Louis-Félix Bersier

QUANTITATIVE DESCRIPTORS OF FOOD‐WEB MATRICES

A food web customarily describes the qualitative feeding relationships in a community. Descriptors have been used to extract ecologically meaningful information from such data, e.g., the proportion of top species (the proportion of taxa without consumers) or vulnerability (the average number of consumers per taxon). Analyses of collections of food webs based on these properties have revealed regularities that fostered the formulation of models of food-web structure. However, it has been shown that most of these qualitative descriptors are highly sensitive to the varying levels of sampling effort used to document a food web. The principal problem is that webs described extensively include trophic links of highly uneven magnitude, with typically few strong/important links and a wealth of weak ones; with qualitative descriptors, the same weight is given to all trophic interactions. To overcome this problem, food webs should be described and analyzed quantitatively. Consequently, we propose here a suite of food-web descriptors, which are built on information-theory indices and take the magnitude of the trophic interactions into account. We define descriptors having a similar meaning as the classical qualitative indices. Two versions of each quantitative descriptor are proposed, one giving the same weight to each taxon, and one weighting each taxon by the total amount of its incoming and outgoing biomass flows. We use a published quantitative food web to exemplify the computation of the new descriptors, and discuss their potential and limitations.

Sown wildflower strips for insect conservation: a review

Abstract.  1. Sown wildflower strips are increasingly being established in European countries within agri‐environmental schemes to enhance biodiversity, especially in intensively used agricultural areas.

The impact of mowing as a management strategy for wet meadows on spider (Araneae) communities

Abstract Meadows are often managed by mowing to prevent succession and brushwood encroachment. This management practice is efficient to maintain plant diversity, but the effect on arthropod groups is less well known. We sampled spiders in the Grande Caricaie (Switzerland), a site of nature conservation importance, in two types of wet meadows, an unmanaged site and one 2-years-old mown conditions. Numbers of individuals for the most abundant families, diversity measures, and indicator taxa were compared among vegetation and treatment types. The results indicate that the less mobile spiders and species linked to litter or dead reeds, including rare species, are reduced by mowing. Present management consists in triennial mowing of 2–4 ha non-contiguous sectors. We investigate the conflict between the need to mow the meadows to maintain them, and the negative effect of mowing on spider communities. We therefore propose a new management scheme aimed at maintain the vegetation while lessening its negative effects on spider communities by providing refuges.

Modeling Food Webs: Exploring Unexplained Structure Using Latent Traits

Several stochastic models have tried to capture the architecture of food webs. This approach is interesting, but it is limited by the fact that different assumptions can yield similar results. To overcome this limitation, we develop a purely statistical approach. Body size in terms of an optimal ratio between prey and predator is used as explanatory variable. In 12 observed food webs, this model predicts, on average, 20% of interactions. To analyze the unexplained part, we introduce a latent term: each species is described by two latent traits, foraging and vulnerability, that represent nonmeasured characteristics of species once the optimal body size has been accounted for. The model now correctly predicts an average of 73% of links. The key features of our approach are that latent traits quantify the structure that is left unexplained by the explanatory variable and that this quantification allows a test of whether independent biological information, such as microhabitat use, camouflage, or phylogeny, explains this structure. We illustrate this method with phylogeny and find that it is linked to one or both latent traits in nine of 12 food webs. Our approach opens the door to the formulation of more complex models that can be applied to any kind of biological network.

SPATIAL ORGANIZATION OF A HERPETOFAUNA ON AN ELEVATIONAL GRADIENT REVEALED BY NULL MODEL TESTS

Five null model tests were applied to the herpetofaunal assemblage on the western slope of Mount Kupe, Cameroon. Based on the pattern of species range boundaries and abundances along the primary forest elevational gradient, ranging from 900 to 2000 m, the relative importance of interspecific competition and ecotones in structuring the assemblage was assessed. Tests were run for (1) all species, (2) amphibians, (3) reptiles, (4) amphibians dependent on streams for reproduction, and (5) amphibians that do not use streams for reproduction. For three null models, the observed patterns did not differ from random expectations. The results indicated that there are very few species whose gradient distributions may be limited by interspecific competition between congeners. Significant discontinuities in abun- dance patterns and range boundary dispersion revealed zonations in all subsets analyzed but neither indicated distinct species groups with sharp exclusion boundaries nor a strong response to vegetational ecotones. Physical factors varying in parallel with the gradient and specific habitat components, particularly water bodies suitable as amphibian breeding sites, are suggested to be the dominant factors limiting gradient distributions of amphibians and reptiles on Mount Kupe. The zonations revealed suggest a pattern of three spatially nonexclusive species groups: physical factors separate distinct lowland and montane species limited by physiological constraints and produce faunal discontinuities in the lower sub- montane forest around 1300 m; this boundary is encompassed by the range of a group of anuran species, whose distributions on the gradient are centered at intermediate elevation and appear to be limited by specific habitat requirements. The response to predominantly abiotic factors suggests a basic difference from endotherms, where biotic factors seem to be of major importance in limiting elevational distributions.

Phylogeny versus body size as determinants of food web structure

Food webs are the complex networks of trophic interactions that stoke the metabolic fires of life. To understand what structures these interactions in natural communities, ecologists have developed simple models to capture their main architectural features. However, apparently realistic food webs can be generated by models invoking either predator–prey body-size hierarchies or evolutionary constraints as structuring mechanisms. As a result, this approach has not conclusively revealed which factors are the most important. Here we cut to the heart of this debate by directly comparing the influence of phylogeny and body size on food web architecture. Using data from 13 food webs compiled by direct observation, we confirm the importance of both factors. Nevertheless, phylogeny dominates in most networks. Moreover, path analysis reveals that the size-independent direct effect of phylogeny on trophic structure typically outweighs the indirect effect that could be captured by considering body size alone. Furthermore, the phylogenetic signal is asymmetric: closely related species overlap in their set of consumers far more than in their set of resources. This is at odds with several food web models, which take only the view-point of consumers when assigning interactions. The echo of evolutionary history clearly resonates through current food webs, with implications for our theoretical models and conservation priorities.

The importance of landscape and spatial structure for hymenopteran-based food webs in an agro-ecosystem

1. Understanding the environmental factors that structure biodiversity and food webs among communities is central to assess and mitigate the impact of landscape changes. 2. Wildflower strips are ecological compensation areas established in farmland to increase pollination services and biological control of crop pests and to conserve insect diversity. They are arranged in networks in order to favour high species richness and abundance of the fauna. 3. We describe results from experimental wildflower strips in a fragmented agricultural landscape, comparing the importance of landscape, of spatial arrangement and of vegetation on the diversity and abundance of trap-nesting bees, wasps and their enemies, and the structure of their food webs. 4. The proportion of forest cover close to the wildflower strips and the landscape heterogeneity stood out as the most influential landscape elements, resulting in a more complex trap-nest community with higher abundance and richness of hosts, and with more links between species in the food webs and a higher diversity of interactions. We disentangled the underlying mechanisms for variation in these quantitative food web metrics. 5. We conclude that in order to increase the diversity and abundance of pollinators and biological control agents and to favour a potentially stable community of cavity-nesting hymenoptera in wildflower strips, more investment is needed in the conservation and establishment of forest habitats within agro-ecosystems, as a reservoir of beneficial insect populations.

Food web structure: from scale invariance to scale dependence, and back again?

Publisher Summary This chapter reviews the effects of rates of flows on stability and dynamics of food webs. Food webs are complex and variable, and a general understanding of their structure and functioning must rely on a careful examination of their regularities. Most natural ecosystems are orders of magnitude more complicated and more variable than planetary motion. Their elements—species or sets of species—are themselves complex objects: they are composed of heterogeneous individuals, each involved in a wealth of interactions between themselves and their biotic and abiotic environment, and each one able to adapt and evolve. Many details have to be discarded to tackle the study of such intricate systems. The interest in the study of trophic interactions within communities stems from the stability–complexity debate: the finding that local stability in random systems is not a mathematical consequence of complexity. The findings raise the question whether the food web collection analyzed here is structurally constrained with respect to the diversity of link magnitude and if so, what mechanisms give rise to such limitations. With regard to the search for regularities in food web structure, the result of scale in variant (quantitative) link density would certainly be of extreme relevance with profound implications for the issue of stability and complexity.

Nestling barn owls beg more intensely in the presence of their mother than in the presence of their father

Nestling begging behaviour may be an honest signal of need used by parents to adjust optimally both feeding rate and within-brood food allocation. Although several studies showed that mothers and fathers can be differentially responsive to nestling begging behaviour with one parent showing a stronger tendency to feed the offspring that beg the most, little information is yet available on whether offspring beg for food at different intensities from the mother than father. In the present study, we investigated in nestling barn owls whether the intensity of vocal begging behaviour in the presence of the mother and in the presence of the father is different. A difference is expected because reproductive tasks are divided between the sexes with fathers bringing more food items to the nest than mothers. The results show that although mothers transfer their prey item to one of the offspring more rapidly than fathers once in their nestbox, nestlings begged more intensely in the presence of their mother than in the presence of their father. To our knowledge, this is the first empirical evidence that offspring vocalize to different levels in the presence of their mother than in the presence of their father.

Ecology of Three Sympatric Species of the Genus Chamaeleo in a Tropical Upland Forest in Cameroon

Abstract The three species of chameleons inhabiting the primary forest on the western slope of Mount Kupe, Cameroon, Chamaeleo montium, Chamaeleo pfefferi, and Chamaeleo quadricornis, are compared with respect to elevational distribution, diet, and body size. Chameleo montium is restricted to lower elevations and markedly separated in elevational distribution from its congeners, which are exclusively found in the submontane forest. The three species prey almost entirely on arthropods, mostly coleopterans, heteropterans, hymenopterans, dipterans, and spiders; niche breadth values that incorpate prey availability are high and suggest opportunistic feeding. Niche overlaps are high with respect to prey type, but the smallest species, C. pfefferi, differs significantly from its congeners in terms of prey volume. Body size difference is largest between the two syntopic species, C. pfefferi and C. quadricornis, and identically low to the allotopic C. montium. The results on dietary segregation and morphological similarity are in accordance with competition limiting elevational distributions of the chameleons on Mount Kupe.