Resetting predator baselines in coral reef ecosystems

What did coral reef ecosystems look before human impacts became pervasive? Early efforts to reconstruct baselines resulted in the controversial suggestion that pristine coral reefs have inverted trophic pyramids, with disproportionally large top predator biomass. The validity of the inverted trophic pyramid has been questioned, but until now, was not resolved empirically. We use extensive field surveys and spatially explicit analytical tools to accurately assess the size and density of the predator community at an unfished coral reef, focusing on the most abundant predator. Our results suggest that the trophic structure of an unexploited reef fish community is not inverted and that even healthy top predator populations may be considerably smaller, and more precarious, than previously thought.

Life history variability of the grey reef shark across its range
For broadly distributed, often overexploited species such as elasmobranchs (sharks and rays), conservation management would benefit from understanding how life history traits change in response to local environmental and ecological factors. However, fishing obfuscates this objective by causing complex and often mixed effects on the life histories of target species. Disentangling the many drivers of life history variability requires knowledge of elasmobranch populations in the absence of fishing, which is rarely available. Here, we describe the growth, maximum size, sex ratios, size at maturity, and offer a direct estimate of survival of an unfished population of grey reef sharks (Carcharhinus amblyrhynchos) using data from an eight year tag-recapture study. We then synthesized published information on the life history of C. amblyrhynchos from across its geographic range, and for the first time, we attempted to disentangle the contribution of fishing from geographic variation in an elasmobranch species. 

Bradley D
, Conklin E, Papastamatiou YP, McCauley DJ, Pollock K, Pollock A, Kendall BE, Gaines SD, Caselle JE. 2017. Resetting predator baselines in coral reef ecosystems. Scientific Reports 7:43131. doi:10.1038/srep43131

Bradley D, Conklin E, Papastamatiou YP, McCauley DJ, Pollock K, Kendall BE, Gaines SD, Caselle JE. 2017. Growth and life history variability of the grey reef shark (Carcharhinus amblyrhynchos) across its range. PLoS ONE 12(2):e0172370. doi:10.1371/journal.pone.0172370


Spatial separation without territoriality in shark communities
Spatial separation within predator communities can arise via territoriality but also from competitive interactions between and within species.  However, linking competitive interactions to predator distribution patterns is difficult and theoretical models predict different habitat selection patterns dependent on habitat quality and how competition manifests itself.  While models generally consider competitors to be either equal in ability, or for one phenotype to have a fixed advantage over the other, few studies consider that an animal may only have a competitive advantage in specific habitats.  We used > 10 years of telemetry data, habitat surveys and behavioral experiments, to show spatial partitioning between and within two species of reef shark (grey reef, Carcharhinus amblyrhinchos, and blacktip reef sharks, C. melanopterus) at an unfished Pacific atoll.  Within a species, sharks remained within small ‘sub-habitats’ with very few movements of individuals between sub-habitats, which previous models have suggested could be caused by intra-specific competition.  Blacktip reef sharks were more broadly distributed across habitat types but a greater proportion used lagoon and backreef habitats, while grey reef sharks preferred forereef habitats.  Grey reef sharks at a nearby atoll where blacktip reef sharks are absent, were distributed more broadly between habitat types than when both species were present.  A series of individual-based models predict that habitat separation would only arise if there are competitive interactions between species that are habitat-specific, with grey reefs having a competitive advantage on the forereefs and blacktips in the lagoons and backreef.  We provide compelling evidence that competition drives distribution patterns and spatial separation of a marine predator community, and highlight that competitive advantages may not be constant but rather dependent on habitats.

Predation risk influences feeding rates but competition structures space use for a common Pacific parrotfish
In terrestrial systems it is well known that the spatial patterns of grazing by herbivores can influence the structure of primary producer communities. On coral reefs, the consequences of varied space use by herbivores on benthic community structure are not well understood, nor are the relative influences of bottom-up (resource abundance and quality), horizontal (competition), and top-down (predation risk) factors in affecting spatial foraging behaviors of mobile herbivorous fishes. In the current study we quantified space use and feeding rates of the parrotfish, Chlorurus spilurus, across a strong gradient of food resources and predator and competitor abundance across two islands with drastically different fisheries management schemes. We found evidence that while feeding rates of this species are affected by direct interference competition and chronic predation risk, space use appears to be primarily related to exploitative competition with the surrounding herbivore community. We found no evidence that predation risk influences diurnal foraging space use in this small bodied parrotfish species. Additionally, we found the influence of chronic predation risk on feeding rates of this species to be less dramatic than the results of recent studies that used model predators to measure acute behavioral responses of other species of herbivorous fishes. Our results indicate that the non-consumptive effects of predators on the foraging behaviors of coral reef herbivores may be less dramatic than previously thought.

Papastamatiou YP, Bodey TW, Friedlander AM, Lowe CG, Bradley D, Weng K, Priestley V, Caselle JC. 2017. Spatial separation without territoriality in shark communitiesOikos. doi: 10.1111/oik.04289

Davis K, Carlson PM, Bradley D, Warner RR, Caselle JE. 2017. Predation risk influences feeding rates but competition structures space use for a common Pacific parrotfish. Oecologia. doi:10.1007/s00442-017-3857-9