Marine subsidies, spatial heterogeneity, and territoriality drive trophic ecology in desert predators

Chloé Warret Rodrigues , Ingrid Wiesel, Christine M Drea, Jane M Waterman, James D. Roth

1. Allochthonous (including marine) subsidies can have far-reaching effects on recipient ecosystems. By altering local resource availability, the combined effects of these subsidies and landscape heterogeneity can structure consumer diets, shape communities and influence ecosystem dynamics.
2. We examined how marine subsidies, landscape heterogeneity, and territoriality drive resource use in desert carnivorans. Specifically, we used stable isotope analysis to determine how patchily distributed colonies of Cape fur seals (Arctocephalus pusillus pusillus) along the coastal Namib Desert influence the diet and dietary niches of territorial and variably social brown hyenas (Parahyaena brunnea) and black-backed jackals (Lupulella mesomelas), which can forego territoriality around rich resource clumps.
3. Dietary niches of brown hyenas expanded with greater distance to the coast, while seal consumption was highest at colonies (64% [50–75]), slightly lower at coastal sites (50% [33–65]) despite their proximity to colonies, and lowest inland (11% [2–26]). Heterogeneity in marine resource distribution thus played a strong role in driving diet at fine spatial scale, suggesting a departure from the resource dispersion hypothesis. By contrast, all studied black-backed jackals resided within travel distance of a seal colony, and exhibited a particularly narrow dietary niche, showing extreme reliance on Cape fur seals (75% [69 - 81]), consistent with a territoriality breakdown in response to abundant, patchy food.
4. The diet and dietary niche variation of brown hyenas likely reflected the effects of landscape heterogeneity, territoriality, opportunistic foraging, and scarcity of terrestrial resources. Consistent with optimal foraging theory, the abundance of seals largely subsidized both terrestrial species, precluding exploitation competition, but offering context for aggression due to higher encounter rates at seal colonies.
5. By integrating landscape and trophic ecology, our study underscores the crucial role of marine subsidies in shaping terrestrial communities and provides new insight into understanding variation in the realized niche of carnivorans.