The ability of climate envelope models to predict the effect of climate change on species distributions

Climate envelope models (CEMs) have been used to predict the distribution of species under current, past, and future climatic conditions by inferring a species' environmental requirements from localities where it is currently known to occur. CEMs can be evaluated for their ability to predict current species distributions but it is unclear whether models that are successful in predicting current distributions are equally successful in predicting distributions under different climates (i.e. different regions or time periods). We evaluated the ability of CEMs to predict species distributions under different climates by comparing their predictions with those obtained with a mechanistic model (MM). In an MM the distribution of a species is modeled based on knowledge of a species' physiology. The potential distributions of 100 plant species were modeled with an MM for current conditions, a past climate reconstruction (21 000 years before present) and a future climate projection (double preindustrial CO₂ conditions). Point localities extracted from the currently suitable area according to the MM were used to predict current, future, and past distributions with four CEMs covering a broad range of statistical approaches: Bioclim (percentile distributions), Domain (distance metric), GAM (general additive modeling), and Maxent (maximum entropy). Domain performed very poorly, strongly underestimating range sizes for past or future conditions. Maxent and GAM performed as well under current climates as under past and future climates. Bioclim slightly underestimated range sizes but the predicted ranges overlapped more with the ranges predicted with the MM than those predicted with GAM did. Ranges predicted with Maxent overlapped most with those produced with the MMs, but compared with the ranges predicted with GAM they were more variable and sometimes much too large. Our results suggest that some CEMs can indeed be used to predict species distributions under climate change, but individual modeling approaches should be validated for this purpose, and model choice could be made dependent on the purpose of a particular study.     

Effects of climate change on species turnover and body mass frequency distributions of South African bird communities

Climate change will lead to substantial shifts in species distributions. Most of the predictions of shifting distributions rely on modelling future distributions with ecological niche models. We used these models to investigate (i) the expected species turnover, loss and gain within bird communities of four South African biomes and (ii) the expected changes in the body mass frequency distributions of these communities. We used distributional data of the Southern African Bird Atlas Project, current climate data and two scenarios of future climate change for 2050 to build ensemble models of bird distributions. Our results indicate that future species loss, gain and turnover within the four biomes will be considerable. Climate change will also have statistically significant effects on body mass frequency distributions, and these effects differ substantially depending on the severity of future climate change. We discuss the possible ecological effects of these predicted changes on ecosystem interactions and functions.     

Modelling the spatial distribution of selected North American woodland mammals under future climate scenarios

1. North America has a diverse array of mammalian species. Model projections indicate significant variations in future climate conditions of North America, and the habitats of woodland mammals of this continent may be particularly sensitive to changes in climate.
2. We report on the potential spatial distributions of 13 wide-ranging, relatively common species of North American woodland mammals under future climate scenarios.
3. We examined the potential influence of the mean and seasonal climate variables on the distribution of species. Presence-only occurrence records of species, four predictor variables, two future climate scenarios (Representative Concentration Pathways 4.5 and 8.5), and two time steps (current and 2070) were used to build species’ distribution models using a maximum entropy algorithm (MaxEnt).
4. Our results suggested that overall, 11 of the 13 species are likely to gain climatically suitable space (regions where climate conditions will be similar to those of area currently occupied) at the continental scale, but American marten Martes americana and ‘woodland’ caribou Rangifer tarandus are likely to lose suitable climate range by 2070. Furthermore, climate space is likely to be expanding northwards under future climate scenarios for most of the mammals, and many jurisdictions in the border region between Canada and the USA are likely to lose iconic species, such as moose Alces alces. We identified regions as potential in situ and ex situ climate change refugia, which are increasingly considered to be important for biodiversity conservation.
5. The model results suggest significant implications for conservation planning for the 13 mammalian species under global climate change, especially at fine spatial scales. Numerous species that are presently common at their southern range edge will be functionally or completely extirpated in 50 years. The potential in situ and ex situ climate change refugia could provide an effective support for adaptive strategies aimed at species conservation planning.

     

Continental and regional ranges of North American mammals: Rapoport's rule in real and null worlds

Aim To assess the relationship between species richness and distribution within regions arranged along a latitudinal gradient we use the North American mammalian fauna as a study case for testing theoretical models. Location North America. Methods We propose a conceptual framework based on a fully stochastic mid‐domain model to explore geographical patterns of range size and species richness that emerge when the size and position of species ranges along a one‐dimensional latitudinal gradient are randomly generated. We also analyse patterns for the mammal fauna of North America by comparing empirical results from a biogeographical data base with predictions based on randomization null models. Results We confirmed the validity of Rapoport's rule for the mammals of North America by documenting gradients in the size of the continental ranges of species. Additionally, we demonstrated gradients of mean regional range size that parallel those of continental range. Our data also demonstrated that mean range size, measured both as a continental or a regional variable, is significantly correlated with the geographical pattern in species richness. All these patterns deviated sharply from null models. Main conclusions Rapoport's statement of an areographic relationship between species distribution and richness is highly relevant in modern discussions about ecological patterns at the geographical scale.     

Predicted impact of climate change on European bats in relation to their biogeographic patterns

There has been considerable recent interest concerning the impact of climate change on a wide range of taxa. However, little is known about how the biogeographic affinities of taxa may affect their responses to these impacts. Our main aim was to study how predicted climate change will affect the distribution of 28 European bat species grouped by their biogeographic patterns as determined by a spatial Principal Component Analysis. Using presence‐only modelling techniques and climatic data (minimum temperature, average temperature, precipitation, humidity and daily temperature range) for four different climate change scenarios (IPCC scenarios ranging from the most extreme A1FI, A2, B2 to the least severe, B1), we predict the potential geographic distribution of bat species in Europe grouped according to their biogeographic patterns for the years 2020–2030, 2050–2060 and 2090–2100. Biogeographic patterns exert a great influence on a species' response to climate change. Bat species more associated with colder climates, hence northern latitudes, could be more severely affected with some extinctions predicted by the end of the century. The Mediterranean and Temperate groups seem to be more tolerant of temperature increases, however, their projections varied considerably under different climate change scenarios. Scenario A1FI was clearly the most detrimental for European bat diversity, with several extinctions and declines in occupied area predicted for several species. The B scenarios were less damaging and even predicted that some species could increase their geographical ranges. However, all models only took into account climatic envelopes whereas available habitat and species interactions will also probably play an important role in delimiting future distribution patterns. The models may therefore generate ‘best case’ predictions about future changes in the distribution of European bats.     

Evidence for non-uniform dispersal in the biological invasions of two naturalized North American bird species

Most previous attempts to model the geographical range expansion of an invading species assume random dispersal of organisms through a homogeneous environment. These models result in a series of uniformly increasing circles radiating out from the centre of origin over time. Although these models often give reasonable fits to available data, they do not typically include mechanisms of dispersal. Alternatively, models that include assumptions of non‐random dispersal and a heterogeneous environment inevitably result in an anisotropic or jagged invasion front. This front will include propagules of pioneer individuals for the expanding species. Existing data from biological invasions reveal that the spatial structure of an invading species usually exhibits these propagules. Using population data gathered from the past century, we investigated the propagules of two North American invading bird species: the European starling (Sturnus vulgaris Linnaeus), and the house finch (Carpodacus mexicanus Müller), and found a correlation between propagule location and habitat quality. These results suggest that dispersing individuals seek out favourable habitat and remain there, thus introducing a possible mechanism for explaining non‐uniform dispersal during invasions. When combined with results from other studies, our results suggest that propagules provide starting points for future population expansion of an invading species.     

Precipitation and winter temperature predict long‐term range‐scale abundance changes in Western North American birds

Predicting biodiversity responses to climate change remains a difficult challenge, especially in climatically complex regions where precipitation is a limiting factor. Though statistical climatic envelope models are frequently used to project future scenarios for species distributions under climate change, these models are rarely tested using empirical data. We used long‐term data on bird distributions and abundance covering five states in the western US and in the Canadian province of British Columbia to test the capacity of statistical models to predict temporal changes in bird populations over a 32‐year period. Using boosted regression trees, we built presence‐absence and abundance models that related the presence and abundance of 132 bird species to spatial variation in climatic conditions. Presence/absence models built using 1970–1974 data forecast the distributions of the majority of species in the later time period, 1998–2002 (mean AUC = 0.79 ± 0.01). Hindcast models performed equivalently (mean AUC = 0.82 ± 0.01). Correlations between observed and predicted abundances were also statistically significant for most species (forecast mean Spearman′s ρ = 0.34 ± 0.02, hindcast = 0.39 ± 0.02). The most stringent test is to test predicted changes in geographic patterns through time. Observed changes in abundance patterns were significantly positively correlated with those predicted for 59% of species (mean Spearman′s ρ = 0.28 ± 0.02, across all species). Three precipitation variables (for the wettest month, breeding season, and driest month) and minimum temperature of the coldest month were the most important predictors of bird distributions and abundances in this region, and hence of abundance changes through time. Our results suggest that models describing associations between climatic variables and abundance patterns can predict changes through time for some species, and that changes in precipitation and winter temperature appear to have already driven shifts in the geographic patterns of abundance of bird populations in western North America.     

Integrating mechanistic and empirical model projections to assess climate impacts on tree species distributions in northwestern North America

Empirical and mechanistic models have both been used to assess the potential impacts of climate change on species distributions, and each modeling approach has its strengths and weaknesses. Here, we demonstrate an approach to projecting climate‐driven changes in species distributions that draws on both empirical and mechanistic models. We combined projections from a dynamic global vegetation model (DGVM) that simulates the distributions of biomes based on basic plant functional types with projections from empirical climatic niche models for six tree species in northwestern North America. These integrated model outputs incorporate important biological processes, such as competition, physiological responses of plants to changes in atmospheric CO₂ concentrations, and fire, as well as what are likely to be species‐specific climatic constraints. We compared the integrated projections to projections from the empirical climatic niche models alone. Overall, our integrated model outputs projected a greater climate‐driven loss of potentially suitable environmental space than did the empirical climatic niche model outputs alone for the majority of modeled species. Our results also show that refining species distributions with DGVM outputs had large effects on the geographic locations of suitable habitat. We demonstrate one approach to integrating the outputs of mechanistic and empirical niche models to produce bioclimatic projections. But perhaps more importantly, our study reveals the potential for empirical climatic niche models to over‐predict suitable environmental space under future climatic conditions.     

Fire regime shifts affect bird species distributions

Aim Fire is a major driver of ecosystem structure and process, and shifts in fire regimes are implicated in the decline of many species. Shifting fire regimes have been documented around the world, and fire frequency and extent is predicted to increase in many areas because of changes in both climate and land management. Here, we evaluate how predicted increases in fire frequency are likely to impact on species distributions. Location The tropical savannas of northern Australia. Methods We developed distribution models for 44 bird species using the modelling algorithm Maxent. Our models incorporated bird locality records and environmental variables including climate, total fire frequency and the subset of fire frequency occurring late in the dry season. We investigated the effect of increasing total fire frequency and increasing fire late in the dry season, on species distributions by projecting species model algorithms onto scenarios of incrementally increased total fire frequency. Results The probability of presence for most species was higher when fire frequency late in the dry season was low. Species showed a mixed response to an overall increase in total fire frequency, with one‐third predicted to increase in distribution. However, almost all species (98%) showed a decrease in predicted range with increased late‐dry season fire, and species distribution area was generally negatively correlated with an increase in late‐dry season fire. Main conclusions Our study highlighted the array of responses of species to increasing fire frequency and suggested that increased fire frequency late in the dry season is detrimental to most savanna‐restricted bird species. The understanding of individual species’ preferences for particular fire frequencies is important for informed conservation planning.     

The performance of models relating species geographical distributions to climate is independent of trophic level

Species–climate ‘envelope’ models are widely used to evaluate potential climate change impacts upon species and biodiversity. Previous studies have used a variety of methods to fit models making it difficult to assess relative model performance for different taxonomic groups, life forms or trophic levels. Here we use the same climatic data and modelling approach for 306 European species representing three major taxa (higher plants, insects and birds), and including species of different life form and from four trophic levels. Goodness‐of‐fit measures showed that useful models were fitted for >96% of species, and that model performance was related neither to major taxonomic group nor to trophic level. These results confirm that such climate envelope models provide the best approach currently available for evaluating reliably the potential impacts of future climate change upon biodiversity.     

Multimodality in the individual size distributions of bird communities

Aim Body size often plays a significant role in community assembly through its impacts on the life history and ecological attributes of species. Insight into the importance of size in structuring communities can be gained by examining the distribution of sizes of individuals [i.e. the individual size distribution (ISD) or size spectrum] in a community. ISDs have been studied extensively in aquatic and tree communities, but have received little attention in terrestrial animal communities. Here, we conduct the first macroecological analysis of ISDs in terrestrial animal communities to determine whether they show broad‐scale consistency in shape. Location North America, north of Mexico. Methods Using likelihood‐based methods and Gaussian mixture modelling, coupled with data from the Breeding Bird Survey and Christmas Bird Count, we determine whether the ISDs for thousands of breeding and wintering North American bird communities are: (1) monotonically decreasing, (2) unimodal or (3) multimodal. Results We find that avian ISDs are consistently multimodal, with most characterized by more than five modes in both breeding and wintering communities from local to continental scales. In addition, the positions of these modes along the size axis are remarkably consistent. Main conclusions The striking consistency in the ISD within bird communities, as with tree and aquatic communities, indicates that the ISD is an important and informative characterization of resource utilization within an ecological assemblage. The differences in shape of the ISD among these groups also suggest that differences in body size‐related constraints affect interactions within a group and with the environment. Our results confirm that avian assemblages do exhibit structure along the body size axis, and therefore it will be fruitful to explore this pattern in greater detail.     

The role of climate in limiting European resident bird populations

Abstract Aim To study densities of eight groups of resident forest bird populations across Europe to examine their association with abiotic (temperature, precipitation) factors. Location Europe. Methods Densities of residents (and migrant birds, which were used as a control group) were extracted from published breeding bird censuses. For each census location we obtained geographical co‐ordinates (latitude and longitude), temperature and precipitation variables describing both breeding and non‐breeding seasons. Resident densities were first examined separately in relation to co‐ordinates and principal component axes, which were extracted from temperature and precipitation variables. The relative impact of each explaining factor (co‐ordinates and principal component axis) on resident densities was checked by using residual examination and partial correlation. Results Densities of resident birds were in general negatively correlated with both latitude and longitude and temperature and precipitation factors, but latitude and temperature proved to be the strongest individual factors along which resident densities varied. The higher the latitude or the lower the temperatures, the lower were the densities of most resident birds. Partial correlation analysis suggested that of those two factors, latitude was the dominant one. Both the density of resident and migrant birds decreased towards the north, but the decrease of residents was steeper, and in the north they comprised only a small fraction of breeding bird numbers, whereas further south they commonly comprise half of the breeding bird numbers. Main conclusions The best explanatory factor for resident densities was latitude. For this reason, it was difficult to separate the relative effect of each individual factor, because latitude partly describes all the original variables (temperature and precipitation). The results suggested that precipitation was of less importance and that the interplay between latitude and ambient temperatures was not the sole factor behind the strong effect of latitude. We suggest that the coupled effect of harshening climate and decreasing amount of available energy with increasing latitude is limiting the population sizes of resident birds. We also propose that the effect of those factors is most apparent in northern Fennoscandia, where resident densities are suggested to be lower than would be expected on the basis of summer time carrying capacity.     

Assessment of the relationships of geographic variation in species richness to climate and landscape variables within and among lineages of North American freshwater fishes

Aim Geographic variation in species richness is a well‐studied phenomenon. However, the unique response of individual lineages to environmental gradients in the context of general patterns of biodiversity across broad spatial scales has received limited attention. The focus of this research is to examine relationships between species richness and climate, topographic heterogeneity and stream channel characteristics within and among families of North American freshwater fishes. Location The United States and Canada. Methods Distribution maps of 828 native species of freshwater fishes were used to generate species richness estimates across the United States and Canada. Variation in species richness was predicted using spatially explicit models incorporating variation in climate, topography and/or stream channel length and stream channel diversity for all 828 species as well as for the seven largest families of freshwater fishes. Results The overall gradient of species richness in North American freshwater fishes is best predicted by a model incorporating variables describing climate and topography. However, the response of species richness to particular climate or landscape variables differed among families, with models possessing the highest predictive ability incorporating data on climate, topography and/or stream channel characteristics within a region. Main conclusions The correlations between species richness and abiotic variables suggest a strong influence of climate and physical habitat on the structuring of regional assemblages of North American freshwater fishes. However, the relationship between these variables and species richness varies among families, suggesting the importance of phylogenetic constraints on the regulation of geographic distributions of species.     

Revisiting projected shifts in the climate envelopes of North American trees using updated general circulation models

Global climate models are constantly being upgraded, but it is often not clear what these changes have on climate change impact projections. We used difference maps to directly compare downscaled projections of temperature and precipitation across North America for two versions (or generations) of three different Atmospheric‐Ocean General Circulation Models (AOGCM)s. We found that AOGCM versions differed in their projections for the end of the current century by up to 4 °C for annual mean temperature and 60% for annual precipitation. To place these changes in an ecological context, we reanalyzed our work on shifts in tree climate envelopes (CEs) using the newer‐generation AOGCM projections. Based on the updated AOGCMs, by the 2071–2100 period, tree CEs shifted up to 2.4 degrees further north or 2.6 degrees further south (depending on the AOGCM) and were about 10% larger in size. Despite considerable differences between versions of a given AOGCM, projections made by the newer version of each AOGCM were in general agreement, suggesting convergence across the three models studied here. Assessing the AOGCM outputs in this way provides insight into the magnitude and importance of change associated with AOGCM upgrades as they continue to evolve through time.