54,804 research outputs found
Modelling Food Webs
We review theoretical approaches to the understanding of food webs. After an
overview of the available food web data, we discuss three different classes of
models. The first class comprise static models, which assign links between
species according to some simple rule. The second class are dynamical models,
which include the population dynamics of several interacting species. We focus
on the question of the stability of such webs. The third class are species
assembly models and evolutionary models, which build webs starting from a few
species by adding new species through a process of "invasion" (assembly models)
or "speciation" (evolutionary models). Evolutionary models are found to be
capable of building large stable webs.Comment: 34 pages, 2 figures. To be published in "Handbook of graphs and
networks" S. Bornholdt and H. G. Schuster (eds) (Wiley-VCH, Berlin
The architecture of predator-prey and the relationship between complexity and stability
Theoretical studies predict that the stability of an ecosystem is negatively correlated with its complexity, measured by the number of interacting species. On the other hand, empirical evidence indicates that food webs are highly interconnected. In this manuscript we present results on the stability two-level predator-prey food webs. We analyzed exhaustively all possible topologies of connections among species. Our findings show that those food webs fall into two classes with clearly distinct stability properties. In one of them stability is negatively correlated with complexity, and in the other group stability is positively correlated. For a positive relationship our results reveals highly structured food webs. The positive or negative relationship is related only to the topological structure of the food web. It is independent of the number of connections, strengths of predator-prey interactions or number of species. We review empirical evidence that corroborates our results
Population Dynamics on Complex Food Webs
In this work we analyse the topological and dynamical properties of a simple
model of complex food webs, namely the niche model. In order to underline
competition among species, we introduce "prey" and "predators" weighted overlap
graphs derived from the niche model and compare synthetic food webs with real
data. Doing so, we find new tests for the goodness of synthetic food web models
and indicate a possible direction of improvement for existing ones. We then
exploit the weighted overlap graphs to define a competition kernel for
Lotka-Volterra population dynamics and find that for such a model the stability
of food webs decreases with its ecological complexity.Comment: 11 Pages, 5 Figures, styles enclosed in the submissio
Food webs in forest and pasture streams in the Waikato region, New Zealand: A study based on analyses of stable isotopes of carbon and nitrogen, and fish gut contents.
Stable isotopes of carbon (C) and nitrogen (N) were studied in 11 stream communities in the Waikato region of New Zealand. From comparisons of mean d13C and d15N values, food webs in the shaded, forest streams were clearly based on allochthonous material (conditioned leaf litter and terrestrial invertebrates). Autotrophs in forest streams were not a significant C source for the food webs. However, the C source of food webs in the unshaded pasture streams appeared to be a mixture of allochthonous and autochthonous material. Conditioned leaf litter appeared to contribute to the pasture stream food webs, and the d13C and d15N of some samples of epilithic diatoms indicated their consumption by invertebrates in pasture streams. Fish ate a wide range of aquatic invertebrates; longfinned eels (Anguilla dieffenbachiai) and banded kokopu (Galaxias fasciatus) also had a large proportion of terrestrial invertebrates in their diet. Filamentous green algae were found only at pasture sites, where they were sometimes abundant. The wide range of d13C values of filamentous green algae (-18.8 to -29.7[[perthousand]]) complicated understanding of their role in the stream food webs. The d13C values of Cladophora were related to water velocity, with more 13C-enriched values in pools than in runs (-23.2[[perthousand]] in pools, mean velocity 0.12 m s-1; -28.1[[perthousand]] in runs, mean velocity 0.24 m s-1). Crayfish and the gastropod mollusc Potamopyrgus appeared to be the only invertebrates to eat filamentous green algae
Classroom Activity: Food Webs
This lesson plan is designed to help students understand the interrelatedness of food webs and to see how populations of organisms affect each other. Students assume the roles of the various organisms in the ecosystem; the ones that are dependent upon each other are symbolically connected by lengths of yarn. A materials list, instructions, assessment ideas, and educational standards are provided. Educational levels: High school, Middle school
Food Webs: Experts Consuming Families of Experts
The question what determines the structure of natural food webs has been
listed among the nine most important unanswered questions in ecology. It arises
naturally from many problems related to ecosystem stability and resilience. The
traditional view is that population-dynamical stability is crucial for
understanding the observed structures. But phylogeny (evolutionary history) has
also been suggested as the dominant mechanism. Here we show that observed
topological features of predatory food webs can be reproduced to unprecedented
accuracy by a mechanism taking into account only phylogeny, size constraints,
and the heredity of the trophically relevant traits of prey and predators. The
analysis reveals a tendency to avoid resource competition rather than apparent
competition. In food webs with many parasites this pattern is reversed.Comment: 16 pages, 3 figures, 1 table + Appendix of 36 pages, 18 figures.
movie available from http://ag.rossberg.net/matching.mp
Scaling Behaviors of Weighted Food Webs as Energy Transportation Networks
Food webs can be regarded as energy transporting networks in which the weight
of each edge denotes the energy flux between two species. By investigating 21
empirical weighted food webs as energy flow networks, we found several
ubiquitous scaling behaviors. Two random variables and defined for
each vertex , representing the total flux (also called vertex intensity) and
total indirect effect or energy store of , were found to follow power law
distributions with the exponents and ,
respectively. Another scaling behavior is the power law relationship, , where . This is known as the allometric scaling
power law relationship because can be treated as metabolism and as
the body mass of the sub-network rooted from the vertex , according to the
algorithm presented in this paper. Finally, a simple relationship among these
power law exponents, , was mathematically derived
and tested by the empirical food webs
Deleting species from model food webs
We use food webs generated by a model to investigate the effects of deleting
species on other species in the web and on the web as a whole. The model
incorporates a realistic population dynamics, adaptive foragers and other
features which allow for the construction of model webs which resemble
empirical food webs. A large number of simulations were carried out to produce
a substantial number of model webs on which deletion experiments could be
performed. We deleted each species in four hundred distinct model webs and
determined, on average, how many species were eliminated from the web as a
result. Typically only a small number of species became extinct; in no instance
was the web close to collapse. Next, we examined how the the probability of
extinction of a species depended on its relationship with the deleted species.
This involved the exploration of the concept of indirect predator and prey
species and the extent that the probability of extinction depended on the
trophic level of the two species. The effect of deletions on the web itself was
studied by searching for keystone species, whose removal caused a major
restructuring of the community, and also by looking at the correlation between
a number of food web properties (number of species, linkage density, fraction
of omnivores, degree of cycling and redundancy) and the stability of the web to
deletions. With the exception of redundancy, we found little or no correlation.
In particular, we found no evidence that complexity in terms of increased
species number or links per species is destabilising.Comment: 30 pages, 9 figure
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