Mathematical models predict that species interactions such as competition and predation can generate chaos. However, experimental demonstrations of chaos in ecology are scarce, and have been limited to simple laboratory systems with a short duration and artificial species combinations. Here, we present the first experimental demonstration of chaos in a long-term experiment with a complex food web. Our food web was isolated from the Baltic Sea, and consisted of bacteria, several phytoplankton species, herbivorous and predatory zooplankton species, and detritivores. The food web was cultured in a laboratory mesocosm, and sampled twice a week for more than 2,300 days. Despite constant external conditions, the species abundances showed striking fluctuations over several orders of magnitude. These fluctuations displayed a variety of different periodicities, which could be attributed to different species interactions in the food web. The population dynamics were characterized by positive Lyapunov exponents of similar magnitude for each species. Predictability was limited to a time horizon of 15-30 days, only slightly longer than the local weather forecast. Hence, our results demonstrate that species interactions in food webs can generate chaos. This implies that stability is not required for the persistence of complex food webs, and that the long-term prediction of species abundances can be fundamentally impossible.
Coupling of several predator-prey oscillations can generate intriguing patterns of synchronization and chaos. Theory predicts that prey species will fluctuate in phase if predator-prey cycles are coupled through generalist predators, whereas they will fluctuate in anti-phase if predator-prey cycles are coupled through competition between prey species. Here, we investigate predator-prey oscillations in a long-term experiment with a marine plankton community. Wavelet analysis of the species fluctuations reveals two predator-prey cycles that fluctuate largely in anti-phase. The phase angles point at strong competition between the phytoplankton species, but relatively little prey overlap among the zooplankton species. This food web architecture is consistent with the size structure of the plankton community, and generates highly dynamic food webs. Continued alternations in species dominance enable coexistence of the prey species through a non-equilibrium 'killing-the-winner' mechanism, as the system shifts back and forth between the two predator-prey cycles in a chaotic fashion.
Vertical profiles of soluble and particulate nutrients were analyzed at the end of summer stratification in two dimictic lakes located in northeast Germany. In addition, irradiance and plankton biomass were determined. The concentrations of particulate organic carbon and phytoplankton biomass in the epilimnion were higher in Lake Tiefer than in Lake Dudinghausen, even though the apparent trophic status of Lake Tiefer was higher than Lake Dudinghausen. In Lake Dudinghausen, phototrophic sulfur bacteria accumulated in the hypolimnion between 8 and 10 m, whereas in Lake Tiefer low light penetration prevented the development of phototrophic bacteria in those horizons in which sulfide might be present. Because both lakes have anoxic hypolimnia, we assumed that in both cases phosphorus was released from the sediment Aquatic Sciences into the hypolimnion. In Lake Tiefer, redox conditions and the presence of nitrate and nitrite limited the water depth range in which P-release occurred. In Lake Dudinghausen, part of the released soluble reactive phosphorus was incorporated into the phototrophic sulfur bacteria biomass and thus transformed to particulate phosphorus. As much as 70% of the particulate phosphorus in the hypolimnion was found in the phototrophic sulfur bacterial layer, with 15 -20 % of this particulate phosphorus consisting of polyphosphate storage compounds. The low ratio of soluble reactive phosphorus to particulate phosphorus in the hypolimnion was, therefore, attributed to phototrophic sulfur bacteria. The phototrophic sulfur bacteria appear to act as an internal nutrient filter and convert soluble reactive phosphorus into particulate phosphorus.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.