Macroscopic ecosystem studies often complete our knowledge based on population-level experiments and models. In this paper, the changed control of ecosystem functioning is reported by analyzing the structure of the energy flow network of a tidal marsh community (Crystal River, Florida). The positional importance of trophic components is characterized by a graph theoretical approach. Then, positional importance of points is compared to the magnitude of fitting carbon flows (i.e., the importance of links) and the congruency is expressed in percents. These results are presented for both an unperturbed (control) and a thermally stressed creek ecosystem of the river. The comparison of average congruency values for the two communities suggests that, first, trophic control may be stronger in the stressed community and, second, the reliability of carbon flows is also higher in the stressed ecosystem.
In complex, modern food webs, the analysis of pairwise interactions gives weak predictions of the behaviour of either single species or the whole community. Indirect effects call for explicit study and quantification. However, just as focusing only on pairwise interactions is incorrect, overemphasising the role of long, indirect pathways also seems to be unrealistic. Thus, a reasonable range of indirect trophic effects spreading through the food web is to be defined and quantified. I suggest a graph theoretical measure for quantifying this range, considering only network position (topology). I call this the trophic field of a species (or trophic group), recalling the idea that field theory could be a fruitful research programme in biology. Further, I propose a measure for the quantification of the indirect component of the trophic field. Finally, the use of introduced concepts and indices is illustrated by analysing the trophic flow network of the Schlei Fjord ecosystem (N. Germany).
Food webs inform about both direct trophic interactions (predation and food supply) and implicitly involved, indirect ones (e. g. exploitative competition). Traditionally, direct links between species are considered stronger and more important. However, as a body of experimental results suggests, indirect links may well be able to surpass their effects. In this paper, after shortly summarizing an interesting case study, I propose a simple structural approach to estimate the role of indirect vs. direct effects
Changes in the trophic structure of communities are good indicators of ecosystem stress or environmental change. Here, we compare the sink webs of two bird species (great tit, Parus major, and European blackbird, Turdus merula) in two habitats (a forest and an orchard), in order to detect the differences in their trophic status. The webs are functionally aggregated in three steps, based on energetics. Our main conclusion is that tits are less sensitive to habitat change from the natural to the agricultural.
Belgrano, A., U.M. Scharler, J. Dunne, and R.E. Ulanowicz (eds.), 2005. Aquatic Food Webs - An Ecosystem Approach. Oxford University Press, Oxford, x+262 pp. ISBN 0-19-856483-X, paperback, price: GBP 39.95, USD 65.00.; Fenner, M. and K. Thompson. 2005. The ecology of seeds. Cambridge University Press, Cambridge, x+250 pp (with 47 figures and 2 tables). ISBN 0-521-65311-8, hardback, price: GBP 55.00, USD 90.00; ISBN 0-521-65368-1, paperback, price: GBP 26.00, USD 45.00.;
In the last decades, many works investigated the trophic structure of communities stressing, in particular, the role played by species in food webs (e.g., their trophic level and, more recently, their centrality). There exist some encouraging applications, but few details are known about the relationships between centrality measurements and trophic levels. In addition, these studies almost refer to unweighted trophic networks, despite the acknowledged need of investigating weighted webs. Here we aim to contribute to the synthetic treatment of these complementary issues by analyzing several indices of centrality and trophic level. Studying 19 ecosystems, we ranked the nodes according to their positional importance values (based on various centrality indices) and we compared the rank order of coefficients with unweighted or weighted trophic levels. Our goal was revealing potential biases in finding high centrality nodes among basal, intermediate and top species. We found that key species occupy intermediate positions of the trophic hierarchy. In case of unweighted data, trophic levels of key nodes do not deviate from trends displayed by the whole dataset. Significant differences were observed when using weighted data. These results contradict the common belief of many ecologists that identified top-predators and charismatic megafauna as main targets of conservation policies. We discuss the potential consequences of the observed features on ecosystem dynamics.
R.W. Sterner and J.J. Elser. 2002. Ecological Stoichiometry - the biology of elements from molecules to biosphere. Princeton University Press, Princeton, New Jersey, 439 pp. (with 117 figures and tables). ISBN: 0-691-07491-9, paperback, price: USD 29.95, GBP 19.95. M. Black and H.W. Pritchard (eds.) 2002. Dessication and survival in plants. Drying without dying. CABI Publishing, Wallingford, x+412 pp. ISBN: 0-85199-534-9, hardback, price: GBP 75.00. B.D. Booth, S.D. Murphy, C.J. Swanton. 2003. Weed ecology in natural and agricultural systems. CABI Publishing, Wallingford, viii+303 pp. ISBN 0-85199-528-4, paperback, price: GBP 35.00, USD 60.00.
Advanced techniques of network analysis allow the quantification of the indirect interactions and the topological importance of components in ecological interaction networks. In current conservation biology, considering indirect causal effects, identifying keystone species and outlining multispecies approaches begin to be high priority goals. We make an attempt to connect these issues within a network context. Our main interest is to determine the positionally most important set of
nodes in a network, to analyse whether a set of nodes for small
is a subset of another for larger
, and to quantify this nestedness. We apply the KeyPlayer software, a novel tool for network analysis in ecology, introduced originally in mathematical sociology. Topological keystone species complexes are defined, we illustrate the use of this method in a case study and analyse a database of 9 plant-pollinator interaction networks. Our main conclusion is that multispecies approaches may give results very different from single species analyses.