After 15 years, no evidence for trophic cascades in marine protected areas

In marine ecosystems, fishing often targets predators, which can drive direct and indirect effects on entire food webs. Marine reserves can induce trophic cascades by increasing predator density and body size, thereby increasing predation pressure on populations of herbivores, such as sea urchins. In California's northern Channel Islands, two species of sea urchins are abundant: the red urchin Mesocentrotus franciscanus , which is targeted by an economically valuable fishery, and the virtually unfished purple urchin Strongylocentrotus purpuratus . We hypothesized that urchin populations inside marine reserves would be depressed by higher predation, but that red urchins would be less affected due to fishing outside reserves. Instead, our analyses revealed that purple urchin populations were unaffected by reserves, and red urchin biomass significantly increased in response to protection. Therefore, urchin biomass overall has increased inside reserves, and we found no evidence that giant kelp is positively affected by reserves. Our results reveal the overwhelming direct effect of protecting fished species in marine reserves over indirect effects that are often predicted but seldom clearly documented. Indirect effects due to marine reserves may eventually occur in some cases, but very effective predators, large reserves or extended time periods may be needed to induce them.

Fishing, Subsistence Change, and Foraging Strategies on Western Santa Rosa Island, California

Shifting from shellfish collecting to fishing as a primary coastal foraging strategy can allow hunter-gatherers to obtain more food and settle in larger populations. On California's northern Channel Islands (NCI), after the development of the single-piece shell fishhook around 2500 cal BP, diet expanded from primarily shellfish to include nearshore fishes in greater numbers. During the Medieval Climatic Anomaly (1150–600 cal BP), settlement on the islands condensed to a small number of large coastal villages with high population densities supported largely by nearshore fish species including rockfishes, surfperches, and señoritas. Faunal data from five sites on western Santa Rosa Island (CA-SRI-15, -31, -97, -313, and -333) demonstrate an increase in nearshore fishing through time. We argue that demographic changes that occurred on the northern Channel Islands were accompanied by changes in subsistence strategies that were related in part to risk of failure when attempting to acquire different resources. As population density increased, the low-risk strategy of shellfish harvesting declined in relative importance as a higher-risk strategy of nearshore fishing increased. While multiple simultaneous subsistence strategies are frequently noted among individual hunter-gatherer communities in the ethnographic record, this study provides a framework to observe similar patterns in the archaeological record.

Submerged paleoshoreline mapping using high-resolution Chirp sub-bottom data, Northern Channel Islands platform, California, USA

High-resolution Chirp sub-bottom data were obtained offshore from the Northern Channel Islands (NCI), California, to image submerged paleoshorelines and assess local uplift rates. Although modern bathymetry is often used for modeling paleoshorelines, Chirp data image paleoshorelines buried beneath sediment that obscures their seafloor expression. The NCI were a unified landmass during the last glacial maximum (LGM; ~20 ka), when eustatic sea level was ~120 m lower than present. We identified seven paleoshorelines, ranging from ~28 to 104 m in depth, across this now-submerged LGM platform. Paleoshoreline depths were compared to local sea-level curves to estimate ages, which suggest that some were reoccupied over multiple sea-level cycles. Additionally, previous studies determined conflicting uplift rates for the NCI, ranging from 0.16 to 1.5 m/ka. Our results suggest that a rate on the lower end of this range better fits the observed submerged paleoshorelines. Using the uplift rate of ~0.16 m/ka, we estimate that paleoshorelines formed during Marine Oxygen Isotope Stage 3, the LGM, and the Younger Dryas stade are preserved on the NCI platform. These results help clarify uplift rates for the NCI and illustrate the importance of sub-bottom data for mapping submerged paleoshorelines.

Differences in Human Population Density between the California Islands and the Coastal Mainland

If the California Islands were marginal environments for the indigenous people who occupied them, human population densities on the islands should be significantly lower than the adjacent mainland coast. Reconstructing population densities at particular times in the past is fraught with methodological difficulties, but data from site sizes and densities, radiocarbon date distributions, and mission records give no indication that island population densities were significantly lower than along the coastal mainland. Population density measures provide little evidence of environmental marginality on the Northern Channel Islands. Human population densities on islands further south may have been lower than the northern islands, but do not appear to have been significantly different than the adjacent mainland coast.

Paleocoastal Landscapes, Marginality, and Early Human Settlement of the California Islands

If the California Islands were marginal for human settlement, why were several of them occupied more or less continuously since Terminal Pleistocene or Early Holocene times? The earliest human history of California's Islands is clouded by sea level rise, coastal erosion, dune building, and differential research intensity. Nonetheless, Paleocoastal sites are abundant on the Northern Channel Islands and Cedros Island, suggesting that they were optimal habitat for early hunter-gatherers, with ample food, freshwater, mineral, and other resources to sustain permanent settlement. Worldwide on islands where late Pleistocene or early Holocene human colonization occurred, climate shifts and massive landscape changes caused by postglacial sea level rise require detailed reconstructions of paleogeography and paleoecology to assess the potential productivity or marginality of islands or archipelagos.