greatly between islands (from a few hundred to nearly 10,000 pairs), as reflected in habitat conditions and management history.

Data were collected at 118 study plots in August-September 2022, i.e. at the end of the breeding season, when guano deposition is highest. Analyses included plant species composition, functional characteristics and stable nitrogen isotope content, with elevated δ15N values indicating the contribution of nutrients from higher trophic levels (guano).

Seabird droppings alter vegetation, but not always its functions

Analyses confirmed that guano significantly affects plant species composition. Bird fertilization promoted the growth of more productive nitrogen-loving species and led to an increase in leaf nitrogen content and a decrease in the carbon:nitrogen (C:N) ratio. This pattern was particularly pronounced in poor, sandy areas, where plants are more dependent on external sources of nutrients. The models showed that the effect of guano on plants is best seen within a radius of about 300 meters from the bird colony’s habitat.

The researchers did not find a clear effect of guano on traits key to substrate stability, such as plant height, biomass or root system depth. This means that while seabird droppings favor certain species, they do not necessarily change the way plants shape the landscape.

Guano and the process of sediment accumulation

The analyses showed that the effect of guano on sedimentation processes is complex and depends on how the vegetation characteristics are measured. In one model, the effect was positive – guano enhanced the ability of plants to retain sediment. In another, a weak negative effect was observed, indicating a more complex relationship.

The direct effect of guano on land elevation changes was positive, but relatively small. Other factors, especially elevation, played a key role in shaping the dunes – higher lying areas were more likely to be eroded.

The researchers also observed that the impact of seabird droppings was not uniform. At the scale of entire islands, its importance was small (it explained on average about 1-3 percent of the variability of sedimentary processes), but locally – in the vicinity of bird colonies – it reached up to 6 percent. Differences could also be seen at the scale of individual islands. On some (e.g., Griend or Zuiderduin) guano clearly supported sediment accumulation, while on others its influence was masked by strong natural sedimentary processes. This shows that averaging the results can lead to erroneous conclusions.

Interestingly, the results of the analysis also suggest an inverse relationship: birds are more likely to establish colonies in stable sites less prone to erosion. This means that the relationship between guano and sediment accumulation may be due to both birds’ environmental influence and habitat preferences.

Importance for coastal protection

According to the authors, seabird droppings are a modifier of vegetation and landscape processes on the coast, but the effects are strongly dependent on the context – elevation, type of substrate, season and local conditions.

The study provides the first quantitative evidence that seabirds can influence landscape development by enhancing vegetation-sediment coupling. Guano thus alters the functioning of coastal ecosystems, but not in a simple or uniform way. It strongly influences vegetation composition and nutrient cycling, while its importance in shaping landforms is moderate and highly spatially variable.

An integrated approach – combining field and remote sensing data – has captured these relationships and shown that both biological and physical processes must be taken into account to understand coastal dynamics.

Bibliography:

van Rees, F. F., Govers, L. L., Guseva, P., Zwarts, M. P. A., Tuijnman, C., Camphuysen, C. J., Ruessink, G., and Reijers, V. C.: Nutrient flows and biogeomorphic feedbacks: linking seabird guano to plant traits and morphological change on sandy islands, Biogeosciences, 23, 1527-1544, https://doi.org/10.5194/bg-23-1527-2026, 2026