Spatial separation without territoriality in shark communities

Abstract

Spatial separation within predator communities can arise via territoriality but also from competitive interactions among 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 helps drive distribution patterns and spatial separation of a marine predator community, and highlight that competitive advantages may not be constant but rather dependent on habitats.

Mobile marine predators: an understudied source of nutrients to coral reefs in an unfished atoll

“…Moreover, it has recently been shown that grey reef sharks demonstrate strong residency within specific areas at the sub-habitat level [27]. In addition, grey reef sharks demonstrate vertical movement [27,40]; thus, the predators may be transporting nutrients vertically within habitats also [9]. Some routes and receiver locations along which the largest inputs of N were estimated to have occurred by the tagged sharks crossed between reef zones, demonstrating the potential for grey reef sharks to contribute to nutrient translocation.…”

“…While the precise effects of this nutrient subsidy on Palmyra's benthic communities remain to be explored, changes in grey reef shark population size will probably lead to disruptions in nutrient transport dynamics on this typical, nutrient-limited coral reef. Interspecific interactions between grey reef sharks and blacktip reef sharks may also alter nutrient dynamics, due to strong spatial partitioning between the two species [27]. Removal or reduction of one species may change the degree of among-habitat movements by the other, potentially altering nutrient deposition.…”

“…The grey reef shark population at Palmyra is probably at carrying capacity [24], and may play a significant role in the transportation and flow of nutrients onto the reef and throughout the atoll. Grey reef sharks are often detected on (and suggested to favour) outer-reef slopes and drop-off habitats, but on occasion are detected within lagoons [25][26][27]. Previous research has also highlighted sexual segregation in some grey reef shark populations, which suggests that males and females may connect habitats differently and thus transfer nutrients in differing quantities [25].…”

Dermal denticle assemblages in coral reef sediments correlate with conventional shark surveys

“…For example, denticle accumulation rates were highest in the East and West Lagoons, where C. melanopterus shows strong site fidelity and small home ranges (Papastamatiou, Friedlander, Caselle, & Lowe, ; Papastamatiou, Lowe, Caselle, & Friedlander, ). In contrast, C. melanopterus occasionally transits through the backreef and Center Lagoon (Papastamatiou et al, , ; Papastamatiou, Caselle, et al, ), where shark abundances and denticle accumulation rates were lower. The denticle record therefore could be recording long‐term spatial variation in the abundance and habitat use of this species.…”

“…Shark density at the atoll is high, spatially heterogeneous and has remained relatively stable over the past decade (Bradley, Conklin, et al, ). Blacktip reef sharks Carcharhinus melanopterus are the most common shark species in the lagoon and backreef habitats (Papastamatiou et al, ; Papastamatiou, Caselle, Friedlander, & Lowe, ), followed by grey reef sharks Carcharhinus amblyrhynchos and whitetip reef sharks Triaenodon obesus . However, at least 15 other shark species have been documented by deep reef cameras or shallow surveys (Mundy et al, ; Papastamatiou et al, ; Y. P. Papastamatiou & J. G. Eurich, pers.…”