How Structured Is the Entangled Bank? The Surprisingly Simple Organization of Multiplex Ecological Networks Leads to Increased Persistence and Resilience (original) (raw)

From Small to Large Ecological Networks in a Dynamic World

2005

Food webs are one of the most useful, and challenging, objects of study in ecology. These networks of predator-prey interactions, conjured in Darwin's image of a "tangled bank," provide a paradigmatic example of complex adaptive systems. While it is deceptively easy to throw together simplified caricatures of feeding relationships among a few taxa as can be seen in many basic ecology text books, it is much harder to create detailed descriptions that portray a full range of diversity of species in an ecosystem and the complexity of interactions among them ( ). Difficult to sample, difficult to describe, and difficult to model, food webs are nevertheless of central practical and theoretical importance. The interactions between species on different trophic (feeding) levels underlie the flow of energy and biomass in ecosystems and mediate species' responses to natural and unnatural perturbations such as habitat loss. Understanding the ecology and mathematics of food webs, and more broadly, ecological networks, is central to understanding the fate of biodiversity and ecosystems in response to perturbations.

Simple prediction of interaction strengths in complex food webs

Proceedings of The National Academy of Sciences, 2009

Darwin's classic image of an ''entangled bank'' of interdependencies among species has long suggested that it is difficult to predict how the loss of one species affects the abundance of others. We show that for dynamical models of realistically structured ecological networks in which pair-wise consumer-resource interactions allometrically scale to the 3 ⁄4 power-as suggested by metabolic theory-the effect of losing one species on another can be predicted well by simple functions of variables easily observed in nature. By systematically removing individual species from 600 networks ranging from 10 -30 species, we analyzed how the strength of 254,032 possible pair-wise species interactions depended on 90 stochastically varied species, link, and network attributes. We found that the interaction strength between a pair of species is predicted well by simple functions of the two species' biomasses and the body mass of the species removed. On average, prediction accuracy increases with network size, suggesting that greater web complexity simplifies predicting interaction strengths. Applied to field data, our model successfully predicts interactions dominated by trophic effects and illuminates the sign and magnitude of important nontrophic interactions. body size ͉ ecological networks ͉ species extinctions ͉ species interaction strengths ͉ systems theory I would not give a fig for simplicity on this side of complexity, but I'd give my life for the simplicity on the other side of complexity.

Degree heterogeneity and stability of ecological networks

Journal of the Royal Society, Interface, 2017

A classic measure of ecological stability describes the tendency of a community to return to equilibrium after small perturbations. While many advances show how the network architecture of these communities severely constrains such tendencies, one of the most fundamental properties of network structure, i.e. degree heterogeneity-the variability of the number of links associated with each species, deserves further study. Here we show that the effects of degree heterogeneity on stability vary with different types of interspecific interactions. Degree heterogeneity consistently destabilizes ecological networks with both competitive and mutualistic interactions, while its effects on networks of predator-prey interactions such as food webs depend on prey contiguity, i.e. the extent to which the species consume an unbroken sequence of prey in community niche space. Increasing degree heterogeneity tends to stabilize food webs except those with the highest prey contiguity. These findings he...

Structural and Dynamical Heterogeneity in Ecological Networks

Recent Advances

Diversity is a key feature of biological systems. In complex ecological systems, which are composed of several components and multiple parallel interactions among them, it is increasingly needed to precisely understand structural and dynamical variability among components. This variability is the basis of adaptability and evolvability in nature, as well as adaptive management-based applications. The authors discuss how to quantify and characterize the structural and dynamical variability in ecological networks. They perform network analysis in order to quantify structure and we provide a process algebra-based stochastic simulation model and sensitivity analysis for better understanding the dynamics of the studied ecological system. They use a large, data-rich, real ecological network for illustration.

Shifting levels of ecological network's analysis reveals different system properties

Philosophical Transactions of the Royal Society B: Biological Sciences

Network analyses applied to models of complex systems generally contain at least three levels of analyses. Whole-network metrics summarize general organizational features (properties or relationships) of the entire network, while node-level metrics summarize similar organization features but consider individual nodes. The network- and node-level metrics build upon the primary pairwise relationships in the model. As with many analyses, sometimes there are interesting differences at one level that disappear in the summary at another level of analysis. We illustrate this phenomenon with ecosystem network models, where nodes are trophic compartments and pairwise relationships are flows of organic carbon, such as when a predator eats a prey. For this demonstration, we analysed a time-series of 16 models of a lake planktonic food web that describes carbon exchanges within an autumn cyanobacteria bloom and compared the ecological conclusions drawn from the three levels of analysis based on...

Complexity and fragility in ecological networks

Proceedings of the Royal Society of London. Series B: Biological Sciences, 2001

A detailed analysis of three species-rich ecosystem food webs has shown that they display skewed distributions of connections. Such graphs of interaction are, in fact, shared by a number of biological and technological networks, which have been shown to display a very high homeostasis against random removals of nodes. Here, we analyse the responses of these ecological graphs to both random and selective perturbations (directed against the most-connected species). Our results suggest that ecological networks are very robust against random removals but can be extremely fragile when selective attacks are used. These observations have important consequences for biodiversity dynamics and conservation issues, current estimations of extinction rates and the relevance and de¢nition of keystone species.

Network topology and biodiversity loss in food webs: robustness increases with connectance

Ecology Letters, 2002

Food-web structure and complexity can mediate effects of species loss such as cascading extinctions. We simulated species loss in 16 food webs from a variety of ecosystems. The food webs experienced much greater secondary extinctions when the most trophically connected species were removed compared to random species removals. These patterns appear related to skewed degree distributions in food webs, which generally display exponential or uniform distributions. Our analyses generalize prior research that found similar patterns of node loss in biological and non-biological networks with power-law distributions. Food web robustness (the level of primary removals required to induce 50% total species loss) to random and mostconnected species loss does not relate to species richness or omnivory, but increases significantly with greater connectance (links/species 2). We also found strong evidence for the existence of thresholds where food webs display greatly increased sensitivity to removal of most-connected species. Higher connectance delays the onset of this threshold. Leastconnected species removal often has little effect, but in several food webs results in dramatic secondary extinctions. We relate these findings to the diversity-stability debate, effects of species richness on ecosystems, keystone species, and extinction rates.

Parallel ecological networks in ecosystems

Philosophical Transactions of the Royal Society B: Biological Sciences, 2009

In ecosystems, species interact with other species directly and through abiotic factors in multiple ways, often forming complex networks of various types of ecological interaction. Out of this suite of interactions, predator-prey interactions have received most attention. The resulting food webs, however, will always operate simultaneously with networks based on other types of ecological interaction, such as through the activities of ecosystem engineers or mutualistic interactions. Little is known about how to classify, organize and quantify these other ecological networks and their mutual interplay. The aim of this paper is to provide new and testable ideas on how to understand and model ecosystems in which many different types of ecological interaction operate simultaneously. We approach this problem by first identifying six main types of interaction that operate within ecosystems, of which food web interactions are one. Then, we propose that food webs are structured among two main axes of organization: a vertical (classic) axis representing trophic position and a new horizontal 'ecological stoichiometry' axis representing decreasing palatability of plant parts and detritus for herbivores and detrivores and slower turnover times. The usefulness of these new ideas is then explored with three very different ecosystems as test cases: temperate intertidal mudflats; temperate short grass prairie; and tropical savannah.

Structure, dynamics, and robustness of ecological networks

2011

Ecosystems are often made up of interactions between large numbers of species. They are considered complex systems because the behaviour of the system as a whole is not always obvious from the properties of the individual parts. A complex system can be represented by a network: a set of interconnected objects. In the case of ecological networks and food webs, the objects are species and the connections are interactions between species. Many complex systems are dynamic and exhibit intricate time series. Time series analysis has been developed to understand a wide range of natural phenomena. This thesis deals with the structure, dynamics, and robustness of ecological networks, the spatial dynamics of fluctuations in a social system, and the analysis of cardiac time series. Biodiversity on Earth is decreasing largely due to human-induced causes. My work looks at the effect of anthropogenic change on ecological networks. In Chapter Two, I investigate predator adaptation on food-web robu...