Climate tolerance and interspecific variation in geographic range size

The fact that climate influences the continental-scale distributions of species is one of the central tenets of ecology and biogeography. Equally elemental is that species exhibit enormous variation in geographic range size, with most occupying comparatively small areas. The degree to which climate can account for this variation remains unclear. Here, I test three alternative climate-based hypotheses for variation in range size using a large sample of tree and shrub species native to North America north of Mexico. I show that the lowest value of January average daily minimum temperature encompassed by a species' geographic range (T(MIN)), representing the 'climate extremes hypothesis', explains almost 80% of the variation in range size. Hypotheses based on seasonality and climate optima find substantially less support. The relationship between range size and T(MIN) does not change across the breadth of latitudes examined, and is general for conifers and hardwoods, and growth form (tree versus shrub). Differential freezing resistance gains support as the mechanism underlying interspecific variation in range size: using 35 species for which data were available, both T(MIN) and range size are shown to be strongly correlated with measures of freezing resistance.     

Microsatellite variation in cod Gadus morhua throughout its geographic range

Previous genetic studies using neutral markers such as allozymes, mtDNA and minisatellite loci have demonstrated varying amounts of population structure in cod Gadus morhua throughout the Atlantic. Microsatellite loci, which are potentially the most informative of presently available neutral genetic markers, have been applied extensively within western and eastern Atlantic areas but not on a range-wide basis. In the present study, six microsatellite DNA loci were used to screen cod samples from nine locations throughout the geographic range from the Scotian Shelf in the West Atlantic to the Barents and Baltic Seas in the east. Overall F _ST value was 0·03 ( P = < 0·001) across all samples. Statistically significant population differences over all loci combined were evident between more geographically distant samples, using either heterogeneity tests or F _ST analysis, with at least one locus showing significant differences between all samples (prior to Bonferroni correction). A significant correlation was observed between genetic and geographical distance, suggesting a higher level of historical and contemporary gene flow between adjacent populations than more distant populations. Samples from either end of the geographic range (Scotian Shelf and Baltic Sea) were particularly distinct when analysed using the STRUCTURE programme and also showed a high level of self-assignment when individuals of either the Scotian Shelf or Baltic Sea were tested against the entire data set. The present microsatellite study demonstrates a high level of geographic population structure between the western Atlantic, middle and eastern Atlantic and Baltic Sea, and thus, the findings should be useful in devising overall management and conservation strategies for the species.     

Incorporating phylogeographic information in alien bird distribution models increases geographic extent but not accuracy of predictions

Biological Invasions - Species distribution models (SDM) have been proposed as valuable first screening tools for predicting species responses to new environmental conditions. SDMs are usually...     

Population structure and pattern of geographic variation in Muscari comosum along its range of distribution

Genetic variation for six loci in 37 populations of Muscari comosum L. (Liliaeeae) is surveyed. One locus is monomorphic and identical in all the populations. The remaining loci are polymorphic. Although the GOT-1 and GOT-3 loci show a pronounced heterozygote deficit explained by selection acting upon these loci (or on genes linked to them), the remaining loci nearly conform to Hardy-Weinberg proportions. The overall pattern shows a low level of heterozygote deficit (F_IS=0.08) explained by the mixed mating system. The organization of genetic variation shows a low level of interpopulation differentiation (F_ST or G_ST=0.04). At the same time, autocorrelation analysis shows no pattern of geographical variation. It is concluded that gene flow and selection interact to produce the overall pattern of genetic variation.     

Incorporating density dependence into the oviposition preference-offspring performance hypothesis

1. Although theory predicts a positive relationship between oviposition preferences and the developmental performance of offspring, the strength of this relationship may depend not only on breeding site quality, but also on the complex interactions between environmental heterogeneity and density-dependent processes. Environmental heterogeneity may not only alter the strength of density dependence, but may also fundamentally alter density-dependent relationships and the preference-performance relationship. 2. Here I present results from a series of field experiments testing the effects of environmental heterogeneity and density-dependent feedback on offspring performance in tree-hole mosquitoes. Specifically, I asked: (i) how do oviposition activity, patterns of colonization and larval density differ among habitats and among oviposition sites with different resources; and (ii) how is performance influenced by the density of conspecifics, the type of resource in the oviposition site, and the type of habitat in which the oviposition site is located? 3. Performance did not differ among habitats at low offspring densities, but was higher in deciduous forest habitats than in evergreen forest habitats at high densities. Oviposition activity and larval densities were also higher in deciduous forests, suggesting a weak preference for these habitats. 4. The observed divergence of fitness among habitats with increasing density may select for consistent but weak preferences for deciduous habitats if regional abundances vary temporally. This would generate a negative preference-performance relationship when population densities are low, but a positive relationship when population densities are high. 5. This study demonstrates that failure to recognize that fitness differences among habitats may themselves be density-dependent may bias our assumptions about the ecological and evolutionary processes determining oviposition preferences in natural systems.     

The evolution of thermal performance can constrain dispersal during range shifting

Organisms can cope with changing temperature under climate change by either adapting to the temperature at which they perform best and/or by dispersing to more benign locations. The evolution of a new thermal niche during range shifting is, however, expected to be strongly constrained by genetic load because spatial sorting is known to induce fast evolution of dispersal. To broaden our understanding of this interaction, we studied the joint evolution of dispersal and thermal performance curves (TPCs) of a population during range shifting by applying an individual-based spatially explicit model. Always, TPCs adapted to the local thermal conditions. Remarkably, this adaptation coincided with an evolution of dispersal at the shifting range front being equally high or lower than at the trailing edge. This optimal strategy reduces genetic load and highlights that evolutionary dynamics during range shifting change when crucial traits such as dispersal and thermal performance jointly evolve.     

Consequences of range contractions and range shifts on molecular diversity

Due to past and current climatic changes, range contractions and range shifts are essential stages in the history of a species. However, unlike range expansions, the molecular consequences of these processes have been little investigated. In order to fill this gap, we simulated patterns of molecular diversity within and between populations for various types of range contractions and range shifts. We show that range contractions tend to decrease genetic diversity as compared with population with stable ranges but quite counterintuitively fast range contractions preserve higher levels of diversity and induce lower levels of genetic differentiation among refuge areas than slow contractions. Contrastingly, fast range shifts lead to lower levels of diversity than slow range shifts. At odds with our expectations, we find that species actively migrating toward refuge areas can only preserve higher levels of diversity in refugia if the contraction is rapid. Under slow range contraction or slow range shift, active migration toward refugia lead to a larger loss of diversity as compared with scenarios with isotropic migration and may thus not be a good evolutionary strategy. These results suggest that the levels of diversity preserved after a climate change both within and between refuge areas will not only depend on the dispersal abilities of a species but also on the speed of the change. It also implies that a given episode of climatic change will impact differently species with different generation times.     

Host specificity and geographic range in haematophagous ectoparasites

A negative interspecific correlation between the degree of habitat specialization and the size of a species' geographic range has been documented for several free living groups of organisms, providing support for the niche breadth hypothesis. In contrast, practically nothing is known about the geographic range sizes of parasitic organisms and their determinants. In the context of the niche breadth hypothesis, parasites represent ideal study systems, because of the well documented variation in host specificity among parasite species. Here, we investigated the relationship between host specificity (a measure of niche breadth) and geographic range size among flea species parasitic on small mammals, using data from seven distinct geographical regions. Two measures of host specificity were used: the number of host species used by a flea species, and a measure of the average taxonomic distance between the host species used by a flea; the latter index provides an evolutionary perspective on host specificity. After correcting for phylogenetic influences, and using either of our two measures of host specificity, the degree of host specificity of fleas was negatively correlated with the size of their geographic range in all seven regions studied here, with only one minor exception. Overall, these results provide strong support for the niche breadth hypothesis, although other explanations cannot be ruled out.     

The response of two butterfly species to climatic variation at the edge of their range and the implications for poleward range shifts

To predict changes in species' distributions due to climate change we must understand populations at the poleward edge of species' ranges. Ecologists generally expect range shifts under climate change caused by the expansion of edge populations as peripheral conditions increasingly resemble the range core. We tested whether peripheral populations of two contrasting butterflies, a small-bodied specialist (Erynnis propertius) and a large-bodied generalist (Papilio zelicaon), respond favorably to warmer conditions. Performance of populations related to climate was evaluated in seven peripheral populations spanning 1.2 degrees latitude (160 km) using: (1) population density surveys, an indirect measure of site suitability; and (2) organismal fitness in translocation experiments. There was evidence that population density increased with temperature for P. zelicaon whose population density declined with latitude in 1 of 3 sample years. On the other hand, E. propertius showed a positive relationship of population density with latitude, apparently unrelated to climate or measured habitat variables. Translocation experiments showed increased larval production at increased temperatures for both species, and in P. zelicaon, larval production also increased under drier conditions. These findings suggest that both species may increase at their range edge with warming but the preference for core-like conditions may be stronger in P. zelicaon. Further, populations of E. propertius at the range boundary may be large enough to act as sources of colonists for range expansions, but range expansion in this species may be prevented by a lack of available host plants further north. In total, the species appear to respond differently to climate and other factors that vary latitudinally, factors that will likely affect poleward expansion.     

Culture and geographic variation in orangutan behavior

Although geographic variation in an organism's traits is often seen as a consequence of selection on locally adaptive genotypes accompanied by canalized development [1], developmental plasticity may also play a role [2, 3], especially in behavior [4]. Behavioral plasticity includes both individual learning and social learning of local innovations ("culture"). Cultural plasticity is the undisputed and dominant explanation for geographic variation in human behavior. It has recently also been suggested to hold for various primates and birds [5], but this proposition has been met with widespread skepticism [6-8]. Here, we analyze parallel long-term studies documenting extensive geographic variation in behavioral ecology, social organization, and putative culture of orangutans [9] (genus Pongo). We show that genetic differences among orangutan populations explain only very little of the geographic variation in behavior, whereas environmental differences explain much more, highlighting the importance of developmental plasticity. Moreover, variation in putative cultural variants is explained by neither genetic nor environmental differences, corroborating the cultural interpretation. Thus, individual and cultural plasticity provide a plausible pathway toward local adaptation in long-lived organisms such as great apes and formed the evolutionary foundation upon which human culture was built.     

Elevated CO2 ameliorates birch response to high temperature and frost stress: implications for modeling climate-induced geographic range shifts

Despite predictions that both atmospheric CO₂ concentrations and air temperature will rise together, very limited data are currently available to assess the possible interactive effects of these two global change factors on temperate forest tree species. Using yellow birch (Betula alleghaniensis) as a model species, we studied how elevated CO₂ (800 vs. 400 μl l⁻¹) influences seedling growth and physiological responses to a 5°C increase in summer air temperatures (31/26 vs. 26/21°C day/night), and how both elevated CO₂ and air temperature during the growing season influence seedling ability to survive freezing stress during the winter dormant season. Our results show that while increased temperature decreases seedling growth, temperature-induced growth reductions are significantly lower at elevated CO₂ concentrations (43% vs. 73%). The amelioration of high-temperature stress was related to CO₂-induced reductions in both whole-shoot dark respiration and transpiration. Our results also show that increased summer air temperature, and to a lesser degree CO₂ concentration, make dormant winter buds less susceptible to freezing stress. We show the relevance of these results to models used to predict how climate change will influence future forest species distribution and productivity, without considering the direct or interactive effects of CO_2. Received: 5 June 1997 / Accepted: 16 December 1997     

Incorporating intraspecific variation in tests of trait-based community assembly

Environmental filtering and niche differentiation are processes proposed to drive community assembly, generating nonrandom patterns in community trait distributions. Despite the substantial intraspecific trait variation present in plant communities, most previous studies of trait-based community assembly have used species mean trait values and therefore not accounted for intraspecific variation. Using a null model approach, I tested for environmental filtering and niche differentiation acting on three key functional traits??vegetative height, specific leaf area (SLA), and leaf dry matter content (LDMC)??in old-field plant communities. I also examined how accounting for intraspecific variation at the among-plot and individual levels affected the detection of nonrandom assembly patterns. Tests using fixed species mean trait values provided evidence of environmental filtering acting on height and SLA and niche differentiation acting on SLA. Including plot-level intraspecific variation increased the strength of these patterns, indicating an important role of intraspecific variation in community assembly. Tests using individual trait data indicated strong environmental filtering acting on all traits, but provided no evidence of niche differentiation, although these signals may have been obscured by the effects of dispersal limitation and spatial aggregation of conspecific individuals. There was also strong evidence of nonrandom assembly of individuals within single species, with the strength of environmental filtering varying among species. This study demonstrates that, while analyses using fixed species mean trait values can provide insights into community assembly processes, accounting for intraspecific variation provides a more complete view of communities and the processes driving their assembly.     

Dissecting components of population-level variation in seed production and the evolution of masting behavior

Theory predicts that at higher trophic levels species are especially vulnerable to habitat fragmentation due to small population size and dependence on particular prey species. Using aphids as model organism, we tested the hypothesis that herbivore abundance increases in fragmented habitats as a result of reduced predator and parasitoid densities. In a 3 year-study, we examined the population dynamics of aphids with respect to host plant abundance and ant nest density in experimentally fragmented calcareous grasslands at two sites in the northern Jura mountains. Fragments of different size (area: 20.25 m², 2.25 m² and 0.25 m²) were isolated by a 5 m wide strip of frequently mown vegetation and corresponding control plots were situated in the adjacent undisturbed grassland. Aphid density was higher in fragments than in control plots. This was a combined result of a higher frequency of aphid-infested plants and larger aphid colonies in fragments than in control plots. Furthermore, a larger proportion of aphid colonies was ant-attended in fragments than in control plots. Aphid colonies were also more frequently visited by ants in fragments than in control plots in one of the 3 years. Parasitoid pressure on aphids was not influenced by the experimental fragmentation. Neither were aphid species richness and diversity affected by the fragmentation. Our study shows that even small-scale habitat fragmentation can have profound effects on the abundance of herbivorous insects. The effect on aphid density was consistent over 3 years and two sites with slightly different aphid communities.     

Genetic variation and geographic distribution of megalocytiviruses

Viruses belonging to the genus Megalocytivirus in the family Iridoviridae have caused mass mortalities in marine and freshwater fish in Asian countries. In this study, partial major capsid protein (MCP) gene of seven Japanese and six Korean megalocytiviruses was sequenced and compared with the known megalocytiviruses to evaluate genetic variation and geographic distribution of the viruses. Comparison of MCP gene nucleotide sequences revealed sequence identity of 92.8% or greater among these 48 isolates. A phylogenetic tree clearly revealed three clusters: genotype I including nine Japanese isolates, thirteen Korean isolates, one Chinese isolates, one Thailand isolate and one South China Sea isolate; genotype II including five freshwater fish isolates in Southeast Asian countries and Australia; and the remaining genotype III mainly consisted of flatfish isolate in Korea and China. This suggests that viruses belonging to the genotype I widely distribute among various fish species in many Asian countries. Conversely, the epidemic viruses belonged to genotype II and III are may be still locally spreading and constrained in their prevalence to the limited host fish species, i.e., genotype II viruses mainly distribute in Southeast Asian countries, whereas genotype III viruses distribute in flatfish species in Korea and China.     

Integrating ecophysiological models into species distribution projections of European reptile range shifts in response to climate change

Uncertainty in projections of global change impacts on biodiversity over the 21st century is high. Improved predictive accuracy is needed, highlighting the importance of using different types of models when predicting species range shifts. However, this is still rarely done. Our approach integrates the outputs of a spatially‐explicit physiologically inspired model of extinction and correlative species distribution models to assess climate‐change induced range shifts of three European reptile species (Lacerta lepida, Iberolacerta monticola, and Hemidactylus turcicus) in the coming decades. We integrated the two types of models by mapping and quantifying agreement and disagreement between their projections. We analyzed the relationships between climate change and projected range shifts. Agreement between model projections varied greatly between species and depended on whether or not they consider dispersal ability. Under our approach, the reliability of predictions is greatest where the predictions of these different types of models converge, and in this way uncertainty is reduced; sites where this convergence occurs are characterized by both current high temperatures and significant future temperature increase, suggesting they may become hotspots of local extinctions. Moreover, this approach can be readily implemented with other types of models.     

Strain variation and geographic endemism in Streptococcus iniae

Twenty-six Israeli isolates of Streptococcus iniae from both marine and fresh/brackish water sources were compared with each other and with 9 foreign isolates. All the isolates were tentatively identified according to their biochemical profile. Direct sequencing of approximately 600 bp PCR products of the 16S rDNA confirmed their identification as S. iniae at the molecular level and revealed a new (one-nucleotide) variant among Israeli isolates, in addition to 2 variants that had been previously reported. Strain variation was further examined by subjecting the isolates to randomly amplified polymorphic DNA (RAPD) and amplified fragment length polymorphism (AFLP) analyses. The RAPD method allowed separation of the isolates into only 2 groups, one including 5 Israeli fresh/brackish water isolates and one including all the other isolates. The AFLP method grouped the Israeli marine isolates into one homogeneous cluster, although they had been obtained in different years (1995 to 2001) from different species of fish, and from wild (Red Sea) as well as cultured (both Mediterranean and Red Sea) sources. The Israeli fresh/brackish water isolates and foreign isolates separated into distinct entities that clustered at generally high degrees of similarity. The distance between the clusters of the Israeli marine and fresh/brackish water isolates indicates that the S. iniae streptococcosis that has been afflicting the aquaculture industries in the 2 environments in recent years was caused by distinct strains. AFLP showed superior discriminative properties over RAPD in detecting intraspecific variation and proved to be an important tool for the characterization of S. iniae. A correlation between strain variation and geographic endemism was established.