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ABSTRACT: The largest uncertainty in forecasting the effects of climate change on eco-systems is in understanding how it will affect the nature of interactions among species. Climate change may have unexpected consequences because different species show unique responses to changes in environmental temperatures. Here we show that increasingly warmer springs since 1962 have disrupted the trophic linkages between phytoplankton and zoo-plankton in a large temperate lake because of differing sensitivity to vernal warming. The timing of thermal stratification and the spring diatom bloom have advanced by more than 20 days during this time period. A long-term decline in Daphnia populations, the keystone herbivore, is associated with an expanding temporal mismatch with the spring diatom bloom and may have severe consequences for resource flow to upper trophic levels.
Reports Ecology. 01/2100; 85:2100-2106.
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Lauren A Rogers, Daniel E Schindler,
Peter J Lisi,
Gordon W Holtgrieve,
Peter R Leavitt,
Lynda Bunting,
Bruce P Finney,
Daniel T Selbie,
Guangjie Chen,
Irene Gregory-Eaves,
Mark J Lisac,
Patrick B Walsh
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ABSTRACT: 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.
Proceedings of the National Academy of Sciences 01/2013; · 9.68 Impact Factor
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ABSTRACT: Connectivity among diverse habitats can buffer populations from adverse environmental conditions, influence the functioning of meta-ecosystems, and ultimately affect the reliability of ecosystem services. This stabilizing effect on populations is proposed to derive from complementarity in growth and survival conditions experienced by individuals in the different habitats that comprise meta-ecosystems. Here we use the fine scale differentiation of salmon populations between diverse lake habitats to assess how rearing habitat and stock of origin affect the body condition of juvenile sockeye salmon. We use genetic markers (single nucleotide polymorphisms) to assign individuals of unknown origin to stock group and in turn characterize ecologically relevant attributes across habitats and stocks. Our analyses show that the body condition of juvenile salmon is related to the productivity of alternative habitats across the watershed, irrespective of their stock of origin. Emigrants and residents with genetic origins in the high productivity lake were also differentiated by their body condition, poor and high respectively. These emigrants represented a substantial proportion of juvenile sockeye salmon rearing in the lower productivity lake habitat. Despite emigrants originating from the more productive lake, they did not differ in body condition from the individuals spawned in the lower productivity, recipient habitat. Genetic tools allowed us to assess the performance of different stocks groups across the diverse habitats comprising their meta-ecosystem. The ability to characterize the ecological consequences of meta-ecosystem connectivity can help develop strategies to protect and restore ecosystems and the services they provide to humans.
PLoS ONE 01/2013; 8(3):e58584. · 4.09 Impact Factor
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ABSTRACT: When resources are spatially and temporally variable, consumers can increase their foraging success by moving to track ephemeral feeding opportunities as these shift across the landscape; the best examples derive from herbivore-plant systems, where grazers migrate to capitalize on the seasonal waves of vegetation growth. We evaluated whether analogous processes occur in watersheds supporting spawning sockeye salmon (Oncorhynchus nerka), asking whether seasonal activities of predators and scavengers shift spatial distributions to capitalize on asynchronous spawning among populations of salmon. Both glaucous-winged gulls and coastal brown bears showed distinct shifts in their spatial distributions over the course of the summer, reflecting the shifting distribution of spawning sockeye salmon, which was associated with variation in water temperature among spawning sites. By tracking the spatial and temporal variation in the phenology of their principal prey, consumers substantially extended their foraging opportunity on a superabundant, yet locally ephemeral, resource. Ecosystem-based fishery management efforts that seek to balance trade-offs between fisheries and ecosystem processes supported by salmon should, therefore, assess the importance of life-history variation, particularly in phenological traits, for maintaining important ecosystem functions, such as providing marine-derived resources for terrestrial predators and scavengers.
Biology letters 01/2013; 9(3):20130048. · 3.76 Impact Factor
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ABSTRACT: Many fish species disperse broadly during juvenile life history stages. While this may enable persistence in variable environments, it may also produce novel competitive interactions in recipient habitats that contain conspecifics from other populations. Here we used genetics techniques to study the stock-specific movement and performance of juvenile sockeye salmon (Oncorhynchus nerka) between July and August of 2008 in an ecosystem characterized by extensive juvenile migration and environmental change: the Chignik Lake system, Alaska. Genetic composition of juvenile sockeye salmon in the lower nursery lake based on 45 single nucleotide polymorphism markers indicated that 2008 was characterized by earlier timing and larger magnitude of emigrations from the upper lake, where rearing conditions have become increasingly unstable in recent decades. However, the larger size of emigrants did not confer a clear advantage in foraging based on comparisons of growth and body condition with juveniles native to the lower lake. These results highlight how shifting environmental conditions may exert pressures on evolved behavior patterns and increase interactions between sympatric populations, a theme of increasing impor-tance where ecological uncertainty is high. Résumé : De nombreuses espèces de poissons connaissent une vaste dispersion durant les stades juvéniles de leur cycle biologique. Si ce phénomène peut leur permettre de persister dans des milieux variables, il peut également engendrer de nouvelles interactions concurrentielles dans les habitats récepteurs contenant des individus conspécifiques issus d'autres populations. Nous avons utilisé des techniques génétiques pour étudier les déplacements et la performance de stocks donnés de saumons sockeye (Oncorhynchus nerka) juvéniles de juillet à août 2008 dans un écosystème caractérisé par une vaste migration de juvéniles et d'importantes modifications du milieu, à savoir le système du lac Chignik, en Alaska. La composition génétique des saumons sockeye juvéniles dans le lac de séjour inférieur établie à la lumière de 45 marqueurs de polymorphisme mononucléo-tidique indique que l'année 2008 était caractérisée pour des émigrations plus précoces et de plus grande ampleur en provenance du lac supérieur, où les conditions d'alevinage sont devenues de plus en plus instables au cours des dernières décennies. Toutefois, des comparaisons de la croissance et de l'embonpoint des émigrants et des juvéniles natifs du lac inférieur indiquent que la grande taille des émigrants ne leur conférait pas d'avantage clair sur le plan de la quête de nourriture. Ces résultats illustrent comment la modification des conditions ambiantes peut exercer des pressions sur les patrons de comportements évolués et accroître les interactions entre populations sympatriques, un sujet d'importance croissante pour les milieux carac-térisés par une forte incertitude écologique. [Traduit par la Rédaction]
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ABSTRACT: Residential development of lakeshores affects the structure and function of riparian and littoral habitats. Organic detritus
in sediments is a critical component of littoral food webs, but the effects of urbanization on sediment characteristics are
unexplored. We characterized the quantity of organic sediments in Pacific Northwest lakes along a development gradient and
found a 10-fold decline in the proportion of detritus in littoral sediments associated with density of lakeshore dwellings.
In a comparison between two fully developed lakes and two undeveloped reference lakes, we examined several possible controls
on sedimentary organic content, including terrestrial inputs, decomposition rates and associated macroinvertebrate communities,
and physical retention by coarse wood. The littoral sediments of undeveloped lakes ranged from 34 to 77% organic by mass,
whereas the range on urban lakes was an order of magnitude less, ranging from 1 to 3% organic. We found no significant differences
in terrestrial litter inputs between the two sets of lakes. Leaf litter decomposition rates did not vary significantly between
the two sets of lakes, and we found higher densities of shredder macroinvertebrate taxa in the littoral zones of undeveloped
lakes. Sedimentary organic matter on undeveloped lakes accumulated in shallow waters and declined with distance from shore,
whereas the opposite pattern existed on urban lakes. Our results suggest that coarse wood physically retains organic matter
in littoral zones where it can enter the detrital energy pathway, and the loss of this feature on urban lakes alters littoral
sediment characteristics, with potentially far-reaching consequences for lake food webs.
Ecosystems 04/2012; 10(7):1057-1068. · 3.49 Impact Factor
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ABSTRACT: Two important themes in ecology include the understanding of how interactions among species control ecosystem processes, and
how habitats can be connected through transfers of nutrients and energy by mobile organisms. An impressive example of both
is the large influx of nutrients and organic matter that anadromous salmon supply to inland aquatic and terrestrial ecosystems
and the role of predation by brown bears (Ursus arctos) in transferring these marine-derived nutrients (MDN) from freshwater to riparian habitats. In spite of the recognition that
salmon-bear interactions likely play an important role in controlling the flux of MDN from aquatic to riparian habitats, few
studies have linked bear predation on salmon to processes such as nitrogen (N) or carbon (C) cycling. We combine landscape-level
survey data and a replicated bear-exclosure experiment to test how bear foraging on salmon affects nitrous oxide (N2O) flux, carbon dioxide (CO2) flux, and nutrient concentrations of riparian soils. Our results show that bears feeding on salmon increased soil ammonium
(NH4
+) concentrations three-fold and N2O flux by 32-fold. Soil CO2 flux, nitrate (NO3
−), and N transformation differences were negligible in areas where bears fed on salmon. Reference areas without concentrated
bear activity showed no detectable change in soil N cycling after the arrival of salmon to streams. Exclosure experiments
showed that bear effects on soil nutrient cycles were transient, and soil N processing returned to background conditions within
1year after bears were removed from the system. These results suggest that recipient ecosystems do not show uniform responses
to MDN inputs and highlight the importance of large mobile consumers in generating landscape heterogeneity in nutrient cycles.
Ecological Research 04/2012; 24(5):1125-1135. · 1.57 Impact Factor
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Gordon W Holtgrieve, Daniel E Schindler,
William O Hobbs,
Peter R Leavitt,
Eric J Ward,
Lynda Bunting,
Guangjie Chen,
Bruce P Finney,
Irene Gregory-Eaves,
Sofia Holmgren,
Mark J Lisac,
Peter J Lisi,
Koren Nydick,
Lauren A Rogers,
Jasmine E Saros,
Daniel T Selbie,
Mark D Shapley,
Patrick B Walsh,
Alexander P Wolfe
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ABSTRACT: Humans have more than doubled the amount of reactive nitrogen (Nr) added to the biosphere, yet most of what is known about its accumulation and ecological effects is derived from studies of heavily populated regions. Nitrogen (N) stable isotope ratios ((15)N:(14)N) in dated sediments from 25 remote Northern Hemisphere lakes show a coherent signal of an isotopically distinct source of N to ecosystems beginning in 1895 ± 10 years (±1 standard deviation). Initial shifts in N isotope composition recorded in lake sediments coincide with anthropogenic CO(2) emissions but accelerate with widespread industrial Nr production during the past half century. Although current atmospheric Nr deposition rates in remote regions are relatively low, anthropogenic N has probably influenced watershed N budgets across the Northern Hemisphere for over a century.
Science 12/2011; 334(6062):1545-8. · 31.20 Impact Factor
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ABSTRACT: Habitat heterogeneity can generate intraspecific diversity through local adaptation of populations. While it is becoming increasingly clear that population diversity can increase stability in species abundance, less is known about how population diversity can benefit consumers that can integrate across population diversity in their prey. Here we demonstrate cascading effects of thermal heterogeneity on trout-salmon interactions in streams where rainbow trout rely heavily on the seasonal availability of anadromous salmon eggs. Water temperature in an Alaskan stream varied spatially from 5 degrees C to 17.5 degrees C, and spawning sockeye salmon showed population differentiation associated with this thermal heterogeneity. Individuals that spawned early in cool regions of the 5 km long stream were genetically differentiated from those spawning in warmer regions later in the season. Sockeye salmon spawning generates a pulsed resource subsidy that supports the majority of seasonal growth in stream-dwelling rainbow trout. The spatial and temporal structuring of sockeye salmon spawn timing in our focal stream extended the duration of the pulsed subsidy compared to a thermally homogeneous stream with a single population of salmon. Further, rainbow trout adopted movement strategies that exploited the multiple pulses of egg subsidies in the thermally heterogeneous stream. Fish that moved to track the resource pulse grew at rates about 2.5 times higher than those that remained stationary or trout in the reference stream with a single seasonal pulse of eggs. Our results demonstrate that habitat heterogeneity can have important effects on the population diversity of dominant species, and in turn, influence their value to species that prey upon them. Therefore, habitat homogenization may have farther-reaching ecological effects than previously considered.
Ecology 11/2011; 92(11):2073-84. · 4.85 Impact Factor
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ABSTRACT: A central challenge for predators is achieving positive energy balance when prey are spatially and temporally heterogeneous. Ecological heterogeneity produces evolutionary trade-offs in the physiological design of predators; this is because the ability to capitalize on pulses of food abundance requires high capacity for food-processing, yet maintaining such capacity imposes energetic costs that are taxing during periods of food scarcity. Recent advances in physiology show that when variation in foraging opportunities is predictable, animals may adjust energetic trade-offs by rapidly modulating their digestive system to track variation in foraging opportunities. However, it is increasingly recognized that foraging opportunities for animals are unpredictable, which should favour animals that maintain a capacity for food-processing that exceeds average levels of consumption (loads). Despite this basic principle of quantitative evolutionary design, estimates of digestive load:capacity ratios in wild animals are virtually non-existent. Here we provide an extensive assessment of load:capacity ratios for the digestive systems of predators in the wild, compiling 639 estimates across 38 species of fish. We found that piscine predators typically maintain the physiological capacity to feed at daily rates 2-3 times higher than what they experience on average. A numerical simulation of the trade-off between food-processing capacity and metabolic cost suggests that the observed level of physiological opportunism is profitable only if predator-prey encounters, and thus predator energy budgets, are far more variable in nature than currently assumed.
Nature 08/2011; 476(7358):84-7. · 36.28 Impact Factor
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Transactions of the American Fisheries Society 05/2011; 140(3):763-782. · 1.59 Impact Factor
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ABSTRACT: Migratory animals, such as Pacific salmon, can significantly shape communities in recipient habitats both by altering the flux of resources, and changing community composition and subsequent trophic interactions. Here we mainly used paleoecological records from natural sockeye salmon nursery lakes to quantify the response of plankton communities to the influx of salmon-derived nutrients and consumers (juvenile salmon). Our long-term data show that increases in the density of spawning salmon often elevated influx of nutrients, and, in turn, zooplankton production over the past few centuries. In contrast, significant correlations were not detected in two lakes with extremely low or high average spawner densities (i.e. 1.5 and 34.7 × 103 spawners km−2 year−1 respectively). With increasing spawner densities across lakes, analysis of the size structure of subfossils in sediments revealed a strong decrease in body size of a main juvenile salmon prey item (Eubosmina longispina; r2= 0.36, p < 0.001, n = 67), consistent with an overriding effect of predation in lakes with high salmon densities. These long-term data not only highlight the key role of salmon-derived nutrients in stimulating plankton communities, but also suggest that the relative effect of nutrient and consumer subsidies varies along gradients of lake production, despite a single ultimate causal mechanism (migrating fish).
Oikos 04/2011; 120(9):1317 - 1326. · 3.06 Impact Factor
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ABSTRACT: Ecological studies relating population parameters to climate conditions are limited by a lack of experimental control systems and rely instead on correlative evidence to draw inferences about how populations respond to environmental forcing. Consequently, some correlations turn out to be spurious and not ecologically meaningful. To strengthen inferences, multiple populations may be examined simultaneously to confirm whether relationships can be generalized across multiple systems; however, this assumes that populations respond similarly to climate drivers, ignoring the potential for ecological complexity. Using data on eight sockeye salmon populations from southwestern Alaska, we constructed a series of models based on ecological hypotheses, relating salmon population productivity to climate factors experienced at different life stages. We modeled populations at a range of organizational scales, from distinct populations, to populations grouped by common nursery lake, to all populations within a watershed, and determined the relative statistical support for climate drivers at each scale. In general, warmer lake and sea surface temperatures in the summer coincided with increased productivity of these populations, but the most sensitive life-stage for climate effects varied among populations, particularly among nursery lakes. The best model when considering all populations together, despite strong statistical support, failed to represent the complexity which became evident when populations were modeled by common nursery lake, or independently. These results emphasize that the most appropriate organizational scale to model salmon stocks will depend on specific management, scientific, or conservation goals.
Global Change Biology 03/2011; 17(8):2546 - 2558. · 6.86 Impact Factor
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ABSTRACT: In coastal areas of the North Pacific Ocean, annual returns of spawning salmon provide a substantial influx of nutrients and organic matter to streams and are generally believed to enhance the productivity of recipient ecosystems. Loss of this subsidy from areas with diminished salmon runs has been hypothesized to limit ecosystem productivity in juvenile salmon rearing habitats (lakes and streams), thereby reinforcing population declines. Using five to seven years of data from an Alaskan stream supporting moderate salmon densities, we show that salmon predictably increased stream water nutrient concentrations, which were on average 190% (nitrogen) and 390% (phosphorus) pre-salmon values, and that primary producers incorporated some of these nutrients into tissues. However, benthic algal biomass declined by an order of magnitude despite increased nutrients. We also measured changes in stream ecosystem metabolic properties, including gross primary productivity (GPP) and ecosystem respiration (ER), from three salmon streams by analyzing diel measurements of oxygen concentrations and stable isotopic ratios (delta O-O2) within a Bayesian statistical model of oxygen dynamics. Our results do not support a shift toward higher primary productivity with the return of salmon, as is expected from a nutrient fertilization mechanism. Rather, net ecosystem metabolism switched from approximately net autotrophic (GPP > or = ER) to a strongly net heterotrophic state (GPP < ER) in response to bioturbation of benthic habitats by salmon. Following the seasonal arrival of salmon, GPP declined to <12% of pre-salmon rates, while ER increased by over threefold. Metabolism by live salmon could not account for the observed increase in ER early in the salmon run, suggesting salmon nutrients and disturbance enhanced in situ heterotrophic respiration. Salmon also changed the physical properties of the stream, increasing air-water gas exchange by nearly 10-fold during peak spawning. We suggest that management efforts to restore salmon ecosystems should consider effects on ecosystem metabolic properties and how salmon disturbance affects the incorporation of marine-derived nutrients into food webs.
Ecology 02/2011; 92(2):373-85. · 4.85 Impact Factor
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ABSTRACT: While the importance of terrestrial linkages to aquatic ecosystems is well appreciated, the degree of terrestrial support of aquatic consumers remains debated. Estimates of terrestrial contributions to lake zooplankton have omitted a key food source, phytoplankton produced below the mixed layer. We used carbon and nitrogen stable isotope data from 25 Pacific Northwest lakes to assess the relative importance of particulate organic matter (POM) from the mixed layer, below the mixed layer and terrestrial detritus to zooplankton. Zooplankton and deep POM were depleted in ¹³C relative to mixed layer POM in lakes that can support deep primary production. A Bayesian stable isotope mixing model estimated that terrestrial detritus contributed <5% to zooplankton production, and confirms the role of lake optical and thermal properties; deep POM accounted for up to 80% of zooplankton production in the clearest lakes. These results suggest terrestrial support of lake zooplankton production is trivial.
Ecology Letters 02/2011; 14(4):364-72. · 17.56 Impact Factor
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ABSTRACT: Climate change affects individual organisms by altering development, physiology, behavior, and fitness, and populations by altering genetic and phenotypic composition, vital rates, and dynamics. We sought to clarify how selection, phenotypic plasticity, and demography are linked in the context of climate change. On the basis of theory and results of recent empirical studies of plants and animals, we believe the ecological and evolutionary issues relevant to population persistence as climate changes are the rate, type, magnitude, and spatial pattern of climate-induced abiotic and biotic change; generation time and life history of the organism; extent and type of phenotypic plasticity; amount and distribution of adaptive genetic variation across space and time; dispersal potential; and size and connectivity of subpopulations. An understanding of limits to plasticity and evolutionary potential across traits, populations, and species and feedbacks between adaptive and demographic responses is lacking. Integrated knowledge of coupled ecological and evolutionary mechanisms will increase understanding of the resilience and probabilities of persistence of populations and species.
Conservation Biology 02/2011; 25(1):56-63. · 4.69 Impact Factor
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ABSTRACT: Evolutionary adaptation affects demographic resilience to climate change but few studies have attempted to project changes in selective pressures or quantify impacts of trait responses on population dynamics and extinction risk. We used a novel individual-based model to explore potential evolutionary changes in migration timing and the consequences for population persistence in sockeye salmon Oncorhynchus nerka in the Fraser River, Canada, under scenarios of future climate warming. Adult sockeye salmon are highly sensitive to increases in water temperature during their arduous upriver migration, raising concerns about the fate of these ecologically, culturally, and commercially important fish in a warmer future. Our results suggest that evolution of upriver migration timing could allow these salmon to avoid increasingly frequent stressful temperatures, with the odds of population persistence increasing in proportion to the trait heritability and phenotypic variance. With a simulated 2°C increase in average summer river temperatures by 2100, adult migration timing from the ocean to the river advanced by ∼10 days when the heritability was 0.5, while the risk of quasi-extinction was only 17% of that faced by populations with zero evolutionary potential (i.e., heritability fixed at zero). The rates of evolution required to maintain persistence under simulated scenarios of moderate to rapid warming are plausible based on estimated heritabilities and rates of microevolution of timing traits in salmon and related species, although further empirical work is required to assess potential genetic and ecophysiological constraints on phenological adaptation. These results highlight the benefits to salmon management of maintaining evolutionary potential within populations, in addition to conserving key habitats and minimizing additional stressors where possible, as a means to build resilience to ongoing climate change. More generally, they demonstrate the importance and feasibility of considering evolutionary processes, in addition to ecology and demography, when projecting population responses to environmental change.
PLoS ONE 01/2011; 6(6):e20380. · 4.09 Impact Factor
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ABSTRACT: Phenotypic plasticity plays a key role in modulating how environmental variation influences population dynamics, but we have only rudimentary understanding of how plasticity interacts with the magnitude and predictability of environmental variation to affect population dynamics and persistence. We developed a stochastic individual-based model, in which phenotypes could respond to a temporally fluctuating environmental cue and fitness depended on the match between the phenotype and a randomly fluctuating trait optimum, to assess the absolute fitness and population dynamic consequences of plasticity under different levels of environmental stochasticity and cue reliability. When cue and optimum were tightly correlated, plasticity buffered absolute fitness from environmental variability, and population size remained high and relatively invariant. In contrast, when this correlation weakened and environmental variability was high, strong plasticity reduced population size, and populations with excessively strong plasticity had substantially greater extinction probability. Given that environments might become more variable and unpredictable in the future owing to anthropogenic influences, reaction norms that evolved under historic selective regimes could imperil populations in novel or changing environmental contexts. We suggest that demographic models (e.g. population viability analyses) would benefit from a more explicit consideration of how phenotypic plasticity influences population responses to environmental change.
Proceedings of the Royal Society B: Biological Sciences 11/2010; 277(1699):3391-400. · 5.41 Impact Factor
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ABSTRACT: Summary1. Pacific salmon are a textbook example of migratory animals that transfer nutrients between ecosystems, but little is known about how salmon-derived nutrients (SDN) affect the biodiversity of recipient freshwater ecosystems. We examined paleolimnological records from six Alaskan lakes to define how changes in SDN from sockeye salmon (Oncorhynchus nerka) influenced sedimentary diatom community structure and beta-diversity among lakes and through time.2. Using an isotopic mixing model, we showed that SDN loading could account for >80% of the lake total nitrogen budgets and strongly regulated diatom community composition. Spatial dissimilarity in diatom communities was positively related to differences in SDN among lakes (r2 = 0.69, P < 0.01, n = 10). Likewise, temporal dissimilarity in diatom communities was positively related to differences in SDN in a sediment core with substantial variation in salmon spawner dynamics between 1700 and 1950 AD (r2 = 0.34, P < 0.01, n = 19). Finally, beta-diversity metrics quantifying temporal turnover within each lake’s sediment record were also positively related to the variance in SDN loading among lakes (r2 = 0.88, P < 0.05, n = 5). Mean SDN was only negatively correlated to temporal diatom beta-diversity.3. Spatially subsidised systems often receive temporally variable resource inputs, and thus, it is not surprising that, unlike previous studies, we found that resource variability was the key driver of community composition and beta-diversity. In habitats that receive strongly fluctuating external nutrient loads, environment heterogeneity may overweigh stochastic community processes. In addition, freshwater diatoms are characterised by great dispersal capabilities and short life cycles and therefore may be a more sensitive indicator for evaluating the role of resource variability than previously used model organisms. These results suggest that productivity–diversity relationship vary with the nature of nutrient loading and the life history of the community studied.4. Overall, our study highlights that the transport of nutrients by sockeye salmon across ecosystem boundaries is a significant driver of algal community and biodiversity in nursery lakes, mainly through changing the magnitude of nutrient variation. As such, freshwater species diversity in regions like the U.S. Pacific Northwest may become impoverished where there have been long-term declines in salmon populations and decreases in nutrient variability among lakes.
Freshwater Biology 09/2010; 56(2):292 - 301. · 3.29 Impact Factor
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ABSTRACT: Fisheries often exert selective pressures through elevated mortality on a nonrandom component of exploited stocks. Selective removal of individuals will alter the composition of a given population, with potential consequences for its size structure, stability and evolution. Gillnets are known to harvest fish according to size. It is not known, however, whether delayed mortality due to disentanglement from gillnets exerts selective pressures that reinforce or counteract harvest selection. We examined gillnet disentanglement in exploited populations of sockeye salmon (Oncorhynchus nerka) in Bristol Bay, Alaska, to characterize the length distribution of fish that disentangle from gillnets and determine whether nonretention mortality reinforces harvest selection and exerts common pressures according to sex and age. We also evaluated discrete spawning populations to determine whether nonretention affects populations with different morphologies in distinct ways. In aggregate, nonretention mortality in fish that disentangle from gillnets counters harvest selection but with different effects by sex and age. At the level of individual spawning populations, nonretention mortality may exert stabilizing, disruptive, or directional selection depending on the size distribution of a given population. Our analyses suggest nonretention mortality exerts significant selective pressures and should be explicitly included in analyses of fishery-induced selection.
Evolutionary Applications 09/2010; 4(3):429 - 443. · 4.92 Impact Factor
