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As metabolism increases with body size, populations of large-sized species can become constrained to relatively low trophic positions within a local community due to their high energetic demands combined with the limited efficiency with which energy is transferred up the food chain from primary producers. In a metacommunity context, dispersal can become a major driver of population dynamics and persistence, having also the potential to ameliorate the aforementioned energetic constraints due to its effect on colonization and energy subsidies from connected patches. Here, we derive a size-dependent model for food chain length in communities subjected to migration and show that populations with higher influx of migrants are able to persist at higher trophic positions. Simulations of random, dendritic, and real aquatic metacommunities corroborate this result and further indicate that community location in the landscape (relative centrality or isolation) may determine local food web structure. More central patches contained larger populations and were less constrained in trophic position. This effect was particularly strong in dendritic metacommunities, which are representative of freshwater watersheds in general. Furthermore, the role of spatial processes is nonlinearly intensified with increases in body size, indicating that larger organisms have a much stronger dependence on landscape attributes. Comme le metabolisme augmente proportionnellement a la taille du corps, les populations de grandes especes peuvent devenir limitees a des positions trophiques relativement basses au sein d'une communaute locale en raison de leurs fortes demandes energetiques combinees a l'efficacite limitee du transfert d'energie provenant de producteurs primaires vers le haut de la chaine trophique. Dans un contexte de metacommunaute, la dispersion peut devenir un important moteur de la dynamique et de la persistance des populations et peut egalement intensifier les contraintes energetiques susmentionnees par son effet sur la colonisation et les apports d'energie provenant de parcelles connectees. Nous elaborons un modele dependant de la taille pour decrire la longueur de la chaine trophique dans les communautes assujetties a la migration et demontrons que les populations caracterisees par les plus grands influx de migrants sont en mesure de persister a des positions trophiques plus elevees. Des simulations de metacommunautes aleatoires, dendritiques et reelles corroborent ces resultats et indiquent en outre que l'emplacement de la communaute dans le paysage (centralite ou isolement relatifs) pourrait determiner la structure du reseau trophique local. Les parcelles plus centrales contenaient des populations plus grandes et leur position dans la chaine trophique etait moins restreinte. Cet effet etait particulierement fort dans les metacommunautes dendritiques, qui sont representatives des bassins hydrographiques d'eau douce en general. En outre, l'augmentation de la taille du corps est reliee de maniere non lineaire a l'intensification du role de processus spatiaux, ce qui indique que les organismes plus grands dependent beaucoup plus fortement des attributs du paysage. [Traduit par la Redaction], Introduction A major change in ecology has been the growing role assigned to the metacommunity--local community connection as a chief determinant of local community structure (Brown and Swan 2010; Chase [...]

We found a segmented pattern, increasing for small sizes and decreasing for larger sizes, in the relationship between trophic position and body size. This pattern provides support for a recently developed theoretical model whose derivation was based on consumers' metabolic requirements and on basic assumptions about feeding relationships. We combined original and published information about stable nitrogen isotopes, a proxy of trophic position, for a broad range of animal body sizes ([10.sup.-3]-[10.sup.5] kg) inhabiting the southwestern Atlantic Ocean. Linear, polynomic, and piecewise segmented models were fit to species trophic position and body mass. The segmented model had the best fit, presenting a positive slope ([β.sub.1] = 0.33 ± 0.08) for small organisms ( Nous avons constate un motif segmente, augmentant pour les petites tailles et diminuant pour les tailles plus grandes, dans la relation entre la position trophique et la taille du corps. Cette distribution appuie un modele theorique recemment etabli dont l'elaboration reposait sur les exigences metaboliques des consommateurs et sur des hypotheses de base touchant aux relations d'alimentation. Nous avons combine de l'information originale et publiee sur les isotopes stables d'azote, une variable substitutive de la position trophique, pour une vaste plage de tailles du corps d'animaux ([10.sup.-3]-[10.sup.5] kg) habitant dans le sud-ouest de l'ocean Atlantique. Des modeles lineaire, polynomial et segmente par morceaux ont ete cales sur la position trophique et la taille du corps des especes. Le modele segmente etait le mieux ajuste aux donnees, presentant une pente positive ([β.sub.1] = 0,33 ±0,08) pour les petits organismes (, Introduction The determinants of food chain length have been studied for approximately 100 years and have well-consolidated theoretical hypotheses but limited empirical evaluations (Arim et al. 2007a). Recent focus on [...]

To authenticate to the full-text of this article, please visit this link: http://dx.doi.org/10.1111/j.1600-0706.2009.17768.x Byline: Matias Arim, Sebastian R. Abades, Gabriel Laufer, Marcelo Loureiro, Pablo A. Marquet Abstract: Ulrich Brose Body size is recognized as an important determinant of trophic structure as it affects individual energetic demands, population density, and the interaction between potential prey and predators. However, its relationship with trophic position remains unclear. It has been hypothesized that a positive relationship between body size and trophic position would be associated to some particular trophic structures, which would allow larger organisms to satisfy their energetic demand and sustain viable population sizes at higher trophic positions, where fewer resources are available. To test this hypothesis, we analyzed the diet of 619 killifishes from four species (Austrolebias cheradophilus, A. luteoflammulatus, A. viarius and Cynopoecilus melanotaenia), collected in temporary ponds occurring in the grasslands of Rocha, Uruguay. Trophic position, diet richness, number of energy sources, and evenness were estimated for 20 size classes, formed by consecutive groups of 31 individuals. Gape limitation and preference for the larger available prey were evaluated as explanations for observed patterns with an individual based model (IBM). In agreement with the hypothesis, killifishes presented a strong positive relationship between trophic position and body size (R.sup.2=0.86), associated with a trophic structure that could allow larger organisms to have access to more energy from the environment. This was reflected in a positive relationship between body size and 1) prey richness, 2) number of basal energy sources (i.e. plants, detritus, phytoplankton and terrestrial prey), and 3) evenness in prey use. IBM results showed that changes in trophic structure with body size are well explained by gape limitation, but not by size preferences. Our results suggest that the fulfilment of the greater energetic demands of larger organism will depend on community diversity, which typically increases with ecosystem size, indicating a novel connection between area, diversity, body size, and food chain length. Article History: Manuscript Accepted 11 June 2009 Article note: Matias Arim (marim@bio.puc.cl), G. Laufer and M. Loureiro, Seccion Zoologia Vertebrados, Facultad de Ciencias, Univ. de la Republica Uruguay, Igua 4225 Piso 9 Sur, Montevideo, Uruguay. - MA, S. R. Abades and P. A. Marquet, Center for Advanced Studies in Ecology and Biodiversity (CASEB) and Depto de Ecologia, Facultad de Ciencias Biologicas, Pontificia Univ. Catolica de Chile, Santiago, CP 6513677, Chile. SRA and PAM also at: Inst. de Ecologia y Biodiversidad (IEB), Casilla 653, Santiago, Chile.

Studies pinniped diets inferred from scats. Biases in prey occurrence; Prey mass, proportion of loss of remains, predator assimilation efficiency and size of scats.