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地行为形成，亦可源于物种间及物种内部的竞争交互。然而，将竞争交互与捕食者分布格局建立关联颇具难度，且理论模型所预测的生境选择模式，会随生境质量及竞争的具体表现形式而有所不同。尽管现有模型通常假定竞争者能力均等，或是某一表型对另一表型具有固定优势，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。我们依托十余年的遥测数据、生境调查与行为实验，揭示了太平洋一处未受捕捞的环礁上两种礁鲨——灰礁鲨（Carcharhinus amblyrhinchos）与黑尾礁鲨（C. melanopterus）——种群内部及种群间的空间分隔现象。在物种内部，礁鲨停留于小型“亚生境”中，个体在亚生境间的移动极少，此前的模型曾提出该现象或由种内竞争所致。黑尾礁鲨的分布范围更广，覆盖更多生境类型，但更多个体使用潟湖与背礁生境；而灰礁鲨则偏好前礁生境。在邻近的一处无黑尾礁鲨分布的环礁中，灰礁鲨的生境分布范围相较于两种鲨共存时更为广泛。一系列基于个体的模型预测，仅当物种间存在生境特异性的竞争互动时，才会产生生境分隔：灰礁鲨在前礁具备竞争优势，而黑尾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，证明竞争驱动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，而是依赖于具体生境。