Detecting spatial homogeneity in the world trade web with Detrended Fluctuation Analysis

In a spatially embedded network, that is a network where nodes can be uniquely determined in a system of coordinates, links' weights might be affected by metric distances coupling every pair of nodes (dyads). In order to assess to what extent metric distances affect relationships (link's weights) in a spatially embedded network, we propose a methodology based on DFA (Detrended Fluctuation Analysis). DFA is a well developed methodology to evaluate autocorrelations and estimate long-range behaviour in time series. We argue it can be further extended to spatially ordered series in order to assess autocorrelations in values. A scaling exponent of 0.5 (uncorrelated data) would thereby signal a perfect homogeneous space embedding the network. We apply the proposed methodology to the World Trade Web (WTW) during the years 1949-2000 and we find, in some contrast with predictions of gravity models, a declining influence of distances on trading relationships.Comment: 15 pages, 7 figure

Sukcesija mediteranske serpentinske vegetacije u središnjoj Italiji

The mam plant communities of ultramafic (serpentine) outcrops of a Mediterranean area in central Italy are described and their successional pathway discussed. Four vegetation stages are distinguished: a) an endemic garigue stage, with high percentage of exposed rocky soil belonging to Armerio-Alyssetum bertolonii; b) a garigue stage similar to the previous one but with several Mediterranean evergreen shrubs; c) a closed evergreen maquis of Mediterranean type belonging to Viburno-Quercetum illicis subass. ericetosum; d) an evolved evergreen forest stage with some deciduous species belonging to the Viburno-Quercetum ilicis subass ornetosum. While the pioneer stages are characterised by several endemic serpentinophytes, adapted to the peculiar environmental conditions of ultramafic shallow soils, the more evolved vegetation stages, on the contrary, are typical of Mediterranean environments and do not show any serpentine characterisation.Opisuju se glavni tipovi vegetacije na serpentinima mediteranskog dijela središnje Italije i prikazuje se njihova sukcesija. Utvrđena su četiri vegetacijska stadija: a) endemičan garig na izrazito stjenovitoj podlozi, koji pripada zajednici Armerio-Alyssetum bertolonii; b) garig sličan prethodnom, ali sa rijetkim vazdazelcnim mediteranskim grmljem; c) vazdazelena makija mediteranskog tipa, koja pripada zajednici Vi- burno-Quercetum ilicis subass. ericetosum; d) vazdazelena šuma sa nešto listopadnih elemenata, koja pripada zajednici Viburno-Quercetum ilicis subass. ornetosum. Pionirski stadiji vegetacije okarakterizirani su sa nekoliko enemičnih serpentinofita, prilagođenih na osebujne uvjete života na plitkim serpentinskim tlima. Suprotno tome, razvijeniji vegetacijski stadiji tipični su za mediteransko područje i ne pokazuju nikakva serpentinska obilježja

Integrative models explain the relationships between species richness and productivity in plant communities

The relationship between plant productivity and species richness is one of the most debated and important issues in ecology. Ecologists have found numerous forms of this relationship and its underlying processes. However, theories and proposed drivers have been insufficient to completely explain the observed variation in the forms of this relationship. Here, we developed and validated integration models capable of combining twenty positive or negative processes affecting the relationship. The integration models generated the classic humped, asymptotic, positive, negative and irregular forms and other intermediate forms of the relationship between plant richness and productivity. These forms were linked to one another and varied according to which was considered the dependent variable. The total strengths of the different positive and negative processes are the determinants of the forms of the relationship. Positive processes, such as resource availability and species pool effects, can offset the negative effects of disturbance and competition and change the relationship. This combination method clarifies the reasons for the diverse forms of the relationship and deepens our understanding of the interactions among processes

Species–area relationships in continuous vegetation : evidence from Palaearctic grasslands

Aim: Species-area relationships (SARs) are fundamental scaling laws in ecology although their shape is still disputed. At larger areas power laws best represent SARs. Yet, it remains unclear whether SARs follow other shapes at finer spatial grains in continuous vegetation. We asked which function describes SARs best at small grains and explored how sampling methodology or the environment influence SAR shape. Location: Palaearctic grasslands and other non-forested habitats. Taxa: Vascular plants, bryophytes and lichens. Methods: We used the GrassPlot database, containing standardised vegetation-plot data from vascular plants, bryophytes, and lichens spanning a wide range of grassland types throughout the Palaearctic and including 2057 nested-plot series with at least seven grain sizes ranging from 1 cm2 to 1024 m². Using non-linear regression, we assessed the appropriateness of different SAR functions (power, power quadratic, power breakpoint, logarithmic, Michaelis-Menten). Based on AICc, we tested whether the ranking of functions differed among taxa, methodological settings, biomes or vegetation types. Results: The power function was the most suitable function across the studied taxonomic groups. The superiority of this function increased from lichens to bryophytes to vascular plants to all three taxonomic groups together. The sampling method was highly influential as rooted-presence sampling decreased the performance of the power function. By contrast, biome and vegetation type had practically no influence on the superiority of the power law. Main conclusions: We conclude that SARs of sessile organisms at smaller spatial grains are best approximated by a power function. This coincides with several other comprehensive studies of SARs at different grain sizes and for different taxa, thus supporting the general appropriateness of the power function for modelling species diversity over a wide range of grain sizes. The poor performance of the Michaelis-Menten function demonstrates that richness within plant communities generally does not approach any saturation, thus calling into question the concept of minimal area

An interspecific variation in rhizosphere effects on soil anti-erodibility

Soil erosion due to underground leakage is a major factor causing land degradation in karst regions. Rhizosphere effects (REs) on soil anti-erodibility (SAE) can alleviate this type of soil erosion by improving soil physical processes such as aggregate stability. However, the magnitudes and causes of interspecific variation in REs on SAE remain unclear. We tested the rhizosphere SAE indices of 42 key woody species distributed worldwide. Biologically active matter (BAM) and analogs of antibiotics (AOAs) that affect the SAE in rhizosphere soils were tested by gas chromatography-mass spectrometry (GC-MS). We then used principal component analysis (PCA) and redundancy analysis (RA) to establish a spectrum of interspecific variability in the REs for the first time. The spectrum shows a gradient of change among species. Eleven species exerted negative REs on the SAE, while the remaining species showed positive effects along the spectrum. The species with large positive effects were mostly deciduous, which have high contents of both BAM and total organic matter and low contents of AOAs in their rhizosphere soil; compared with the other species tested, these species also have more leaves and roots and are better adapted to barren soils. The botanical characteristics of species with negative REs on the SAE differed from those with large positive effects. The contents of BAM in the rhizosphere accounted for 16\u201323% of the total variation in REs on the SAE. This study quantified interspecific variation in REs and identified root exudates with negative REs

Optimizing sampling effort and information content of biodiversity surveys: a case study of alpine grassland

Aims: Current rates of biodiversity loss do not allow for inefficient monitoring. Optimized monitoring maximizes the ratio between information and sampling effort (i.e., time and costs). Sampling effort increases with the number and size of sampling units. We hypothesize that an optimal size and number of sampling units can be determined providing maximal information via minimal effort. We apply an approach that identifies the optimal size and number of sampling quadrats. The approach can be adapted to any study system. Here we focus on alpine grassland, a diverse but threatened ecosystem. Location: Gran Paradiso National Park, Italy. Methods: We sampled nine 20 m 7 20 m-plots. Each plot consisted of 100 2 m 7 2 m-subplots. Species richness and Shannon diversity were quantified for different sizes and quantities of subplots. We simulated larger subplot sizes by unifying adjacent 2 m 7 2 m-subplots. Shannon's information entropy was used to quantify information content among richness and diversity values resulting from different subplot sizes and quantities. The optimal size and number of subplots is the lowest size and number of subplots returning maximal information. This optimal subplot size and number was determined by Mood's median test and segmented linear regression, respectively. Results: The information content among richness values increased with subplot size, irrespective of the number of subplots. Therefore, the largest subplot size available is the optimal size for information about richness. Information content among diversity values increased with subplot size if 18 or less subplots were considered, and decreased if at least 27 subplots were sampled. The subplot quantity consequently determined whether the smallest or largest subplot size available is the optimal size, and whether the optimal size can be generalized across richness and diversity. Given a 2 m 7 2 m size, we estimated an optimal quantity of 54. Given a size of 4 m 7 4 m, we estimated an optimal number of 36. The optimal number of plots can be generalized across both indices because it barely differed between the indices given a fixed subplot size. Conclusions: The information content among richness and diversity values depends on the sampling scale. Shannon's information entropy can be used to identify the optimal number and size of plots that return most information with least sampling effort. Our approach can be adapted to other study systems to create an efficient in-situ sampling design, which improves biodiversity monitoring and conservation under rapid environmental change

The nature of vegetation science

Fil: Partel, Meelis. University of Tartu. Institute of Ecology and Earth Sciences; EstoniaFil: Chiarucci, Alessandro. University of Siena. Department of Environmental Science ‘‘G. Sarfatti’’. BIOCONNET, Biodiversity and Conservation Network; ItaliaFil: Diaz, Sandra Myrna. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Córdoba. Instituto Multidisciplinario de Biología Vegetal (p); Argentina; Argentina. Universidad Nacional de Córdoba; ArgentinaFil: Wilson, J. Bastow. University of Otago. Botany Department ; Nueva Zeland

There is room for everyone: Invasion credit cannot be inferred from the species–area relationship in fragmented forests

Questions Land use change, habitat fragmentation and biological invasion represent major drivers of global change that strongly interact to alter ecosystems. Following the breaking apart of forests into smaller fragments or the afforestation of former agricultural lands, biodiversity experiences drastic changes due to species loss and turnover over time. This leads to two important outcomes, namely extinction debt and invasion (colonization) credit, which both reflect the inertia of the system's response to environmental changes. Our study investigated the following questions: Is it possible to infer invasion credit from species–area relationship (SAR) residuals both for native and alien plants? Is there any trend linked with the degree of habitat fragmentation through time? Location Somme, Oise and Aisne departments, northern France. Methods We analyzed the pattern of SARs' residuals for native and alien vascular plant species separately across nine sets of forest fragments that differ by the landscape matrix they are embedded in (i.e., open field, bocage, forest), while considering plant richness, area and age of the 355 forest patches. Results The relationship between alien and native SARs’ residuals is positive across all landscapes, suggesting a lack of invasion credit. Instead, these results support the “rich get richer” hypothesis, that is a high environmental heterogeneity allows colonization by new species, be they native or alien. Interestingly, the relationship between alien and native residuals depends upon fragment age (i.e., time since patch creation) in the most intensively managed landscapes (i.e., open fields). In the latter, recent forest patches are more prone to alien invasion, as a likely consequence of increased alien propagule pressure (i.e., more sources and vectors for alien plants), increased forest invasibility (i.e., disturbance-induced environmental heterogeneity), and decreased matrix permeability (i.e., natives are more dispersal-limited than aliens). Conclusions Our study provides new insights into alien species ecology, by showing that (i) it is not possible to infer “invasion credit” from the SAR's residuals; (ii) the invasion rate by alien species in forest fragments increases with their native species richness, and (iii) this relationship depends upon patch age in intensively managed landscapes

There is room for everyone: Invasion credit cannot be inferred from the species–area relationship in fragmented forests

Questions: Land use change, habitat fragmentation and biological invasion represent major drivers of global change that strongly interact to alter ecosystems. Following the breaking apart of forests into smaller fragments or the afforestation of former agricultural lands, biodiversity experiences drastic changes due to species loss and turnover over time. This leads to two important outcomes, namely extinction debt and invasion (colonization) credit, which both reflect the inertia of the system's response to environmental changes. Our study investigated the following questions: Is it possible to infer invasion credit from species–area relationship (SAR) residuals both for native and alien plants? Is there any trend linked with the degree of habitat fragmentation through time?. Location: Somme, Oise and Aisne departments, northern France. Methods: We analyzed the pattern of SARs' residuals for native and alien vascular plant species separately across nine sets of forest fragments that differ by the landscape matrix they are embedded in (i.e., open field, bocage, forest), while considering plant richness, area and age of the 355 forest patches. Results: The relationship between alien and native SARs’ residuals is positive across all landscapes, suggesting a lack of invasion credit. Instead, these results support the “rich get richer” hypothesis, that is a high environmental heterogeneity allows colonization by new species, be they native or alien. Interestingly, the relationship between alien and native residuals depends upon fragment age (i.e., time since patch creation) in the most intensively managed landscapes (i.e., open fields). In the latter, recent forest patches are more prone to alien invasion, as a likely consequence of increased alien propagule pressure (i.e., more sources and vectors for alien plants), increased forest invasibility (i.e., disturbance-induced environmental heterogeneity), and decreased matrix permeability (i.e., natives are more dispersal-limited than aliens). Conclusions: Our study provides new insights into alien species ecology, by showing that (i) it is not possible to infer “invasion credit” from the SAR's residuals; (ii) the invasion rate by alien species in forest fragments increases with their native species richness, and (iii) this relationship depends upon patch age in intensively managed landscapes

Potential natural vegetation and pre-anthropic pollen records on the Azores Islands in a Macaronesian context

This paper discusses the concept of potential natural vegetation (PNV) in the light of the pollen records available to date for the Macaronesian biogeographical region, with emphasis on the Azores Islands. The classical debate on the convenience or not of the PNV concept has been recently revived in the Canary Islands, where pollen records of pre-anthropic vegetation seemed to strongly disagree with the existing PNV reconstructions. Contrastingly, more recent PNV model outputs from the Azores Islands show outstanding parallelisms with pre-anthropic pollen records, at least in qualitative terms. We suggest the development of more detailed quantitative studies to compare these methodologies as an opportunity for improving the performance of both. PNV modelling may benefit by incorporating empirical data on past vegetation useful for calibration and validation purposes, whereas palynology may improve past reconstructions by minimizing interpretative biases linked to differential pollen production, dispersal and preservation.Spanish Ministry of Economy and Competititvity, project PaleoNAO [CGL2010-15767]Spanish Ministry of Economy and Competititvity, project RapidNAO [CGL2013-40608-R]Spanish Ministry of Economy and Competititvity, project PaleoMODES [CGL2016-75281-C2-1-R]info:eu-repo/semantics/publishedVersio