What constrains food webs? A maximum entropy framework for predicting their structure with minimal biases

Food webs are complex ecological networks whose structure is both ecologically and statistically constrained, with many network properties being correlated with each other. Despite the recognition of these invariable relationships in food webs, the use of the principle of maximum entropy (MaxEnt) in network ecology is still rare. This is surprising considering that MaxEnt is a renowned and rigorous statistical tool precisely designed for understanding and predicting many different types of constrained systems. Precisely, this principle asserts that the least-biased probability distribution of a system's property, constrained by prior knowledge about that system, is the one with maximum information entropy. Here we show how MaxEnt can be used to derive many food-web properties both analytically and heuristically. First, we show how the joint degree distribution (the joint probability distribution of the numbers of prey and predators for each species in the network) can be derived analytically using the number of species and the number of interactions in food webs. Second, we present a heuristic and flexible approach of finding a network's adjacency matrix (the network's representation in matrix format) based on simulated annealing and SVD entropy. We built two heuristic models using the connectance and the joint degree sequence as statistical constraints, respectively. We compared both models' predictions against corresponding null and neutral models commonly used in network ecology using open access data of terrestrial and aquatic food webs sampled globally. We found that the heuristic model constrained by the joint degree sequence was a good predictor of many measures of food-web structure, especially the nestedness and motifs distribution. Specifically, our results suggest that the structure of terrestrial and aquatic food webs is mainly driven by their joint degree distribution

Sommes-nous bien informés? : écarts entre la couverture du changement climatique et de la biodiversité par les médias et la littérature scientifique

Les changements climatiques et de biodiversité sont deux enjeux environnementaux de premier plan, souvent liés l’un à l’autre. Ils se produisent actuellement à un rythme supérieur à ce qui est acceptable pour maintenir les processus biophysiques de la planète (Rockström et al., 2009). La réduction de l’impact des activités humaines sur l’environnement nécessite au préalable une compréhension de ces problématiques, d’où l’importance des découvertes scientifiques qui améliorent nos connaissances sur ces enjeux et ouvrent des pistes de réflexion menant à l’élaboration de solutions temporaires ou durables

Effect of a major canopy disturbance on the coexistence of Acer saccharum and Fagus grandifolia in the understorey of an old-growth forest

In forest communities, species coexistence can be favoured by disturbance-related variations in light regime coupled with rank reversal in species performance. The objective of this study was to determine if a major canopy disturbance, resulting from an ice storm, would favour the coexistence of sugar maple (Acer saccharum) and beech (Fagus grandifolia) in an old-growth forest located near the northern limit of the species' range. The growth, density and frequency of occurrence of understorey stems were evaluated 7 years after the ice storm and compared with pre-disturbance values to determine if sugar maple, a slightly less shade-tolerant species, would be favoured over beech. Although height and radial growth increased three- to five-fold, and > 70% of the 5-10 cm d.b.h. stems showed a release, sugar maple did not benefit more from the opening of the canopy than beech. The inability of sugar maple to outgrow beech might be related to the high proportion of beech root sprouts at our site, suggesting that the potential for the disturbancemediated mechanism to slow the competitive exclusion of maple might decrease as the importance of vegetative reproduction increases in beech. Prior to the disturbance, sugar maple was more abundant and occurred in a larger proportion of the plots among small size classes, while beech dominated in the larger size classes. Although some increases in the density of both species were detected after the disturbance, the general trends of relative abundance and distribution of the species were not modified significantly. Our results do not indicate that a single canopy disturbance such as the major 1998 ice storm significantly favoured sugar maple over beech. When a disturbance opens the canopy, competition for light in gaps may tend to maintain or reinforce a pre-existing hierarchy instead of reversing it, especially when interspecific differences in postdisturbance growth are small or absent, as in this study. This study shows that the dynamics of sugar maple - beech communities are more complex than previously envisaged and that revised models should incorporate additional factors (e.g. vegetative reproduction) that might also play an important role in regulating community dynamics