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.

No abstract

Mortality during the egg and larval stages is thought to play a major role in de termining year-class strength of many marine fish. Predation of eggs and larvae is normally considered to be a major factor but the full suite of predators responsible has rarely been identified. Potential predators on a patch of plaice Pleuronectes platessa eggs located in the eastern Irish Sea were mapped using acoustics and sampled by trawl and a plankton multi-net. Gut contents of 3373 fish, crustacea and cephalopods sampled in the area were screened using a plaice-specific TaqMan DNA probe. Herring Clupea harengus and sprat Sprattus sprattus dominated trawl catches and showed high positive TaqMan responses (77 and 75% of individuals tested respectively). Locations of clupeid schools also broadly corresponded with the distribution of fish eggs in the plankton. Whiting Merlangius merlangus were also reasonably abundant in trawl hauls and 86% of their stomachs tested positive for plaice DNA. Species showing lower levels of positive TaqMan response included mackerel Scomber scombrus, poor cod Trisopterus minutus, squid Loligo spp., dogfish Scyliorhinus canicula and weever Trachinus vipera. Nonre activity of all negative controls precluded the occurrence of cross-contamination, and positive reactions from demersal species, such as dogfish and weever, may have resulted from secondary predation. No benthic macro-crustaceans tested positive. Samples of planktonic organisms yielded 13% positive TaqMan reactions, mainly from clupeoid or sandeel (Ammodytidae) larvae, but also included some Malacostraca and Amphipoda. Use of the molecular approach allowed rapid screening of a large number of potential predators of plaice eggs, and the results provide a more holistic description of the predator community than has previously been achieved.