Idiosyncratic species effects confound size-based predictions of responses to climate change

Understanding and predicting the consequences of warming for complex ecosystems and indeed individual species remains a major ecological challenge. Here, we investigated the effect of increased seawater temperatures on the metabolic and consumption rates of five distinct marine species. The experimental species reflected different trophic positions within a typical benthic East Atlantic food web, and included a herbivorous gastropod, a scavenging decapod, a predatory echinoderm, a decapod and a benthic-feeding fish. We examined the metabolism–body mass and consumption–body mass scaling for each species, and assessed changes in their consumption efficiencies. Our results indicate that body mass and temperature effects on metabolism were inconsistent across species and that some species were unable to meet metabolic demand at higher temperatures, thus highlighting the vulnerability of individual species to warming. While body size explains a large proportion of the variation in species'' physiological responses to warming, it is clear that idiosyncratic species responses, irrespective of body size, complicate predictions of population and ecosystem level response to future scenarios of climate change.     

Variation in responses to spawning Pacific salmon among three south-eastern Alaska streams

1. Pacific salmon are thought to stimulate the productivity of the fresh waters in which they spawn by fertilising them with marine‐derived nutrients (MDN). We compared the influence of salmon spawners on surface streamwater chemistry and benthic biota among three south‐eastern Alaska streams. Within each stream, reaches up‐ and downstream of barriers to salmon migration were sampled during or soon after spawners entered the streams. 2. Within streams, concentrations of dissolved ammonium and soluble reactive phosphorus (SRP), abundance of epilithon (chlorophyll a and ash‐free dry mass) and biomass of chironomids were significantly higher in reaches with salmon spawners. In contrast, biomass of the mayflies Epeorus spp. and Rhithrogena spp. was significantly higher in reaches lacking spawners. 3. Among streams, significant differences were found in concentrations of dissolved ammonium, dissolved organic carbon, nitrate and SRP, abundance of epilithon, and the biomass of chironomids and Rhithrogena. These differences did not appear to reflect differences among streams in spawner density, nor the changes in water chemistry resulting from salmon spawners. 4. Our results suggest that the ‘enrichment’ effect of salmon spawners (e.g. increased streamwater nutrient concentrations) was balanced by other concurrent effects of spawners on streams (e.g. sediment disturbance). Furthermore, the collective effect of spawners on lotic ecosystems is likely to be constrained by conditions unique to individual streams, such as temperature, background water chemistry and light attenuation.     

Evolutionary responses by native species to major anthropogenic changes to their ecosystems: Pacific salmon in the Columbia River hydropower system

The human footprint is now large in all the Earth's ecosystems, and construction of large dams in major river basins is among the anthropogenic changes that have had the most profound ecological consequences, particularly for migratory fishes. In the Columbia River basin of the western USA, considerable effort has been directed toward evaluating demographic effects of dams, yet little attention has been paid to evolutionary responses of migratory salmon to altered selective regimes. Here we make a first attempt to address this information gap. Transformation of the free-flowing Columbia River into a series of slack-water reservoirs has relaxed selection for adults capable of migrating long distances upstream against strong flows; conditions now favour fish capable of migrating through lakes and finding and navigating fish ladders. Juveniles must now be capable of surviving passage through multiple dams or collection and transportation around the dams. River flow patterns deliver some groups of juvenile salmon to the estuary later than is optimal for ocean survival, but countervailing selective pressures might constrain an evolutionary response toward earlier migration timing. Dams have increased the cost of migration, which reduces energy available for sexual selection and favours a nonmigratory life history. Reservoirs are a benign environment for many non-native species that are competitors with or predators on salmon, and evolutionary responses are likely (but undocumented). More research is needed to tease apart the relative importance of evolutionary vs. plastic responses of salmon to these environmental changes; this research is logistically challenging for species with life histories like Pacific salmon, but results should substantially improve our understanding of key processes. If the Columbia River is ever returned to a quasinatural, free-flowing state, remaining populations might face a Darwinian debt (and temporarily reduced fitness) as they struggle to re-evolve historical adaptations.     

Identifying regional differences in threshold responses of aquatic invertebrates to land cover gradients

Conversion of land from natural to urban or agricultural cover degrades stream ecosystems and results in loss of biodiversity. We compared cumulative frequency distributions to measure responses to land use gradients for aquatic invertebrate taxa to agricultural, urban, and impervious surface cover gradients across the state of Maryland, USA. The technique identifies the upper limit threshold above which taxa cease to occur as well as a lower limit of detection of effect for negatively affected taxa. Urban development and impervious surface cover negatively affected the distributions of 44–56% of the 180 taxa tested, depending on region. Across similar taxa, negative responses occurred at lower levels of urban land covers in the Piedmont compared to the Coastal Plain physiographic province, which suggests that Piedmont aquatic biodiversity may be more vulnerable to urbanization. Most taxa were capable of tolerating high levels of agricultural development, although a number of common taxa in the Coastal Plain and Highlands regions were found to be agriculture-sensitive. Some taxa traditionally used as indicators were tolerant of very high levels of human-altered land uses, suggesting that such taxa require examination prior to use as indicators of landscape stressors. Our analysis method appears to be sufficiently flexible and sensitive to be used for a variety of taxa and systems for stressor detection, ecosystem monitoring, and spatially explicit forecasts of taxa loss as watershed land cover changes.     

Exploring the relationship between land cover and the distribution of water beetle species (Coleoptera) at the regional scale

The advent of remote-sensed satellite land cover data has provided the opportunity to assess the relationship between invertebrate species distributions and individual land cover types. Water beetle species occur in habitats within specific land cover types and the relationship between the distribution of water beetle species and land covers at the regional scale was investigated using records of 154 species from 1018 sites in north-east England. The land covers of tilled land and urban in the lowlands and of shrub heath and heath grassland in the upland areas proved to be most important in explaining the distribution of species. There were both positive and negative associations between some species and other covers such as woodland and the coast. However, a considerable number of species, generally those with a large number of records, showed no strong relationships with any land cover types. The integration of water beetle species recording data and remote-sensed land cover data as a basis for predicting and monitoring both species distribution and environmental change is discussed.     

Frequency responses of age-structured populations: Pacific salmon as an example

Increasing evidence of the effects of changing climate on physical ocean conditions and long-term changes in fish populations adds to the need to understand the effects of stochastic forcing on marine populations. Cohort resonance is of particular interest because it involves selective sensitivity to specific time scales of environmental variability, including that of mean age of reproduction, and, more importantly, very low frequencies (i.e., trends). We present an age-structured model for two Pacific salmon species with environmental variability in survival rate and in individual growth rate, hence spawning age distribution. We use computed frequency response curves and analysis of the linearized dynamics to obtain two main results. First, the frequency response of the population is affected by the life history stage at which variability affects the population; varying growth rate tends to excite periodic resonance in age structure, while varying survival tends to excite low frequency fluctuation with more effect on total population size. Second, decreasing adult survival strengthens the cohort resonance effect at all frequencies, a finding that addresses the question of how fishing and climate change will interact.     

The distribution of juvenile Pacific salmon and associated fish species in Lake Sopochnoye,Iturup Island

A single survey using a minnow seine net showed that juvenile chum and sockeye salmon are distributed extremely unevenly along the shoreline of Lake Sopochnoye, on Iturup Island. Young-of-the-year chum salmon aggregate mainly near the estuaries of the rivers that flow into the lake; young-of-the-year and 2-year-old sockeye salmon aggregate closer to the central part of the lake. The distribution of associated fish species is also very uneven, but none of them dominates within the zone of catches; the abundance of each of these species is comparable to that of the juvenile Pacific salmon.     

Chemosensory responses of juvenile Coho salmon, Oncorhynchus kisutch, Dolly Varden, Salvelinus malma, and sculpins (Cottus spp.) to eggs and other tissues from adult Pacific salmon

The carcasses of semelparous Pacific salmon (Oncorhynchus spp.) provide nutrients that enter aquatic ecosystems by various pathways, including direct consumption of tissue by fishes. Salmonids and other species frequently eat eggs and other tissues from dead salmon but the roles of vision and olfaction are unclear, as is the relative attraction to different tissues. Accordingly, we conducted a series of in situ experiments using minnow traps in two natural streams in Alaska to test the relative roles of chemosensory and visual cues in attraction of fishes to eggs from adult Pacific salmon, and then compared catch rates of traps baited with eggs, muscle, liver, and testis. Experiments indicated that chemical traces were necessary and sufficient to attract juvenile Dolly Varden (Salvelinus malma), coho salmon (Oncorhynchus kisutch), and sculpins (Cottus spp.) into traps. Combining both sites, 70 salmonids and 19 sculpins were trapped using visual and chemical cues, and 53 and 21, respectively, for traps with only chemical cues. Traps with only the sight of eggs caught no salmonids and only 5 sculpins, comparable to empty control traps. In addition, eggs were markedly more attractive than the other tissues, trapping 68?% of the salmonids and 69?% of the sculpins, compared to 14?% and 15?% for muscle tissue, 12?% and 11?% for liver, and 6?% and 5?% for the testis. Visual cues undoubtedly play a role in egg consumption in streams, but these experiments indicated a very important role of chemical traces in attracting fish to the vicinity of the eggs, and selective attraction of eggs over other salmon tissues.     

Host recognition and selection by Aphytis species: Response to California red, oleander, and cactus scale cover extracts

One or more chemicals associated with the host's cover appear to play a major role in host recognition by Aphytis species. Aphytis melinus DeBach, A. lingnanensis Compère, A. coheni DeBach and Comperiella bifasciata Howard, all parasitoids of diaspidid scale-insects, were found to respond to water extracts of California red scale covers, Aonidiella aurantii (Mask.); A. melinus and A. lingnanensis responded to water extracts of cactus scale covers, Diaspis echinocacti (Bouché); and A. chilensis Howard to water extracts of oleander scale covers, Aspidiotus nerii Bouché. A. lingnanensis responded to water extracts of cactus scale covers only if it had been reared on this scale. A. melinus also responded to ethanol extracts of California red scale covers while A. lingnanensis did not. A. melinus failed to respond to water extracts of California red scale covers after a single ovipositional experience on California red scale. Such kairomones might prove useful in screening natural enemies as potential biological control agents for specific hosts as well as in elucidating the mechanisms by which parasitoids of scale-insects differentiate among potential hosts.

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Résumé Aphytis lingnanensis, A. melinus, A. coheni et Comperiella bifasciata réagissent tous à des extraits aqueux d'Aonidiella aurantii, tandis qu'A. chiliensis réagit aux extraits aqueux d'Aspidiotus nérii. La réaction de A. coheni aux extraits aqueux d'A. aurantii confirme les écrits de Quednau et Hübsche (1964). Nous n'avons pas été capables d'observer une réaction de A. chrysomphali aux extraits aqueux d'A. aurantii. Comme nous n'avons pas essayé de traiter les boucliers avec d'autres solvants, nous ne pouvons pas affirmer l'absence de réponse de cet aphélinide aux kairomones de cette cochenille.Que A. melinus et A. lingnanensis aient été élevés sur Aspidiotus nerii ou sur Aonidiella aurantii, ils ne réagissent pas aux extraits aqueux d'A. nerii; qui est accepté comme hôte par ces deux aphélinides au laboratoire. Ces résultats suggèrent que d'autres éléments, comme la forme ou la texture, jouent aussi un rôle important dans la détermination des hôtes. A. melinus et A. lingnanensis réagissent tous deux aux extraits aqueux de boucliers de Diaspis echinocacti; cependant, après élevage sur D. echinocacti, seul A. lingnanensis réagit à l'extrait. La réaction de A. melinus à l'extrait de D. echinocacti suggère que cet aphélinide est préadapté chimiosensoriellement à déceler cette cochenille, dont la distribution est néotropicale tandis que celle de A. melinus orientale. A. melinus a été élevé à partir de D. echinocacti récoltés dans la nature. La réaction de A. lingnanensis aux extraits de D. echinocacti uniquement lorsqu'il a élévé dessus, suggère qu'un conditionnement préimaginal peut aussi jouer un rôle dans la détermination de l'hôte.

     

Inter-annual variation in responses of water chemistry and epilithon to Pacific salmon spawners in an Alaskan stream

1. Pacific salmon (Oncorhynchus spp.) deliver salmon‐derived nutrients (SDN) to the streams in which they spawn. However, many stream parameters, such as discharge and spawner abundance, can vary from year to year, which could alter the quantity and flux of SDN. 2. Over six consecutive years, we studied responses in streamwater chemistry and epilithon (i.e. the microbial community on submerged rocks) to salmon spawners in Fish Creek, southeastern Alaska, U.S.A. The lower reach of Fish Creek receives spawners of several salmon species, while the upper reach does not receive spawners because of an intervening waterfall. 3. We estimated salmon spawner biomass, analysed water chemistry [ammonium, nitrate, soluble reactive phosphorus (SRP) and dissolved organic carbon (DOC)], and measured epilithon abundance [as chlorophyll a (chl a) and ash‐free dry mass (AFDM)] in Fish Creek. Measurements were made in both the upper and lower reaches, before, during and after the major salmon runs. 4. Absolute values and relative differences indicated that the presence of salmon spawners consistently increased dissolved ammonium (by 58 μg L⁻¹ on average, 41× over background), SRP (by 6 μg L⁻¹, 14×), epilithon chl a (by 35 mg m⁻², 16×), and epilithon AFDM (by 3 g m⁻², 8×). Salmon spawners did not increase nitrate or DOC in either absolute or relative amounts. The persistence and magnitude of spawner effects varied among years and appeared to reflect weather‐driven hydrology as well as spawner biomass. 5. Salmon‐derived nutrients can stimulate the growth of primary producers by increasing streamwater nutrient concentrations, but this positive influence may be modulated by other factors, such as water temperature and discharge. To better assess the ecological influence of SDN on stream biota, future studies should explicitly consider the role of key environmental factors and their temporal and spatial dynamics in stream ecosystems.     

Relating land cover to stream properties in southern Chilean watersheds: trade-off between geographic scale, sample size, and explicative power

Several studies relating land cover to stream properties have used sample sizes of more than 100 watersheds, but the variance that they explain is moderate to low (R ² less than 50%), limiting the predictive value of these studies when their models are applied to watersheds that were not included in the models’ development. We hypothesize that this is due to the increases in variation that occur with increases in sample size and in the geographic scales of the areas in which the watersheds are distributed. Land cover alone cannot explain all of that variation; more predictors must be considered. Conversely, models with high explicative power would require relatively small sample sizes distributed over small areas. This hypothesis was evaluated sampling 17 watersheds from southern Chile’s Lake Region, for which we developed regressive models between land cover/watershed area/precipitation/geomorphology and stream properties (i.e., conductivity, temperature). With a maximum n = 15 watersheds, on a regional scale, a poorly explained variation in hydrologic variables (mean 37–49%) was obtained. The R ² increased slightly, to 45–52%, when precipitation was included as a predictor. In half of the cases analyzed, the models improved when geomorphology was considered as an additional predictor (60–66%), supporting our hypothesis. Furthermore, when our analysis was restricted to a narrower latitudinal span (n = 9), the R ² was much stronger (68–87%) when only land cover and watershed area were included as predictors. These percentages also increased when more predictors were incorporated. Nevertheless, a portion of unexplained variance remained that would require the consideration of more predictors, such as geology and edaphology. The documented trade-off provides evidence that argues against the spatial generality of land cover/stream property models.     

Comparative landscape genetic analysis of three Pacific salmon species from subarctic North America

We examined the assumption that landscape heterogeneity similarly influences the spatial distribution of genetic diversity in closely related and geographically overlapping species. Accordingly, we evaluated the influence of watershed affiliation and nine habitat variables from four categories (spatial isolation, habitat size, climate, and ecology) on population divergence in three species of Pacific salmon (Oncorhynchus tshawytscha, O. kisutch, and O. keta) from three contiguous watersheds in subarctic North America. By incorporating spatial data we found that the three watersheds did not form the first level of hierarchical population structure as predicted. Instead, each species exhibited a broadly similar spatial pattern: a single coastal group with populations from all watersheds and one or more inland groups primarily in the largest watershed. These results imply that the spatial scale of conservation should extend across watersheds rather than at the watershed level which is the scale for fishery management. Three independent methods of multivariate analysis identified two variables as having influence on population divergence across all watersheds: precipitation in all species and subbasin area (SBA) in Chinook. Although we found general broad-scale congruence in the spatial patterns of population divergence and evidence that precipitation may influence population divergence in each species, we also found differences in the level of population divergence (coho > Chinook and chum) and evidence that SBA may influence population divergence only in Chinook. These differences among species support a species-specific approach to evaluating and planning for the influence of broad-scale impacts such as climate change.     

Asynchrony, fragmentation, and scale determine benefits of landscape heterogeneity to mobile herbivores

Understanding the ways that resource heterogeneity shapes the performance of individuals and the dynamics of populations offers a central challenge in contemporary ecology. Emerging evidence shows that herbivores track heterogeneity in nutritional quality of vegetation by responding to phenological differences in plants, differences that result from spatial and temporal variation in conditions favoring plant growth. Theory predicts that when spatial variation in temperature, nutrients, or moisture results in spatially asynchronous pulses of plant growth, herbivores are able to prolong the period during which they have access to forage of peak nutritional value. Although this idea has substantial support from observational and modeling studies, it has not been examined experimentally. We hypothesized that access to asynchronous resources enhances nutritional status and growth of herbivores and that the magnitude of this effect depends on the scale of access relative to the grain of resources. We tested these hypotheses in mesocosm experiment using the migratory grasshopper, Melanoplus sanguinipes, feeding on young wheat and protein-rich bran as a model system. We demonstrated access to asynchronous pulses in resources enhanced the efficiency of use of high quality resource use and increased growth of individuals by 13%. Disruption of this mechanism when landscapes were fragmented lowered efficiency of resource use and caused growth of individuals to decline by 15%. However, the strength of the effects of fragmentation on herbivore performance depended on the spatial extent of fragmentation relative to the spatial and temporal grain of resource emergence. Our findings add experimental support to modeling and observational studies that have linked herbivore performance to spatial and temporal variation in plant phenology. We also offer evidence that fragmentation can impair herbivore performance, even when the total amount and quality of resources on landscapes remains unchanged.     

Experimental effects of habitat fragmentation on old-field canopy insects: community, guild and species responses

We examined the effects of habitat fragmentation on the species distributions, guild membership, and community structure of old-field insects using a fine-scale experimental approach. A continuous 1-ha goldenrod field was fragmented into four treatments that varied in both patch size and degree of isolation. Each treatment was replicated four times and arranged in a Latin square design. Canopy insects in fragmented patches were sampled with sweep nets during early and late summer 1995. The species richness of insects was significantly lower in fragmented than in unfragmented treatments during July, but was similar among treatments in September. Overall community abundance showed no treatment effect during either month. We also found significant row and column effects, suggesting there was spatial heterogeneity in species richness and abundance apart from treatment effects. Differences in species richness during July were primarily due to the loss of rare species in highly fragmented plots. Overall abundance was less responsive to community change because deletions of rare species in fragmented areas were not detected in abundance analyses. Four feeding guilds showed different responses to fragmentation: the species richness of sucking herbivores and the abundance of parasitoids were significantly reduced by fragmentation but predators and chewing herbivores were largely unaffected. Analyses of a subset of individual species within guilds suggest that the greater effects of fragmentation on sucking herbivores and parasitoids may be due to the degree of habitat specificity of guild members. The effects of small-scale habitat fragmentation were therefore detectable at the level of community, guild, and individual species. Changes in species richness, guild structure and species distributions were likely due to differential effects of habitat alteration on individual movements and patch selection rather than dispersal or demographic change. Nonetheless, the selective loss of rare species, differential guild effects and changes in species occupancy that we found in this small-scale experiment are also factors that are likely to operate in fragmented habitats over broader spatial scales. Received: 11 May 1998 / Accepted: 27 September 1998     

Daylight responses to overhead cover in stream channels for fry of four salmonid species

Brown trout ( Salmo trutta ), Atlantic salmon ( Salmo salar ), brook trout ( Salvelinus fontinalis ) and lake trout ( Salvelinus namaycush ) fry entering the free-feeding stage, were tested for overhead cover preferences in stream channels at different temperatures and water velocities. Atlantic salmon showed strong preferences for overhead cover, brown trout moderate preferences, whereas lake trout had preferences only at high temperatures, i.e. 12.4–19.2°C. Brook trout showed no cover preferences. Temperature influenced cover preferences of Atlantic salmon and brown trout considerably. The fry tended to seek more cover at low temperatures, i.e. 6.0–8.3°C