La dinàmica temporal de les xarxes ecològiques

Els estudis de xarxes tenen una llarga tradició en ecologia i amb els anys s'ha anat veient que hi ha una sèrie de propietats comunes que s'observen reiteradament en tots els sistemes ecològics. Per exemple les xarxes ecològiques són sempre asimètriques, és a dir, unes poques espècies concentren un gran nombre d'interaccions i ocupen un lloc central en el fluxe d'energia mentre que la resta es situa en posicions més perifèriques i tenen un nombre més reduït d'interaccions. Un estudi presentat per un equip de la UAB demostra que l'estructura de les xarxes depèn alhora de diversos processos, ecològics i evolutius, que varien amb diferents ritmes temporals, incloent alhora variacions estacionals en el comportament de les espècies i canvis morfològics adaptatius que requereixen moltes generacions.Los estudios de redes tienen una larga tradición en ecología y con los años se ha ido viendo que hay una serie de propiedades comunes que se observan en todos los sistemas ecológicos. Por ejemplo, las redes ecológicas son siempre asimétricas; es decir, unas pocas especies concentran un gran número de interacciones y ocupan un lugar central en el flujo de energía mientras que el resto se sitúa en posiciones más periféricas y tienen un número más reducido de interacciones. Un estudio presentado por un equipo de la UAB demuestra que la estructura de las redes depende a la vez de diversos procesos, ecológicos y evolutivos, que varían con diferentes ritmos temporales, incluyendo variaciones estacionales en el comportamiento de las especies y cambios morfológicos adaptativos que requieren muchas generaciones

La necessitat de descarbonitzar la societat

Aquest article estudia la dependència dels recursos fòssils en la nostra societat com a motor del seu creixement, i considera la necessitat de desempallegar-se d'aquest vell model de creixement tecnològic i social. Ja sigui per interessos ecològics, econòmics o laborals, les possibilitats que ofereix un canvi de règim energètic són desitjables i beneficioses, sempre i quan estiguem disposats a adaptar sectors com el del transport o l'energètic.Este artículo estudia la dependencia de los recursos fósiles en nuestra sociedad como motor de su crecimiento, y considera la necesidad de deshacerse de este viejo modelo de crecimiento tecnológico y social. Ya sea por intereses ecológicos, económicos o laborales, las posibilidades que ofrece un cambio de régimen energético son deseables y beneficiosas, siempre y cuando estemos dispuestos a adaptar sectores como el del transporte o el energético

Butterfly diversity in the Montseny Mountains : patterns and processes

Aim: To describe spatial patterns of Mediterranean butterfly species richness by functional groups, analyze their main landscape and climatic drivers, test the existence of extinction debt effects, and predict past species richness distributions using past climatic and landscape data. Location: A transect of 186 1-km2 quadrats in the Montseny region and surrounding plains in the area of Catalonia (NE Spain), located between the Mediterranean Sea and the Pyrenees mountains. Methods: From 2003 to 2010 we systematically recorded the presence of butterfly species in each quadrat for a total 123 different butterfly species. Times series data were analyzed for average monthly temperature and average monthly precipitation to determine significant trends in climate. Proportions of landscape types per quadrat were calculated for years 1956, 1993 and 2005. Using these geographic, climatic and landscape data for each quadrat, a generalized linear model was built to determine the significant factors affecting butterfly species richness patterns. Results: Butterfly species richness followed a hump-shaped pattern along the altitudinal gradient. The highest species richness occurred at an average elevation range between 600 and 800 m. Of the landscape and climatic data, species richness was best explained by the interaction of temperature and precipitation (quadratic effect) as well as the amount of artificial unproductive land (negative effect), natural unproductive land (positive effect), and meadows and pastures present (positive effect). No extinction debt was found using past climatic and landscape data from the 1950s and 1990s. Main conclusions: Significant increases in temperature and large increases in artificial unproductive land may be attributable for the change in the predicted distribution of species from 1956 to 2005. These effects could also be filtering out certain functional groups, selecting for species most suited to higher temperatures and urbanized areas (i.e. species with high temperature preference, high dispersal ability and most generalist in habitat specialization), particularly at lower elevations

Factors influencing the foliar elemental composition and stoichiometry in forest trees in Spain

Concentrations of nutrient elements in organisms and in the abiotic environment are key factors influencing ecosystem structure and function. We studied how concentrations and stoichiometries of nitrogen (N), phosphorus (P) and potassium (K) in leaves of forest trees are related to phylogeny and to environmental factors (mean annual precipitation, mean annual temperature, forest type, and nitrogen deposition). Using data for 4691 forest plots from across Spain, we tested the following hypotheses: (i) that foliar stoichiometries of forest trees are strongly influenced by phylogeny, (ii) that climate, as an important driver of plant uptake and nutrient use efficiency, affects foliar stoichiometry, (iii) that long-term loads of N influence N, P and K concentrations and ratios in natural vegetation, and (iv) that sympatric species are differentiated according to their foliar stoichiometry, thereby reducing the intensity of resource competition. Our analyses revealed that several factors contributed to interspecific variation in elemental composition and stoichiometry. These included phylogeny, forest type, climate, N deposition, and competitive neighborhood relationships (probably related to niche segregation effect). These findings support the notion that foliar elemental composition reflects adaptation both to regional factors such as climate and to local factors such as competition with co-occurring species

Global biodiversity, stoichiometry and ecosystem function responses to human-induced C-N-P imbalances

Global change analyses usually consider biodiversity as a global asset that needs to be preserved. Biodiversity is frequently analysed mainly as a response variable affected by diverse environmental drivers. However, recent studies highlight that gradients of biodiversity are associated with gradual changes in the distribution of key dominant functional groups characterized by distinctive traits and stoichiometry, which in turn often define the rates of ecosystem processes and nutrient cycling. Moreover, pervasive links have been reported between biodiversity, food web structure, ecosystem function and species stoichiometry. Here we review current global stoichiometric gradients and how future distributional shifts in key functional groups may in turn influence basic ecosystem functions (production, nutrient cycling, decomposition) and therefore could exert a feedback effect on stoichiometric gradients. The C-N-P stoichiometry of most primary producers (phytoplankton, algae, plants) has been linked to functional trait continua (i.e. to major axes of phenotypic variation observed in inter-specific analyses of multiple traits). In contrast, the C-N-P stoichiometry of higher-level consumers remains less precisely quantified in many taxonomic groups. We show that significant links are observed between trait continua across trophic levels. In spite of recent advances, the future reciprocal feedbacks between key functional groups, biodiversity and ecosystem functions remain largely uncertain. The reported evidence, however, highlights the key role of stoichiometric traits and suggests the need of a progressive shift towards an ecosystemic and stoichiometric perspective in global biodiversity analyses

Foliar and soil concentrations and stoichiometry of N and P across European Pinus sylvestris forests: relationships with climate, N deposition and tree growth

1. This study investigated the factors underlying the variability of needle and soil elemental composition and stoichiometry and their relationships with growth in Pinus sylvestris forests throughout the species' distribution in Europe by analyzing data from 2245 forest stands. 2. Needle N concentrations and N:P ratios were positively correlated with total atmospheric N deposition, whereas needle P concentrations were negatively correlated. These relationships were especially pronounced at sites where high levels of N deposition coincided with both higher mean annual temperature and higher mean annual precipitation. Trends toward foliar P deficiency were thus more marked when high N deposition coincided with climatic conditions favorable to plant production. 3. Atmospheric N deposition was positively correlated with soil-solution NO3-, SO42-, K+, P, and Ca2+ concentrations, the soil-solution NO3-:P ratio, total soil N, and the total soil N:Olsen P ratio, and negatively correlated with soil Olsen P concentration. 4. Despite these nutrient imbalances, during the period studied (1990-2006), N deposition was positively related with Pinus sylvestris absolute basal diameter (BD) growth, although only accounting for the 10% of the total variance. However, neither N deposition nor needle N concentration were related with relative annual BD growth. In contrast, needle P concentration was positively related with both absolute and relative annual BD growth. 5. These results thus indicate a tendency of European P. sylvestris forests to store N in trees and soil in response to N deposition and unveil a trend toward increased nutrient losses in runoff as a consequence of higher soil-solution N concentrations. Overall, the data show increasing ecosystem nutrient imbalances with increasingly limiting roles of P and other nutrients such as K in European P. sylvestris forests, especially in the center of their distribution where higher levels of N deposition are observed. Thus, although the data show that N deposition has had an overall positive effect on P. sylvestris growth, the effect of continuous N deposition, associated with decreasing P and K and increasing N:P in leaves and in soil, may in the future become detrimental for the growth and competitive ability of P. Sylvestris trees

Foliar and soil concentrations and stoichiometry of N and P across European Pinus sylvestris forests : relationships with climate, N deposition and tree growth

1. This study investigated the factors underlying the variability of needle and soil elemental composition and stoichiometry and their relationships with growth in Pinus sylvestris forests throughout the species' distribution in Europe by analyzing data from 2245 forest stands. 2. Needle N concentrations and N:P ratios were positively correlated with total atmospheric N deposition, whereas needle P concentrations were negatively correlated. These relationships were especially pronounced at sites where high levels of N deposition coincided with both higher mean annual temperature and higher mean annual precipitation. Trends toward foliar P deficiency were thus more marked when high N deposition coincided with climatic conditions favorable to plant production. 3. Atmospheric N deposition was positively correlated with soil-solution NO3-, SO42-, K+, P, and Ca2+ concentrations, the soil-solution NO3-:P ratio, total soil N, and the total soil N:Olsen P ratio, and negatively correlated with soil Olsen P concentration. 4. Despite these nutrient imbalances, during the period studied (1990-2006), N deposition was positively related with Pinus sylvestris absolute basal diameter (BD) growth, although only accounting for the 10% of the total variance. However, neither N deposition nor needle N concentration were related with relative annual BD growth. In contrast, needle P concentration was positively related with both absolute and relative annual BD growth. 5. These results thus indicate a tendency of European P. sylvestris forests to store N in trees and soil in response to N deposition and unveil a trend toward increased nutrient losses in runoff as a consequence of higher soil-solution N concentrations. Overall, the data show increasing ecosystem nutrient imbalances with increasingly limiting roles of P and other nutrients such as K in European P. sylvestris forests, especially in the center of their distribution where higher levels of N deposition are observed. Thus, although the data show that N deposition has had an overall positive effect on P. sylvestris growth, the effect of continuous N deposition, associated with decreasing P and K and increasing N:P in leaves and in soil, may in the future become detrimental for the growth and competitive ability of P. Sylvestris trees

Forest resilience to global warming is strongly modulated by local-scale topographic, microclimatic and biotic conditions

Resilience of endangered rear edge populations of cold-adapted forests in the Mediterranean basin is increasingly altered by extreme heatwave and drought pressures. It remains unknown, however, whether microclimatic variation in these isolated forests could ultimately result in large intra-population variability in the demographic responses, allowing the coexistence of contrasting declining and resilient trends across small topographic gradients. Multiple key drivers promoting spatial variability in the resilience of rear edge forests remain largely unassessed, including amplified and buffered thermal exposure induced by heatwaves along topographic gradients, and increased herbivory pressure on tree saplings in defaunated areas lacking efficient apex predators. Here we analysed whether indicators of forest resilience to global warming are strongly modulated by local-scale topographic, microclimatic and biotic conditions. We studied a protected rear edge forest of sessile oak Quercus petraea, applying a suite of 20 indicators of resilience of tree secondary growth, including multidecadal and short-term indices. We also analysed sapling recruitment success, recruit/adult ratios and sapling thermal exposure across topographic gradients. We found large within population variation in secondary growth resilience, in recruitment success and in thermal exposure of tree saplings to heatwaves, and this variability was spatially structured along small-scale topographical gradients. Multidecadal resilience indices and curves provide useful descriptors of forest vulnerability to climate warming, complementing assessments based in the analysis of short-term resilience indicators. Species-specific associations of trees with microclimatic variability are reported. Biotic factors are key in determining long-term resilience in climatically stressed rear edge forests, with strong limitation of sapling recruitment by increased roe deer and wild boar herbivory. Our results also support non-stationary effects of climate determining forest growth responses and resilience, showing increased negative effects of warming and drought over the last decades in declining stands. Synthesis. Our findings do not support scenarios predicting spatially homogeneous distributional shifts and limited resilience in rear edge populations, and are more supportive of scenarios including spatially heterogeneous responses, characterised with contrasting intra-population trends of forest resilience. We conclude that forest resilience responses to climate warming are strongly modulated by local-scale microclimatic, topographic and biotic factors. Accurate predictions of forest responses to changes in climate would therefore largely benefit from the integration of local-scale abiotic and biotic factors

Impacts of global change on Mediterranean forests and their services

The increase in aridity, mainly by decreases in precipitation but also by higher temperatures, is likely the main threat to the diversity and survival of Mediterranean forests. Changes in land use, including the abandonment of extensive crop activities, mainly in mountains and remote areas, and the increases in human settlements and demand for more resources with the resulting fragmentation of the landscape, hinder the establishment of appropriate management tools to protect Mediterranean forests and their provision of services and biodiversity. Experiments and observations indicate that if changes in climate, land use and other components of global change, such as pollution and overexploitation of resources, continue, the resilience of many forests will likely be exceeded, altering their structure and function and changing, mostly decreasing, their capacity to continue to provide their current services. A consistent assessment of the impacts of the changes, however,remains elusive due to the difficulty of obtaining simultaneous and complete data for all scales of the impacts in the same forests, areas and regions. We review the impacts of climate change and other components of global change and their interactions on the terrestrial forests of Mediterranean regions, with special attention to their impacts on ecosystem services. Management tools for counteracting the negative effects of global change on Mediterranean ecosystem- services are finally discussed

Species richness, interaction networks, and diversification in bird communities : a synthetic ecological and evolutionary perspective/

Consultable des del TDXTítol obtingut de la portada digitalitzadaAquesta tesi examina els mecanismes ecològics i evolutius que mantenen els gradients de riquesa específica en ocells. S'examinen en primer lloc el patrons de diversitat de l'avifauna de Nord Amèrica. (capitol 1). Tot seguit es testen mecanismes a escala regional, tot estudiant el cas de l'avifauna de Catalunya (capitols 2 i 3). Finalment, es testen diferents mecanismes que regulen la diversitat a escala local en una comunitat d'ocells frugívors al Parc nacional de Doñana. El capitol 6 integra tots els anteriors i els relaciona tot fent una revisió bibliogràfica en profunditat.Species richness gradients have been analyzed during many decades and they have progressively emerged as a central topic in community ecology (Darwin 1859, Wallace 1878, Willis 1922, Dobzhansky 1950, Fisher 1960, Hillebrand 2004, Riclkefs 2004, Mittelbach et al. 2007, among others). Historically, species richness gradients have been analyzed from two main points of view: the ecological and the evolutionary perspective (Ricklefs 2004, 2006b). The ecological perspective assumes that populations are evolutionarily fixed and studies species richness gradients as the result of regional colonization and extinction processes, and ecological interactions (MacArthur and Wilson 1967b, Scheiner and Willig 2005). Contrarily, the evolutionary approach states that species richness gradients are the result of geographic differences in the long-term evolutionary processes of speciation and lineage extinction (Rohde 1978, 1992, Mittelbach et al. 2007). For instance, according to the evolutionary view, tropical regions achieve higher species numbers due to increased diversification rates in low latitude areas (i.e. increased speciation and/or reduced extinction rates). Interestingly enough, recent empirical evidence derived from phylogenetic studies suggests that bird diversification rates are effectively higher in the tropics thus providing empirical support for the evolutionary view (Cardillo 1999, Cardillo et al. 2005, Golberg et al. 2005, Ricklefs 2006a, Jablonski et al. 2006, Weir and Schluter 2007). In contrast with the evolutionary approach, the ecological approach assumes that populations are evolutionary stable units and focus on the effect of regional colonization-extinction dynamics, local conditions and local interactions in determining species richness gradients (Hutchinson 1959, MacArthur and Wilson 1963, 1967ab, MacArthur and Levins 1967, May 1975, Riclkefs 2006b). The ecological approach highlights that species richness gradients can be generated by ecological mechanisms independently of the evolutionary processes of speciation and lineage extinction that conform the regional pool of available species. For instance, an environmental gradient can cause a species richness gradient by limiting the number of successful colonizations in environmentally severe localities without the participation of any speciation process in the regional pool (Wright 1983, Hanski 1997, Boulinier et al. 1998). These two contrasting and complementary views, the evolutionary and ecological approaches, are progressively being merged in a unified framework (Ricklefs 2006b, Johnson and Stinchcombe 2007). For instance, Johnson and Stinchcombe have recently proposed that two general hypotheses (H1, H2) are underlying a new synthesis between community ecology and evolutionary biology. On one hand (H1), evolutionary processes explain present-day community patterns and the ecological dynamics of species interactions. For instance, extinction and colonization processes that generate species richness gradients might be shaped by species' adaptations, history and phylogenetic relationships (Wiens and Donoghue 2004, Wiens and Graham 2005, Kraft et al. 2007). On the other hand (H2), Johnson and Stinchcombe highlighted that both species interactions and community context strongly influence the direction, rate and outcome of present-day evolutionary processes (Benkman 1999, Thompson 2005). Overall, the emerging synthesis of community ecology and evolutionary ecology highlights that ecological and evolutionary views are profoundly interlinked. Here we applied an evolutionary and ecological synthetic perspective to the study of the generation and maintenance of species richness in bird communities (Johnson and Stinchcombe 2007). As a first preliminary step, we examined geographical patterns of bird species richness in North America in several functional groups and the associated environmental correlates at the continental scale (Chapter 1). Our results showed that global large-scale patterns of avian diversity in temperate regions were best viewed as the overlayed response of distinct species groups to diverse ecological factors. These results strongly suggested the convenience of choosing specific functional groups in order to examine specific macroecological evolutionary and ecological hypotheses. Consequently, in Chapter 2, we next chose forest birds as a study group for the analysis of macroecological patterns, because they are the largest group in Nearctic and Palaeoarctic regions among terrestrial birds, and hold strong species-energy relationships (Mönkkönnen et al. 2006). Regional data with colonization and extinction estimates for forest birds were available in Catalonia along an altitudinal gradient (Estrada et al. 2004) providing an exceptional dataset to study the ecological and evolutionary processes behind species richness gradients (Chapters 2 and 3). We examined the altitudinal species richness gradient in Catalonia from an evolutionary and ecological synthetic perspective. First, the role of evolutionary processes in the gradient was assessed studying changes in community phylogenetic structure along the altitudinal gradient. Secondly, the role of ecological processes was examined by studying colonization and extinction dynamics at an ecological time-scale (20 years). These analyses allowed us to examine the role of phylogeny, colonizations, extinctions, community size, productivity, habitat availability, and dispersal limitation in generating bird species richness gradients in Catalonia. However, due to the coarse structure of macroecological data used, the role of local species interactions in such processes remained yet obscure and elusive. Understanding the role of species interactions in the processes of coexistence and diversification of bird communities thus remained as a big challenge. Since long ago, ecological interactions have been hypothesized to play a role in the maintenance of diversity (Hutchinson 1959, MacArthur and Levins 1967, McPeek 1997, Mittelbach et al. 2007). Therefore, the quantitative study of species interactions was expected to provide insights on the processes that ultimately generate species richness in bird communities (Cattin et al. 2006, Rezende et al. 2007). However, data on species interactions is usually available only for specific local communities. Therefore, a macroecological approach is precluded because the bulk of the studies of community interaction networks are carried out at local scales. Accordingly, we examined the role of species interactions in the generation of species richness in a local Mediterranian bird community for which interaction good-quality data was available (Chapters 4 and 5) (Jordano 1984, 1987). We addressed the two general hypotheses delineated by Johnson and Stinchcombe (2007). First, in chapter 4, we analyzed if evolutionary processes causally affect present-day community interaction patterns and the dynamics of species interactions (H1). Secondly, in chapter five, we assessed if species interactions and community context might influence the direction, rate and outcome of present-day diversification processes that ultimately generate species richness (H2). Overall, our results provide strong support for the two hypotheses examined, thus highlighting the idea that evolutionary and ecological processes are effectively profoundly interlinked. On one hand, we show that long-term evolutionary processes effectively modulate present-day community interaction patterns and dynamics (Webb et al. 2002, Cavender-Bares and Wilczek 2003, Johnson and Stinchcombe 2007, Chapter 4). Likewise, we demonstrate that community context can potentially shape bird morphological diversification processes and drive speciation (Thompson 2005, Abrams 2006, Johnson and Stinchcombe 2007, Chapter 5). Overall, Chapters 4 and 5 provide some new empirical and theoretical insights dealing with the processes that generate and maintain local bird diversity from an integrated evolutionary and ecological perspective