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- Synthesizing mechanisms of density dependence in reef fishes: Behavior, habitat configuration, and observational scale,
- , , , Coral and rocky reef fish populations are widely used as model systems for the experimental exploration of density-dependent vital rates, but patterns of density-dependent mortality in these systems are not yet fully understood. In particular, the paradigm for strong, directly density-dependent (DDD) postsettlement mortality stands in contrast to recent evidence for inversely density-dependent (IDD) mortality. We review the processes responsible for DDD and IDD per capita mortality in reef fishes, noting that the pattern observed depends on predator and prey behavior, the spatial configuration of the reef habitat, and the spatial and temporal scales of observation. Specifically, predators tend to produce DDD prey mortality at their characteristic spatial scale of foraging, but prey mortality is IDD at smaller spatial scales due to attack-abatement effects (e.g., risk dilution). As a result, DDD mortality may be more common than IDD mortality on patch reefs, which tend to constrain predator foraging to the same scale as prey aggregation, eliminating attack-abatement effects. Additionally, adjacent groups of prey on continuous reefs may share a subset of refuges, increasing per capita refuge availability and relaxing DDD mortality relative to prey on patch reefs, where the patch edge could prevent such refuge sharing. These hypotheses lead to a synthetic framework to predict expected mortality patterns for a variety of scenarios. For nonsocial, nonaggregating species and species that aggregate in order to take advantage of spatially clumped refuges, IDD mortality is possible but likely superseded by DDD refuge competition, especially on patch reefs. By contrast, for species that aggregate socially, mortality should be IDD at the scale of individual aggregations but DDD at larger scales. The results of nearly all prior reef fish studies fit within this framework, although additional work is needed to test many of the predicted outcomes. This synthesis reconciles some apparent contradictions in the recent reef fish literature and suggests the importance of accounting for the scale-sensitive details of predator and prey behavior in any study system., ,
- Wilson White, Samhouri, Stier, Wormald, Hamilton, Sandin
- Quantifying patterns of fish herbivory on Palmyra Atoll (USA), an uninhabited predator-dominated central Pacific coral reef
- On many coral reefs, herbivorous fish play an essential role in regulating algal growth and influencing the outcome of coral-algal competition. Working on a remote predator-dominated coral reef on Palmyra Atoll, USA, we used behavioral foraging observations to quantify the roles of common parrotfish and surgeonfish in the roving herbivore guild. We recorded species-specific bite rates on different benthic organisms, quantified the relative abundance of those benthic organisms, and estimated benthos-specific grazing intensities as a function of bite rates, fish abundance, and percent cover. These grazing metrics were compared between the exposed fore reef (∼10 m depth) and protected reef terrace (∼5 m depth) habitats. We observed large differences in feeding rates and substrate selectivity among fish species. Most species fed predominately on algal turfs; however, some species foraged broadly among fleshy macroalgal taxa, while others specialized on calcified green algae of the genus Halimeda. The highest bite rates were recorded from species targeting algal turfs, while the highest rates of defecation were recorded from species targeting Halimeda. Per capita bite rates of all species were higher in the fore reef habitat (mean 45% more bites min-1); however, overall grazing intensities on turf algae (bites cm-2 d-1) were 5 times higher on the reef terrace than on the fore reef. Despite habitat-specific differences in the herbivore assemblages, the estimated distribution of total bites showed consistency among habitats, with strong guild-level positive foraging selectivity for algal turf. Comparisons of bite and defecation rate data for these herbivores across the Indo-Pacific highlight phylogenetic constraints on grazing activities. Overall, this study illustrates the importance of herbivore functional redundancy, variability in species-specific grazing, and provides a framework for assessing guildwide grazing impacts on coral reefs. © Inter-Research 2014.
- Hamilton, Smith, Price, Sandin