The role of vegetation cover and diet in explaining long-term changes in the breeding population of little terns (Sternula albifrons) in Ria Formosa, Algarve

摘要

Seabird populations are naturally regulated and their demographic fluctuations are explained by mainly two factors: the availability of suitable breeding habitats and the presence of foraging habitats with appropriate food resources. An important characteristic in determining suitability of breeding habitats for ground nesting seabirds is their vegetative cover. Suitable foraging habitats determine the availability of food resources, and are influenced by environmental variables, such as the climatic proxy, North Atlantic Oscillation (NAO) index. Environmental variables are known to affect the distribution of food resources in the marine environment which can cause declines in the availability of prey fish for seabirds.Little Terns (Sternula albifrons) are known to avoid nesting on densely vegetated areas to avoid predation, however the habitats with low vegetative cover preferred by these birds to nest are subjected to the overgrowth of vegetation which leads to the abandonment of the breeding site. Little Terns are also highly susceptible to fluctuations in food availability, especially during the breeding season, and abrupt changes in their diet can lead to variations in their breeding performance which will ultimately affect the breeding population size.This study aimed to first, evaluate if the fluctuations in Little Tern breeding population size in Ria Formosa, Algarve, could be explained by changes in vegetation cover. Secondly, it also intended to assess the role of the environmental variables and annual variations in diet, in Little Tern breeding parameters and in the number of breeding pairs in Ria Formosa, Algarve.The percentage of vegetation cover in each sandy beach was calculated from aerial pictures of Ria Formosa barrier islands from 1976 to 2012, and related with census data from the same period. A linear regression model was performed between these two variables, after controlling for the effect of environmental variation (NAO index), and predictions of the percentage of the number of breeding pairs nesting on sandy beaches of Ria Formosa for a given percentage of vegetation cover were made. To better address this relation between the number of breeding pairs and vegetation cover, a habitat experiment was made on a site with vegetation overgrowth, with the removal of part of the vegetation before the beginning of Little Tern’s breeding season. A comparison of the percentage of vegetation cover between breeding sites in salinas and on sandy beaches was also made. Little Tern diet and breeding parameters (clutch size, timing of breeding and egg size) in Ria Formosa were studied in 2012, 2013 and2014 and added to previous published data (2002 to 2011). Annual variations in diet and the NAO index were related with breeding parameters and with census data from 1979 to 2014, to assess whether environmental variables and, as a consequence, diet could explain the fluctuations in the number of breeding pairs.A strong negative relation was found between the number of Little Tern breeding pairs nesting on sandy beaches and the percentage of vegetation cover in this breeding habitat. In salinas, the same pattern was observed in the site subjected to the habitat experiment: the site was abandoned by breeding Little Terns when the vegetation cover became unsustainable to breed, however, birds returned to that site following vegetation removal. The comparison of the levels of vegetation between the two types of breeding habitats showed that vegetation cover in salinas was greater than that on sandy beaches, and also that sandy beaches with smaller vegetation cover had a higher number of breeding pairs. The breeding population size fluctuations over the years were also related with the environmental conditions, as captured by the NAO index. There was a relation between the negative NAO conditions and (1) earlier breeding, (2) larger clutch size, and, as a consequence, (3) higher number of breeding pairs. Diet was also related with breeding parameters and, consequently, with the number of breeding pairs.The climatic variation measured by the NAO index did not show a significant relation with the number of breeding pairs nesting in each sandy beach colony, so we suggest that the percentage of vegetation cover is more important in explaining the breeding population size on a local scale. Similarly, in a more regional scale the food availability, determined by the climatic conditions and the NAO index, should play an important role in explaining the number of breeding Little Terns.