Birds that live and breed in vegetated coastal areas, such as dunes and small islands, not only build nests but also – unintentionally – shape their own surroundings. This was discovered by Utrecht-based earth scientist Floris van Rees. He studied five uninhabited islands in the Dutch Wadden Sea region, comparable to the barrier islands off North Carolina and in the Chesapeake Bay, and observed how seabirds have a major impact on plant growth. 'Plant species that retain sand better benefit from the presence of bird excrement, which in turn is beneficial for dune formation.' This is particularly important now that sea level rise and coastal erosion are putting pressure on the habitats of many coastal birds.
Seabirds bring a lot of nutrients with their droppings (guano). These are not produced locally, but are brought in by the birds from elsewhere. On small, uninhabited islands – where there are few other sources of nutrition – guano can therefore have a major impact on plant growth. 'In places where there is a lot of breeding, and where a lot of guano ends up in the soil, you see vegetation growing much faster than in other places,' Van Rees observed from satellite images. 'And so-called dune-building grasses such as marram grass and sand couch can retain sand well, so they are extremely important for dune formation.' The plants in breeding areas also grow faster early in the season, thanks to the nutrients from the guano. 'And that also allows them to retain more sand.' In addition to the grass species mentioned above, annual herbaceous plants also benefit from guano fertilisation on sandy soils. 'In more organically rich soils, it is species such as sea couch and Atriplex that thrive in bird breeding sites.'
Resilience
Van Rees' research once again demonstrates the dynamics of the Wadden Sea region. "We are, of course, talking about low-lying islands that are vulnerable to high waves. Sometimes, after a storm, there is a temporary setback in the form of erosion, but nature is resilient: for example, you see faster greening in the spring at the breeding sites, which means that the landscape recovers more quickly. If the number of breeding pairs changes, this can have consequences not only for the birds themselves, but also for the landscape in which they breed and rest."
Nitrogen
A crucial component of guano is nitrogen. "There are two variants or isotopes: a light and a heavier variant. Coastal plants are often full of the light variant; they obtain their nitrogen from the air and water. But plants can also absorb nitrogen via bird droppings, which mainly contains the heavier variant. And that is what we found in the vegetation on the islands." This nutrient flow is part of a natural ecosystem process. "Whereas a large-scale supply of nutrients in other nature reserves can lead to undesirable enrichment and loss of biodiversity."
Measurements
Van Rees grew up in the northern Dutch province of Groningen and has been fascinated by the Wadden Sea region since childhood. For his research at Utrecht University and the Royal Netherlands Institute for Sea Research (NIOZ), he inventoried the effects of guano on the flora of Rottumeroog, Rottumerplaat, Richel, Griend and the Zuiderduintjes – all islands that are normally inaccessible to humans. He combined field measurements (such as plant growth and nitrogen measurements) with satellite images and elevation maps. Once back in the office, he also used computer models to analyse patterns.
Barrier islands
The Wadden or Frisian Islands are barrier islands: long, narrow deposits of sand or sediment parallel to the coast. Due to their composition, they are dynamic, "malleable" islands, unlike volcanic islands, for example. Sandy barrier islands are mainly found along gently sloping, sandy coasts, not only along the east coast of North America and the Wadden Sea of the Netherlands, Germany and Denmark, but also along the east coast of Australia and parts of the south-east coast of South America. On these sandy islands, where millions of migratory birds breed or rest each year, these birds probably play an important but underexposed role in shaping the landscape. This research thus underscores the importance of protecting the birds and their sandy breeding grounds.
