Bernat C. López

Novel communities from climate change

Climate change is generating novel communities composed of new combinations of species. These result from different degrees of species adaptations to changing biotic and abiotic conditions, and from differential range shifts of species. To determine whether the responses of organisms are determined by particular species traits and how species interactions and community dynamics are likely to be disrupted is a challenge. Here, we focus on two key traits: body size and ecological specialization. We present theoretical expectations and empirical evidence on how climate change affects these traits within communities. We then explore how these traits predispose species to shift or expand their distribution ranges, and associated changes on community size structure, food web organization and dynamics. We identify three major broad changes: (i) Shift in the distribution of body sizes towards smaller sizes, (ii) dominance of generalized interactions and the loss of specialized interactions, and (iii) changes in the balance of strong and weak interaction strengths in the short term. We finally identify two major uncertainties: (i) whether large-bodied species tend to preferentially shift their ranges more than small-bodied ones, and (ii) how interaction strengths will change in the long term and in the case of newly interacting species.

STRUCTURE AND DYNAMICS OF THE ROOT SYSTEM

The belowground component of terrestrial ecosystems is much less understood than any of the aboveground components, yet important ecosystem processes such as nutrient recycling, water storage, and long-term carbon accumulation occur largely in this compartment. For instance, belowground structures accounted for up to 83% of the total biomass in 13 Mediterranean woody communities (Hilbert and Canadell 1995), and belowground primary production was 60–80% of the total net primary production in a variety of woody systems (Coleman 1976; Agren et al. 1980; Fogel 1985). Yet both root biomass and production are infrequently studied and technical difficulties make the measurements often inaccurate. Furthermore, plant root distribution and maximum rooting depths play important roles in overall ecosystem function, but it was not until recently that ecosystem-level and global comprehensive studies have been undertaken (Canadell et al. 1996; Jackson et al. 1996).

Climate change impacts on body size and food web structure on mountain ecosystems

The current distribution of climatic conditions will be rearranged on the globe. To survive, species will have to keep pace with climates as they move. Mountains are among the most affected regions owing to both climate and land-use change. Here, we explore the effects of climate change in the vertebrate food web of the Pyrenees. We investigate elevation range expansions between two time-periods illustrative of warming conditions, to assess: (i) the taxonomic composition of range expanders; (ii) changes in food web properties such as the distribution of links per species and community size-structure; and (iii) what are the specific traits of range expanders that set them apart from the other species in the community—in particular, body mass, diet generalism, vulnerability and trophic position within the food web. We found an upward expansion of species at all elevations, which was not even for all taxonomic groups and trophic positions. At low and intermediate elevations, predator : prey mass ratios were significantly reduced. Expanders were larger, had fewer predators and were, in general, more specialists. Our study shows that elevation range expansions as climate warms have important and predictable impacts on the structure and size distribution of food webs across space.

Declining hydraulic performances and low carbon investments in tree rings predate Scots pine drought-induced mortality

Key messageThe retrospective analysis of wood anatomical features evidences how a long-term deterioration of hydraulic performance and carbon use portend drought-induced mortality in Scots pine.AbstractWidespread episodes of drought-induced tree mortality are predicted to become more frequent as climate becomes warmer and drier. Nevertheless, growth trends and their links to changes in wood anatomy before tree dies are still poorly understood. Wood anatomical features provide valuable information that can be extracted to infer the mechanisms leading to tree death. In this study, we characterize drought-induced mortality affecting two Scots pine (Pinus sylvestris) sites (Prades and Arcalís) located in the North Eastern Iberian Peninsula. Co-occurring now-dead and living Scots pine trees were sampled and their wood anatomical features were measured and compared. We aimed to detect differences in anatomical features between living and dead trees, and to infer past physiological performances that might have determined their subsequent death or survival. Now-dead trees showed lower tracheid and resin duct production, and smaller radial lumen diameters than co-occurring living trees. At the more xeric Prades site, these anatomical differences were larger and chronic, i.e. were observed over the three studied decades, whilst they were less pronounced at the other, more mesic Arcalís site, where tree mortality episodes were more recent. This indicates that dead trees’ hydraulic conductivity was severely affected and that carbon investment in xylem formation and resin duct production was constrained prior to tree death. Our findings show that both hydraulic deterioration and low carbon allocation to xylem formation were associated to drought-induced mortality in Scots pine. Nevertheless, the temporal dynamics of these processes differed between populations as a function of site climatic conditions.

Network complexity and species traits mediate the effects of biological invasions on dynamic food webs

Biological invasions are a major threat to natural communities worldwide. While several species traits have been identified as important determinants of invasion success, a systematic exploration of the effects of invasions on native communities, and the role of species and community features on community robustness in the face of invasion is lacking. We present a theoretical approximation considering food web structure and species population dynamics to study the effects of invasions on complex food webs. We find that less complex (i.e. less connected) food webs are more resistant to invasions. Simulated invasions promote profound changes in several food web properties and stability measures, such as decreases in modularity and the number of food chains from basal to top species; and a decoupling of community- and population-level temporal variability. Additionally, species traits such as body size and diet breadth are strong determinants of invasion success across several trophic levels, with larger and more generalist species being more successful invaders in general. Our work complements species-centred invasion studies by adding a more holistic and systematic perspective to the study of invasions on species interaction networks.

Drought-induced mortality selectively affects Scots pine trees that show limited intrinsic water-use efficiency responsiveness to raising atmospheric CO2

Widespread drought-induced tree mortality has been documented around the world, and could increase in frequency and intensity under warmer and drier conditions. Ecophysiological differences between dying and surviving trees might underlie predispositions to mortality, but are poorly documented. Here we report a study of Scots pines (Pinus sylvestris L.) from two sites located in north-eastern Iberian Peninsula where drought-associated mortality episodes were registered during the last few decades. Time trends of discrimination against 13C (Δ13C) and intrinsic water-use efficiency (WUEi) in tree rings at an annual resolution and for a 34 year period were used to compare co-occurring now-dead and surviving pines. Results indicate that both surviving and now-dead pines significantly increased their WUEi over time, although this increase was significantly lower for now-dead individuals. These differential WUEi trends corresponded to different scenarios describing how plant gas exchange responds to increasing atmospheric CO2 (Ca): the estimated intercellular CO2 concentration was nearly constant in surviving pines but tended to increase proportionally to Ca in now-dead trees. Concurrently, the WUEi increase was not paralleled by a growth enhancement, regardless of tree state, suggesting that in water-limited areas like the Mediterranean, it cannot overcome the impact of an increasingly warmer and drier climate on tree growth.

Do pre- and post-copulatory sexually selected traits covary in large herbivores?

BackgroundIn most species, males compete to gain both matings (via pre-copulatory competition) and fertilizations (via post-copulatory competition) to maximize their reproductive success. However, the quantity of resources devoted to sexual traits is finite, and so males are predicted to balance their investment between pre- and post-copulatory expenditure depending on the expected pay-offs that should vary according to mating tactics. In Artiodactyla species, males can invest in weapons such as horns or antlers to increase their mating gains or in testes mass/sperm dimensions to increase their fertilization efficiency. Moreover, it has been suggested that in these species, males with territory defence mating tactic might preferentially increase their investment in post-copulatory traits to increase their fertilization efficiency whereas males with female defence mating tactic might increase their investment in pre-copulatory sexually selected traits to prevent other males from copulating with females. In this study, we thus test the prediction that male’s weapon length (pre-copulatory trait) covaries negatively with relative testes size and/or sperm dimensions (post-copulatory traits) across Artiodactyla using a phylogenetically controlled framework.ResultsSurprisingly no association between weapon length and testes mass is found but a negative association between weapon length and sperm length is evidenced. In addition, neither pre- nor post-copulatory traits were found to be affected by male mating tactics.ConclusionsWe propose several hypotheses that could explain why male ungulates may not balance their reproductive investment between pre- and post-copulatory traits.

Explaining the successful introduction of the alpine marmot in the Pyrenees.

Alpine marmots were introduced into the northern Pyrenees between 1948 and 1988 from individuals captured in the French Alps, in order to bolster food sources for the golden eagle and brown bear. The marmot’s subsequent occupation of the southern Pyrenees has been extremely fast. From an initial population of ~400 individuals, the present population in the southern Pyrenees is estimated to be of more than 10,000 individuals. The objective of this study was to assess what were the mechanisms that have enabled such a fast occupation of the territory. We studied habitat preferences and habitat selection of the alpine marmot in the southern Pyrenees both at the micro- and meso-scale, and compared our results with similar data in the bibliography on their native region. We also compared climatic data from both the native and introduction sites. Our results indicate relatively low climate (precipitation and temperature) matching between the two sites but a relatively high habitat matching. Marmots negatively select high woody cover and the presence of conifers in their home range, while they choose alpine and sub-alpine meadows close to rivers with boulders. Furthermore, the marmot population is independent of snow cover duration. We conclude that the successful establishment in the Pyrenees by the alpine marmot is explained both by the habitat- and climate-matching mechanisms. In both aspects, marmots show a generalist response. Meso-scale GIS-derived variables were non significant when analyzed together with local, micro-scale variables from field measurements.

Nest-Box Use by Boreal Owls (Aegolius funereus) in the Pyrenees Mountains in Spain

Abstract The Boreal Owl (Aegolius funereus) is a nocturnal forest-dwelling species widely distributed throughout the world. One of the least studied and most southerly populations of this species lives in the Pyrenees Mountains. This population, 500–600 pairs, probably suffered a slight decline during recent decades due to forestry management practices. The use of nest boxes may become necessary to protect this species in southern Europe. The objective of this study was to establish a standard protocol for nest-box installation, based on the analysis of occupation rates of nest boxes installed during the last 20 yr and the comparison of reproductive data for nest boxes and natural nest cavities. Our results indicated that nest boxes should be installed at high elevations (above 2000 m asl), below 4 m aboveground, and preferably facing south or southeast. We also found that most territories are located where maximum July temperatures are <17°C, which suggests that increasing temperatures due to climate change may have a negative effect on this population.

Potential distribution of the alpine marmot in southern Pyrenees.

Alpine marmots were introduced in the French Pyrenees between 1948 and 1988. The exact number of re-introduced individuals is unknown, but it oscillated around 400. The likely preference of marmots for the southern sunny slopes rapidly facilitated their expansion to the southern Pyrenees, where the lack of both natural predators and of important interspecific competitors also likely facilitated an important expansion of this species. There is only one attempt to broadly calculate the population of marmots in the southern Pyrennes, estimating a population of around 10,000 individuals. However, there exist no reliable data to calculate the potential distribution of this new colonizing species in the southern Pyrenees, and a map of the potential distribution of the species is necessary to see whether alpine marmots can potentially establish in sites where it might be necessary to manage its populations for various reasons. We developed a map of potential distribution based on census carried out in summer of 2007 in an area of more than 600 km. We censused more around 300 colonies together with around 300 random points to characterized habitat selection variables. A map with a pixel size of 15 × 15 m has been obtained based on preferred habitats and also on distances to other habitats for the whole southeastern Pyrenees.