Information on predator and prey distributions is integral to our understanding of migratory connectivity, food web dynamics and ecosystem structure. In marine systems, although large animals that return to land can be fitted with tracking devices, minimum instrument sizes preclude deployments on small seabirds that may nevertheless be highly abundant and hence major consumers. An increasingly popular approach is to use N and C stable isotope analysis of feathers sampled at colonies to provide information on distribution and trophic level for the preceding, and generally little-known, nonbreeding period. Despite the burgeoning of this research, there have been few attempts to verify such relationships. In this study, we demonstrate a clear correspondence between isotope ratios of feathers and nonbreeding distributions of seven species from South Georgia tracked using loggers. This generated a rudimentary isoscape that was used to infer the habitat preferences of eight other species ranging in size from storm petrels to albatrosses, and which could be applied, with caveats, in other studies. Differences in inferred distribution within and between species had major implications for relative exposure to anthropogenic threats, including climate change and fisheries. Although there were no differences in isotope values between sexes in any of the smaller petrels, mean stable C (delta(13)C), but not stable N isotope ratios (delta(15)N), tended to be greater in females than males of the larger, and more sexually size-dimorphic species. This indicates a difference in C source (distribution), rather than trophic level, and a correspondence between the degree of sexual size dimorphism in Procellariiformes and the level of between-sex niche segregation.

An important aspect of foraging ecology is the extent to which different individuals or genders within a population exploit food resources in a different manner. For diving seabirds, much of this information relates either to short-term dietary data or indirect measures such as time budgets. Moreover, dietary specialisation can be difficult to detect due to biases associated with conventional sampling techniques. We used stable isotope ratios in blood and feathers to infer trophic and habitat specialisations among 4 diving seabird taxa -the gentoo penguin Pygoscelis papua, the macaroni penguin Eudyptes chrysolophus, the South Georgian shag Phalacrocorax (atriceps) georgianus and the Kerguelen shag P. (atriceps) verrucosus. This allowed us to investigate foraging specialisation and assess whether social dominance or differences in foraging preferences explained the observed patterns. In all taxa where sexes were known we found that males foraged at a higher trophic level (δ 15 N values) than females, although this was not significant in macaroni penguins. We believe that this is linked to a dual foraging strategy among female macaroni penguins. For South Georgian shags, we found that sex-related dietary differences persisted for long periods (inferred from stable isotope analyses of feathers and blood). We suggest that the trophic differences are driven by differences in physiological performance, with males tending to dive deeper than females because of their larger size, and hence able to access higher trophic level prey items. Moreover, male and female shags tend to forage at different times of day; therefore, social dominance by males is unlikely to be driving the observed differences. We also recorded highly significant relationships between stable isotope signatures in blood (representing the breeding season diet) and those in feathers (mostly representing the previous non-breeding season diet) in both the South Georgian and Kerguelen shags. This strongly suggests that these 2 taxa include individuals with distinct foraging specialisation (and most probably foraging locations) that are maintained over long periods.KEY WORDS: Penguin · Shag · Trophic level · δ 13 C · δ 15 N Resale or republication not permitted without written consent of the publisher Editorial responsibility: Rory P. Wilson

Until recently, the study of negative and antagonistic interactions (for example, competition and predation) has dominated our understanding of community structure, maintenance and assembly. Nevertheless, a recent theoretical model suggests that positive interactions (for example, mutualisms) may counterbalance competition, facilitating long-term coexistence even among ecologically undifferentiated species. Müllerian mimics are mutualists that share the costs of predator education and are therefore ideally suited for the investigation of positive and negative interactions in community dynamics. The sole empirical test of this model in a Müllerian mimetic community supports the prediction that positive interactions outweigh the negative effects of spatial overlap (without quantifying resource acquisition). Understanding the role of trophic niche partitioning in facilitating the evolution and stability of Müllerian mimetic communities is now of critical importance, but has yet to be formally investigated. Here we show that resource partitioning and phylogeny determine community structure and outweigh the positive effects of Müllerian mimicry in a species-rich group of neotropical catfishes. From multiple, independent reproductively isolated allopatric communities displaying convergently evolved colour patterns, 92% consist of species that do not compete for resources. Significant differences in phylogenetically conserved traits (snout morphology and body size) were consistently linked to trait-specific resource acquisition. Thus, we report the first evidence, to our knowledge, that competition for trophic resources and phylogeny are pivotal factors in the stable evolution of Müllerian mimicry rings. More generally, our work demonstrates that competition for resources is likely to have a dominant role in the structuring of communities that are simultaneously subject to the effects of both positive and negative interactions.

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