Marine Subsidies Produce Cactus Forests on Desert Islands

Marine nutrient subsidies can shape terrestrial plant biodiversity. In island systems, nitrogen-rich seabird guano is a large component of such marine subsidies. In zones of nutrient upwelling such as the Gulf of California, copious seabird guano is commonplace on bird islands. Several bird islands host regionally unique cactus forests, especially of the large columnar cactus, cardón (Pachycereus pringlei). We propose that a chain of interactions across the land-sea interface yields an allochthonous input of nitrogen in the form of seabird guano, fueling the production of some of the densest cactus populations in the world. Fish, seabird, guano, soil, and cactus samples were taken from Isla San Pedro Mártir for nitrogen stable isotope ratio measurements, which were compared to soil and cactus samples from other seabird and non-seabird Gulf islands and terrestrial ecosystems throughout the range of the cardón. Isla San Pedro Mártir δ15N values of the food/nutrient cycle are distinctively high, ranging from fish +17.7, seabird +19.7, guano +14.8, soil +34.3 and cactus +30.3. These δ15N values are among the highest ever reported for plants. Seabird island soil and cactus δ15N values were consistently enriched relative to mainland and non-bird islands. Our findings demonstrate that seabird mediated marine nutrient deposits provide the source for solubilized N on desert islands, which stimulate terrestrial plant production in the cardón cactus significantly beyond that seen in either mainland ecosystems or non-seabird islands. These results elucidate the integral nature of nutrient movement across the land-sea interface.

Marine subsidies mediate patterns in avian island biogeography

The classical theory of island biogeography , which predicts species richness using island area and isolation, has been expanded to include contributions from marine subsidies, i.e. subsidized island biogeography (SIB) theory . We tested the effects of marine subsidies on species diversity and population density on productive temperate islands, evaluating SIB predictions previously untested at comparable scales and subsidy levels. We found that the diversity of terrestrial breeding bird communities on 91 small islands (approx. 0.0001–3 km 2 ) along the Central Coast of British Columbia, Canada were correlated most strongly with island area, but also with marine subsidies. Species richness increased and population density decreased with island area, but isolation had no measurable influence. Species richness was negatively correlated with marine subsidy, measured as forest-edge soil δ 15 N. Density, however, was higher on islands with higher marine subsidy, and a negative interaction between area and subsidy indicates that this effect is stronger on smaller islands, offering some support for SIB. Our study emphasizes how subsidies from the sea can shape diversity patterns on islands and can even exceed the importance of isolation in determining species richness and densities of terrestrial biota.

Coyote (Canis latrans) use of marine resources in coastal California: A new behavior relative to their recent ancestors

Coyotes ( Canis latrans) are known to consume marine foods, but the importance and persistence of marine subsidies to coyotes is unknown. Recent access to a marine subsidy, especially if gained following apex predator loss, may facilitate coyote expansion along coastal routes and amplify the effects of mesopredator release. Our goal was to quantify and contextualize past and present marine resource use by coyotes on the central coast of California via stable isotope analysis. We measured δ13C and δ15N values in coyotes, their competitors, and their food resources at two modern sites, seven archaeological sites spanning in age from ~3000 to 750 BP, and from historical (AD 1893–1992) coyote and grizzly bear hair and bone sourced from coastal counties. We found evidence for marine resource use by modern coastal California coyotes at one site, Año Nuevo, which hosts a mainland northern elephant seal ( Mirounga angustirostris) breeding colony. Seals and sea lions account for ~20% of Año Nuevo coyote diet throughout the year and this marine subsidy likely positively impacts coyote population size. Isotopic data suggest that neither historic nor prehistoric coyotes consumed marine-derived foods, even at sites near ancient mainland seal rookeries. Marine resource use by some contemporary California coyotes is a novel behavior relative to their recent ancestors. We hypothesize that human alteration of the environment through extirpation of the California grizzly bear and the more recent protection of marine mammals likely enabled this behavioral shift.

Tracking data and retrospective analyses of diet reveal the consequences of loss of marine subsidies for an obligate scavenger, the Andean condor

Over the last century, marine mammals have been dramatically reduced in the world's oceans. We examined evidence that this change caused dietary and foraging pattern shifts of the Andean condor ( Vultur gryphus ) in Patagonia. We hypothesized that, after the decrease in marine mammals and the increase in human use of coastlines, condor diet changed to a more terrestrial diet, which in turn influenced their foraging patterns. We evaluated the diet by means of stable isotope analysis ( δ 13 C, δ 15 N and δ 34 S) of current (last decade) and historical (1841–1933) feathers. We further evaluated the movement patterns of 23 condors using satellite tracking of individuals. Condors reduced their use of marine-derived prey in recent compared with historical times from 33 ± 13% to less than 8 ± 3% respectively; however, they still breed close to the coast. The average distance between the coast and nests was 62.5 km, but some nests were located close to the sea (less than 5 km). Therefore, some birds must travel up to 86 km from nesting sites, crossing over the mountain range to find food. The worldwide reduction in marine mammal carcasses, especially whales, may have major consequences on the foraging ecology of scavengers, as well as on the flux of marine inputs within terrestrial ecosystems.

Utilisation of stranded marine fauna washed ashore on K’gari (Fraser Island), Australia, by dingoes

Stranded marine fauna have been identified as a potentially significant food resource for terrestrial carnivores, but how such subsidisation influences terrestrial species ecology is not well understood. We describe the dietary and behavioural responses of dingoes (Canis familiaris) to the occurrence of large-animal marine strandings (e.g. dead cetaceans, marine turtles and pinnipeds) between 2006 and 2016 on K’gari (Fraser Island), Australia, to better understand the trophic links between marine and terrestrial systems. A total of 309 strandings were recorded during this period (~3.1 strandings per month), yielding an annual average of 30.3 tons of available carrion to the 100–200 dingoes present on the island. Carcass monitoring with camera traps showed that dingoes used carcasses almost daily after a short period of decomposition. Whole packs of up to seven dingoes of all age classes at a time were observed visiting carcasses for multiple successive days. These data demonstrate that large-animal marine subsidies can be a common, substantial and important food source for dingoes, and that the estimated daily dietary needs of roughly 5–10% of the island’s dingo population were supported by this food source. Our data suggest that marine subsidisation can influence terrestrial carnivore diet, behaviour and abundance, which may produce cascading indirect effects for terrestrial ecosystems in contexts where subsidised carnivores interact strongly with other species.

Short-term effects of hunting on naïve black-tailed deer (Odocoileus hemionus sitkensis): behavioural response and consequences on vegetation growth

Hunting is commonly used to reduce overabundant deer populations that are of socioeconomic and ecological concerns. Recently, it has been suggested that “hunting for fear” might help maximize nonconsumptive effects of hunting and their cascading impacts. We investigated how a predator-free black-tailed deer (Odocoileus hemionus sitkensis Merriam, 1898) population introduced to the Haida Gwaii archipelago (Canada) responded to short-term hunting for fear. We conducted a hunt that minimized killing and maximized scaring on an island where some deer were marked. Between control and hunted sites, we compared deer use of bait stations and of shoreline areas (i.e., an open and risky area that provided marine subsidies). We also compared the browsing pressure and growth of two grasses and two shrubs. Hunting for fear had no effect on the overall use of bait stations and shorelines. However, in the area exposed to hunting, unmarked deer, which were assumed to be less tolerant of human disturbance, avoided bait stations at the outset of hunting. We concluded that individuals perceive and respond to risk differently. Furthermore, two of the four plant species were less likely to be browsed and had better growth in the experimental area than in the control area, suggesting an indirect effect of hunting on plants mediated by deer behaviour.