Abiotic Environmental Gradients Research Articles

Above‐ and belowground drivers of intraspecific trait variability across subcontinental gradients for five ubiquitous forest plants in North America

AbstractIntraspecific trait variability (ITV) provides the material for species' adaptation to environmental changes. To advance our understanding of how ITV can contribute to species' adaptation to a wide range of environmental conditions, we studied five widespread understorey forest species exposed to both continental‐scale climate gradients, and local soil and disturbance gradients. We investigated the environmental drivers of between‐site leaf and root trait variation, and tested whether higher between‐site ITV was associated with increased trait sensitivity to environmental variation (i.e. environmental fit).We measured morphological (specific leaf area: SLA, specific root length: SRL) and chemical traits (Leaf and Root N, P, K, Mg, Ca) of five forest understorey vascular plant species at 78 sites across Canada. A total of 261 species‐by‐site combinations spanning ~4300 km were sampled, capturing important abiotic and biotic environmental gradients (neighbourhood composition, canopy structure, soil conditions, climate). We used multivariate and univariate linear mixed models to identify drivers of ITV and test the association of between‐site ITV with environmental fit.Between‐site ITV of leaf traits was primarily driven by canopy structure and climate. Comparatively, environmental drivers explained only a small proportion of variability in root traits: these relationships were trait specific and included soil conditions (Root P), canopy structure (Root N) and neighbourhood composition (SRL, Root K). Between‐site ITV was associated with increased environmental fit only for a minority of traits, primarily in response to climate (SLA, Leaf N, SRL).Synthesis. By studying how ITV is structured along environmental gradients among species adapted to a wide range of conditions, we can begin to understand how individual species might respond to environmental change. Our results show that generalisable trait–environment relationships occur primarily aboveground, and only accounted for a small proportion of variability. For our group of species with broad ecological niches, variability in traits was only rarely associated with higher environmental fit, and primarily along climatic gradients. These results point to promising research avenues on the various ways in which trait variation can affect species' performance along different environmental gradients.

Depth-Dependent Diversity Patterns of Rocky Subtidal Macrobenthic Communities Along a Temperate Fjord in Northern Chilean Patagonia

Understanding the distribution of biodiversity along environmental gradients allows us to predict how communities respond to natural and anthropogenic impacts. In fjord ecosystems, the overlap of strong salinity and temperature gradients provides us with the opportunity to assess the spatial variation of biodiversity along abiotic environmental gradients. However, in Northern Chilean Patagonia (NCP), a unique and at the same time threatened fjord system, the variation of macrobenthic communities along abiotic environmental gradients is still poorly known. Here, we tested whether macrobenthic species diversity and community structure followed systematic patterns of variation according to the spatial variation in salinity and temperature in Comau Fjord, NCP. A spatially extensive nested sampling design was used to quantify the abundance of subtidal macrobenthic species along the fjord axis (fjord sections: head, middle, and mouth) and a depth gradient (0–21 m). The vertical structure of the water column was strongly stratified at the head of the fjord, characterized by a superficial (depth to ca. 5 m) low-salinity and relatively colder layer that shallowed and decayed toward the mouth of the fjord. The biotic variation followed, in part, this abiotic spatial pattern. Species richness peaked at high salinities (>27 psu) between 5 and 10 m in the head section and between 15 and 21 m in the middle and mouth sections. Diversity and evenness were also highest at these salinities and depth ranges in the head and middle sections, but at shallower depth ranges in the mouth. Information theory-based model selection provided a strong empirical support to the depth- and section-dependent salinity, but not temperature, effects on the three biodiversity metrics. Erect algae and the edible mussel Aulacomya atra numerically dominated in shallow water (0–3 m) at the head and the middle of the fjord, coinciding with the horizontal extension of the low-density water layer—these taxa were further replaced by the crustose algae Lithothamnion sp. and deep-dwelling suspension filters (e.g., corals, polychaetes, and sponges) along depth gradient. Macrobenthic biodiversity correlated, therefore, with the influence of freshwater inputs and the density-driven stratification of the water column in this ecosystem. The spatially variable (across both, horizontal and vertical fjord axes) thresholds observed in our study question the widely accepted pattern of increasing biodiversity with increasing distance from the head of estuarine ecosystems. Finally, non-linear environmental stress models provide us a strong predictive power to understand the responses of these unique ecosystems to natural and anthropogenic environmental changes.

Distribution of deep-water scleractinian and stylasterid corals across abiotic environmental gradients on three seamounts in the Anegada Passage

In the Caribbean Basin the distribution and diversity patterns of deep-sea scleractinian corals and stylasterid hydrocorals are poorly known compared to their shallow-water relatives. In this study, we examined species distribution and community assembly patterns of scleractinian and stylasterid corals on three high-profile seamounts within the Anegada Passage, a deep-water throughway linking the Caribbean Sea and western North Atlantic. Using remotely operated vehicle surveys conducted on the E/V Nautilus by the ROV Hercules in 2014, we characterized coral assemblages and seawater environmental variables between 162 and 2,157 m on Dog Seamount, Conrad Seamount, and Noroît Seamount. In all, 13 morphospecies of scleractinian and stylasterid corals were identified from video with stylasterids being numerically more abundant than both colonial and solitary scleractinians. Cosmopolitan framework-forming species including Madrepora oculata and Solenosmilia variabilis were present but occurred in patchy distributions among the three seamounts. Framework-forming species occurred at or above the depth of the aragonite saturation horizon with stylasterid hydrocorals being the only coral taxon observed below Ωarag values of 1. Coral assemblage variation was found to be strongly associated with depth and aragonite saturation state, while other environmental variables exerted less influence. This study enhances our understanding of the factors that regulate scleractinian and stylasterid coral distribution in an underreported marginal sea and establishes a baseline for monitoring future environmental changes due to ocean acidification and deoxygenation in the tropical western Atlantic.

Moss-inhabiting diatoms as ecological indicators in Neora Valley National Park (Eastern Himalaya), India

Terrestrial mosses which retain moisture provide a suitable ambience for diatoms colonization and suitable experimental platforms for observing climate change impact. The distribution and diversity patterns of such epibryic diatom communities are governed by various biotic and abiotic factors of moss habitats. The present study is first of ever attempt to investigate the moss-inhabiting diatoms in relation to various abiotic environmental variables from the Indian Himalayan Region (IHR). Samples were collected from 23 different sites in the Neora Valley National Park in Eastern Himalayas and analyzed through standard methods. Two-way indicator species analysis (TWINSPAN) and cluster analysis (CA) performed to classify the samples and habitats while direct canonical correspondence analysis (CCA) was used to determine the linkage between moss-inhabiting diatom species from various sample and abiotic environmental gradients. A total of 27 diatoms found colonizing on terrestrial moss patches from 23 sites of the park. CCA ordination plots showed that pH, conductivity and relative humidity were the primary factors influencing the diatom diversity and distribution, but altitude and temperature had no influence on the species distribution. But, correlation study on the average valve length of the most abundant and frequent taxa of Neora Valley National Park, i.e. Eunotia paludosa, E. bigibba and Hantzschia amphioxys, in relation to the environmental variables like, altitude, moisture content, relative humidity and temperature revealed a significant inference. The average valve length of these three diatoms decreases in warmer and drier climatic conditions at lower altitudes. Thus, these can be considered as potential indicator species to observe any climate change.

Biodiversity of an Antarctic rocky subtidal community and its relationship with glacier meltdown processes

Glacier meltdown is a major environmental response to climate change in the West Antarctic Peninsula. Yet, the consequences of this process for local biodiversity are still not well understood. Here, we analyse the diversity and structure of a species-rich marine subtidal macrobenthic community (consumers and primary producers) across two abiotic environmental gradients defined by the distance from a glacier (several km) and depth (between 5 and 20 m depth) in Fildes Bay, King George Island. The analysis of spatially extensive records of seawater turbidity, high-frequency temperature and salinity data, and suction dredge samples of macrobenthic organisms revealed non-linear and functional group-dependent associations between biodiversity, glacier influence, and depth. Turbidity peaked in shallow waters and in the nearby of the glacier. Temperature and salinity, on the other hand, slightly decreased in the proximity of the glacier relative to reference sites. According to the spatial pattern in turbidity, species richness of consumers was lowest in shallow waters and near to the glacier. Also, Shannon's diversity of consumers significantly decreased in the nearby of glacier across depths. Moreover, the spatial variation in community structure of consumers and primary producers depended on both glacier distance and depth. These results suggest that glacier melting can have significant effects on diversity and community structure. Therefore, the accelerated glacier meltdown may have major consequences for the biodiversity in this ecosystem.

Species range shifts along multistressor mosaics in estuarine environments under future climate

AbstractRange shifts are a key mechanism that species employ in response to climate change. Increasing global temperatures are driving species redistributions to cooler areas along three main spatial axes: increasing latitudes, altitudes and water depths. Climate‐mediated range shift theory focuses on temperature as the primary ecological driver, but global change alters other environmental factors as well, and these rarely work in isolation. Ecosystems are often characterized as mosaics of overlapping environmental stressors, resulting in temporal and spatial heterogeneity which differs between stable, low complexity mosaics (e.g. open ocean) and highly variable, highly complex mosaic environments (e.g. estuaries). We propose a multistressor mosaic of climate‐mediated species range shift across abiotic environmental gradients, typical for mobile species (e.g. fish) in variable coastal environments. We conceptualize how climate‐driven changes in salinity, temperature, dissolved oxygen and pH can drive redistribution of estuarine species in a future world. Non‐thermal drivers are a critical component of species range shifts and when not considered, underestimate the impact of global change on species populations and ecosystem services.

Mowing may bring about vegetation change, but its effect is strongly modified by hydrological factors

We assessed the effect of long-term (11 years) mowing with a hand scythe on the distributions of plant species and species turnover within meadow communities dominated by Carex acutiformis and C. acuta in a small lowland river valley subjected to annual flooding. We hypothesized that mowing would trigger the process of species exchange toward multispecies communities according to the abiotic environmental gradients, as has been reported for traditionally used wet meadows. We found that mowing had a much greater impact on the increase in plant species abundance in wetter, subjected to deeper and longer flooding, zone of the valley than in its drier part. The treatment and hydrology had no substantial effect on the sedges and cryptogams while the graminoids and the forbs were the least stable components of the plant communities. We found that annual management was conducive to the appearance of numerous seedlings of Alnus glutinosa in the part of wetland situated close to the valley edge. Surface flooding was found to be a driving force modifying the impact of mowing on sedge meadows. This “hydrological resetting impulse” created a specific floristic reset of the community, bringing it to the more simplified forms. Mowing every 4–5 years at the beginning of August is advisable to protect sedge meadows distributed in flooded small lowland river valleys.

Trait plasticity is more important than genetic variation in determining species richness of associated communities

Abstract Intraspecific variation can be an important driver of ecological interactions in species‐rich communities. Predicting the effects of intraspecific variation in different environments, however, remains a major challenge. This is because we often do not quantify both the effects of functional traits on associated communities and the extent to which trait variation is due to genetics (genotype effects) vs. plasticity (environment effects). As a consequence, the relative importance of trait plasticity vs. genetic variation in structuring associated communities remains unclear. We sought to fill this gap by conducting common garden experiments with the plant Salix hookeriana across biotic (ant–aphid interactions) and abiotic (wind exposure) environmental gradients in a coastal dune ecosystem. In each experiment, we simultaneously measured plant traits and species richness of associated above‐ and below‐ground communities. We then used statistical models to quantify the relative importance of trait plasticity vs. genetic variation in structuring communities. Our major finding was that trait plasticity was more important than genetic variation in determining the number of species in associated communities. This result was consistent across different environmental contexts (experimental manipulations of ant–aphid interactions and wind exposure), multiple years, and for above‐ground arthropods and root microbes. This occurred because the traits that had the largest effect on species richness were also the most plastic. Synthesis. These results indicate that trait plasticity can be a dominant driver of above‐ and below‐ground biodiversity.

Phenolic concentrations of brown seaweeds and relationships to nearshore environmental gradients in Western Australia

Phenolic compounds are found in all brown macroalgae and function as cell wall structure, UV protection and as herbivore deterrents. The concentrations of phenolic compounds vary among taxa and between temperate and tropical ecosystems. Australasia has high concentrations of soluble phenolics compared to other regions. Presently, relationships between phenolic concentrations and environmental gradients are unclear. The purpose of this study was to determine the soluble phenolic concentrations of brown seaweeds along temperate and tropical ecosystems of the Western Australia coastline. We tested the hypothesis that phenolic concentrations are related to local and broad-scale abiotic environmental gradients. Strong environmental gradients of coastal Western Australia provided the opportunity to characterize phenolic compounds across one large gradient. Phenolic concentrations of brown seaweeds at seven study locations varied across latitude with higher concentrations found at higher latitudes and were comparable to seaweeds from similar latitudes in Australia. This trend coincided with a negative relationship between photosynthetically active radiation and phenolic compounds, and a positive relationship with salinity. We also found phenolic concentrations were positively related to salinity in tropical Shark Bay but this was dependent on species. Environmental conditions are important in regulating concentrations of phenolic compounds. Multiple factors influence the concentrations of macroalgal phenolic compounds creating unique distributions among geographical regions. This study highlighted the importance of considering multiple factors when studying phenolic ecology and suggests photosynthetically active radiation and salinity as important drivers of phenolic compound distribution in Western Australia.

Floral size and shape evolution following the transition to gender dimorphism.

Floral morphology is expected to evolve following the transition from cosexuality to gender dimorphism in plants, as selection through male and female function becomes dissociated. Specifically, male-biased dimorphism in flower size can arise through selection for larger flowers through male function, selection for smaller flowers through female function, or both. The evolutionary pathway to floral dimorphism can be most effectively reconstructed in species with intraspecific variation in sexual system. We examined the evolution of flower size and shape in Lycium californicum, whose populations are either gender dimorphic with male and female plants, or cosexual with hermaphroditic plants. Floral morphology was characterized in populations spanning the species' complete range. For a subset of the range where cosexual and dimorphic populations are in close proximity, we compared the size and shape of flowers from female and male plants in dimorphic populations to hermaphrodites in cosexual populations, accounting for variation associated with abiotic environmental conditions. The magnitude of flower size dimorphism varied across dimorphic populations. After controlling for environmental variation across cosexual and dimorphic populations, flowers on males were larger than flowers on females and hermaphrodites, whereas flower size did not differ between females and hermaphrodites. Flower shape differences were associated with mating type, sexual system, and environmental variation. While abiotic environmental gradients shape both overall flower size and shape, male-biased flower size dimorphism in L. californicum appears to arise through selection for larger flowers in males but not smaller flowers in females.

Hydrodynamical and hydrochemical groundwater controls on abiotic environmental gradients in a nature reserve in Flanders (Belgium)

The biological value of natural and protected areas like nature reserves is often reflected in the diversity and combination of different vegetation associations. In settings where the vegetation is strongly groundwater dependent, the specific interrelation between groundwater flow characteristics (hydrodynamics) and groundwater composition (hydrochemistry) is usually the most important factor for controlling the ecological value and occurrence of site-specific vegetation. Consequently, small-scale variations in ecological gradients within the site rather than its overall size are the key features that control the biodiversity of groundwater-dependent ecosystems. This is demonstrated for a small nature reserve called "Gulke Putten" in the northwest of Belgium which is only a few km 2 large, but shows an interesting variety in groundwater types the occurrence and genesis of which are due to local and highly seasonal flow conditions. As a result, the site has a high ecological value which is rare in this part of the country. This paper shows how multi-year daily water level measurements were used to quantify seasonal and inter-yearly variation in water levels and flow situations and how these are related to the condition in groundwater chemistry. Different hydrochemical facies are found, ranging from evaporated rainwater and waters influenced in varying degree by calcite dissolution, pyrite oxidation, cation exchange and some marine influence originating from the marine sediments of Tertiary age in the phreatic aquifer. This paper relates the variations in flow conditions and groundwater seepage intensity to the groundwater chemistry and helps to understand the origin of the abiotic ecological gradients that are related to the specific ecohydrological value of the site.

Cushion plant morphology controls biogenic capability and facilitation effects of Silene acaulis along an elevation gradient

Summary The stress‐gradient hypothesis (SGH) predicts that the balance of plant–plant interactions shifts along abiotic environmental gradients, with facilitation becoming more frequent under stressful conditions. However, recent studies have challenged this perspective, reporting that positive interactions are, in some cases, more common at the intermediate level of environmental severity gradients. Here, we test whether and how neighbour effects by Silene acaulis cushions vary along a 700 m wide altitudinal transect, in relation to cushion morphological traits and environmental severity. Field measurements along the gradient, within and outside cushions, included (i) species richness and cover of coexisting vascular plants; (ii) cushion morphology; (iii) above‐ and below‐ground microclimate; and (iv) soil quality. We used the relative interaction index to decouple neighbour trait effects and environmental severity effects on plant diversity at different elevations. The ability of the cushion plant to facilitate heterospecifics shifts considerably along the elevation gradient, being greatest at the intermediate level. On the other hand, Silene morphological traits steadily change along the gradient, from lax, soft and flat‐shaped cushion habits at low elevation to tightly knit and dome‐shaped habits at high elevation. Cushion morphological changes are associated with mitigating effects on microclimate, indicating that cushions effectively act as a heat‐trap at medium and high elevations, while at low elevations the soft and flat cushions avoid excessive heat accumulation by tight coupling with the surrounding atmosphere. At the upper end of the gradient, cushion cespitose–pulvinate compactness and high stem density appear to be critical traits in modulating the net effect of plant–plant interaction, since the space available for hosting other vascular species is considerably reduced. In conclusion, this work provides a mechanistic link between plant morphological traits, associated biogenic microclimate changes and variation in net plant–plant interactions along the explored severity gradient. Our findings support an alternative conceptual model to SGH, with plant facilitation collapsing at the upper extreme of the abiotic stress gradient.

Survival Mechanism of Psammophytes in arid desert

Species traits, environmental gradients, and distribution pattern could be three main tools providing mechanistic understanding of ecosystems, predicting the key issues in ecological stability, balance, and succession orientation. Present study is focused on a desert-oasis area, characterized by possible two abiotic environmental gradients and the distribution of seven plant species dominating four types of sand dunes. Aim of this study is to find the desert survival mechanisms for sand-binding application. Within the accessions of this transitional belt, three major types of water use characteristics could be defined. Dominant species including shrubs and herbs were significantly negatively correlated with their diurnal transpiration at rhizosphere moisture (P<0.01,R2=0.94). The results indicate pioneer species surviving arid desert by lavish water use strategies, with less cover, high Pr, high Tr, high WUE and preceding cresting of WUE. Soil property gradient played an important role in affecting the fixed and semi-fixed dunes, while soil moisture gradient was more important in shifting and semi-shifting dunes.

Endemic species have highly integrated phenotypes, environmental distributions and phenotype–environment relationships

AbstractAimWhy are some species geographically restricted? Ecological explanations suggest that endemic species may have restricted distributions because limited phenotypic variability results in narrow niches. However, studying variability of traits independently may not fully explain the interactions within and between complex phenotypes and environments. Here, we hypothesize that endemic species are restricted to a narrow range of habitats due to strong phenotypic integration (i.e. strong correlations among traits), strong environmental integration (i.e. strong correlations among the environments occupied) and strong correlations among trait–environment combinations.LocationThe Kerguelen Islands, sub‐Antarctic.MethodsWe measured flowering phenology, multiple morphological characters, and species distribution along three abiotic environmental gradients (elevation, soil moisture and soil salinity) in 14 plant species whose distributions range from strictly endemic to cosmopolitan.ResultsWe found that for individual species, trait means and variances were independent of endemism, but that endemics occupied higher and less variable microhabitats. However, phenotypic integration, environmental integration along the three gradients, and the strength of trait–environment correlations all increased with the level of species endemism.Main conclusionsHigher levels of integration within and between phenotypes and environments are associated with more restricted geographical ranges in the species studied. In endemic species phenotypic integration may explain range contraction during the taxon cycle and reduce the ability to adapt to novel microhabitats formed as a result of environmental change.

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The inventory of fish species of the rio Mamore sub-drainage in the Bolivian Amazon is far from being complete. This article informs about a small scale species inventory in the close vicinity of the town Santa Ana del Yacuma (drained by the rio Yacuma, a left side tributary of the rio Mamore). Sampling four habitat types, 615 fish specimens belonging to 101 species were collected. Four species were reported for the first time from the rio Mamore sub-drainage: Aphyocharax rathbuni, Apistogramma erythrura, Apistogramma similis and Hyphessobrycon elachys. Differences in species composition among sampled habitats stress the importance of including a high number of collecting sites across biotic and abiotic environmental gradients to reliably survey species diversity.

Disparate relatives: Life histories vary more in genera occupying intermediate environments

Species within clades are commonly assumed to share similar life history traits, but within a given region some clades show much greater variability in traits than others. Are variable clades older, allowing more time for trait diversification? Or do they occupy particular environments, providing a wider range of abiotic or biotic opportunities for the establishment and maintenance of diverse trait attributes? Does environmental opportunity increase trait variability across all species, or is it specific to species belonging to the same clade, increasing only within-clade trait variability? We studied the variability of six life-history traits (initiation of flowering, duration of flowering, plant life span, seed mass, stress tolerance, type of reproduction) within 383 angiosperm genera from Central Europe distributed along six abiotic gradients. We compared patterns of within-genus variability to those present in the entire dataset, independent of genus membership. We found that trait variability differed strongly between genera, but did not depend on their age. Trait variability was higher within genera occupying intermediate positions along regional abiotic environmental gradients, compared with patterns across the entire dataset (and unbiased by geographical sampling, family membership or species richness). Increasing trait variability within genera reflected increasing independence of traits from the abiotic environment. We conclude that intermediate abiotic environments play an important role in maintaining and possibly generating the striking diversity of life history traits present within certain clades. They may do so by relaxing the abiotic constraints on the evolution and maintenance of species traits within clades.

Evaluating Relationships between Spatial Heterogeneity and the Biotic and Abiotic Environments

While most studies of heterogeneity have focused on describing patterns of species or communities, few have focused on the relationships between biotic and abiotic environmental landscape-level gradients. Our study was designed to determine relationships between grazing (heavy, moderate, ungrazed), topographic position (upland vs. riparian), vegetation structure and the thermal environment (i.e., soil-surface temperatures) and determine the influence on landscape patterns of heterogeneity. Biotic and abiotic patterns of heterogeneity were evaluated by establishing 200-m transects that were centered on and perpendicular to a riparian zone so that spatial patterns of variability could be determined along each transect which transcends the maximum level of landscape heterogeneity. Vegetation cover and structure and soil-surface temperatures were recorded at 1-m intervals along the transect. Bare ground increased and leaf litter, grass cover, vegetation heights and angle of obstruction decreased with grazing intensity. However, mean soil-surface temperatures did not differ between grazing treatments. Tree canopy cover associated with riparian areas generally reduced soil-surface temperatures 20 C below that of upland temperatures. In fact, 96% of observations of riparian soil-surface temperature were ≤ 39 C, while 94% of upland soil-surface temperatures were ≥ 40 C regardless of grazing intensity. Vegetation characteristics and soil-surface temperatures were correlated (P < 0.05), but correlation coefficients were small because soil-surface temperature was highly variable. Grazing influenced patterns of landscape heterogeneity, but effects were inconsistent among biotic and abiotic variables. Although grazing had little influence on moderating mean soil-surface temperatures, results suggest that grazing intensity influences thermal heterogeneity at a variety of spatial scales. For instance, thermal heterogeneity (in moderately grazed treatments) is highest at smaller (lag distances ≤ 20 m) and larger (lag distances ≥ 48 m) spatial scales but was lowest at moderate scales (lag distances 22–45 m). For all variables, other than soil-surface temperature and forb cover, semi-variances of moderately grazed sites generally lie intermediate between heavy and ungrazed sites. Nearly all ungrazed vegetation characteristics, except leaf litter, fit a spherical model that reached a sill at a lag distance ≤ 20 m and became spatially independent thereafter, while heavily and moderately grazed sites typically fit an exponential model, indicating a high degree of continuity. Patterns of thermal variability (on uplands) are not related directly to any one vegetation variable, hence, landscape patterns based on vegetation parameters alone are of limited value since patterns of thermal variability are effected by the integration of vegetation and environmental variables within the ecosystem.

Convergent life-history shifts: toxic environments result in big babies in two clades of poeciliids

The majority of studies on ecological speciation in animals have investigated the divergence caused by biotic factors like divergent food sources or predatory regimes. Here, we examined a system where ecological speciation can clearly be ascribed to abiotic environmental gradients of naturally occurring toxic hydrogen sulfide (H(2)S). In southern Mexico, two genera of livebearing fishes (Poeciliidae: Poecilia and Gambusia) thrive in various watercourses with different concentrations of H(2)S. Previous studies have revealed pronounced genetic differentiation between different locally adapted populations in one species (Poecilia mexicana), pointing towards incipient speciation. In the present study, we examined female reproductive life-history traits in two species pairs: Gambusia sexradiata (from a nonsulfidic and a sulfidic habitat) and Gambusia eurystoma (sulfide-endemic), as well as P. mexicana (nonsulfidic and sulfidic) and Poecilia sulphuraria (sulfide endemic). We found convergent divergence of life-history traits in response to sulfide; most prominently, extremophile poeciliids exhibit drastically increased offspring size coupled with reduced fecundity. Furthermore, within each genus, this trend increased with increasing sulfide concentrations and was most pronounced in the two endemic sulfur-adapted species. We discuss the adaptive significance of large offspring size in toxic environments and propose that divergent life-history evolution may promote further ecological divergence through isolation by adaptation.

Stream amphibians as metrics of critical biological thresholds in the Pacific Northwest, U.S.A.: a response to Kroll et al.

Summary1. Kroll, Hayes &amp; MacCracken (in press) Concerns regarding the use of amphibians as metrics of critical biological thresholds: a comment on Welsh and Hodgson 2008. Freshwater Biology, criticised our paper [Welsh &amp; Hodgson (2008) Amphibians as metrics of critical biological thresholds in forested headwater streams of the Pacific Northwest. Freshwater Biology, 53, 1470–1488] proposing the use of headwater stream amphibians as metrics of stream status in the Pacific Northwest (PNW). They argued that our analysis of previously published data reflected circular reasoning because we reached the same conclusions as the earlier studies. In fact, we conducted a meta‐analysis to address new questions about the optimum values and thresholds (based on animal densities) for abiotic stream attributes that were found to be important to these amphibians in earlier studies. This is analogous to determining blood pressure thresholds or fat‐to‐weight ratios that facilitate predicting human health based on meta‐analyses of earlier data from studies that found significant correlations between these variables and relative health.2. Kroll et al. argued that we should not make inference to environmental conditions across the PNW from data collected in California. We collected data from northern California and southern Oregon, the southern extent of the PNW. We made inference to the Klamath‐Siskiyou and North Coast bioregions, and argued that available research on these headwater species indicates that our results have the potential to be applied throughout the PNW with minimal regional adjustments.3. Kroll et al. contended that we need reproductive success, survival estimates and density estimates, corrected for detection probabilities, to establish relationships between animal density and stream attributes. Reproductive success and survival estimates are important for demographic modelling and life tables, but they are not necessary to demonstrate meaningful relationships with abiotic conditions. Both corrected occupancy estimates and individual detection probabilities are unnecessary, and take multiple sampling efforts per site, or onerous mark release and re‐capture studies, respectively, to determine accurately.4. Kroll et al. questioned the use of stream amphibians as a surrogate for measuring physical parameters, such as water temperature, claiming that measuring the physical parameters directly is more efficient. Here they misinterpreted the main point of our paper: stream organisms are integrators of what happens in a catchment, and carefully selected species can serve as surrogates for the biotic community and the relative condition of the network environment.5. Kroll et al. claimed that we demonstrated weak inferences regarding ecosystem processes. We argue that by relating densities of stream amphibians with changes along abiotic environmental gradients that are commonly affected by anthropogenic activities, we are establishing biological links to gradients that represent important ecosystem processes and identifying biometrics that can be used to quantify the status (health) of these gradients.

The Relative Importance of Biotic and Abiotic Controls on Young Conifer Growth After Fire in the Klamath-Siskiyou Region

In the Klamath-Siskiyou region of northern California and southwest Oregon, the mixed-severity fire regime and short fire return interval has created diverse vegetative landscape with frequent regenerating patches of forest vegetation. Considerable evidence from this region suggests that intense competition from neighborhood vegetation can greatly reduce the survival and growth of young conifers following severe site disturbance, including fire. However, relatively little information exists regarding the growing conditions one to three decades following disturbance, particularly in unmanaged stands. We measured the effects of neighbor-hood vegetation on regenerating Douglas-fir (Pseudotsuga menziesii) growth in northern California, 10-20 years following severe wildfires. Steep climatic gradients owing to complex topography in the region can cause significant variation in local temperatures, levels of precipitation, and solar radiation; thus our measurements were dispersed across two abiotic environmental gradients, elevation and heat load. We used a nested sampling design to elicit the tree-level effects of neighborhood vegetation, the plot-level effects of elevation and heat load, and the interactions among tree-level and plot-level variables on Douglas-fir growth. We used a combination of of hierarchical regression models and non-parametric, multiplicative regression modeling for 3-dimensional graphical data presentation. Our results indicate that conifers are exposed to both positive (facilitative) and negative (competitive) neighborhood interactions depending on location in the abiotic environmental landscape. However, growth appears to be much more sensitive to abiotic environmental conditions than to the effects of neighborhood interactions. Considering the spatially explicit patterns in our data, we suggest that site-specific conditions play a significant role in the development of post-fire forest management strategies in the Klamath-Siskiyou region.