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The indirect impact of landfills on waterbird communities.

It is obvious that human activities have an impact on nature and all its biodiversity, but the impact factors are not always well defined or the effects are direct or imminent. In some cases, the source of the impact may be far away from the ecosystem or entity that is affected.

Over the years, the San Pedro del Pinatar saltworks have undergone various alterations to the landscape and ecosystem, most of which have been directly caused by human activity. However, the biodiversity of this natural enclave has also shown changes in response to human activities outside the environment of the Regional Park. For example, the fishing activity carried out in the Mar Menor and the Mediterranean sea may have conditioned the settlement of breeding populations of fish-eating species that use the discards from these boats, such as the Audouin’s Gull (Larus audouinii) or the Sandwich Tern (Thalasseus sandvicensis). But, without a doubt, among the main activities that have brought about changes in this natural space, the organic waste dumps stand out.

Despite the fact that the nearest landfills to the salt flats of San Pedro del Pinatar are dozens of kilometres away, even in inland areas of the Region of Murcia, some of the waterbirds that settle in the saltworks make use of these “new habitats” as an alternative and inexhaustible source of food. The exposure of organic waste to the open air has led generalist species to travel dozens of kilometres day after day over land in search of food, the most important of these species being the Yellow-legged Gull (Larus michahellis) (see figure). The use of this food source completely modifies their biological parameters, with a productivity (number of chicks that manage to take flight) and juvenile survival rate much higher than the theoretical rate under natural conditions. Evidently, this fact has triggered an exponential growth of the wintering and breeding population of yellow-legged gulls in the San Pedro del Pinatar saltworks, reaching more than 2,000 wintering individuals and around 1,000 breeding pairs.

Graphic of the movements of GPS-tagged yellow-legged gulls in the San Pedro del Pinatar saltworks. Source: Seabird Ecology Lab.
(https://twitter.com/SeabirdEcology/status/1263780484590559234/photo/1)

This overpopulation promoted by the landfills has in turn triggered modifications in the nesting habitat of the rest of the waterfowl species that breed in the salt marshes. The excrement provided by the concentrations of these birds to roost during the winter months or in their breeding areas has modified the properties of the substrate of the saltworks motes, increasing the nitrogen in the soil and developing a dense vegetation cover unsuitable for these breeding areas. In this sense, the space available for nesting by other waterbirds has been reduced by 79% in 20 years, which has led to a decline in the populations of certain species.

Overgrowth of vegetation cover in former breeding sites.

Since the beginning of the LIFE SALINAS Project, in parallel to the construction of green infrastructures as new breeding habitats, actions have been carried out to control the yellow-legged gull population by removing nests. Three visits are made every year during the breeding period to remove the nests of this species, removing approximately 1,000 nests annually. The map below shows the density of the nesting areas of this species in the Regional Park.

Density map of Yellow-legged Gull breeding areas in the Regional Park

The LIFE SALINAS project is aware that this population control activity is palliative and must be maintained over time in order to try to regulate the populations of the Yellow-legged Gull. As applied in other areas of Europe, and as some regional administrations in Spain are trying to implement, the priority measure is to act at the source of the problem, limiting the availability of organic waste in open areas and its accessibility to the fauna, in this case regulating the populations in a natural way.

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