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Community Ecology
Authors: F. Jordán, T. Kalapos, and P. Csontos

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.

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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 n nodes in a network, to analyse whether a set of nodes for small n is a subset of another for larger n , 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.

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We present a new method to asses the strength of indirect interactions and to indentify candidate keystone species in quantitative food webs. We apply this method to the structural analysis of a host-parasitoid community. The strength and symmetry of indirect interactions between 12 leaf-miner hosts and their 27 hymenopteran parasitoids are quantified. It is shown that (1) quantifying longer pathways helps in determining which species have more important direct or indirect effects on others, (2) a keystone pattern of relative species importance, based on positionality in the interaction network, seems to characterize this community, (3) considering longer pathways results in a characteristic “few strong - many weak” distribution of interaction strength, and (4) between the majority of species pairs the interaction is weakly asymmetrical. We emphasise that a very simple network algebra approach may offer important predictions on both species- and community-level patterns.

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Network models are traditional in community ecology. For example, they provide a rich analytical toolkit to put higher predators into a multispecies context. Better understanding their top-down effects and the potential bottom-up control on them would be of key importance for predictive ecosystem management. Food web architecture may be used to predict community dynamics, but it is an old question how reliable are the studies considering only static information. A general and intuitive assumption is that stronger links (with larger weights) mediate stronger effects. We study this statement and use an illustrative case study. We investigate the trophic structure of the Prince William Sound food web in terms of biomass flows, and study its simulated dynamics in a stochastic modelling framework. We aim to understand bottom-up effects of preys on consumers: we focus on the fluctuations of top predator populations, following disturbance on their prey. Several disturbance regimes are studied and compared. Food web structure and link weight generally predict well the average impacts of preys on top-predators, with larger flows mediating stronger effects. Most exceptions appear for weak links: these are less predictable, some of them can be surprisingly important.

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Community Ecology
Authors: Á. Móréh, F. Jordán, A. Szilágyi, and I. Scheuring

There is increasing evidence that regime shifts occur at several scales in ecosystems (from the spatiotemporal alternation of two species to large-scale, ecosystem-level rearrangements). Yet, the theoretical background for understanding these changes is far from clear. Since fishing down in marine ecosystems is well-documented trend, and its top-down cascading effects in food webs have been richly documented, it is a current question whether overfishing, in general, can also influence regime shifts at lower levels. We model simple marine ecosystems by dynamical food webs and investigate the probability of regime shifts emerging among primary consumers. We considered cases where only one of the primary consumers is persistent in the stationary state. By perturbing the death rates in the food web, we studied the circumstances when the previously persistent primary producer is indirectly changed by the previously non-persistent one. Whether and how regime shifts (e.g., change in primary consumers) can occur depends on (1) food web topology (presence of top-predator and alternative producer), (2) the relative strength of perturbation of primary consumers’ death rates, and (3) the dynamical parameters of the recovering consumer. We found that overfishing, food web topology and dynamical parameters together determine the probability of regime shifts. Thus, integrative and complex models are needed in multispecies fisheries.

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Link distribution is an important architectural feature of ecological networks, since it is thought to influence community dynamics. Several attempts have been made in order to characterize the typical link distribution of food webs, but the number of webs studied thus far is low and their quality is unbalanced. Comparability is a rarely asked methodological question, and as far as we see only two data bases are available which allow reliable comparison of food webs: one for terrestrial, high resolution, host-parasitoid webs and another for highly aggregated, marine trophic networks. We present an analysis of a set of food webs belonging to the latter type, since the host-parasitoid networks are only subgraphs and therefore uninformative on the structure of the entire community. We address the following three questions: (1) how to characterize the link distribution of these small networks which cannot always be fitted statistically to well-known distributions (such as the exponential or the Poisson, etc.), (2) are these distributions of more or less similar shape or they belong to different „architectural classes”, and (3) if there are different classes, then what are their distinctive topological and biological properties. We suggest that link distribution of such small networks can be compared to each other by principal coordinates ordination and clustering. We conclude that (1) the webs can be categorized into two different classes, and (2) one of the classes contains significantly larger and topologically more heterogeneous webs for which net output of material is also of higher variance. We emphasize that link distribution is an interesting and important property not only in case of complex, speciose food webs, but also in highly aggregated, low-resolution webs.

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Community Ecology
Authors: M.K. Singh, Z. Tuba, F. Jordán, I. Scheuring, and J. Podani

K. J. Gaston and J. I. Spicer (eds.) 2004. Biodiversity: An Introduction (Second Edition). Blackwell Science Ltd, a Blackwell Publishing Company, Padstow, Cornwall, 191 pp. ISBN 1-4051-1857-1, paperback, price: USD 49.95, GBP 19.99; U. Sommer and B. Worm (eds.) 2002. Competition and Coexistence. Ecological Studies, Vol. 161. Springer Verlag, Berlin, 221 pp. (with 69 figures, 5 in color, and 2 tables). ISBN 3-540-43311-2, hardback, price: EUR 69.95; J. Kolbek, M. Šrùtek and E. O. Box. (eds.) 2003. Forest Vegetation of Northeast Asia. Geobotany 28. Kluwer Academic Publishers, Dordrecht, xii+462 pp. ISBN 1-4020-1370-1, hardback, price: EUR 200.00;

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