The responses of grassland plants to experimentally simulated climate change depend on land use and region

Macroclimatic niche properties derived from species distribution ranges are fundamental for projections of climate change impacts on biodiversity. However, it has been recognized that changes in regional or local distribution patterns also depend on interactions with land use. The reliability and transferability of large scale geographic predictions to small scale plant performance need to be tested experimentally. Thus, we asked how grassland plant species pairs with different macroclimatic niche properties respond to increased spring temperature and decrease summer precipitation in three different land‐use types. An experiment was carried out in the framework of the German Biodiversity Exploratories simulating climate change in 45 experimental plots in three geographical regions (Schorfheide‐Chorin, Hainich‐Dün, Schwäbische Alb) and three grassland management types (meadow, pasture, mown pasture). We planted six plant species as phytometers, each two of them representing congeneric species with contrasting macroclimatic niches and recorded plant survival and growth over 1 year. To quantify the species macroclimatic niches with respect to drought tolerance, the species’ distribution ranges were mapped and combined with global climate data. The simulated climate change had a general negative effect on plant survival and plant growth, irrespective of the macroclimatic niche characteristics of the species. Against expectation, species with ranges extending into drier regions did not generally perform better under drier conditions. Growth performance and survival was best in mown pastures, representing a quite intensive type of land use in all study regions. Species with higher macroclimatic drought tolerance were generally characterized by lower growth rates and higher survival rates in land‐use types with regular mowing regimes, probably because of reduced competition in the growing season. In conclusion, plant species with similar climatic niche characteristics cannot be expected to respond consistently over different regions owing to complex interactions of climate change with land use practices.     

Range and niche shifts in response to past climate change in the desert horned lizard Phrynosoma platyrhinos

During climate change, species are often assumed to shift their geographic distributions (geographic ranges) in order to track environmental conditions – niches – to which they are adapted. Recent work, however, suggests that the niches do not always remain conserved during climate change but shift instead, allowing populations to persist in place or expand into new areas. We assessed the extent of range and niche shifts in response to the warming climate after the Last Glacial Maximum (LGM) in the desert horned lizard Phrynosoma platyrhinos, a species occupying the western deserts of North America. We used a phylogeographic approach with mitochondrial DNA sequences to approximate the species range during the LGM by identifying populations that exhibit a genetic signal of population stability versus those that exhibit a signal of a recent (likely post‐LGM) geographic expansion. We then compared the climatic niche that the species occupies today with the niche it occupied during the LGM using two models of simulated LGM climate. The genetic analyses indicated that P. platyrhinos persisted within the southern Mojave and Sonoran deserts throughout the latest glacial period and expanded from these deserts northwards, into the western and eastern Great Basin, after the LGM. The climatic niche comparisons revealed that P. platyrhinos expanded its climatic niche after the LGM towards novel, warmer and drier climates that allowed it to persist within the southern deserts. Simultaneously, the species shifted its climatic niche towards greater temperature and precipitation fluctuations after the LGM. We concluded that climatic changes at the end of the LGM promoted both range and niche shifts in this lizard. The mechanism that allowed the species to shift its niche remains unknown, but phenotypic plasticity likely contributes to the species ability to adjust to climate change.     

Patterns of niche filling and expansion across the invaded ranges of an Australian lizard

Studies of realized niche shifts in alien species typically ignore the potential effects of intraspecific niche variation and different invaded‐range environments on niche lability. We incorporate our detailed knowledge of the native‐range source populations and global introduction history of the delicate skink Lampropholis delicata to examine intraspecific variation in realized niche expansion and unfilling, and investigate how alternative niche modelling approaches are affected by that variation. We analyzed the realized niche dynamics of L. delicata using an ordination method, ecological niche models (ENMs), and occurrence records from 1) Australia (native range), 2) New Zealand, 3) Hawaii, 4) the two distinct native‐range clades that were the sources for the New Zealand and Hawaii introductions, and 5) the species’ global range (including Lord Howe Island, Australia). We found a gradient of realized niche change across the invaded ranges of L. delicata: niche stasis on Lord Howe Island, niche unfilling in New Zealand (16%), and niche unfilling (87%) and expansion (14%) in Hawaii. ENMs fitted to native‐range data generally identified suitable climatic conditions at sites where the species has established non‐native populations, whereas ENMs based on native‐range source clades and non‐native populations had lower spatial transferability. Our results suggest that the extent to which realized niches are maintained during invasion does not depend on species‐level traits. When realized niche shifts are predominately due to niche unfilling, fully capturing species’ responses along climatic gradients by basing ENMs on native distributions may be more important for accurate invasion forecasts than incorporating phylogenetic differentiation, or integrating niche changes in the invaded range.     

The ‘Hutchinsonian niche’ as an assemblage of demographic niches: implications for species geographic ranges

Hutchinson defined the ecological niche as a hypervolume shaped by the environmental conditions under which a species can ‘exist indefinitely’. Although several authors further discussed the need to adopt a demographic perspective of the ecological niche theory, very few have investigated the environmental requirements of different components of species’ life cycles (i.e. vital rates) in order to examine their internal niche structures. It therefore remains unclear how species’ demography, niches and distributions are interrelated. Using comprehensive demographic data for two well‐studied, short‐lived plants (Plantago coronopus, Clarkia xantiana), we show that the arrangement of species’ demographic niches reveals key features of their environmental niches and geographic distributions. In Plantago coronopus, opposing geographic trends in some individual vital rates, through different responses to environmental gradients (demographic compensation), stabilize population growth across the range. In Clarkia xantiana, a lack of demographic compensation underlies a gradient in population growth, which could translate in a directional geographic range shift. Overall, our results highlight that occurrence and performance niches cannot be assumed to be the same, and that studying their relationship is essential for a better understanding of species’ ecological niches. Finally, we argue for the value of considering the assemblage of species’ demographic niches when studying ecological systems, and predicting the dynamics of species geographical ranges.     

Population trends influence species ability to track climate change

Shifts of distributions have been attributed to species tracking their fundamental climate niches through space. However, several studies have now demonstrated that niche tracking is imperfect, that species’ climate niches may vary with population trends, and that geographic distributions may lag behind rapid climate change. These reports of imperfect niche tracking imply shifts in species’ realized climate niches. We argue that quantifying climate niche shifts and analyzing them for a suite of species reveal general patterns of niche shifts and the factors affecting species’ ability to track climate change. We analyzed changes in realized climate niche between 1984 and 2012 for 46 species of North American birds in relation to population trends in an effort to determine whether species differ in the ability to track climate change and whether differences in niche tracking are related to population trends. We found that increasingly abundant species tended to show greater levels of niche expansion (climate space occupied in 2012 but not in 1980) compared to declining species. Declining species had significantly greater niche unfilling (climate space occupied in 1980 but not in 2012) compared to increasing species due to an inability to colonize new sites beyond their range peripheries after climate had changed at sites of occurrence. Increasing species, conversely, were better able to colonize new sites and therefore showed very little niche unfilling. Our results indicate that species with increasing trends are better able to geographically track climate change compared to declining species, which exhibited lags relative to changes in climate. These findings have important implications for understanding past changes in distribution, as well as modeling dynamic species distributions in the face of climate change.     

Analyzing niche stability and biogeography of Late Ordovician brachiopod species using ecological niche modeling

Gauging the potential impacts of environmental change on the geographic distributions of species is a central area of modern biogeographic analysis, often involving complex models of species–environment interactions. The geographic distribution of fossil species can also provide a framework to test the impact of environmental change on biogeography and ecological niches of species, yet few paleontological analyses have attacked this question in deep time. Herein we present a quantitative biogeographic analysis to examine the stability of ecological niches and geographic ranges of rhynchonelliform brachiopods during an interval of sea level change preserved in Upper Ordovician strata of the Cincinnati Arch.The intensive sampling, excellent preservation, and numerous prior paleoecological and sedimentological analyses within the tri-state region of Kentucky, Indiana, and Ohio provide a robust framework for detailed paleobiogeographic study. Quantitative biogeographic modeling methods incorporating GIS (Geographic Information Systems) are utilized in order to spatially analyze the geographic ranges of brachiopod species of the Corryville and Mt. Auburn Formations of the C3 (uppermost Maysvillian) depositional sequence.This study employs the ecological niche modeling program GARP (Genetic Algorithm using Rule-set Prediction) to predict the geographic distribution of eight brachiopod species during three time slices within the C3 sequence. This method estimates a species’ geographic range by modeling the ecological niche of the species based on a set of known species occurrence data coupled with environmental data inferred from sedimentologic proxies. Once environmental tolerances for a species are modeled; the species is predicted to occur wherever its preferred set of environmental conditions occurs within the study region.Distributional patterns were reconstructed for three time slices during the C3 sequence. Recovered range predictions were quantitatively analyzed for evidence of temporal range changes. Results indicate that average species range within the study area decreased and species tracked their preferred niche with high fidelity during the transition from the early to middle portions of the C3 depositional sequence, an interval of rapid relative sea level change. However, during the transition from the middle to late portions of the sequence, gradual shallowing within the basin and development of discontinuous habitat patches correlates with niche evolution of five of the eight species modeled. The average area a species occupied within the basin increased during this interval, but there is a mixed response including both increases and decreases in range size within the study group. In general, the species that exhibit niche evolution increased their geographic range size while those that continue to track their niche with high fidelity experience a decrease in geographic range size. During the latter half of the C3 sequence, previously continuous habitats become fragmented, thereby isolating individual populations and providing a mechanism for niche evolution. The rate of sea level change and the corresponding fragmentation of previously continuous habitats into isolated patches appear to be the primary controls on both mean geographic range size and relative degree of niche evolution.     

Human-dominated land uses favour species affiliated with more extreme climates,especially in the tropics

Rapid human population growth has driven conversion of land for uses such as agriculture, transportation and buildings. The removal of natural vegetation changes local climate, with human-dominated land uses often warmer and drier than natural habitats. Yet, it remains an open question whether land-use changes influence the composition of ecological assemblages in a direction consistent with the mechanism of local climatic change. Here, we used a global database of terrestrial vertebrates (mammals, birds, reptiles and amphibians) to test whether human-dominated land uses systematically favour species with distinctive realised climatic niches. We 1) explored the responses of community-average temperature and precipitation niches to different types of land use, 2) quantified the abundances of species with distinctive climatic niches across land uses and 3) tested for differences in emergent patterns in communities from tropical versus temperate latitudes. We found that, in comparison to species from undisturbed natural habitats, the average animal found in human-altered habitats lives in areas with higher maximum and lower minimum temperatures and higher maximum and lower minimum precipitation levels. We further found that tropical assemblages diverged more strongly than temperate assemblages between natural and human-altered habitats, possibly because tropical species are more sensitive to climatic conditions. These results strongly implicate the role of land-use change in favouring species affiliated with more extreme climatic conditions, thus systematically reshaping the composition of terrestrial biological assemblages. Our findings have the potential to inform species' vulnerability assessments and highlight the importance of preserving local climate refugia.     

Ecological Niche Transferability Using Invasive Species as a Case Study

Species distribution modeling is widely applied to predict invasive species distributions and species range shifts under climate change. Accurate predictions depend upon meeting the assumption that ecological niches are conserved, i.e., spatially or temporally transferable. Here we present a multi-taxon comparative analysis of niche conservatism using biological invasion events well documented in natural history museum collections. Our goal is to assess spatial transferability of the climatic niche of a range of noxious terrestrial invasive species using two complementary approaches. First we compare species’ native versus invasive ranges in environmental space using two distinct methods, Principal Components Analysis and Mahalanobis distance. Second we compare species’ native versus invaded ranges in geographic space as estimated using the species distribution modeling technique Maxent and the comparative index Hellinger’s I. We find that species exhibit a range of responses, from almost complete transferability, in which the invaded niches completely overlap with the native niches, to a complete dissociation between native and invaded ranges. Intermediate responses included expansion of dimension attributable to either temperature or precipitation derived variables, as well as niche expansion in multiple dimensions. We conclude that the ecological niche in the native range is generally a poor predictor of invaded range and, by analogy, the ecological niche may be a poor predictor of range shifts under climate change. We suggest that assessing dimensions of niche transferability prior to standard species distribution modeling may improve the understanding of species’ dynamics in the invaded range.     

Quantification of climatic niches in birds: adding the temporal dimension

Quantification of the climatic niche from geographic occurrences is an increasingly important tool for studying species’ relationships to their environment, for example to predict responses to climate change. However, as the geographic distributions of birds are seasonally dynamic, they pose a challenge to carrying out comparable and appropriate quantification of climatic niches. In this review, we first assess how relevant seasonal dynamics are across birds as a whole by compiling a database of migratory behaviour for 10 443 bird species. Second, we examine how studies have quantified climatic niches of birds. Finally, using Australia as a case study, we investigate how well existing distribution datasets represent temporal dynamics by comparing seasonal patterns of species richness obtained from point‐occurrence data with those from range maps and assess the consequences for niche quantification. We provide a consistent classification of migratory behaviour across all birds, and find that a huge variety exists between and within species that should be considered when quantifying climatic niches. Despite this, our review of the literature revealed that seasonal dynamics have often not been accounted for. For future studies, we provide a framework for selecting appropriate occurrence data depending on migratory behaviour and data availability. Our comparison of seasonal species richness patterns obtained from extent‐of‐occurrence range maps and point‐occurrence data suggests that range maps are less able to detect temporal dynamics of bird distributions than point‐occurrence data. We conclude that seasonally explicit range maps combined with climatic data for the corresponding time period can be used to adequately quantify climatic niches for resident birds, but are not adequate to quantify the climatic niches of migratory and nomadic species. Therefore, consistent quantification of climatic niches across all birds requires temporally explicit occurrence points. As such, increasing the availability of occurrence data and methods correcting biases should be a priority.     

Cryptic niche conservatism among evolutionary lineages of an invasive lizard

Aim There is increasing evidence that the quality and breadth of ecological niches vary among individuals, populations, evolutionary lineages and therefore also across the range of a species. Sufficient knowledge about niche divergence among clades might thus be crucial for predicting the invasion potential of species. We tested for the first time whether evolutionary lineages of an invasive species vary in their climate niches and invasive potential. Furthermore, we tested whether lineage‐specific models show a better performance than combined models. Location Europe. Methods We used species distribution models (SDMs) based on climatic information at native and invasive ranges to test for intra‐specific niche divergence among mitochondrial DNA (mtDNA) clades of the invasive wall lizard Podarcis muralis. Using DNA barcoding, we assigned 77 invasive populations in Central Europe to eight geographically distinct evolutionary lineages. Niche similarity among lineages was assessed and the predictive power of a combination of clade‐specific SDMs was compared with a combined SDM using the pooled records of all lineages. Results We recorded eight different invasive mtDNA clades in Central Europe. The analysed clades had rather similar realized niches in their native and invasive ranges, whereas inter‐clade niche differentiation was comparatively strong. However, we found only a weak correlation between geographic origin (i.e. mtDNA clade) and invasive occurrences. Clades with narrow realized niches still became successful invaders far outside their native range, most probably due to broader fundamental niches. The combined model using data for all invasive lineages achieved a much better prediction of the invasive potential. Conclusions Our results indicate that the observed niche differentiation among evolutionary lineages is mainly driven by niche realization and not by differences in the fundamental niches. Such cryptic niche conservatism might hamper the success of clade‐specific niche modelling. Cryptic niche conservatism may in general explain the invasion success of species in areas with apparently unsuitable climate.     

Temperature‐related geographical shifts among passerines: contrasting processes along poleward and equatorward range margins

Climate change is causing widespread geographical range shifts, which likely reflects different processes at leading and trailing range margins. Progressive warming is thought to relax thermal barriers at poleward range margins, enabling colonization of novel areas, but imposes increasingly unsuitable thermal conditions at equatorward margins, leading to range losses from those areas. Few tests of this process during recent climate change have been possible, but understanding determinants of species’ range limits will improve predictions of their geographical responses to climate change and variation in extinction risk. Here, we examine the relationship between poleward and equatorward range margin dynamics with respect to temperature‐related geographical limits observed for 34 breeding passerine species in North America between 1984–1988 and 2002–2006. We find that species’ equatorward range margins were closer to their upper realized thermal niche limits and proximity to those limits predicts equatorward population extinction risk through time. Conversely, the difference between breeding bird species’ poleward range margin temperatures and the coolest temperatures they tolerate elsewhere in their ranges was substantial and remained consistent through time: range expansion at species’ poleward range margins is unlikely to directly reflect lowered thermal barriers to colonization. The process of range expansion may reflect more complex factors operating across broader areas of species’ ranges. The latitudinal extent of breeding bird ranges is decreasing through time. Disparate responses observed at poleward versus equatorward margins arise due to differences in range margin placement within the realized thermal niche and suggest that climate‐induced geographical shift at equatorward range limits more strongly reflect abiotic conditions than at their poleward range limits. This further suggests that observed geographic responses to date may fail to demonstrate the true cost of climate change on the poleward portion of species’ distributions. Poleward range margins for North American breeding passerines are not presently in equilibrium with realized thermal limits.     

Species’ thermal ranges predict changes in reef fish community structure during 8 years of extreme temperature variation

Aim

To assess whether observed thermal bounds in species’ latitudinal ranges (i.e., realized thermal niches) can be used to predict patterns of occurrence and abundance changes observed during a marine heatwave, relative to other important life history and functional traits.

Location

Rottnest Island, Western Australia.

Methods

A time series of standardized quantitative surveys of reef fishes spanning 8 years of pronounced ocean temperature change is used to test whether accurate predictions on shifts in species occupancy and abundance are possible using species traits.

Results

Species‐level responses in occurrence and abundance were closely related to the mid‐point of their realized thermal niche, more so than body size, range size or trophic level. Most of the species that disappeared from survey counts during the heatwave were characterized by geographic ranges that did not extend to latitudes with temperatures equivalent to the ocean temperature peak during the heatwave. We thus find support for the hypothesis that current distribution limits are set directly or indirectly by temperature and are highly responsive to ocean temperature variability.

Main conclusions

Our study shows that reef fish community structure can change very quickly when exposed to extreme thermal anomalies, in directions predicted from the realized thermal niche of the species present. Such predictions can thus identify species that will be most responsive to changing ocean climate. Continued warming, coupled with periodic extreme heat events, may lead to the loss of ecosystem services and ecological functions, as mobile species relocate to more hospitable climes, while less mobile species may head towards extinction.

     

Habitat suitability and the constraints of migration in New World warblers

Identifying the factors that influence geographic range limits can illustrate the various ecological, physiological, and evolutionary constraints imposed on a species. The range limits of migratory birds are particularly challenging to study as they occur in disjunct regions at different times of the year, which can impose different constraints. Travel between breeding and wintering regions poses a significant energetic and navigational challenge to birds, although it is not clear how these movements influence breeding dispersal and range expansion. Here I ask whether the possible costs of migration limit the breeding ranges of wood warblers, a group of birds with an extensive history of ecological and evolutionary studies. I used occurrence records for multiple wood warbler species, breeding primarily in the boreal forest of North America, to generate environmental niche models. I tested for over‐prediction of habitat suitability into the western boreal forest, where most these species do not have occurrence records but where there is presumably suitable habitat. I found that some of these vagile taxa, primarily found east of the Rocky Mountains, also have predicted habitat suitability that extends into the north and west, where they have little to no occurrence records. I discuss several possible explanations for this discordance. In particular, the patterns are consistent with the costs of a long‐distance migration limiting the benefits of range expansion, as migration may become too onerous as the distance between breeding and wintering areas increases. These results speak to the process of niche filling more generally and call attention to an under‐appreciated explanation for why migratory species may not fully occupy their fundamental niche.     

Different climatic envelopes among invasive populations may lead to underestimations of current and future biological invasions

Aim We explore the impact of calibrating ecological niche models (ENMs) using (1) native range (NR) data versus (2) entire range (ER) data (native and invasive) on projections of current and future distributions of three Hieracium species. Location H. aurantiacum, H. murorum and H. pilosella are native to Europe and invasive in Australia, New Zealand and North America. Methods Differences among the native and invasive realized climatic niches of each species were quantified. Eight ENMs in BIOMOD were calibrated with (1) NR and (2) ER data. Current European, North American and Australian distributions were projected. Future Australian distributions were modelled using four climate change scenarios for 2030. Results The invasive climatic niche of H. murorum is primarily a subset of that expressed in its native range. Invasive populations of H. aurantiacum and H. pilosella occupy different climatic niches to those realized in their native ranges. Furthermore, geographically separate invasive populations of these two species have distinct climatic niches. ENMs calibrated on the realized niche of native regions projected smaller distributions than models incorporating data from species’ entire ranges, and failed to correctly predict many known invasive populations. Under future climate scenarios, projected distributions decreased by similar percentages, regardless of the data used to calibrate ENMs; however, the overall sizes of projected distributions varied substantially. Main conclusions This study provides quantitative evidence that invasive populations of Hieracium species can occur in areas with different climatic conditions than experienced in their native ranges. For these, and similar species, calibration of ENMs based on NR data only will misrepresent their potential invasive distribution. These errors will propagate when estimating climate change impacts. Thus, incorporating data from species’ entire distributions may result in a more thorough assessment of current and future ranges, and provides a closer approximation of the elusive fundamental niche.     

Climate change and habitat conversion favour the same species

Land‐use change and climate change are driving a global biodiversity crisis. Yet, how species' responses to climate change are correlated with their responses to land‐use change is poorly understood. Here, we assess the linkages between climate and land‐use change on birds in Neotropical forest and agriculture. Across > 300 species, we show that affiliation with drier climates is associated with an ability to persist in and colonise agriculture. Further, species shift their habitat use along a precipitation gradient: species prefer forest in drier regions, but use agriculture more in wetter zones. Finally, forest‐dependent species that avoid agriculture are most likely to experience decreases in habitable range size if current drying trends in the Neotropics continue as predicted. This linkage suggests a synergy between the primary drivers of biodiversity loss. Because they favour the same species, climate and land‐use change will likely homogenise biodiversity more severely than otherwise anticipated.     

Global realized niche divergence in the African clawed frog Xenopus laevis

Although of crucial importance for invasion biology and impact assessments of climate change, it remains widely unknown how species cope with and adapt to environmental conditions beyond their currently realized climatic niches (i.e., those climatic conditions existing populations are exposed to). The African clawed frog Xenopus laevis, native to southern Africa, has established numerous invasive populations on multiple continents making it a pertinent model organism to study environmental niche dynamics. In this study, we assess whether the realized niches of the invasive populations in Europe, South, and North America represent subsets of the species’ realized niche in its native distributional range or if niche shifts are traceable. If shifts are traceable, we ask whether the realized niches of invasive populations still contain signatures of the niche of source populations what could indicate local adaptations. Univariate comparisons among bioclimatic conditions at native and invaded ranges revealed the invasive populations to be nested within the variable range of the native population. However, at the same time, invasive populations are well differentiated in multidimensional niche space as quantified via n‐dimensional hypervolumes. The most deviant invasive population are those from Europe. Our results suggest varying degrees of realized niche shifts, which are mainly driven by temperature related variables. The crosswise projection of the hypervolumes that were trained in invaded ranges revealed the south‐western Cape region as likely area of origin for all invasive populations, which is largely congruent with DNA sequence data and suggests a gradual exploration of novel climate space in invasive populations.     

Combining physiological threshold knowledge to species distribution models is key to improving forecasts of the future niche for macroalgae

Species distribution models (SDM) are a useful tool for predicting species range shifts in response to global warming. However, they do not explore the mechanisms underlying biological processes, making it difficult to predict shifts outside the environmental gradient where the model was trained. In this study, we combine correlative SDMs and knowledge on physiological limits to provide more robust predictions. The thermal thresholds obtained in growth and survival experiments were used as proxies of the fundamental niches of two foundational marine macrophytes. The geographic projections of these species’ distributions obtained using these thresholds and existing SDMs were similar in areas where the species are either absent‐rare or frequent and where their potential and realized niches match, reaching consensus predictions. The cold‐temperate foundational seaweed Himanthalia elongata was predicted to become extinct at its southern limit in northern Spain in response to global warming, whereas the occupancy of southern‐lusitanic Bifurcaria bifurcata was expected to increase. Combined approaches such as this one may also highlight geographic areas where models disagree potentially due to biotic factors. Physiological thresholds alone tended to over‐predict species prevalence, as they cannot identify absences in climatic conditions within the species’ range of physiological tolerance or at the optima. Although SDMs tended to have higher sensitivity than threshold models, they may include regressions that do not reflect causal mechanisms, constraining their predictive power. We present a simple example of how combining correlative and mechanistic knowledge provides a rapid way to gain insight into a species’ niche resulting in consistent predictions and highlighting potential sources of uncertainty in forecasted responses to climate change.     

The role of climate for the range limits of parapatric European land salamanders

Abrupt range limits of parapatric species may serve as a model system to understand the factors that determine species’ range borders. Theory suggests that parapatric range limits can be caused by abiotic conditions along environmental gradients, biotic interactions or a combination of both. Geographic ranges of the parapatric salamanders, Salamandra salamandra and S. atra, meet in small contact zones in the European Alps and to date, the cause of parapatry and the restricted range of S. atra remain elusive. We combine multivariate approaches and climatic data analysis to explore niche differentiation among the two salamanders with respect to the available climatic environment at their contact zones. Our purpose is to evaluate whether climatic conditions explain the species’ sharp range limits or if biotic interactions may play a role for range delimitation. Analyses were carried out in three contact zones in Switzerland to assess possible geographic variation. Our results indicate that both species occur at localities with different climatic conditions as well as the presence of a strong climatic gradient across the species’ range limits. Although the species’ climatic niches differ moderately (with a wider niche breadth for S. atra), interspecific niche overlap is found. Comparisons among the contact zones confirm geographic variation in the species’ climatic niches as well as in the conditions within the geographically available space. Our results suggest that the change in climatic conditions along the recognized gradient represents a determining factor for species’ range limits within contact zones. However, our analyses of geographic variation in climatic conditions reveal that both salamander species can occur in a much wider range of conditions than observed within contact zones. This finding and the interspecific climatic niche overlap within each contact zone provides indirect evidence that biotic interactions (likely competition) between the two species may also determine their range limits.     

Does probability of occurrence relate to population dynamics?

Hutchinson defined species' realized niche as the set of environmental conditions in which populations can persist in the presence of competitors. In terms of demography, the realized niche corresponds to the environments where the intrinsic growth rate (r) of populations is positive. Observed species occurrences should reflect the realized niche when additional processes like dispersal and local extinction lags do not have overwhelming effects. Despite the foundational nature of these ideas, quantitative assessments of the relationship between range‐wide demographic performance and occurrence probability have not been made. This assessment is needed both to improve our conceptual understanding of species' niches and ranges and to develop reliable mechanistic models of species geographic distributions that incorporate demography and species interactions. The objective of this study is to analyse how demographic parameters (intrinsic growth rate r and carrying capacity K ) and population density (N ) relate to occurrence probability (P_occ ). We hypothesized that these relationships vary with species' competitive ability. Demographic parameters, density, and occurrence probability were estimated for 108 tree species from four temperate forest inventory surveys (Québec, western USA, France and Switzerland). We used published information of shade tolerance as indicators of light competition strategy, assuming that high tolerance denotes high competitive capacity in stable forest environments. Interestingly, relationships between demographic parameters and occurrence probability did not vary substantially across degrees of shade tolerance and regions. Although they were influenced by the uncertainty in the estimation of the demographic parameters, we found that r was generally negatively correlated with P_occ, while N, and for most regions K, was generally positively correlated with P_occ. Thus, in temperate forest trees the regions of highest occurrence probability are those with high densities but slow intrinsic population growth rates. The uncertain relationships between demography and occurrence probability suggests caution when linking species distribution and demographic models.