Species response curves along environmental gradients. A case study from SE Norwegian swamp forests

Abstract. Vegetation science has relied on untested paradigms relating to the shape of species response curves along environmental gradients. To advance in this field, we used the HOF approach to model response curves for 112 plant species along six environmental gradients and three ecoclines (as represented by DCA ordination axes) in SE Norwegian swamp forests. Response curve properties were summarized in three binary response variables: (1) model unimodal or monotonous (determinate) vs. indeterminate; (2) for determinate models, unimodal vs. monotonous and (3) for unimodal models, skewed vs. symmetric. We used logistic regression to test the influence, singly and jointly, of seven predictor variables on each of three response variables. Predictor variables included gradient type (environmental or ecocline) and length (compositional turnover); species category (vascular plant, moss, Sphagnum or hepatic), species frequency and richness, tolerance (the fraction of the gradient along which the species occurs) and position of species along each gradient. The probability for fitting a determinate model increased as the main occurrence of species approached gradient extremes and with increasing species tolerance and frequency and gradient length. Appearance of unimodal models was favoured by low species tolerance and disfavoured by closeness of species to gradient extremes. Appearance of skewed models was weakly related to predictors but was slightly favoured by species optima near gradient extremes. Contrary to the results of previous studies, species category, gradient type and variation in species richness along gradients did not contribute independently to model prediction. The overall best predictors of response curve shape were position along the gradient (relative to extremes) and tolerance; the latter also expressing gradient length in units of compositional turnover. This helps predicting species responses to gradients from gradient specific species properties. The low proportion of skewed response curves and the large variation of species response curves along all gradients indicate that skewed response curves is a smaller problem for the performance of ordination methods than often claimed. We find no evidence that DCA ordination increases the unimodality, or symmetry, of species response curves more than expected from the higher compositional turnover along ordination axes. Thus ordination axes may be appropriate proxies for ecoclines, applicable for use in species response modelling.     

Patterns of species richness and turnover along the pH gradient in deciduous forests: testing the continuum hypothesis

Question: (i) How do species richness and species turnover change along a pH gradient? (ii) What are possible driving factors behind these patterns? (iii) Can the observed patterns be explained by an individualistic continuum concept that postulates independence of species responses and constant turnover rates? Location: Semi‐natural, deciduous hardwood forests in NW Germany (558 plots). Methods: The instantaneous rate of compositional turnover is measured by the sum of slope angles of modelled response curves (119 understorey species) at any point along the pH gradient. Total turnover rate, positive turnover rate (species increasing in probability of occurrence) and negative turnover rate (species decreasing in probability of occurrence) are calculated separately. Species richness is modelled using Poisson regression and by calculating the sum of predicted probabilities at any gradient point. Turnover rates are compared with those calculated from a null model based on a Gaussian community model. Soil chemical analyses of 49 plots are used to interpret biodiversity patterns. Results: Species richness shows a hump‐shaped relation to pH(CaCl₂) with a minor decline at approximately pH>5.0. The decline is possibly due to the confounding influence of water regime and local species pool effects. Increasing richness from pH 2.5 to 4.7 can be traced back to positive turnover exceeding negative turnover. Peaks in turnover rates, dominated by positive turnover, are located at pH 3.7 and 2.8, where turnover rates considerably exceed rates derived from the null model. The turnover pattern can be related to soil chemical conditions, e.g. decreasing base saturation, Al and H⁺ toxicity and the occurrence of mor. Conclusions: The high turnover rates and the massive imbalance in positive and negative turnover rates found in deciduous forests cannot be explained by the individualistic continuum concept. Physiological constraints at the gradient limits and species pool effects could be responsible for this. Their role should be considered more explicitly in vegetation concepts dealing with the continuum‐discontinuum controversy. The presented approach can be regarded as a comprehensive analytical tool for a better understanding of biodiversity patterns along environmental gradients by linking species richness, turnover and response curve types.     

Measuring compositional change along gradients

A new procedure for measuring compositional change along gradients is proposed. Given a matrix of species-by-samples and an initial ordering of samples on an axis, the ‘gradient rescaling’ method calculates 1) gradient length (beta diversity), 2) rates of species turnover as a function of position on the gradient, and 3) an ecologically meaningful spacing of samples along the gradient. A new unit of beta diversity, the gleason, is proposed. Gradient rescaling is evaluated with both simulated and field data and is shown to perform well under many ecological conditions. Applications to the study of succession, phenology, and niche relations are briefly discussed.     

Rescaling of ecological gradients. I. Calculation of ecological distance between vegetation stands by means of their floristic composition

Geometric models of vegetation (conceptual spaces) are reviewed. Spaces with samples or species as axes are termed flortistic spaces, as opposed to ecological space with environmental gradients as axes. The terms floristic and ecological relationships are defined as relationships in floristic and ecological spaces, respectively. Compositional turnover is pointed out as the essence of ecological gradients, and arguments in favour of measuring ecological distance in units of compositional turnover are given. The most important criteria for evaluation of ecological distance measures are considered to be linear response to separation along ecological gradients and robustness. Theoretical disadvantages of measures of floristic relationships used as ecological distance measures are discussed. A new measure of ecological distance, separation along a DCA ordination axis, is proposed. This measure and four measures of floristic relationships were tested on four simulated coenoclines (high and low beta diversity, high and low noise) using four weighting functions. The new measure was generally superior, particularly with noisy data. The distance measures generally performed best with intermediate weighting of a percentage cover scale. Application of DCA to calculation of ecological distances in multi-gradient systems is briefly discussed. The potential of DCA for rescaling of ecological gradients is emphasized, and some possible applications of rescaling are suggested.     

Steady-state characteristics of the proton receptor in the somatic membrane of rat sensory neurons

Steady-state characteristics of proton-activated permeability of the somatic membrane of rat sensory neurons were studied. Transmembrane ionic currents arising in response to application of solutions with low pH were measured during intracellular perfusion and simultaneous voltage clamping. A rapid change in proton concentration in the external medium was produced by a modified rapid application method. Steady-state desensitization of proton-induced permeability was found to develop in the region of weakly alkaline values (pK_a 7.15). The mechanism of desensitization is highly cooperative: Hill's parameter for dose-effect dependences was close to 7. Activation also is cooperative, but its Hill's parameters is about 2. Calcium and magnesium cations compete with protons, changing pK_a of dose-effect relationships for desensitization and activation, but they have only a weak effect on the value of Hill's parameter.     

Patterns of plant species turnover along grazing gradients

Questions: How are plant species distributed along grazing gradients? What is the shape of species richness patterns? How can we test for the existence of potential discontinuities in species turnover pattern? Location: Semi‐deserts in the eastern Caucasus, Azerbaijan, Gobustan district. Methods: We studied the distribution of vascular plant species along transects 900‐m long, perpendicular to five farms, and estimated grazing intensity as current livestock units per distance. We modelled species response curves with Huismann–Olff–Fresco (HOF) models and calculated species turnover by accumulating the first derivatives of all response curves. To test for potential discontinuities in changes of vegetation composition along the grazing gradient, we introduce a new null model based on the individualistic continuum concept that uses permutations of the observed pattern of species responses. Results: Most species show a sigmoidal negative response to grazing intensity, while a few species respond with a unimodal pattern. The monotonic decrease in species richness with increasing grazing intensity marks a process of overgrazing that leads to the complete extirpation of plant species. Although the species turnover pattern shows a clear peak, it does not deviate significantly from the null model of individualistic continuous changes. Conclusions: Our approach offers a method for differentiating between transition zones and continuous shifts in species composition along ecological gradients. It also provides a valuable tool for rangeland management to test state‐and‐transition concepts and gives deeper insights into ecological processes affected by grazing.     

The generality of empirical and theoretical explanations of behavior

For theoretical explanations of data, parameter values estimated from a single dependent measure from one procedure are used to predict alternative dependent measures from many procedures. Theoretical explanations were compared to empirical explanations of data in which known functions and principles were used to fit only selected dependent measures. The comparison focused on the ability of theoretical and empirical explanations to generalize across samples of the data, across dependent measures of behavior, and across different procedures. Rat and human data from fixed-interval and peak procedures, in which principles (e.g., scalar timing) are well known, were described and fit by a theory with independent modules for perception, memory, and decision. The theoretical approach consisted of fitting closed-form equations of the theory to response rate gradients calculated from the data, simulating responses using parameter values previously estimated, and comparing theoretical predictions with dependent measures not used to estimate parameters. Although the empirical and theoretical explanations provided similar fits to the response rate gradients that generalized across samples and had the same number of parameters, only the theoretical explanation generalized across procedures and dependent measures.     

Turning Up the Heat on a Hotspot: DNA Barcodes Reveal 80% More Species of Geometrid Moths along an Andean Elevational Gradient

We sampled 14,603 geometrid moths along a forested elevational gradient from 1020–3021 m in the southern Ecuadorian Andes, and then employed DNA barcoding to refine decisions on species boundaries initially made by morphology. We compared the results with those from an earlier study on the same but slightly shorter gradient that relied solely on morphological criteria to discriminate species. The present analysis revealed 1857 putative species, an 80% increase in species richness from the earlier study that detected only 1010 species. Measures of species richness and diversity that are less dependent on sample size were more than twice as high as in the earlier study, even when analysis was restricted to an identical elevational range. The estimated total number of geometrid species (new dataset) in the sampled area is 2350. Species richness at single sites was 32–43% higher, and the beta diversity component rose by 43–51%. These impacts of DNA barcoding on measures of richness reflect its capacity to reveal cryptic species that were overlooked in the first study. The overall results confirmed unique diversity patterns reported in the first investigation. Species diversity was uniformly high along the gradient, declining only slightly above 2800 m. Species turnover also showed little variation along the gradient, reinforcing the lack of evidence for discrete faunal zones. By confirming these major biodiversity patterns, the present study establishes that incomplete species delineation does not necessarily conceal trends of biodiversity along ecological gradients, but it impedes determination of the true magnitude of diversity and species turnover.     

Measurement of oscillatory flow pressure gradient in an elastic artery model

In vitro experiments were conducted to measure the oscillatory flow pressure gradient along an elastic tube in order to assess the recent nonlinear theory of Wang and Tarbell. According to this theory, in an elastic tube with oscillatory flow, the mean (time-averaged) pressure gradient cannot be calculated using Poiseuille's law. The effect of wall motion creates a nonlinear convective acceleration, and an induced mean pressure gradient is required to balance the convective acceleration. The induced mean pressure gradient depends on the diameter variation over a cycle, the pulsatility and unsteadiness of the flow, and the phase difference between the pressure wave form and the flow wave form. The amplitude of the pressure gradient also depends on these parameters and may deviate significantly from Womersley's rigid tube theory. A flow loop was constructed to produce oscillatory flow in an elastic tube. Flow wave forms were measured with an ultrasonic flow probe, and ultrasonic diameter crystals were used to measure wall movement. A special device for pressure drop measurement was constructed using Millar catheter tip transducers to obtain both forward and backward pressure drops that were then averaged. This averaging method eliminated the static error of the pressure transducers. The pressure-flow phase angle was varied by clamping a distal elastic section at various locations. Pressure gradients were obtained for a range of phase angles between −55 ° and +49 °. The mean and amplitude of the measured pressure gradient were compared to theoretical values. Both positive and negative induced mean pressure gradients were measured over the range of phase angles. The measured pressure gradient amplitudes were always lower than predicted by Womersley's rigid tube theory. The experimental means and amplitudes are in good agreement with the elastic tube theoretical values. Thus, the experiments verify the theory of Wang and Tarbell.     

Do commonly used indices of β‐diversity measure species turnover?

Abstract. Indices of β‐diversity are of two major types, (1) those that measure among‐plot variability in species composition independently of the position of individual plots on spatial or environmental gradients, and (2) those that measure the extent of change in species composition along predefined gradients, i.e. species turnover. Failure to recognize this distinction can lead to the inappropriate use of some β‐diversity indices to measure species turnover. Several commonly‐used indices of β‐diversity are based on Whittaker's β_W (β_W = γ/α, where γ is the number of species in an entire study area and α is the number of species per plot within the study area). It is demonstrated that these indices do not take into account the distribution of species on spatial or environmental gradients, and should therefore not be used to measure species turnover. The terms ‘β‐diversity’ and ‘species turnover’ should not be used interchangeably. Species turnover can be measured using matrices of compositional similarity and physical or environmental distances among pairs of study plots. The use of indices of β‐diversity and similarity‐distance curves is demonstrated using simulated data sets.     

Scaling hierarchy of factors controlling riparian vegetation patterns of the Fynbos Biome at the Western Cape,South Africa

Question: How can vegetation gradients be described in riparian zones located in a species‐rich mountain range and how do these gradients explain the variation found in the vegetation? Location: Hottentots Holland Mountains, Western Cape, South Africa. Methods: Three gradients (geographic, longitudinal, lateral) were defined to describe the complex vegetation patterns found here. The gradients are related as follows: (1) the geographic gradient: related to the climatic and topographical changes across the entire mountain range; (2) the longitudinal gradient related to the changes along the different river reaches; (3) the lateral gradient related to the processes along the profile of the riverbed. These three gradients operated on three different hierarchical levels. Partial Canonical Correspondence Analysis (pCCA) was used to determine the amount of variation that is explained on each of the hierarchical levels. Results: The geographical gradient explained the highest fraction (more than 50%) of the total variation explained. This can be ascribed to the high species turnover across landscapes in the Fynbos Biome; this is most likely an outcome that is specific for this region. The second most important gradient was the lateral gradient, which reflects stream power and inundation frequencies of the river. This gradient is represented by ca. 48% of the explained variation and this gradient explains the major disturbances occurring in a riverine ecosystem. The longitudinal gradient was the least important of the gradients and shows overlap with the geographical gradient. Conclusions: In the species‐rich environment of the Fynbos Biome geographical factors do not only account for variation in zonal vegetation but also for variation within azonal vegetation, like riparian corridors.     

Floristic variation,chorological types and diversity: do they correspond at broad and local scales?

Abstract. The relationships between biogeographical patterns and local‐scale patterns based on microscale features, such as topoclimate, are well known in plant biogeography. Here we present a method of determining this correspondence using constrained ordination and correlations. We examined compositional gradients at two different scales, biogeographical chorotypes, and diversity. Compositional data (124 taxa × 113 plots) were sampled at four regularly spaced sites in south‐eastern Spain. Longitude (LONGI) was used as a spatial variable representing an east–west climate gradient, together with a radiation index (RADIN), elevation, and a disturbance indicator. All factors correlated with the compositional gradients, but the local‐topoclimate factor (RADIN) and the broad‐scale factor (LONGI) were most important. These two, spatially independent factors were both correlated with the two first ordination axes, and therefore should relate to the same general trend in species‐turnover. There was a significant Spearman's rank correlation between the species order along these two gradients. This is interpreted as an ecological self‐similar pattern, i.e. coenoclines repeating at different scales. A consistent order of species along local‐ and broad‐scale coenoclines may indicate that similar operational factors act at several scales, here related to moisture and temperature. The distribution of Mediterraneo–Macaronesian, Mediterraneo–Saharo–Arabian and Ibero–Maghribian species confirmed the correspondence between the broad‐ and local‐scale gradients. The former group decreases in number with increasing aridity along both gradients, whereas the two latter groups increase. A discordant pattern was found with south‐eastern Iberian endemics, but this may be explained by several of them being edaphic (saxicolous) specialists. There is a significant decrease in species richness with high radiation, but the expected increase along the longitudinal gradient from west (dry) to east (moist) was not statistically significant. This may be due to the correspondence between high richness and disturbance, both occurring in the middle of the broad‐scale gradient.     

A comparison between desert and Mediterranean antlion populations: differences in life history and morphology

We performed a transplant experiment to compare the life histories and morphologies of five geographically representative antlion Myrmeleon hyalinus populations along a sharp climatic gradient, from a Mediterranean climate in Israel's north to a desert climate in the south. Larvae were raised in two environmental chambers simulating Mediterranean and desert climates to investigate the extent to which the different populations exhibit phenotypic plasticity. Along the north-to-south climatic gradient, we observed a gradient in body mass prior to pupation and in pupation rate. Mediterranean populations suffered higher mortality rate when exposed to desert conditions, whereas the mortality rate of desert populations was consistent between Mediterranean and desert conditions. Our results regarding body mass, pupation rate and mortality rate suggest that Mediterranean populations had a more flexible response compared with desert populations. An analysis of digital photographs was used to measure population morphological differences, which were usually indicative of a decrease in trait size along the north-to-south gradient. We show how climatic gradients translate into phenotypic differences in an antlion population and provide a morphometric tool to distinguish between instar stages.     

Disentangling the relative importance of species occurrence,abundance and intraspecific variability in community assembly: a trait‐based approach at the whole‐plant level in Mediterranean forests

Understanding which factors and rules govern the process of assembly in communities constitutes one of the main challenges of plant community ecology. The presence of certain functional strategies along broad environmental gradients can help to understand the patterns observed in community assembly and the filtering mechanisms that take place. We used a trait‐based approach, quantifying variations in aboveground (leaf and stem) and belowground (root) functional traits along environmental gradients in Mediterranean forest communities (south Spain). We proposed a new practical method to quantify the relative importance of species turnover (distinguishing between species occurrence and abundance) versus intraspecific variation, which allowed us to better understand the assemblage rules of these plant communities along environmental gradients. Our results showed that the functional structure of the studied plant communities was highly determined by soil environment. Results from our modelling approach based on maximum likelihood estimators showed a predominant influence of soil water storage on most of the community functional traits. We found that changes in community functional structure along environmental gradients were mainly promoted by species turnover rather than by intraspecific variability. Specifically, our new method of variance decomposition demonstrated that between‐site trait variation was the result of changes in species occurrence rather than in the abundance of certain dominant species. In conclusion, this study showed that water availability promoted the predominance of specific trait values (both in above and belowground fractions) associated to a resource acquisition or conservation strategy. In addition, we provided evidence that changes on community functional structure along the environmental gradient were mainly promoted by a process of species replacement, which represent a crucial step towards a more general understanding of the relative importance of intraspecific versus interspecific trait variation in these woody Mediterranean communities.     

An application of canonical correspondence analysis for developing ecological quality assessment metrics for river macrophytes

1. Aquatic macrophyte composition and abundance is required by the European Union's Water Framework Directive for determining ecological status. Five metrics were produced that can be combined to determine the deviation of aquatic macrophytes from reference conditions in Northern Ireland's rivers. 2. Species optima and niche breadths along silt, nitrate, pH, conductivity and dissolved oxygen gradients were generated from aquatic macrophyte and water quality surveys conducted at 273 sites throughout Northern Ireland using Canonical Correspondence Analysis (CCA). Five metric scores based on these environmental gradients were determined at new monitoring sites using the mean optima of the species occurring at the site, weighted by percentage cover and niche breadth of each species. 3. A preliminary reference network of 32 sites of high physico‐chemical and hydromorphological quality, and representative of the range of river types in Northern Ireland, enabled reference metric scores to be produced for each river type. Five unimpacted and twenty impacted sites were used for testing the performance of the metrics. By subtracting reference metric scores from metric scores at a monitoring site measures of ecological impact could be determined along five different impact gradients. Metrics were also combined to give a measure of total ecological change. 4. The metrics system distinguished unimpacted from impacted sites and correctly identified 77% of the known impacts. The metrics distinguished different types of impact, e.g. silt and nitrate. 5. Aquatic macrophyte occurrence and abundance has high natural variability at a site, both temporally and spatially. This method was designed to be sensitive to ecological change whilst reducing noise caused by natural variation.     

A guide through a family of phylogenetic dissimilarity measures among sites

Ecological studies have now gone beyond measures of species turnover towards measures of phylogenetic and functional dissimilarity. This change of perspective has a main objective: disentangling the processes that drive species distributions from local to broad scales. A fundamental difference between phylogenetic and functional analyses is that phylogeny is intrinsically dependent on a tree‐like structure whereas functional data can, most of time, only be forced to adhere a tree structure, not without some loss of information. When the branches of a phylogenetic tree have lengths, then each evolutionary unit on these branches can be considered as a basic entity on which dissimilarities among sites should be measured. Several of the recent measures of phylogenetic dissimilarities among sites thus are traditional dissimilarity indices where species are replaced by evolutionary units. The resulting indices were named PD‐dissimilarity indices, in reference to early work on the phylogenetic diversity (PD) measure. Here I review and compare indices and ordination approaches that, although first developed to analyse the differences in the species compositions of sites, can be adapted to describe PD‐dissimilarities among sites. Using simulations of species distributions along environmental gradients, I compare indices, associated with permutation tests and null models, in their ability to reveal existing phylogenetic patterns along the gradients. As an illustration, I show that the amount of bat PD‐dissimilarities along a disturbance gradient in Selva Lacandona of Chiapas, Mexico is dependent on whether species' abundance is considered, and on the PD‐dissimilarity index used. Overall, the family of PD‐dissimilarity indices has a critical potential for future analyses of phylogenetic diversity as it benefits from decades of research on the measure of species dissimilarity. I provide clues to help to choose among many potential indices, identifying which indices satisfy minimal basic properties, and analysing their sensitivity to abundance, size, diversity and joint absences.     

Current problems of environmental gradients and species response curves in relation to continuum theory

Abstract. Comparisons of the positions of species on Grimes'C-S-R triangular ordination model with their responses to individual environmental gradients indicates that the C-S-R model does not necessarily predict species ecological behaviour. The importance of the stress, productivity and disturbance gradients relative to other environmental gradients needs to be determined. In studies of species behaviour along a biomass/productivity gradient the collective vegetation property, biomass, has been confused with the environmental factor, fertility. Patterns of responses to biomass gradients e.g. Keddy's centrifugal model, should be examined in a two-dimensional environmental space to avoid such confounding effects. Assumptions regarding the shapes of species responses to environmental gradients remain untested. A recent model of species response functions to environmental gradients suggested that skewed responses curves show a pattern in the direction of the skew, always with the tail towards the presumed most mesic position on the gradient. Further evidence is presented to support this model for a temperature gradient in eucalypt forest in south-eastern Australia. 21 out of 24 species tested conform to the model.     

Climatic and geometric constraints as driving factors of butterfly species richness along a Neotropical elevational gradient

We tested the effects of temperature, humidity and geographical constraints upon butterfly species richness along an elevational gradient covering an altitude ranging from 117 to 3,104 m above sea level (m. a.s.l.), in Southern Mexico. Ten transect sites were sampled 219 times from May 2010 to May 2011, along the elevational gradient to estimate range and population abundance of butterfly species. The effects of temperature, humidity and geometric constraints (mid-domain effects) on species richness along the study gradient were assessed using ordinary least squares regression. A total of 7,005 specimens representing 193 species were recorded. Species richness was relatively higher at elevations between 117 and 1,000 m. a.s.l. with an observed decline in richness values as elevation increased. Butterfly species richness along the study environmental gradient was predominantly determined by climatic constraints, rather than geometric constraints—a mid-domain model fit well only for large-ranged Pieridae species. Temperature and humidity explained the variation species richness for the entire butterfly community and for the three families evaluated; however the effect of predictor variables varied according to the measure of species richness and taxonomic family. This discrepancy in the response of butterfly richness to temperature, humidity and geometric constraints emphasizes the need to evaluate the response of different taxa to elevational gradients, to establish general patterns that help us to prioritize conservation measures that reduce population declines and local extinctions predicted by climate change in highly diverse tropical mountain ecosystems.     

Increased forest carbon storage with increased atmospheric CO2 despite nitrogen limitation: a game‐theoretic allocation model for trees in competition for nitrogen and light

Changes in resource availability often cause competitively driven changes in tree allocation to foliage, wood, and fine roots, either via plastic changes within individuals or through turnover of individuals with differing strategies. Here, we investigate how optimally competitive tree allocation should change in response to elevated atmospheric CO₂ along a gradient of nitrogen and light availability, together with how those changes should affect carbon storage in living biomass. We present a physiologically‐based forest model that includes the primary functions of wood and nitrogen. From a tree's perspective, wood is an offensive and defensive weapon used against neighbors in competition for light. From a biogeochemical perspective, wood is the primary living reservoir of stored carbon. Nitrogen constitutes a tree's photosynthetic machinery and the support systems for that machinery, and its limited availability thus reduces a tree's ability to fix carbon. This model has been previously successful in predicting allocation to foliage, wood, and fine roots along natural productivity gradients. Using game theory, we solve the model for competitively optimal foliage, wood, and fine root allocation strategies for trees in competition for nitrogen and light as a function of CO₂ and nitrogen mineralization rate. Instead of down‐regulating under nitrogen limitation, carbon storage under elevated CO₂ relative to carbon storage at ambient CO₂ is approximately independent of the nitrogen mineralization rate. This surprising prediction is a consequence of both increased competition for nitrogen driving increased fine root biomass and increased competition for light driving increased allocation to wood under elevated CO₂.