Ecological opportunity is considered a crucial factor for adaptive radiation. Here, we combine genetic, morphological and ecological data to assess species and ecomorphological diversity of Artic charr in six lakes of a catchment in southernmost Greenland, where only charr and stickleback occur. Because the diversity of habitats and resources increases with lake size, we predict a positive association between lake size and the extent of ecomorphological diversity. The largest lake of the catchment harbours the largest Arctic charr assemblage known today. It consists of six genetically differentiated species belonging to five ecomorphs (anadromous, littoral benthic, profundal dwarf, planktivorous, piscivorous), of which the latter comprises two ecomorphologically extremely similar species. Lakes of intermediate size contain two ecomorphologically and genetically distinct species. Small lakes harbour one genetically homogeneous, yet sometimes ecomorphologically variable population. Supporting our prediction, lake size is positively correlated with the extent of ecomorphological specialization towards profundal, pelagic and piscivorous lifestyle. Furthermore, assemblagewide morphospace increases sharply when more than one genetic cluster is present. Our data suggest that ecological opportunity and speciation jointly determine phenotypic expansion in this charr radiation.
Summary1. Many anti-predator benefits of group living are predicted to scale with prey density. Nevertheless, evidence for a general density-dependent increase of prey survival is scarce. A possible reason for this discrepancy is the reduction of costly anti-predator behaviour of prey with increasing density, which may offset density-dependent survival gains. Benefits of group living might hence accrue by saved investment into anti-predator behaviours rather than by increased survival rates. 2. Here, we experimentally presented predators in a colony of the cooperatively breeding cichlid fish Neolamprologus pulcher to study density dependence of their anti-predator defences. Predation is a driver of the formation and stability of breeding groups in this species, but potential benefits of coloniality are yet unclear. We hypothesised that increased density of breeding groups would either increase total anti-predator behaviour or allow individuals to reduce their anti-predator effort due to enhanced predator deterrence from neighbours. 3. Confirming predictions from the second hypothesis, our results show that focal groups invested less into anti-predator behaviour at higher densities, while neighbouring groups' behaviour compensated for this reduced effort. This resulted in stable levels of anti-predator behaviours over the entire range of natural densities. Thus, aggregating in colonies allows these fish to save investment in anti-predator behaviour. 4. These results suggest that the formation of both breeding groups and colonies reflects adaptive responses to high predation pressure in this species. Two different levels of sociality seem to be favoured by the same selective force. 5. Our study provides experimental evidence in nature for an important benefit of coloniality that may explain the concomitant existence of different levels of sociality in many highly social taxa.
The frequent occurrence of adaptive radiations on oceanic islands and in lakes is often attributed to ecological opportunity resulting from release from competition where arrival order among lineages predicts which lineage radiates. This occurs when the lineage that arrives first expands its niche breadth and diversifies into a set of ecological specialists with associated monopolization of the resources. Later-arriving species do not experience ecological opportunity and do not radiate. While theoretical support and evidence from microbial experiments for priority effects are strong, empirical evidence in nature is difficult to obtain. Lake Victoria (LV) is home to an exceptional adaptive radiation of haplochromine cichlid fishes, where 20 trophic guilds and several hundred species emerged in just 15 000 years, the age of the modern lake that was preceded by a complete desiccation lasting several thousand years. However, while about 50 other lineages of teleost fish also have established populations in the lake, none of them has produced more than two species and most of them did not speciate at all. Here, we test if the ancestors of the haplochromine radiation indeed arrived prior to the most competent potential competitors, 'tilapias' and cyprinids, both of which have made rapid radiations in other African lakes. We assess LV sediment core intervals from just before the desiccation and just after refilling for the presence of fossil fish teeth. We show that all three lineages were present when modern LV began to fill with water. We conclude that the haplochromines' extraordinary radiation unfolded in the presence of potentially competing lineages and cannot be attributed to a simple priority effect.
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