Quantitative Descriptors of Food-Web Matrices

Abstract

Abstract. 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.

Interaction frequency, network position, and the temporal persistence of interactions in a plant–pollinator network

“…We calculated several descriptive metrics for each yearly network: plant species richness, pollinator species richness, connectance (the proportion of all possible interactions realized); links per species (a binary measure of interaction richness: the mean number of unique binary links per species, where a link is the connection of two species in the network through interaction events), linkage density (a quantitative measure of interaction diversity, weighted by the total number of interactions of each species; see details and formulas in Bersier et al. , Dormann et al. ), interaction diversity (Shannon diversity index calculated for interactions), interaction evenness (Shannon evenness of interaction matrix), network specialization H 2 ′ (ranging from 0, indicating no specialization, to 1, indicating maximum specialization; Blüthgen et al.…”

Structural properties of mutualistic networks withstand habitat degradation while species functional roles might change

“…To estimate interaction diversity we used connectance (fill) of each networks as the number of realized links divided by the size (Tylianakis et al 2010). To also incorporate the quantitative component of each link (the interaction strengths) we calculated quantitative link density (LD q ) following Bersier et al (2002) by weighing each species by their relative frequency.…”

Evaluating factors that predict the structure of a commensalistic epiphyte–phorophyte network

“…We use network statistics that characterize several aspects of network structure [40] to describe the observed network and to compare observed values with the values obtained from models of network determinants using functions in R [12,[40][41][42] (see the electronic supplementary material, appendix S2): (i) connectance (C ¼ number of links/IJ); (ii) nestedness [4], two metrics: N ¼ 100 2 T, where T is the matrix temperature [43], and nestedness based on overlap and decreasing fill (NODF) [44]; (iii) interaction evenness [45]; (iv) H 0 2 ; a quantitative metric of specialization that controls for the interaction frequencies expected from the total observations per species, i.e. the effect of the differences in the abundance of species, that results in abundant species interacting more frequently and with more partners, is removed [46]; (v) generality and vulnerability, the weighted mean number of phorophyte species per epiphyte species, and epiphyte species per phorophyte species, respectively [47]; and (vi) the average interaction strength asymmetry for phorophytes and for epiphytes [8]. The first two statistics are based on unweighted links, whereas the remaining are based on weighted links.…”