BioND — Dynamics of Biological Networks

Food web stability

Identifying the decisive factors that lend ecosystems their stability, robustness, and resilience is one of the central challenges in community ecology. Addressing this challenge may not only lead to more efficient strategies for the protection of natural ecosystems, but may also reveal design principles for building robust technical systems and organizational structures. An important step toward this goal is investigating the stability of food webs, the complex networks of who-eats-who, which form the backbone of most natural ecosystems.

Until the 1970s there was a widespread belief among ecologists that the diversity and complexity of natural food webs stabilized them against perturbations. However, Robert May showed mathematically that large complex networks are less likely to be stable than simpler or smaller nets. The observed stability of natural ecosystems therefore provides a strong indication that the food webs realized in nature are not randomly assembled, but exhibit specific structural features conveying a high dynamical stability. Identifying these decisive features is difficult because of the inherent complexity of large food webs and the long catalog of empirically observed features of natural food webs which may or may not have an impact on stability.

Because of numerical constraints previous studies of food web stability were forced to consider either highly simplified models with questionable relevance or investigate only few model replicates, which often precluded statistically significant predictions. We proposed a new class of food web models, which capture the complexity of natural systems to the same degree as conventional models, but can be explored much more efficiently. In a recent paper (Gross et al. 2009) we used this approach to investigate the stability of a total of 100 billion replicates of complex food webs comprising up to 50 species. The large number of replicates allowed for a sound statistical analysis that confirmed and quantified the impact of several previously identified factors for food webs stability. However, our analysis only partially confirmed the stabilizing effect of weak links in food webs – the factor that has been most prominently discussed in the recent literature. We showed that in small webs, such as those that were previously considered, weak links have a stabilizing influence, whereas in larger webs they tend to destabilize the system. Finally, our investigation pointed to a very strong effect of the distribution of links within the network: Stability is strongly increased if the network contains top-predators which prey on many different species of specialized predators.

Our present work focuses on extending these past results in several different directions. For instance we consider the effect of other types of interactions (e.g. mutualism, competition, parasitism), a spatial environment (e.g., metacommunities) and other biological insights (e.g., stoichiometric constraints). Another important challenge for the future will be to apply our food web model for analyzing specific empirically observed food webs.

Key Publications

A general consumer-resource population model
Kevin Lafferty, Giulio DeLeo, Cheryl J. Briggs, Andrew P. Dobson, Thilo Gross, and Armand M. Kuris
Science 349(6250), 854-857, 2015.
(abstract) (supporting material) (link to publisher) (media coverage)

Collapse of an ecological network in Ancient Egypt
Justin D. Yeakel, Mathias M. Pires, Lars Rudolf, Nathaniel J. Dominy, Paul L. Koch, Paulo L. Guimaraes, and Thilo Gross
Proceedings of the National Academy of Sciences 111(40), 14472–14477, 2014.
(abstract) (link to publisher) (arXiv) (media coverage)

Generalized models reveal stabilizing factors in food webs
Thilo Gross, Lars Rudolf, Simon A. Levin, and Ulf Dieckmann
Science 325, 747-750, 2009.
(abstract) (supporting material) (link to publisher) (download preprint) (media coverage)

Additional Publications

The influence of dispersal on a predator-prey system with two habitats
Philipp Gramlich, Sebastian Plitzko, Lars Rudolf, Barbara Drossel, and Thilo Gross
Journal of Theoretical Biology 398, 150, 2016.
(abstract) (link to publisher) (arXiv) (download preprint)

Meta-food-chains as a many-layer epidemic process on networks
Edmund Barter and Thilo Gross
Physical Review E 93, 022303, 2016.
(abstract) (link to publisher) (arXiv)

Mesoscale symmetries explain dynamical equivalence of food webs
Helge Aufderheide, Lars Rudolf, and Thilo Gross
New Journal of Physics 14, 105014, 2012.
(abstract) (link to publisher) (arXiv) (download preprint)

The invisible niche: Weakly density-dependent mortality and the coexistence of species
Thilo Gross, Andrew M. Edwards, and Ulrike Feudel
Journal of Theoretical Biology 258(1), 148-155, 2009.
(abstract) (supporting material) (link to publisher) (download preprint)

Local dynamical equivalence of certain food webs
Thilo Gross and Ulrike Feudel
Ocean Dynamics 59(2), 417-427, 2009.
(abstract) (link to publisher) (arXiv) (download preprint)

Instabilities in spatially extended predator-prey systems: spatio-temporal patterns in the neighborhood of Turing-Hopf bifurcations
Martin Baurmann, Thilo Gross, and Ulrike Feudel
Journal of Theoretical Biology 245(2), 220-229, 2007.
(abstract) (link to publisher) (download preprint)

Generalized models as an universal approach to the analysis of nonlinear dynamical systems
Thilo Gross and Ulrike Feudel
Physical Review E 73, 016205-14, 2006.
(abstract) (link to publisher) (arXiv) (download preprint)

Long food chains are in general chaotic
Thilo Gross, Wolfgang Ebenhöh, and Ulrike Feudel
Oikos 109(1), 135-155, 2005.
(abstract) (link to publisher) (download preprint)

Enrichment and foodchain stability: the impact of different functional forms
Thilo Gross, Wolfgang Ebenhöh, and Ulrike Feudel
Journal of Theoretical Biology 227(3), 349-358, 2004.
(abstract) (link to publisher) (download preprint)

Media Coverage

Kevin Knudson, Forbes, 2015-10-15
Scientists love unifying theories – a single equation (or set of equations) that explains everything in the discipline. Physicists have been searching for theirs for decades, running into difficulty unifying quantum theory with relativity. Economists work very hard at trying to model the global economy via complicated regression models. Until recently, however, ecologists thought that different food webs operated under distinct conditions. That is, the dynamics of foxes and squirrels must be different from those of parasites eating hosts.
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Just Cebrian, Science Magazines, 2015-09-04
All organisms in an ecosystem can be placed on a trophic level, depending on whether they are producers or consumers of energy within the food chain (see the photo). Ecologists have long debated what regulates the trophic structure and dynamics of ecosystems (1). This is important because trophic structure and dynamics regulate many of the goods and services that ecosystems provide to wildlife and humankind, such as the production of harvestable food and energy, carbon sequestration and modulation of climate change, and nutrient uptake and control of global biogeochemical cycles (2). A study by Hatton et al. on page 1070 of this issue (3) and a recent report by Lafferty et al. (4) represent important advances toward a unified theory of trophic structure that captures observed trends across all ecosystems.
(more)

American Scientist, 2015-05-15
From the Cambrian Burgess Shale to ancient Egypt, food webs share surprising structural attributes. When redundancy is lost, the threat of extinction grows.
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Clive Cookson and Tyler Shendruk, Financial Times Weekend Magazine, 2014-10-03
Research concludes that the region is more vulnerable to environmental perturbations today than at any previous time in recorded history
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Tajirul Hague, New Historian, 2014-09-15
According to a new study that was published in the Proceedings of the National Academy of Sciences, a number of creatures that have been depicted in Ancient Egyptian artifacts have helped scientists to create a record of mammal extinctions that date back to 6000 years. The study might not offer comprehensive proof that population pressures and droughts forced these animals away, but they do depict an interesting pattern.
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Archeology, 2014-09-14
Ancient Egyptian images of the natural world have helped quantitative ecologist Justin Yeakel of the University of California, Santa Cruz, determine that the drying climate and growing human population have probably made Egypt’s ecosystem progressively less stable.
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Robert Czepel, Austrian Broadcasting, 2014-09-09
Vor 6.000 Jahren lebten in Ägypten noch Löwen, Elefanten und Giraffen. Heute sind sie längst aus dem Niltal verschwunden. Forscher haben die Wellen des Aussterbens mit Hilfe von Tierdarstellungen auf Kunstwerken rekonstruiert. Ihr Fazit: Verantwortlich dafür waren der Klimawandel - und der Mensch.
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Sarah Griffith, Daily Mail, 2014-09-09
Rock inscriptions and tomb carvings have shed light on the large beasts that thrived in Egypt before they were wiped out around 6,000 years ago.
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Popular Archaeology, 2014-09-08
Ancient Egyptian artworks help scientists reconstruct how animal communities changed as climate became drier and human populations grew.
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Sarah Zielinski, Smithsonian Magazine, 2014-09-08
Ancient Egypt’s highly decorated tombs and funerary objects—meant to ensure a safe trip into the afterlife—also hold a rich record of the region’s wildlife. Now scientists have used that art, along with other paleontological, archaeological and historical evidence, to map out the rise and fall of Egypt’s large mammals and match those patterns to changes in climate and human interactions.
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Jessica Ruvinsky, Science, 2014-09-08
Six thousand years ago, Egyptian lions hunted wildebeests and zebras in a landscape that resembled the Serengeti more than the Sahara. Since then, the number of large mammal species has decreased from 37 to eight, says quantitative ecologist Justin Yeakel of the Santa Fe Institute. New research using ancient animal depictions tracks the collapse of Egypt’s ecological networks one extinction at a time, offering a glimpse into how climate change and human impacts have altered the structure and stability of ecosystems over millennia.
(more)

John Roach, NBC News, 2014-09-08
Images of lions, giraffes, wildebeests and other creatures depicted on ancient Egyptian artifacts have helped scientists create a 6,000-year record of local mammal extinctions, according to a new study. Several of the extinction episodes correlate with known periods of drought and rapid human population growth.
(more)

Virginia Gewin, nature, 2013-08-08
Ancient Egyptian rock inscriptions and carvings on pharaonic tombs chronicle hartebeest and oryx — horned beasts that thrived in the region more than 6,000 years ago. Researchers have now shown that those mammal populations became unstable in concert with significant shifts in Egypt’s climate.
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Sabiene Sütterlin, Science Blogs, 2010-08-26
Seit ich mit der Netzwerk-Gruppe zu tun habe, sehe ich überall Netzwerke. Die Leser dieses Blogs sind teilweise untereinander vernetzt, zum Beispiel. Und die Nervenzellen in unseren Gehirnen knüpfen stets neue Verbindungen
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Natur + Kosmos, 2009-08-20
Was hält eine Lebensgemeinschaft im Innersten zusammen? Wissenschaftler aus Deutschland, Österreich und den USA haben mithilfe von Computersimulationen fundamentale Gesetzmäßigkeiten aufgedeckt, die die Stabilität von Ökosystemen mitbestimmen
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Austrian Broadcasting (ORF), 2009-08-07
Vielseitige Raubtiere, die sich nicht auf eine bestimmte Beute-Art spezialisiert haben, stabilisieren Ökosysteme
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ND TV, 2009-08-07
Using computer models, scientists from Germany, Austria, and the United States have discovered fundamental rules that determine the stability of ecosystems. The computations reveal that small ecosystems follow other rules than large ecosystems
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Science Daily, 2009-08-07
New findings, published in the journal Science, conclude that food-web stability is enhanced when many diverse predator-prey links connect high and intermediate trophic levels
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Natalie Bachl, Der Standard, 2009-08-07
Forscher finden Gesetzmäßigkeiten, wie Nahrungsketten stabil bleiben - Löwen gehen mit gutem Beispiel voran
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VET-Magazin, 2009-08-07
Wissenschaftler aus Deutschland, Österreich und den USA haben mithilfe von Computersimulationen fundamentale Gesetzmäßigkeiten aufgedeckt, die die Stabilität von Ökosystemen mitbestimmen. Nahrungsnetze sind demnach stabiler, wenn Raubtierarten an der Spitze der Nahrungskette sich von verschiedenen Beutetieren ernähren und Beutearten in der Mitte der Nahrungskette vielen Räubern ausgesetzt sind.
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Scinexx, 2009-08-07
Das Nahrungsnetz eines Ökosystems ist stabiler, wenn die Top-Raubtierarten keine Nahrungsspezialisten sind, sondern sich von vielen verschiedenen Beutetieren ernähren. Das haben Wissenschaftler mithilfe eines neuen Modells festgestellt. Die Berechnungen haben zudem ergeben, dass kleine Ökosysteme anderen Regeln gehorchen als große, wie die Forscher jetzt in "Science" berichten
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