Niche partitioning and individual specialisation in resources and space use of sympatric fur seals at their range margin

Abstract Ecological theory predicts niche partitioning between high level predators living in sympatry as a strategy to minimise the selective pressure of competition. Accordingly, male Australian fur seals Arctocephalus pusillus doriferus and New Zealand fur seals A. forsteri that live in sympatry should partition their broad niches (in habitat and trophic dimensions) in order to coexist. However, at the northern end of their distributions in Australia both are recolonising their historic range after a long absence due to over-exploitation, and their small population sizes suggest competition should be weak and allow overlap in niche space. We found some niche overlap, yet clear partitioning in diet trophic level (δ 15 N values from vibrissae), movement space (horizontal and vertical telemetry data) and circadian activity patterns (timing of dives) between males of each species, suggesting competition remained an active driver of niche partitioning among individuals in these small, peripheral populations. Consistent with individual specialisation theory, broad niches of populations were associated with high levels of individual specialisation for both species, despite putative low competition. Specialists in isotopic space were not necessarily specialists in movement space, further emphasising their diverse individual strategies for niche partitioning. Males of each species displayed distinct foraging modes, with Australian fur seals primarily benthic and New Zealand fur seals primarily epipelagic, though unexpectedly high individual specialisation for New Zealand fur seals might suggest marginal populations provide exceptions to the pattern generally observed among other fur seals.