How local geomorphic and hydrologic features mediate the sensitivity of stream thermal regimes to variation in climatic conditions remains a critical uncertainty in understanding aquatic ecosystem responses to climate change. We used stable isotopes of hydrogen and oxygen to estimate contributions of snow and rainfall to 80 boreal streams and show that differences in snow contribution are controlled by watershed topography. Time series analysis of stream thermal regimes revealed that streams in rain‐dominated, low‐elevation watersheds were 5–8 times more sensitive to variation in summer air temperature compared to streams draining steeper topography whose flows were dominated by snowmelt. This effect was more pronounced across the landscape in early summer and less distinct in late summer. Thus, the impact of climate warming on freshwater thermal regimes will be spatially heterogeneous across river basins as controlled by geomorphic features. However, thermal heterogeneity may be lost with reduced snowpack and increased ratios of rain to snow in stream discharge.
Observational data from the past century have highlighted the importance of interdecadal modes of variability in fish population dynamics, but how these patterns of variation fit into a broader temporal and spatial context remains largely unknown. We analyzed time series of stable nitrogen isotopes from the sediments of 20 sockeye salmon nursery lakes across western Alaska to characterize temporal and spatial patterns in salmon abundance over the past ∼500 y. Although some stocks varied on interdecadal time scales (30-to 80-y cycles), centennial-scale variation, undetectable in modern-day catch records and survey data, has dominated salmon population dynamics over the past 500 y. Before 1900, variation in abundance was clearly not synchronous among stocks, and the only temporal signal common to lake sediment records from this region was the onset of commercial fishing in the late 1800s. Thus, historical changes in climate did not synchronize stock dynamics over centennial time scales, emphasizing that ecosystem complexity can produce a diversity of ecological responses to regional climate forcing. Our results show that marine fish populations may alternate between naturally driven periods of high and low abundance over time scales of decades to centuries and suggest that management models that assume time-invariant productivity or carrying capacity parameters may be poor representations of the biological reality in these systems.Oncorhynchus nerka | nitrogen stable isotopes | fisheries | paleolimnology L arge fluctuations in abundance are a hallmark of fish stocks (1, 2); fish abundance can fluctuate substantially over interannual to centennial time scales (3-5). Recent short-term variation in stock abundance can be characterized by fisheries catch records and scientific survey data, but to characterize low-frequency variability requires novel ecological approaches (6). Knowing how fish stocks have varied over long time scales can provide an important context for recently observed shifts in abundance, provide insight into the potential effects of climate change, and inform management frameworks (7). In this study, we describe and analyze time series of nitrogen (N) stable isotopes in sediments from 20 lakes as a proxy for the abundance of anadromous sockeye salmon, Oncorhynchus nerka, to quantify how salmon stocks have varied in abundance during the past ∼500 y. This analysis provides a comprehensive synthesis of the natural patterns of variability in salmon abundance before the onset of commercial fishing and encompasses a study region that currently produces over 70% of global sockeye salmon catches (8).Low-frequency variation in fish abundance is often attributed to climatic forcing (2, 9) or harvesting (10, 11). At a broad scale, persistent multidecadal shifts in North Pacific salmon production throughout the 20th century have been linked to the Pacific Decadal Oscillation (PDO), with total salmon production in Alaska high when the PDO is in a warm phase and vice versa for cold phases (12). At finer spatial ...
Brown bear Ursus arctos population density was estimated for a 21,178-km2 study area in southwest Alaska. Estimates were obtained using an aerial line-transect method that allows for peak detection to be both off the transect line and less than 100%. Data collection required five small aircraft with 2-person crews. Surveys were flown in 10-d windows to capture the period after den emergence but prior to full green-up. Surveys were flown in two consecutive years in order to detect sufficient bear groups to support the estimation. The study detected 197 bear groups (330 bears) in 969 aerial transects averaging 24.8 km long and with a strip width of 728 m. Estimated population density in the study area was 40.4 bears/1,000 km2 (95% CI = 31.4–54.5); estimated density of independent bears was 27.3 bears/1,000 km2 (95% CI = 21.4–34.4). Assuming similar estimate precision, repeating the survey could detect population changes of approximately 38% or larger with a power of 80%. We find the method described here suitable for regions of relatively high bear population densities or detection rates.
We investigated mortality among nestling eastern bluebirds (Sialia sialis) in Polk and Highlands counties, Florida (USA) in 1999-2001. At least six species of maggots from three families of muscoid flies, Calliphoridae, Sarcophagidae, and Muscidae were found associated with the nestlings. Philornis porteri, the only species of obligate bird parasite collected, was found in the contents of two nests, in the ear canal and the musculature of the jaw of one nestling, and in the abdominal subcutis of another. This is the first record of bluebird parasitism by P. porteri. Although some nestlings were infested by tissue-invading fly larvae antemortem, the role of these maggots in the overall mortality was not clear.
Habitat alteration and species exploitation are fundamental issues in conservation, yet their interacting effects on food webs are rarely considered. We used a foraging model based on the Wood River basin (Alaska, USA) to explore how watershed development and commercial fisheries affect energy flow from sockeye salmon to brown bears. We found that, where salmon are abundant, fisheries can harvest large fractions of runs without substantially reducing bear consumption of salmon, but that watershed development could strongly reduce bear consumption if it shortens the duration of foraging opportunities by reducing population-level variation in salmon spawn timing. Habitats with the lowest resource abundance (small streams) were particularly profitable for bear foraging because they offer salmon at unique times of the season. This result challenges environmental impact assessments that assume ecological effects respond solely to changes in resource abundance. -based management, fisheries, phenology, predation, trade-off, watershed development This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. K E Y W O R D S ecosystem
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