Geographic range limits of species

Understanding the forms that the geographic range limits of species take, their causes and their consequences are key issues in ecology and evolutionary biology. They are also topics on which understanding is advancing rapidly. This themed issue of Proc. R. Soc. B focuses on the wide variety of current research perspectives on the nature and determinants of the limits to geographic ranges. The contributions address important themes, including the roles and influences of dispersal limitation, species interactions and physiological limitation, the broad patterns in the structure of geographic ranges, and the fundamental question of why at some point species no longer evolve the ability to overcome the factors constraining their distributions and thus fail to continue to spread. In this introduction, these contributions are placed in the wider context of these broad themes.     

Positive species interactions shape species' range limits

The relationship between niche and distribution, and especially the role of biotic interactions in shaping species' geographic distributions, has gained increasing interest in the last two decades. Most ecological research has focused on negative species interactions, especially competition, predation and parasitism. Yet the relevance of positive interactions – mutualisms and commensalisms – have been brought to the fore in recent years by an increasing number of empirical studies exploring their impact on range limits. Based on a review of 73 studies from a Web of Science search, we found strong evidence that positive interactions can influence the extent of species' geographic or ecological ranges through a diversity of mechanisms. More specifically, we found that while obligate interactions, and especially obligate mutualisms, tend to constrain the ranges of one or both partners, facultative positive interactions tend to widen ranges. Nonetheless, there was more variation in effects of facultative interactions on range limits, pointing to important context-dependencies. Therefore, we propose that conceptual development in this field will come from studying ecological interactions in the context of networks of many species across environmental gradients, since pairwise interactions alone might overlook the indirect and environmentally-contingent effects that species have on each other in communities of many interacting species. Finally, our study also revealed key data gaps that limit our current understanding of the pervasiveness of effects that positive interactions have on species' ranges, highlighting potential avenues for future theoretical and experimental work.     

How diversification rates and diversity limits combine to create large-scale species-area relationships

Species-area relationships (SARs) have mostly been treated from an ecological perspective, focusing on immigration, local extinction and resource-based limits to species coexistence. However, a full understanding across large regions is impossible without also considering speciation and global extinction. Rates of both speciation and extinction are known to be strongly affected by area and thus should contribute to spatial patterns of diversity. Here, we explore how variation in diversification rates and ecologically mediated diversity limits among regions of different sizes can result in the formation of SARs. We explain how this area-related variation in diversification can be caused by either the direct effects of area or the effects of factors that are highly correlated with area, such as habitat diversity and population size. We also review environmental, clade-specific and historical factors that affect diversification and diversity limits but are not highly correlated with region area, and thus are likely to cause scatter in observed SARs. We present new analyses using data on the distributions, ages and traits of mammalian species to illustrate these mechanisms; in doing so we provide an integrated perspective on the evolutionary processes shaping SARs.     

Increased seed dispersal potential towards geographic range limits in a Pacific coast dune plant

Dispersal may be favoured at geographic range edges by unstable population and metapopulation dynamics. However, dispersal may also evolve in response to geographic variation in other life-history traits, especially the mating system. Here, increased dispersal at range margins was tested for with a range-wide analysis of seed dispersal and mating system traits in Abronia umbellata, a plant endemic to Pacific coastal dunes of North America. Seeds disperse within winged anthocarps. Anthocarps from 34 populations varied widely in wing size (mass-corrected wing index). Wing index correlated negatively with threshold wind velocity for dispersal in wind tunnel tests, suggesting that wings facilitate tumbling over open sandy substrate. As predicted, wing index increased and threshold velocity decreased towards both range limits. Flower size, herkogamy and self-incompatibility declined towards range limits, indicating a shift to self-fertilization, and flower size and wing index correlated negatively. However, the increase in wing index towards range limits remained after statistically controlling flower size. These results are consistent with selection favouring dispersal at range margins. The evolutionary lability of dispersal across the range may affect the interaction between selection and gene flow in the establishment and maintenance of geographic range limits.     

Geographic range limits: achieving synthesis

Understanding of the determinants of species'' geographic range limits remains poorly integrated. In part, this is because of the diversity of perspectives on the issue, and because empirical studies have lagged substantially behind developments in theory. Here, I provide a broad overview, drawing together many of the disparate threads, considering, in turn, how influences on the terms of a simple single-population equation can determine range limits. There is theoretical and empirical evidence for systematic changes towards range limits under some circumstances in each of the demographic parameters. However, under other circumstances, no such changes may take place in particular parameters, or they may occur in a different direction, with limitation still occurring. This suggests that (i) little about range limitation can categorically be inferred from many empirical studies, which document change in only one demographic parameter, (ii) there is a need for studies that document variation in all of the parameters, and (iii) in agreement with theoretical evidence that range limits can be formed in the presence or absence of hard boundaries, environmental gradients or biotic interactions, there may be few general patterns as to the determinants of these limits, with most claimed generalities at least having many exceptions.     

Visualizing environmental correlates of species geographical range limits

Although many studies have treated aspects of species geographical distributions and numerous approaches exist for understanding overall ecological correlates of distributions, software tools for exploring environmental correlates of distributional limits are relatively few. We focused on the challenge of understanding spatial correlates of distributional limits, and developed an extension to arcview that provides a simple, univariate test and visualization for such explorations. The ‘Boundary U‐test’ seeks out environmental variables that show steep gradients associated with user‐defined boundaries across geography. We illustrate the tool and its applications with an example of the likely historical distribution of Mexican wolf (Canis lupus).     

Climatic niche conservatism and the evolutionary dynamics in species range boundaries: global congruence across mammals and amphibians

Aim Comparative evidence for phylogenetic niche conservatism – the tendency for lineages to retain their ancestral niches over long time scales – has so far been mixed, depending on spatial and taxonomic scale. We quantify and compare conservatism in the climatic factors defining range boundaries in extant continental mammals and amphibians in order to identify those factors that are most evolutionarily conserved, and thus hypothesized to have played a major role in determining the geographic distributions of many species. We also test whether amphibians show stronger signals of climatic niche conservatism, as expected from their greater physiological sensitivity and lower dispersal abilities. Location Global; continental land masses excluding Antarctica. Methods We used nearly complete global distributional databases to estimate the climatic niche conservatism in extant continental mammals and amphibians. We characterized the climatic niche of each species by using a suite of variables and separately investigate conservatism in each variable using both taxonomic and phylogenetic approaches. Finally, we explored the spatial, taxonomic and phylogenetic patterns in recent climatic niche evolution. Results Amphibians and mammals showed congruent patterns of conservatism in cold tolerance, with assemblages of escapee species (i.e. those escaping most from the climatic constraints of their ancestors) aggregated in the North Temperate Zone. Main conclusions The relative strength of climatic niche conservatism varies across the variables tested, but is strongest for cold tolerance in both mammals and amphibians. Despite the apparent conservatism in this variable, there is also a strong signal of recent evolutionary shifts in cold tolerance in assemblages inhabiting the North Temperate Zone. Our results thus indicate that distribution patterns of both taxa are influenced by both niche conservatism and niche evolution.     

Evaluating the potential causes of range limits of birds of the Colombian Andes

Aim To evaluate how factors acting at different spatial scales influence range limits in bird species of the Colombian Andes. Location Andes Mountains of Colombia. Methods We used Maxent , a climate envelope model (CEM), and environmental and geographic information to study range‐filling (i.e. the extent to which a species occurs in all the areas in which it is predicted to occur) in 70 range‐restricted bird species of the Colombian Andes. Environmental data were taken from the WorldClim database, and species occurrence data were taken from museum data collated by the BioMap project, an observational database, and the literature. We evaluated how climate and geographic barriers may shape range limits at two scales. Results At a broad extent (i.e. across the three main cordilleras within the Colombian Andes), we find that CEMs predict there to be suitable environmental conditions for particular species in regions where the species is absent, possibly as a result of dispersal limitation or biotic interactions. In contrast, at a finer scale (within a given cordillera), species generally occur across the entire area predicted to be suitable by a given CEM. Geographic discontinuities within cordilleras do not generally correspond to range limits; instead, range limits correspond to changes in environmental conditions. Main conclusions Our results suggest that different mechanisms influence the presence of species at different scales. Dispersal limitation, potentially combined with species interactions, may influence range limits at a broad extent (the entire Colombian Andes), while strong environmental gradients correspond to range limits at a finer scale (within a cordillera).     

Detecting the macroevolutionary signal of species interactions

Species interactions lie at the heart of many theories of macroevolution, from adaptive radiation to the Red Queen. Although some theories describe the imprint that interactions will have over long timescales, we are still missing a comprehensive understanding of the effects of interactions on macroevolution. Current research shows strong evidence for the impact of interactions on macroevolutionary patterns of trait evolution and diversification, yet many macroevolutionary studies have only a tenuous relationship to ecological studies of interactions over shorter timescales. We review current research in this area, highlighting approaches that explicitly model species interactions and connect them to broad‐scale macroevolutionary patterns. We also suggest that progress has been made by taking an integrative interdisciplinary look at individual clades. We focus on African cichlids as a case study of how this approach can be fruitful. Overall, although the evidence for species interactions shaping macroevolution is strong, further work using integrative and model‐based approaches is needed to spur progress towards understanding the complex dynamics that structure communities over time and space.     

Trophic interactions and range limits: the diverse roles of predation

Interactions between natural enemies and their victims are a pervasive feature of the natural world. In this paper, we discuss trophic interactions as determinants of geographic range limits. Predators can directly limit ranges, or do so in conjunction with competition. Dispersal can at times permit a specialist predator to constrain the distribution of its prey—and thus itself—along a gradient. Conversely, we suggest that predators can also at times permit prey to have larger ranges than would be seen without predation. We discuss several ecological and evolutionary mechanisms that can lead to this counter-intuitive outcome.     

Population genetic analysis of a recent range expansion: mechanisms regulating the poleward range limit in the volcano barnacle Tetraclita rubescens

As range shifts coincident with climate change have become increasingly well documented, efforts to describe the causes of range boundaries have increased. Three mechanisms—genetic impoverishment, migration load, or a physical barrier to dispersal—are well described theoretically, but the data needed to distinguish among them have rarely been collected. We describe the distribution, abundance, genetic variation, and environment of Tetraclita rubescens, an intertidal barnacle that expanded its northern range limit by several hundreds of kilometres from San Francisco, CA, USA, since the 1970s. We compare geographic variation in abundance with abiotic and biotic patterns, including sea surface temperatures and the distributions of 387 co‐occurring species, and describe genetic variation in cytochrome c oxidase subunit I, mitochondrial noncoding region, and nine microsatellite loci from 27 locations between Bahia Magdalena (California Baja Sur, Mexico) and Cape Mendocino (CA, USA). We find very high gene flow, high genetic diversity, and a gradient in physical environmental variation coincident with the range limit. We infer that the primary cause of the northern range boundary in T. rubescens is migration load arising from flow of maladapted alleles into peripheral locations and that environmental change, which could have reduced selection against genotypes immigrating into the newly colonized portion of the range, is the most likely cause of the observed range expansion. Because environmental change could similarly affect all taxa in a region whose distributional limits are established by migration load, these mechanisms may be common causes of range boundaries and largely synchronous multi‐species range expansions.     

Environmental changes define ecological limits to species richness and reveal the mode of macroevolutionary competition

Co‐dependent geological and climatic changes obscure how species interact in deep time. The interplay between these environmental factors makes it hard to discern whether ecological competition exerts an upper limit on species richness. Here, using the exceptional fossil record of Cenozoic Era macroperforate planktonic foraminifera, we assess the evidence for alternative modes of macroevolutionary competition. Our models support an environmentally dependent macroevolutionary form of contest competition that yields finite upper bounds on species richness. Models of biotic competition assuming unchanging environmental conditions were overwhelmingly rejected. In the best‐supported model, temperature affects the per‐lineage diversification rate, while both temperature and an environmental driver of sediment accumulation defines the upper limit. The support for contest competition implies that incumbency constrains species richness by restricting niche availability, and that the number of macroevolutionary niches varies as a function of environmental changes.     

Height growth rate tradeoffs determine northern and southern range limits for trees

Abstract. Identifying the biological determinants of range limits of trees is an unsolved problem of critical importance for predicting the effects of climate change on forests. Data showing that many boreal trees can grow in temperate climates indicate that southern range limits do not necessarily result from excessive temperature per se. A growth tradeoff could exist between freezing tolerance and height growth rate if adaptations to tolerate cold climates interfered with growth. Analysis of height growth rate versus freezing tolerance for twenty-two North American trees provided evidence for such a tradeoff. Provenance trials of numerous tree species also showed that a tradeoff exists within species, indicating a genetic basis for these traits. The result of this tradeoff is that at their southern range margins most species do not suffer from too much heat but rather face competitors with a faster growth rate. The implication for future climate change is that forests will not suffer catastrophic dieback due to increased temperatures but will rather be replaced gradually by faster growing types, perhaps over hundreds of years.     

Intraspecific range dynamics and niche evolution in Candidula land snail species

The range dynamics of a species can either be governed by the spatial tracing of the fundamental environmental niche or by adaptation that allows to occupy new niches. Therefore, the investigation of spatial variation in the realized environmental niche is central to the understanding of species range limit dynamics. However, the study of intraspecific niche variation has been neglected in most phylogeographical studies. We studied the spatial distribution of the realized environmental niche in three land snail species of the genus Candidula , integrating phylogeographical methods, morphometrics, and spatial biodiversity informatics . The phylogeographical analyses showed significant range expansions in all species. These expansions were accompanied in Candidula gigaxii by a shift in the realized environmental niche, the species Candidula unifasciata followed its ancestral niche during expansion while the climate changed in the area of origin and Candidula rugosiuscula tracked the ancestral environmental conditions. The significant niche shifts were associated with potentially adaptive changes of shell morphology. We propose our presented approach as a practicable framework to test hypotheses on intraspecific niche evolution. © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 90 , 303–317.     

The demography of range boundaries versus range cores in eastern US tree species

Regional species–climate correlations are well documented, but little is known about the ecological processes responsible for generating these patterns. Using the data from over 690 000 individual trees I estimated five demographic rates—canopy growth, understorey growth, canopy lifespan, understorey lifespan and per capita reproduction—for 19 common eastern US tree species, within the core and the northern and southern boundaries, of the species range. Most species showed statistically significant boundary versus core differences in most rates at both boundary types. Differences in canopy and understorey growth were relatively small in magnitude but consistent among species, being lower at the northern (average −17%) and higher at the southern (average +12%) boundaries. Differences in lifespan were larger in magnitude but highly variable among species, except for a marked trend for reduced canopy lifespan at the northern boundary (average −49%). Differences in per capita reproduction were large and statistically significant for some species, but highly variable among species. The rate estimates were combined to calculate two performance indices: R₀ (a measure of lifetime fitness in the absence of competition) was consistently lower at the northern boundary (average −86%) whereas Z^* (a measure of competitive ability in closed forest) showed no sign of a consistent boundary–core difference at either boundary.     

Linking population‐level and microevolutionary processes to understand speciation dynamics at the macroevolutionary scale

Although speciation dynamics have been described for several taxonomic groups in distinct geographic regions, most macroevolutionary studies still lack a detailed mechanistic view on how or why speciation rates change. To help partially fill this gap, we suggest that the interaction between the time taken by a species to geographically expand and the time populations take to evolve reproductive isolation should be considered when we are trying to understand macroevolutionary patterns. We introduce a simple conceptual index to guide our discussion on how demographic and microevolutionary processes might produce speciation dynamics at macroevolutionary scales. Our framework is developed under different scenarios: when speciation is mediated by geographical or resource‐partitioning opportunities, and when diversity is limited or not. We also discuss how organismal intrinsic properties and different overall geographical settings can influence the tempo and mode of speciation. We argue that specific conditions observed at the microscale might produce a pulse in speciation rates even without a pulse in either climate or physical barriers. We also propose a hypothesis to reconcile the apparent inconsistency between speciation measured at the microscale and macroscale, and emphasize that diversification rates are better seen as an emergent property. We hope to bring the reader''s attention to interesting mechanisms to be further studied, to motivate the development of new theoretical models that connect microevolution and macroevolution, and to inspire new empirical and methodological approaches to more adequately investigate speciation dynamics either using neontological or paleontological data.     

Phylogenetic affinities and species limits within the genus Megadontomys (Rodentia: Cricetidae) based on mitochondrial sequence data

The evolutionary history of the genus Megadontomys, a group of mice allopatrically distributed along the cool‐humid forest in the highlands of México, is controversial. In this study, we examined phylogenetic relationships within the genus using sequences data from the complete mitochondrial cytochrome b gene. This information also allowed us to corroborate species limits, geographic boundaries of taxonomic entities and assess genetic variation within each taxon. The results of the phylogenetic analyses based on maximum likelihood, Bayesian inference and maximum parsimony were largely congruent in that M. nelsoni and M. thomasi were more closely related relative to M. cryophilus. These results are concordant with previous studies based on morphology and allozyme variation. However, testing of the alternative hypothesis of a closer evolutionary affinity between M. nelsoni–M. cryophilus did not produce a significantly less likely tree. The lack of unambiguous support towards one of these previously proposed contending hypotheses is congruent with the alternative scenario of an almost simultaneous diversification of the three species. Application of the phylogenetic species concept and the genetic species concept supports the recognition of three distinct taxonomic entities at the specific level. M. nelsoni inhabits the Sierra Madre Oriental (Hidalgo, Veracruz, and Puebla) including the Sierra Mazateca (Oaxaca); M. cryophilus is restricted to the Sierra de Juárez (Oaxaca); and M. thomasi occurs in portions of the Sierra Madre del Sur (Guerrero) and the Sierra Mixteca (Oaxaca). Our data show that M. thomasi is formed by two genetically distinct lineages that potentially may represent distinct Evolutionary Significant Units.