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The mode and tempo of forest compositional change during periods of rapid climate change, including the potential for the fire regime to produce nonlinear relationships between climate and vegetation, is a long-standing theme of forest ecological research. In the old conifer forests of the coastal Pacific Northwest, fire disturbances are sufficiently rare that their relation to climate and their ecological effects are poorly understood. We used a 14-700-year high-resolution sediment record from Yahoo Lake on the Olympic Peninsula, Washington, USA, to examine vegetation (landscape vegetation from pollen and local vegetation from macrofossils) and fire (landscape fire from total charcoal and local fire from charcoal peaks) in conjunction with independent records of climate. We hypothesized that the successional stage of the local forest will exhibit alternate stable states over a range of fire activity, that species turnover will increase abruptly above a certain level of fire activity and that both responses would be more gradual at the landscape scale than the local scale. Supporting these hypotheses, at the local scale, we found strong evidence for alternate stable states of late vs. early successional communities and inertia of species turnover to changing fire activity. At the landscape scale, vegetation responded more gradually to changing fire activity. From 14-700 to 7000 years ago, high landscape vegetation turnover occurred along with high landscape fire activity, especially during the warm summers of the early Holocene. In several instances, local species turned over completely following fire events but several centuries after climate change. In contrast, during the last 7000 years, the local forest composition was dominated by late-successional species with little species turnover, despite periods of moderate fire activity. We suggest that the relatively minor climate fluctuations of the past 7000 years were not sufficient to cause large-scale species turnover after fire. The Yahoo Lake fire and vegetation record of the early Holocene provides a model for dramatic ecosystem change following an anticipated shift to warmer summer temperatures. [ABSTRACT FROM AUTHOR]

Marine benthic invertebrates living in dense, intraspecific aggregations are important community members because they provide structural habitat for other species. Here, we determined the mechanisms that facilitate gregarious larval settlement and promote group living. Using suspension-feeding oysters (Crassostrea gigas) residing in large assemblages (‘reefs’), experiments were conducted under laboratory conditions that simulated critical aspects of natural estuarine habitats. Oyster larvae were attracted to the scent of their conspecific elders. In still-water trials, they moved downward and settled after contacting a waterborne, adult chemical cue. Yet, mortality of larvae placed in the adult pallial cavity was very high (mean of 91.3%). This seeming paradox of larval attraction to adult cannibals was resolved via laboratory flume (2 cm/s and 6 cm/s flows) experiments. Suspension-feeding activity did not significantly affect flow speeds or directions. Moreover, weak (mean of 1.65 mm/s) adult ciliary currents effectively entrained phytoplankton but rarely captured larvae. In fact, only a small percentage (≤4.6%) of settlers was cannibalized in flume trials, even when they passed within 1 mm of the inhalant opening, or ‘gape’ (a narrow slit between two valves). Larvae cued by conspecifics potentially attach to any portion of the shell surface, but there is a low probability that they will land in or near the inhalant opening. On juvenile and adult oysters, for example, the mean ratio of gape to shell surface area was only 0.025. Furthermore, in surveys of juvenile/adult oysters at nine field sites (Hood Canal and eastern Olympic Peninsula, Washington, USA), the gape was ≤5.2% of the total plane surface area of the reef. Thus, an oyster larva settling onto a reef of suspension-feeding adults is unlikely to be cannibalized. Given this low mortality risk at settlement, future fitness payoffs (e.g., improved fertilization success) may drive the evolution of a gregarious settlement cue that promotes group living. [ABSTRACT FROM AUTHOR]