Data from: 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.捕食者群落内的空间分隔既可通过领域行为形成，也可源于物种间与物种内的竞争相互作用。然而，将竞争相互作用与捕食者分布格局关联起来颇具难度，且理论模型预测的生境选择模式会因生境质量以及竞争的表现形式而异。尽管现有模型通常假设竞争者能力均等，或是某一表型（phenotype）对另一表型存在固定优势，但鲜有研究考虑到动物可能仅在特定生境中具备竞争优势。本研究依托十余年的遥测（telemetry）数据、生境调查与行为实验，揭示了未受捕捞的太平洋环礁上两种礁鲨（灰礁鲨，Carcharhinus amblyrhinchos；黑鳍礁鲨，C. melanopterus）的物种间及物种内空间分隔格局。在同一物种内，鲨类仅活动于小型‘亚生境’中，个体在亚生境间的移动极少——此前的模型曾指出，这种现象可能由种内竞争（intra-specific competition）引发。黑鳍礁鲨在各类生境中的分布范围更广，且有更高比例的个体栖息于潟湖与背礁生境，而灰礁鲨则偏好前礁生境。在附近一处无黑鳍礁鲨栖息的环礁中，灰礁鲨在各类生境间的分布范围比两种鲨共存时更广。一系列基于个体的模型（individual-based models）预测，仅当物种间存在生境特异性的竞争相互作用时，才会出现生境分隔：灰礁鲨在前礁生境具备竞争优势，而黑鳍礁鲨则在潟湖与背礁生境中占据优势。本研究提供了有力证据，证明竞争塑造了海洋捕食者群落的分布格局与空间分隔，并指出竞争优势并非恒定不变，而是依赖于生境类型。