The ecology of opportunistic foragers can be highly dependent on anthropogenic food sources, such as landfills, resulting in changes in several ecological and demographic aspects. The total closure of several landfill sites and the use of deterrence systems to prevent access to the remaining open landfill sites in a region in the northern Iberian Peninsula provided an excellent opportunity to evaluate the consequences of landfills on the trophic ecology of an opportunistic forager, the Yellow‐legged Gull Larus michahellis, using these resources. Based on stable isotope analyses, we used mixing models to estimate changes in diet before and after the closure of the majority of landfills in the breeding and the non‐breeding season. We found a decrease in the consumption of food from landfills after their closure, which was accompanied by an increase in feeding on terrestrial prey (mostly earthworms), but only in the breeding season. Interestingly, we observed no increase in marine prey consumption after the landfill closures. In winter there was a decrease in terrestrial prey consumption, whereas the consumption of marine and, despite their reduce availability, landfill resources increased. Thus, and unlike when all the landfills were open, we detected significant trophic differences between breeding and non‐breeding seasons. Additionally, we found significant trophic differences among colonies that were quite close to each other, but not between breeding adults and chicks. In conclusion, landfill closure or the use of deterrence systems had a direct impact on the trophic ecology of Yellow‐legged Gulls; loss of refuse was mainly compensated for by prey of terrestrial origin when breeding, but not in winter. Thus, this species may experience foraging constraints in winter with potential effects on other life‐history aspects including their dispersal, breeding and survival that needs further evaluation.
Understanding the use of feeding sources at the local scale is crucial in comprehending the factors driving population dynamics, dispersal and territory use. Many gull (Larus spp.) populations have increased sharply, which is partly promoted by their use of landfills as a food resource. Although at the large scale it is known that birds from mainland colonies feed more on landfills than those from offshore colonies, at the local scale, this distance‐dependent exploitation has been little studied. Here, then, we study whether the extent of gulls’ use of landfill is distance‐dependent through the study of 3 different gull colonies and five separate landfill sites within a relatively small geographical area. After controlling for bird numbers by both age cohort and colony, we observed that the number of gulls found at each landfill was colony dependent and that it decreased non‐linearly with increased distance to place of birth.
The management of mixed municipal waste can have an impact on wildlife and ecosystems. Previous studies have investigated how opportunistic species like gulls can react very fast to new landfills; however, the impact of landfill closure on bird populations is less investigated. Yet, there is a need to understand how fast and to what extent, animal populations can be adapted to new scenarios where the waste will not be deposited in landfill sites anymore. The aim is to determine the influence of landfill closures on apparent survival of a resident Yellow‐legged Gull (Larus michahellis) population, used as a model species showing short‐distance foraging movements, and with a high dependence on local food subsidies. Complementarily, we built some basic population growth models in order to determine how potential changes in survival (before/after landfill closure) will impact on population growth rate. Using a data set of 4,437 Yellow‐legged Gull chicks ringed in four colonies over a period of 13 years, we obtained evidence supporting that the apparent survival was affected by landfill closure, especially if the landfill was located within a buffer of 10 km around the colony. Landfill closure affected the survival of first‐year gulls (with a mean decrease of ~ 0.5–0.36), but not of older birds. Consequently, we did not detect a remarkable effect of landfill closures on population growth rate, probably due to the lack of effect on adult survival rates except for one of the surveyed colonies, where we found an annual decline of 7%.
The availability of food subsidies of anthropogenic origin has strong consequences on the trophic ecology and population dynamics of several opportunistic species. However, evidence suggests that the use of certain feeding sources is very local, and hence, potentially important food subsidies may have a relatively small area of influence, resulting in only limited impact from the large geographic-scale perspective. In the south-eastern part of the Bay of Biscay, the Yellow-legged Gull Larus michahellis population is distributed across several colonies, which are located close to each other but have variable dependence on the landfills and fishing harbours in the area. This population is therefore a good model to test the influence of anthropogenic food subsidies on the trophic ecology of an opportunistic species at the local scale. Distance to the nearest fishing harbour showed a stronger effect on the diet of Yellow-legged Gull chicks than distance to the nearest landfill site. Gulls notably reduced their consumption of marine prey (mostly comprised of fishing discards) in relation to decreasing distance of their colony to the nearest fishing harbour. This result has direct implications from a management standpoint, since fish discards were also found to have only a limited effect on diet at regional scales, but a very high impact at the local scale. Moreover, those colonies that consumed a higher proportion of marine prey showed better population trends. In line with European Union policies, a reduction in the availability of fish discards, together with the closure of landfill sites, will occur in the coming years, probably leading to a change in the trophic ecology and dynamics of Yellow-legged Gull populations.
ResumenEn la actualidad, con el progresivo cierre de vertederos, nos situamos en un marco experimental ideal para estudiar cómo afecta la reducción drástica de un recurso trófico en la ecología, dinámica poblacional y movimientos de una especie con alto interés en la gestión, como es la gaviota patiamarilla Larus michahellis Naumann, 1840. En este contexto, en 2016 se equiparon con GPS un total de 10 ejemplares adultos de gaviota patiamarilla en una de las colonias de cría de Gipuzkoa, de los que sólo 4 proporcionaron datos hasta el 31.07.2016. El análisis preliminar aporta información básica sobre el uso del territorio durante una parte del periodo reproductor. El porcentaje de localizaciones se distribuyó de esta manera: 60% en la colonia; 18% en otro tipo de hábitats; 12% en prados y cultivos; y 6.5% en vertederos. Se detecta, así, un relativo alto uso de los prados y cultivos del entorno de la colonia, así como de dos de los vertederos más importantes de la región. Por otro lado, parece existir alta variabilidad individual en el uso del territorio que podría reflejar diferentes estrategias en la búsqueda de alimento.Palabras clave: Dispositivos GPS, ecología espacial, especies oportunistas, Gipuzkoa, vertederos. AbstractToday, the progressive closure of landfills provides a good opportunity to study how this affects the drastic reduction of a food resource on the ecology, population dynamics and movement patterns of opportunistic species, such as the yellow-legged gull Larus michahellis Naumann, 1840. Against this background, in 2016, we captured 10 adult breeding gulls in one of the main breeding colonies in Gipuzkoa. These individuals were equipped with a GPS device. Only four of them provided data up to the end of the breeding period (31.07.2016). Preliminary analyses provide basic information about spatial use and trophic habitat during the breeding period (colony: 60%; other habitat types: 18%; grasslands/crops: 12%; dumps: 6,5%). An apparently relatively high use of grasslands and crops is confirmed, as well as the use of two of the largest landfills in the region. We also observed a very high individual variation in territory use, which might reflect different foraging strategies.Key words: GPS device, spatial ecology, opportunistic species, Gipuzkoa, dumps. LaburpenaEgun, urratsez urrats zabortegiak ixten ari direlarik, testuinguru esperimental ezin hobean gaude ikertzeko nola eragiten dion baliabide trofiko baten erabateko murrizketak maila ekologikoan, populazio dinamikan eta mugimenduetan, baliabide horren kudeaketarekin lotura handia duen kaio hankahoriari, Larus michahellis Naumann, 1840.
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