Key features of intertidal food webs that support migratory shorebirds.

The migratory shorebirds of the East Atlantic flyway land in huge numbers during a migratory stopover or wintering on the French Atlantic coast. The Brouage bare mudflat (Marennes-Oléron Bay, NE Atlantic) is one of the major stopover sites in France. The particular structure and function of a food web affects the efficiency of carbon transfer. The structure and functioning of the Brouage food web is crucial for the conservation of species landing within this area because it provides sufficient food, which allows shorebirds to reach the north of Europe where they nest. The aim of this study was to describe and understand which food web characteristics support nutritional needs of birds. Two food-web models were constructed, based on in situ measurements that were made in February 2008 (the presence of birds) and July 2008 (absence of birds). To complete the models, allometric relationships and additional data from the literature were used. The missing flow values of the food web models were estimated by Monte Carlo Markov Chain--Linear Inverse Modelling. The flow solutions obtained were used to calculate the ecological network analysis indices, which estimate the emergent properties of the functioning of a food-web. The total activities of the Brouage ecosystem in February and July are significantly different. The specialisation of the trophic links within the ecosystem does not appear to differ between the two models. In spite of a large export of carbon from the primary producer and detritus in winter, the higher recycling leads to a similar retention of carbon for the two seasons. It can be concluded that in February, the higher activity of the ecosystem coupled with a higher cycling and a mean internal organization, ensure the sufficient feeding of the migratory shorebirds.

Empirical relationships between predator and prey size among terrestrial vertebrate predators.

In an effort to make complex food web relations more tractable, published data on the food habits of terrestrial vertebrate predators were analyzed for patterns in the use of prey by size. Regressions of prey weight on predator weight were run and provided descriptions of both the relationship between mean prey weight and predator weight and the variation in prey weights taken by the predators. Separate models proved necessary for three trophic specializations: insectivores, piscivores and carnivores. Insectivores were found to take proportionately much smaller prey than carnivores. Mean prey weight tends to decrease slightly relative to predator weight among larger insectivores, while the ratio of prey weight to predator weight tends to increase with carnivore size. On average, insectivores also take a relatively wider range, of prey sizes than carnivores. In all respects, piscivores were intermediate to insectivores and carnivores. These models produce log-normal approximations to the frequency distributions (by weight) of prey sizes in the ration of predators from knowledge of the predators's body mass. Combined with allometric models of the bionergetics and productivity of animals, these relations are used to predict, that: (1) the daily kill rate declines with predator weight and (2) the upper limit to predator biomass is independent of predator weight.