Influence of environmental heterogeneity on genetic diversity and structure in an endemic southern Californian oak

Understanding how specific environmental factors shape gene flow while disentangling their importance relative to the effects of geographical isolation is a major question in evolutionary biology and a specific goal of landscape genetics. Here, we combine information from nuclear microsatellite markers and ecological niche modelling to study the association between climate and spatial genetic structure and variability in Engelmann oak (Quercus engelmannii), a wind-pollinated species with high potential for gene flow. We first test whether genetic diversity is associated with climatic niche suitability and stability since the Last Glacial Maximum (LGM). Second, we use causal modelling to analyse the potential influence of climatic factors (current and LGM niche suitability) and altitude in the observed patterns of genetic structure. We found that genetic diversity is negatively associated with local climatic stability since the LGM, which may be due to higher immigration rates in unstable patches during favourable climatic periods and/or temporally varying selection. Analyses of spatial genetic structure revealed the presence of three main genetic clusters, a pattern that is mainly driven by two highly differentiated populations located in the northern edge of the species distribution range. After controlling for geographic distance, causal modelling analyses showed that genetic relatedness decreases with the environmental divergence among sampling sites estimated as altitude and current and LGM niche suitability. Natural selection against nonlocal genotypes and/or asynchrony in reproductive phenology may explain this pattern. Overall, this study suggests that local environmental conditions can shape patterns of genetic structure and variability even in species with high potential for gene flow and relatively small distribution ranges.     

Beyond climate control on species range: The importance of soil data to predict distribution of Amazonian plant species

Aim

To evaluate the relative importance of climatic versus soil data when predicting species distributions for Amazonian plants and to gain understanding of potential range shifts under climate change.

Location

Amazon rain forest.

Methods

We produced species distribution models (SDM) at 5‐km spatial resolution for 42 plant species (trees, palms, lianas, monocot herbs and ferns) using species occurrence data from herbarium records and plot‐based inventories. We modelled species distribution with Bayesian logistic regression using either climate data only, soil data only or climate and soil data together to estimate their relative predictive powers. For areas defined as unsuitable to species occurrence, we mapped the difference between the suitability predictions obtained with climate‐only versus soil‐only models to identify regions where climate and soil might restrict species ranges independently or jointly.

Results

For 40 out of the 42 species, the best models included both climate and soil predictors. The models including only soil predictors performed better than the models including only climate predictors, but we still detected a drought‐sensitive response for most of the species. Edaphic conditions were predicted to restrict species occurrence in the centre, the north‐west and in the north‐east of Amazonia, while the climatic conditions were identified as the restricting factor in the eastern Amazonia, at the border of Roraima and Venezuela and in the Andean foothills.

Main conclusions

Our results revealed that soil data are a more important predictor than climate of plant species range in Amazonia. The strong control of species ranges by edaphic features might reduce species’ abilities to track suitable climate conditions under a drought‐increase scenario. Future challenges are to improve the quality of soil data and couple them with process‐based models to better predict species range dynamics under climate change.     

The geography and ecology of plant speciation: range overlap and niche divergence in sister species

A goal of evolutionary biology is to understand the roles of geography and ecology in speciation. The recent shared ancestry of sister species can leave a major imprint on their geographical and ecological attributes, possibly revealing processes involved in speciation. We examined how ecological similarity, range overlap and range asymmetry are related to time since divergence of 71 sister species pairs in the California Floristic Province (CFP). We found that plants exhibit strikingly different age-range correlation patterns from those found for animals; the latter broadly support allopatric speciation as the primary mode of speciation. By contrast, plant sisters in the CFP were sympatric in 80% of cases and range sizes of sisters differed by a mean of 10-fold. Range overlap and range asymmetry were greatest in younger sisters. These results suggest that speciation mechanisms broadly grouped under ‘budding’ speciation, in which a larger ranged progenitor gives rise to a smaller ranged derivative species, are probably common. The ecological and reproductive similarity of sisters was significantly greater than that of sister–non-sister congeners for every trait assessed. However, shifts in at least one trait were present in 93% of the sister pairs; habitat and soil shifts were especially common. Ecological divergence did not increase with range overlap contrary to expectations under character displacement in sympatry. Our results suggest that vicariant speciation is more ubiquitous in animals than plants, perhaps owing to the sensitivity of plants to fine-scale environmental heterogeneity. Despite high levels of range overlap, ecological shifts in the process of budding speciation may result in low rates of fine-scale spatial co-occurrence. These results have implications for ecological studies of trait evolution and community assembly; despite high levels of sympatry, sister taxa and potentially other close relatives, may be missing from local communities.     

Species richness and food web structure of soil decomposer community as affected by the size of habitat fragment and habitat corridors

While most ecologists agree that the effects of fragmentation on diversity of organisms are predominantly negative and that the scale of fragmentation defines their severity, the role of habitat corridors in mitigating those effects still remains controversial. This ambiguousness rests largely on various difficulties in experimentation, a problem partially solved in the present paper by the use of easily manipulated soil communities. In this 2.5‐year‐long field experiment, we investigated the responses of soil decomposer organisms (from microbes to mesofaunal predators) to habitat fragment size, in the presence or absence of habitat corridors connecting the fragments. The habitat fragments and corridors, composed of forest humus soil, were embedded in mineral soil representing an uninhabitable (or nonpreferred) matrix for the decomposer organisms. The results demonstrate that soil decomposer organisms do respond to changes in their habitat size: the species richness of microarthropods (mites and collembolans) increased as the size of the fragments increased. Especially collembolan species and predatory mites proved to be sensitive to the restricted habitat size, which is suggested to be a consequence of the large proportion of rare species and small and fluctuating population sizes in these groups. Contrary to our expectations, the presence of corridors had no positive effects on species richness or abundance of any of the studied faunas, possibly because of the low quality of the corridors. On the other hand, the biomass of soil fungi increased in the presence of corridors, which apparently provided a preferred pathway for vegetative dispersal of the fungi. Our results indicate that despite their characteristic underground environment, the response of soil decomposer organisms – in particular that of microarthropods – to habitat size is not unlike to that of the larger organisms in aboveground habitats.     

Molecular systematics of the Amazonian endemic genus Hylexetastes (Aves: Dendrocolaptidae): taxonomic and conservation implications

Hylexetastes woodcreepers are endemic to the terra firme forests of the Amazon basin. Currently, most taxonomic sources recognize two species of Hylexetastes (H. perrotii and H. stresemanni), each divided into three subspecies. Some authors maintain that the H. perrotii subspecies should be elevated to full species status. In particular, Hylexetastes perrotii brigidai is endemic to the eastern Amazon, the second Amazonian area of endemism (Xingu) most affected by deforestation and habitat degradation. Consequently, the taxonomic status of H. p. brigidai is of particular concern for conservation. Thus far, only morphological characters have been evaluated for the taxonomic delimitation of species and subspecies of Hylexetastes. We present a molecular phylogenetic analysis of all subspecies to help delimit Hylexetastes interspecific limits. Fragments of two mitochondrial (Cytb and ND2) and three nuclear genes (FGB5, G3PDH and MUSK) from 57 Hylexetastes specimens were sequenced. An ecological niche model was estimated to describe more accurately the potential distributions of taxa and to evaluate their vulnerability to ongoing deforestation. Phylogenetic analyses support the paraphyly of the polytypic H. perrotii as currently delimited and the elevation of Hylexetastes perrotii uniformis to full species rank, as well as the presence of three evolutionary significant units (ESUs) within this newly delimited species, including one grouping all H. p. brigidai specimens. Alternatively, under lineage-based species concepts, our results support at least five evolutionary species in Hylexetastes: H. stresemanni, H. undulatus, H. perrotii, H. uniformis and H. brigidai. Each of these taxa and ESUs are distributed in different interfluvial areas of the Amazon basin, which have different degrees of disturbance. Because they occupy the most heavily impacted region among all Hylexetastes ESUs, regular assessments of the conservation statuses of H. p. brigidai and both H. uniformis ESUs are paramount.     

Tree species distribution in temperate forests is more influenced by soil than by climate

Knowledge of the ecological requirements determining tree species distributions is a precondition for sustainable forest management. At present, the abiotic requirements and the relative importance of the different abiotic factors are still unclear for many temperate tree species. We therefore investigated the relative importance of climatic and edaphic factors for the abundance of 12 temperate tree species along environmental gradients. Our investigations are based on data from 1,075 forest stands across Switzerland including the cold‐induced tree line of all studied species and the drought‐induced range boundaries of several species. Four climatic and four edaphic predictors represented the important growth factors temperature, water supply, nutrient availability, and soil aeration. The climatic predictors were derived from the meteorological network of MeteoSwiss, and the edaphic predictors were available from soil profiles. Species cover abundances were recorded in field surveys. The explanatory power of the predictors was assessed by variation partitioning analyses with generalized linear models. For six of the 12 species, edaphic predictors were more important than climatic predictors in shaping species distribution. Over all species, abundances depended mainly on nutrient availability, followed by temperature, water supply, and soil aeration. The often co‐occurring species responded similar to these growth factors. Drought turned out to be a determinant of the lower range boundary for some species. We conclude that over all 12 studied tree species, soil properties were more important than climate variables in shaping tree species distribution. The inclusion of appropriate soil variables in species distribution models allowed to better explain species' ecological niches. Moreover, our study revealed that the ecological requirements of tree species assessed in local field studies and in experiments are valid at larger scales across Switzerland.     

Determinants of intra-annual population dynamics in a tropical soil arthropod

Studies on inter-annual dynamics of populations from temperate regions have shown that density dependence and climate effects are relatively common, albeit weak. Yet, for short-lived organisms, intra-annual variation may be at least as important. Furthermore, tropical species commonly experience temperatures close to their upper thermal limit and thus may be more likely to experience heat stress. Here, we used the soil mite Rostrozetes ovulum to investigate the drivers of intra-annual population dynamics in an Amazonian rain forest. We sampled 3,600 soil cores from 20 transects during 13 months, obtaining 180 mite counts. Next, we built a dynamic N-mixture model accounting for different detection probabilities between soil types. In a Bayesian framework, we used this model to estimate (a) the strength of density dependence and (b) per capita growth rates, which were then tested against environmental variables. We found that the intra-annual population dynamics of R. ovulum were weakly density dependent. Further, per capita growth rates increased with resource supply (litterfall) and decreased with maximum temperature over much of the observed thermal range, although these effects explained relatively little variance. Yet, the seasonal correlation between these factors created a trade-off, so that realized population growth was highest when neither resource supply nor thermal suitability was optimal. Overall, our results suggest that the mechanisms shaping soil animal population dynamics may be surprisingly similar across latitudes. Our model offers a starting point for analyses of soil animal counts when extraction from soil samples is imperfect.     

城市嵌草式铺装植被和大型土壤动物群落组成特征——以新泰市为例

嵌草式铺装近年来广泛应用于城市停车场及道路,研究其群落组成特征对城市生态的综合治理具有重要意义。以新泰市为例,通过现场采样调研分析探讨了嵌草式铺装对植被和土壤动物群落组成和多样性的影响。结果显示,植被调查中共记录草本植物14科22属22种。牛筋草(Eleusine indica)为新泰市嵌草式铺装中植被的优势种。植被的平均密度和生物量分别为(21.87±2.15)株/m²和(4.15±0.45)g/m²。不同站点间植株密度和高度有显著空间差异(P<0.05),生物量和盖度差异不显著。不同站点的植被物种多样性特征也有所差异。大型土壤动物调查共记录7目8科10种,其中蚁科为优势类群。嵌草式铺装中土壤动物的丰度、生物量均显著高于不透水路面(P<0.05)。逐步回归分析表明,嵌草式铺装中植被和土壤动物的丰度和生物量与土壤的容积密度和含水率等理化性质显著相关(P<0.05)。土壤动物丰度与植被盖度、土壤动物生物量与植被生物量间也具有显著相关关系(P<0.05)。结果表明,嵌草式铺装能够有效提高城市生态系统中植被和土壤动物群落的丰...     

干热河谷土壤酶活性对碳氮添加的响应

碳氮是生态系统物质循环中的关键元素,然而碳氮对土壤生态系统功能是否具有交互作用还缺乏深入研究。通过干热河谷土壤的葡萄糖和硝酸铵的交互添加实验,研究了氮、碳添加及其交互作用对干热河谷土壤酶活性及其化学计量学特征的影响。结果表明:碳氮交互作用显著影响了葡萄糖苷酶、酸性磷酸酶和亮氨酸胺肽酶活性。在对照样方的土壤中,碳添加使葡萄糖苷酶活性降低了31.4%;在施氮样方的土壤中,碳添加使葡萄糖苷酶活性增加了54.4%。对照样方中的土壤中,碳添加对酸性磷酸酶和亮氨酸胺肽酶活性促进作用较小(分别增加了102.4%和28.8%);相比之下,在施氮样方的土壤中,碳添加使酸性磷酸酶和亮氨酸胺肽酶活性分别增加了302.2%和68.8%。碳添加对葡萄糖苷酶和酸性磷酸酶活性的促进作用与土壤初始有效氮含量显著正相关。几丁质酶活性在碳添加下增加了53.7%,对氮添加没有响应。碳氮交互作用也显著影响了C∶N和C∶P的水解酶化学计量关系。在对照样方中碳添加使C∶N水解酶化学计量比降低了16.9%,C∶P水解酶化学计量比降低了19.9%;而在施氮样方土壤中,碳添加的影响较小C∶N和C∶P水解酶化学计量比分别降低了1.9%和5.8%。碳氮交互作用对干热河谷土壤酶活性的影响表明碳氮在土壤生态系统功能具有重要作用。     

Environmental data sets matter in ecological niche modelling: an example with Solenopsis invicta and Solenopsis richteri

Aim  In response to a recent paper suggesting the failure of ecological niche models to predict between native and introduced distributional areas of fire ants ( Solenopsis invicta ), we sought to assess methodological causes of this failure.
Location  Ecological niche models were developed on the species' native distributional area in South America, and projected globally.
Methods  We developed ecological niche models based on six different environmental data sets, and compared their respective abilities to anticipate the North American invasive distributional area of the species.
Results  We show that models based on the 'bioclimatic variables' of the WorldClim data set indeed fail to predict the full invasive potential of the species, but that models based on four other data sets could predict this potential correctly.
Main conclusions  The difference in predictive abilities appears to centre on the complexity of the environmental variables involved. These results emphasize important influences of environmental data sets on the generality and ability of ecological niche models to anticipate novel phenomena, and offer a simpler explanation for the lack of predictive ability among native and invaded distributional areas than that of niche shifts.     

High‐resolution distribution modeling of a threatened short‐range endemic plant informed by edaphic factors

1. Short‐range endemic plants often have edaphic specializations that, with their restricted distributions, expose them to increased risk of anthropogenic extinction.
2. Here, we present a modeling approach to understand habitat suitability for Ricinocarpos brevis R.J.F.Hend. & Mollemans (Euphorbiaceae), a threatened shrub confined to three isolated populations in the semi‐arid south‐west of Western Australia. The model is a maximum entropy species distribution projection constructed on the basis of physical soil characteristics and geomorphology data at approximately 25 m² (1 arc‐second) resolution.
3. The model predicts the species to occur on shallow, low bulk density soils that are located high in the landscape. The model shows high affinity (72.1% average likelihood of occurrence) for the known populations of R. brevis, as well as identifying likely locations that are not currently known to support the species. There was a strong relationship between the likelihood of R. brevis occurrence and soil moisture content that the model estimated at a depth of 20 cm.
4. We advocate that our approach should be standardized using publicly available data to generate testable hypotheses for the distribution and conservation management of short‐range endemic plant species for all of continental Australia.

     

Biogeography of the Amazon molly: ecological niche and range limits of an asexual hybrid species

Aim To understand the relative contributions of environmental factors, dispersal limitations and the presence of sperm donors in determining the distribution of the Amazon molly (Poecilia formosa), a sperm‐dependent unisexual fish species of hybrid origin. To explore niche similarities and/or differences between the hybrid and parental species. To evaluate whether large‐scale abiotic factors can explain a successful introduction of both P. formosa and Poecilia latipinna. Location South‐east United States, Mexico and Central America. Methods We used abiotic variables in ecological niche modelling (ENM) to identify regions with suitable conditions for the presence of the Amazon molly and its two parental species (P. latipinna and Poecilia mexicana). We also used a recently developed metric to calculate the degree of niche overlap between the hybrid and its parental species. Results ENM produced highly significant models [all area under the curve (AUC) > 0.99 for the three species]. Annual mean temperature and minimum temperature of the coldest month were the variables that best explained the distribution of the Amazon molly. With the exception of south Florida, few areas beyond the known distribution of the species were predicted to have suitable environmental conditions. The hybrid species niche overlaps partially with the parental species. However, given the available data, it is neither more similar nor more different than expected by chance. Main conclusions Two different processes are acting to limit the distribution of P. formosa. At the northern limit, although a sperm donor species is present further north, suitable environmental conditions are absent from nearby locations. At the southern limit, a sperm donor species is present and areas with good environmental conditions are present at nearby locations, suggesting that dispersal ability is the limiting factor. We found that the hybrid species overlaps in a similar way with both parental species while still having its own niche identity. This result may be explained by the fact that hybrid species inherit characteristics of two ecologically divergent species, which can result in intermediate or even transgressive phenotypes. These results support recent work on the role of hybridization in diversification.     

Effects of an invasive crayfish on trophic relationships in north‐temperate lake food webs

1. The introduction of invasive species is one of the main threats to global biodiversity, ecosystem structure and ecosystem processes. In freshwaters, invasive crayfish alter macroinvertebrate community structure and destroy macrophyte beds. There is limited knowledge on how such invasive species‐driven changes affect consumers at higher trophic levels. 2. In this study, we explore how the invasive rusty crayfish Orconectes rusticus, a benthic omnivore, affects benthic macroinvertebrates, as well as the broader consequences for ecosystem‐level trophic flows in terms of fish benthivory and trophic position (TP). We expected crayfish to decrease abundance of benthic macroinvertebrates, making most fish species less reliant on benthic resources. We expected crayfish specialists (e.g. Lepomis sp. and Micropterus sp.) to increase their benthic dependence. 3. In 10 northern Wisconsin lakes, we measured rusty crayfish relative abundance (catch per unit effort, CPUE), macroinvertebrate abundance, and C and N stable isotope ratios of 11 littoral fish species. We used stable isotope data and mixing models to characterise the trophic pathways supporting each fish species, and related trophic structure to crayfish relative abundance, fish body size and abiotic predictors using hierarchical Bayesian models. 4. Benthic invertebrate abundance was negatively correlated with rusty crayfish relative abundance. Fish benthivory increased with crayfish CPUE for all 11 fish species; posterior probabilities of a positive effect were >95%. TP also increased slightly with crayfish CPUE for some species, particularly smallmouth bass, largemouth bass, rock bass and Johnny darter. Moreover, both fish body size and lake abiotic variables explained variation in TP, while their effects on benthivory were small. 5. Rusty crayfish abundance explained relatively little of the overall variation in fish benthivory and TP. Although rusty crayfish appear to have strong effects on abundances of benthic macroinvertebrates, energy flow pathways and trophic niches of lentic fishes were not strongly influenced by invasive rusty crayfish.     

Patterns of diversity and range size of selected plant groups along an elevational transect in the Bolivian Andes

Patterns of point, -, and -diversity, and of an index of range size were studied for Acanthaceae, Araceae, Bromeliaceae, Melastomataceae, Palmae, and Pteridophyta in 204 vegetation plots of 400 m² along an elevational gradient in humid montane forest at 220–3950 m in central Bolivia. Zonal forest point-diversity (mean species number per plot) and -diversity (extrapolated total species number) showed either hump-shaped curves or constant values from lowlands to mid-elevations, followed by a steep decline. These patterns correspond to the hypothesis of maximum diversity at intermediate levels of productivity, while for pteridophytes and epiphytic Bromeliaceae they also fitt a null-model of random distribution within bounded geographical ranges. The ratio of point to -diversity was surprisingly constant across study groups and elevation with values of 0.2–0.4. Range size index (mean inverse range size of all species recorded at a given elevation) either increased with elevation, showed hump-shaped patterns, or remained roughly constant. Non-zonal habitats (ravines, ridge-tops, rock faces, roadsides, pastures) contributed significantly to overall species number for terrestrial herbs and shrubs of Bromeliaceae, Melastomataceae, and Pteridophyta, but not for Acanthaceae, Araceae, and epiphytic taxa. With few exceptions, endemism was most pronounced in zonal forests. These results imply that conservation measures should focus on zonal forests in a few large reserves at low and mid-elevations and in many smaller reserves at higher elevations.     

Nucleotide diversity and demographic history of Pinus bungeana,an endangered conifer species endemic in China

Orographic and climatic oscillations have played crucial roles in shaping the nucleotide diversity and evolutionary history of many species across the Northern Hemisphere. In this study, based on 10 nuclear loci and a chloroplast DNA marker, we analyzed the nucleotide polymorphisms and demographic history of the endangered conifer species Pinus bungeana in Northwest China and investigated the phylogenetic relationships between P. bungeana and two related species, that is, Pinus gerardiana and Pinus squamata. We found that P. bungeana exhibited an extremely low level of nucleotide diversity (π_sil = 0.00159). Demographic simulations based on DIYABC analysis showed that P. bungeana underwent demographic expansion and contraction during the Miocene. According to ecological niche modeling, we found that this species survived in situ during the glacial period and was not restricted to southern refugia. We speculate that P. bungeana may have experienced widespread population shrinkage from the Last Interglacial to the Last Glacial Maximum due to geological or climatic events. Isolation‐with‐migration analysis revealed that the divergence (~2.4–4.2 Ma) among P. bungeana and its related species was significantly associated with the Qinghai–Tibetan Plateau uplift events in the mid‐to‐late Tertiary period. Species tree analyses suggested that these three related Pinus species formed a monophyletic clade with high bootstrap support. These results suggest that the Miocene–Pliocene and Pleistocene geological and climatic fluctuations might have profoundly affected the nucleotide diversity and demography of this psychrotolerant conifer species in western China.     

An evaluation of a GARP model as an approach to predicting the spatial distribution of non-vagile invertebrate species

One of the primary goals of any systematic, taxonomic or biodiversity study is the characterization of species distributions. While museum collection data are important for ascertaining distributional ranges, they are often biased or incomplete. The Genetic Algorithm for Rule-set Prediction (GARP) is an ecological niche modelling method based on a genetic algorithm that has been argued to provide an accurate assessment of the spatial distribution of organisms that have dispersal capabilities. The primary objective of this study is to evaluate the accuracy of a GARP model to predict the spatial distribution of a non-invasive, non-vagile invertebrate whose full distributional range was unknown. A GARP predictive model based on seven environmental parameters and 42 locations known from historical museum records for species of the trapdoor spider genus Promyrmekiaphila was produced and subsequently used as a guide for ground truthing the model. The GARP model was neither a significant nor an accurate predictor of spider localities and was outperformed by more simplistic BIOCLIM and GLM models. The isolated nature of Promyrmekiaphila populations mandates that environmental layers and their respective resolutions are carefully chosen for model production. Our results strongly indicate that, for modelling the spatial distribution of low vagility organisms, one should employ a modelling method whose results are more conducive to interpretation than models produced by a 'black box' algorithm such as GARP.     

Ecology and conservation of Pseudolestes mirabilis (Odonata: Zygoptera), a damselfly endemic to Hainan Island of China

Although some efforts have addressed oriental dragonfly conservation, knowledge on the ecology and geographic distribution of such dragonflies remains scant. The phoenix damselfly Pseudolestes mirabilis Kirby, 1900 is the single species in the family Pseudolestidae that is endemic to Hainan Island of China. This damselfly was recommended by the International Union for the Conservation of Nature to be of priority for further study and conservation. In this work, we use ecological niche modeling techniques to estimate the dimensions of the realized niches of this damselfly and to predict its potential distribution. Our findings suggest that the phoenix damselfly possessed a small climate space characterized by low temperature and high precipitation. Highly suitable areas are mainly distributed in the low‐altitude regions of southern central tree‐covered mountains in Hainan. Caution is warranted when considering the potential habitat loss attributed to human activity and climate change.     

Mixed population genomics support for the central marginal hypothesis across the invasive range of the cane toad (Rhinella marina) in Australia

Understanding factors that cause species' geographic range limits is a major focus in ecology and evolution. The central marginal hypothesis (CMH) predicts that species cannot adapt to conditions beyond current geographic range edges because genetic diversity decreases from core to edge due to smaller, more isolated edge populations. We employed a population genomics framework using 24 235–33 112 SNP loci to test major predictions of the CMH in the ongoing invasion of the cane toad (Rhinella marina) in Australia. Cane toad tissue samples were collected along broad‐scale, core‐to‐edge transects across their invasive range. Geographic and ecological core areas were identified using GIS and habitat suitability indices from ecological niche modelling. Bayesian clustering analyses revealed three genetic clusters, in the northwest invasion‐front region, northeast precipitation‐limited region and southeast cold temperature‐limited region. Core‐to‐edge patterns of genetic diversity and differentiation were consistent with the CMH in the southeast, but were not supported in the northeast and showed mixed support in the northwest. Results suggest cold temperatures are a likely contributor to southeastern range limits, consistent with CMH predictions. In the northeast and northwest, ecological processes consisting of a steep physiological barrier and ongoing invasion dynamics, respectively, are more likely explanations for population genomic patterns than the CMH.     

Transformations of carbon and nitrogen in a Calciorthid soil amended with a range of organic residues

A calcareous soil was treated with an organic fertilizer mixture of grape debris and peat, or with pig manure, chicken manure, city refuse or with aerobic or anaerobic sewage sludges. Changes in different fractions of carbon and nitrogen were followed by analysis of the soil, immediately after these additions, after they had decomposed for six months in the soil, after a maize harvest, and after a subsequent barley harvest. The various forms of carbon had decreased after six months. The rate of decrease varied with the nature of the organic materials. In all samples, the fulvic acids fraction decreased most. The main transformations happened during the first six months whether there was a crop growing or not, but the crops had no influence on the breakdown of the organic materials. The ratio of humic acids/fulvic acids had increased after six months of humification and, in general terms, the final values of the ratio oxidizable carbon/extractable carbon were lower than the initial ones, indicating a higher degree of humification in the organic matter by the end of the experiment. The total nitrogen level remained practically constant after the first six months of humification even after the second crop. Part of the organic nitrogen of the soil samples amended with chicken manure, city refuse or the two sludges was transformed into mineral nitrogen during the humification process. The extractable nitrogen values increased because of the fertilizer applied during the pot trials.