All animals have an effect on the environment, but some key species – called ecosystem engineers – can have a particularly large influence. Their impact can be positive, by creating and maintaining habitats and increasing biodiversity, but they can also be negative, by destroying habitats and reducing biodiversity. Interestingly, the magnitude of the effect that an ecosystem engineer has on the environment is often not a reflection of their size. Some are huge, such as elephants, which provide water for other animals during the dry season by digging up desiccated water holes until water flows. But others are tiny, such as plankton in the oceans, which influence the amount of light reaching the plants down below. In particular, the transportation of marine-derived nutrients by animals is vital in shaping not only oceans but also freshwater and terrestrial ecosystems. Ivan González-Bergonzoni from the Universidad de la República, Uruguay, and his international team of researchers decided to observe this phenomenon from a seabird's perspective.
The seabird in question is the little auk (Alle alle), which constitutes the largest population of seabirds in the North Atlantic region, where they consume enormous amounts of zooplankton on a daily basis. The team travelled to the North Water Polynya between Northwest Greenland and Canada, which is home to 60–70 million breeding pairs, to collect samples from terrestrial and freshwater habitats at sites with and without colonies in order to measure the content of marine-derived nutrients in the form of nitrogen and carbon stable isotopes. These samples came from a wide range of sources, including soil, algae, scats and fur. In addition, the researchers also tracked the flight paths of a number of little auks, as the birds also distribute nutrients via their droppings during their foraging trips. The team found a huge increase in marine-derived nitrogen levels, almost 10-fold, in the terrestrial and freshwater samples from colony sites as well as the associated flight paths; this is greater than the nitrogen transfer provided by fish. However, the levels of marine-derived carbon only increased in the freshwater ecosystems and not in terrestrial areas.
Next, to determine the effect of the additional nutrient load, the team measured the species richness and physical–chemical characteristics of freshwater communities while also measuring the species richness and the productivity of the vegetation in the terrestrial habitats, which allowed them to quantify what the increase in nitrogen levels actually means for the ecosystems. The higher nutrient levels at little auk sites and flight paths contributed to an increased algal biomass in the freshwater ecosystems and increased vegetation in the terrestrial areas, suggesting much better primary productivity as a result of these small birds. In addition, many more animals, such as hares and muskoxen, were seen at terrestrial sites near the colonies, in contrast to the more barren areas that were not colonised by the little auks. Conversely, the team found that the species richness in the freshwater ecosystems near little auk colonies was reduced, probably because of the increase in water acidity and the algae that flourished as a result of nutrient enrichment.
González-Bergonzoni and his group have unequivocally shown that a single tiny bird species, the little auk, has a massive impact on the habitats of the North Water Polynya. By shaping these environments, the little auk is truly an ecosystem engineer and its loss would probably result in a very different landscape. It is vital that we understand which animals are significant ecosystem engineers in order to conserve habitats and protect biodiversity.
