Plankton paradox

from Wikipedia, the free encyclopedia

The plankton paradox is the discrepancy between the expected and observed number of species of planktonic algae, the phytoplankton , in freshwater and marine ecosystems.

Water is a homogeneous medium. Algae species live in it which have very similar needs for abiotic resources . In addition to light, they need nutrients , essentially nitrogen, phosphorus and a number of metal ions, most of them in extremely low concentrations. The nutrient concentrations are so low in most places and at most times that there is strong competition between the algae species. The principle of the exclusion of competition , which is firmly anchored in ecological theory , predicts that several species that have the same resource requirements should not be able to coexist in the same habitat; the stronger competitor would displace the inferior to extinction. Yet an extremely large number, sometimes hundreds, of planktonic algae species live in the same body of water.

This apparent paradox can be explained by the fact that the basic assumptions of the scientific model that were used to describe it are incorrect or incomplete. The plankton paradox is thus an important test case for ecological theory, especially the modeling of competition in general and the theory of ecological niche . Accordingly, it is also used as a test case for the explanation of biodiversity in other habitats. The term plankton paradox was introduced by the American limnologist and ecologist George Evelyn Hutchinson in 1961. Several theories exist to explain the paradox, some of which could be supported in special cases by laboratory tests, field experiments or observation data. These can be divided into several categories. Some theories predict that the apparent homogeneity and simplicity of the system is an illusion, but in reality there are other niche dimensions that allow species to coexist. Other theories are based on the fact that the principle of exclusion of competition only applies in the case of equilibrium. If the system never comes into equilibrium (either for external or for intrinsic reasons), even a species that is superior in the case of equilibrium cannot displace its competitors.

Resources and their proportions

An influential theory in plant ecology, the "resource ratio theory", looks at the possible relationships when two (or more) species compete for two (or more) resources. Accordingly, two species that compete for two resources can stably coexist with each other if one of the resources is limiting for each of the species. As each species grows, it lowers the level of resources in the habitat until a minimum resource level is reached that is just enough for it to survive. The kind that gets by with less of the resource wins. With two species and two resources, a range of values ​​can exist (depending on the demands of the species and the concentrations in the habitat) in which each of the species is prevented by the other resource from lowering the level to such an extent that it can compete with the other species. A key prediction of the theory is that through this mechanism, the number of species that can achieve stable equilibrium can never exceed the number of limiting resources. In simple laboratory experiments it could be shown that algae species can behave according to the predictions of the theory. Since the number of resources in natural water bodies that can limit growth because their supply is limited is relatively small, only very few coexisting species can be directly explained according to this mechanism. However, the species can be mixed up with one another along a resource gradient in such a way that one of them is competitive in a narrow range of values. As a result, the number of possibly coexisting species increases in a (spatially or temporally) heterogeneous habitat, even if there is actually only one or very few stable coexisting species for each value point. It is crucial for the theory that it only leads to the coexistence of species if their strengths and weaknesses are balanced in a certain way through trade-off effects.

Such segregation on the basis of a single factor has been convincingly demonstrated in planktonic algae, for example for the factor light. In marine habitats, tiny photoautotrophic cyanobacteria (formerly: "blue algae") of the genus Synechococcus coexist with different light-absorbing pigments, each of which works most effectively in different wavelength ranges. Depending on the depth of the water, cloudiness and ingredients, a different type is superior in each case. Contrary to appearances, water is not a homogeneous medium. The water molecule has defined absorption maxima for different frequencies of radiation, while frequencies in between are not filtered and can therefore act as niche spaces.

Interactions with other species

The division of resources among several species can be modified by interactions with other species in ways other than competition itself. In principle, this case is no different from resource allocation through competition. Here, too, the niche space between the competitors is divided differently due to the additional types, so that the superior competitor in the simple, isolated case cannot actually exploit this superiority. This gives the system further degrees of freedom in which it can differentiate itself. It is particularly important to modify the competitive relationships through antagonistic species such as predators and pathogens (pathogens). It has been known within limnology for decades that phytoplankton predators, especially zooplankton , can reduce the algae density to such an extent that waters that were previously heavily clouded by algae become clear again. The consumption rate of the algae by the zooplankton can be many orders of magnitude higher than the consumption of land plants by their herbivores . In contrast to phytophages from land plants, many zooplankton are relatively unspecific in their choice of food. B. with a higher number of algae species that of the zooplankton does not increase. However, it is important to differentiate according to size. This means that large phytoplankton can compete with small species to varying degrees, depending on the density of the zooplankton. Planktonic bacteria and, in particular, viruses, whose density and biomass can be several times greater than that of algae, also play a role that has long been underestimated. The presence of the (relatively few) toxic algae species may also play a role.

Spatial and temporal heterogeneity: succession and compartmentalization

It obviously plays an important role in the coexistence of algae species that the environmental conditions in a water body are not constant. They change both predictably over the course of the year and unpredictably and chaotically due to weather phenomena. A competitive species under a certain combination of factors usually has to adjust to the fact that the conditions which promote it are not permanent. You will likely be superseded by conditions that encourage a different species. For planktonic species with their short generation times, the changes over the course of the year are comparable in magnitude to changes over centuries or millennia for long-lived organisms such as forest trees. The regular and predictable sequence of communities within a biotope is called succession in ecology . Such successions, in which species follow one another in waves over the course of the year, are typical for almost all types of waters. During each phase, one species or a few species has a maximum, due to the presence of the species in the preceding or following phases in lower density, the number of species increases sharply. A similar mechanism has now been demonstrated for marine bacterial communities. It was GE Hutchinson himself who first pointed out that the high number of species here could primarily be related to the fact that the period of environmental fluctuation and the generation time of the organisms are of the same order of magnitude. Both shorter and longer phases would make exclusion more likely.

In addition to the temporal sequence, surprisingly there also seems to be something like a spatial compartmentalization of water bodies. Stable eddies and fronts can remain stable for a long time, up to weeks, and thus isolate different bodies of water from one another. Such water bodies, each with different phytoplankton communities, were also directly detected in the sea (where they are presumably of particular importance) by remote sensing methods. Although only one or a few species are dominant in each area due to minor environmental differences (or simply by chance), the resulting intermingling results in a high level of diversity.

Systems without balance: chaos

In addition to the models listed so far, which have in common that they are ultimately based on deterministic predictions, there are serious indications that in systems made up of numerous species and numerous resources there may be no state of equilibrium that could be reached after such a long time. They are possibly (deterministic) chaotic systems. Their properties were explored by Jef Huisman and Franz J. Weissing in a series of works. Chaotic systems are characterized by the fact that even small changes in the initial parameters result in completely different system states, which ultimately make the system unpredictable. Chaos does not occur with interactions of a few species (in Lotka-Volterra models only from four species, and here only in a small parameter range) and is therefore overlooked in overly simplistic models. If systems are subject to chaotic dynamics, an exclusion of competition is not to be expected (or only after extremely long periods of time). Of course, it is problematic to distinguish actually chaotic systems from "only" very complex ones with stochastic environmental fluctuations. But there are indications that natural plankton communities could actually behave in a chaotic manner. Also some laboratory miniature ecosystems ("microcosms") with numerous species ran z. Sometimes more than ten years under constant environmental conditions without a state of equilibrium having come about.

Systems without competition: neutral theory

Some models attempt to explain species number by simply denying the fact that competition is structuring the system. If competition as a regulating principle is unimportant or does not exist, its absence no longer requires any special explanation. This so-called "neutral theory" was worked out primarily by the American ecologist Stephen P. Hubbell . The neutral theory is either introduced as an actual explanation or it only serves as a null model to be able to describe what a hypothetical ecosystem without competition would look like. According to the neutral theory, all species are equivalent to one another. Accordingly, each of them can simply become more or less frequent by chance. Ultimately, each species will become extinct after (possibly very) long periods of time due to a random fluctuation. The neutral theory can easily explain the coexistence of numerous species in each habitat. However, many of their other predictions cannot be observed in real plankton societies. According to most researchers, they do not correctly describe both species change and dominance relationships.

Conclusions

The plankton paradox is still an active and fruitful field of research. At the moment it looks like that there is not one explanation for its existence, but many that are "correct" in part and in relation to certain situations. Although there is no unified theory, it is now in principle explainable how many species with similar requirements can coexist in one habitat. The numerous explanatory approaches, however, cannot yet be assessed in terms of their respective significance to one another. Although numerous researchers, each with good arguments, claim to have solved the problem, there are still numerous possible solutions for each individual system, without it being clear which is relevant in each case.

literature

  • Shovonlal Roy, J. Chattopadhyay (2007): Towards a resolution of 'the paradox of the plankton': A brief overview of the proposed mechanisms. Ecological Complexity 4: 26-33. doi: 10.1016 / j.ecocom.2007.02.016

Individual evidence

  1. ↑ George E. Hutchinson (1961): The paradox of the plankton . American Naturalist 95: 137-145.
  2. ↑ an overview z. B. in: Ray Dybzinski & David Tilman (2009): Competition and coexistence in plant communities. In: Simon A. Levin (editor): The Princeton Guide to Ecology. Princeton University Press (Princeton / Oxford), pp.186-193.
  3. ^ David Tilman (1977): Resource competition between planctonic algae: an experimental and theoretical approach. Ecology 58: 338-348.
  4. ↑ Maayke Stomp, Jef Huisman, Floris de Jongh, Annelies J. Veraart, Daan Gerla, Machteld Rijkeboer, Bas W. Ibelings, Ute IA Wollenzien, Lucas J. Stal (2004): Adaptive divergence in pigment composition promotes phytoplankton biodiversity. Nature 432: 104-107.
  5. ↑ Maayke Stomp, Jef Huisman, Lajos Voros, Frances R. Pick, Maria Laamanen, Thomas Haverkamp, ​​Lucas J. Stal (2007): Colorful coexistence of red and green picocyanobacteria in lakes and seas. Ecology Letters 10: 290-298. doi: 10.1111 / j.1461-0248.2007.01026.x
  6. ↑ Maayke Stomp, Jef Huisman, Lucas J Stal, Hans CP Matthijs (2007): Colorful niches of phototrophic microorganisms shaped by vibrations of the water molecule. ISME Journal 2007: 1-12. (International Society for Microbial Ecology)
  7. ↑ Xabier Irigoien, Jef Huisman, Roger P. Harris (2004): Global biodiversity patterns of marine phytoplankton and zooplankton. Nature 429: 863-867.
  8. ↑ Kai W. Wirtz (2012): Who is eating whom? Morphology and feeding type determine the size relation between planktonic predators and their ideal prey. Marine Ecology Progress Series Vol. 445: 1-12. doi: 10.3354 / meps09502
  9. ↑ Ulrich Sommer (1996): Plankton Ecology: The Past Two Decades of Progress. Science 83: 293-301.
  10. ↑ Sean Nee & Graham Stone (2009): Plankton - not so paradoxical after all. In: Jon D. Witman, Kaustuv Roy (editors): Marine Macroecology. University of Chicago Press, pp. 195-204.
  11. ↑ Shovonlal Roy (2009): Do phytoplankton communities evolve through a self-regulatory abundance – diversity relationship? BioSystems 95: 160-165. doi: 10.1016 / j.biosystems.2008.10.001
  12. ↑ CS Reynolds, J. Padisak, U. Sommer (1993): Intermediate disturbance in the ecology of phytoplankton and the maintenance of species diversity: a synthesis. Hydrobiologia 249: 183-188.
  13. ↑ for Central European lakes cf. z. B. Ulrich Sommer (1986): The periodicity of phytoplankton in Lake Constance (Bodensee) in comparison to other deep lakes of central Europe. Hydrobiologia 138: 1-7.
  14. ↑ Hanno Teeling, Bernhard M. Fuchs, Dörte Becher, Christine Klockow, Antje Gardebrecht, Christin M. Bennke, Mariette Kassabgy, Sixing Huang, Alexander J. Mann, Jost Waldmann, Marc Weber, Anna Klindworth, Andreas Otto, Jana Lange, Jörg Bernhardt, Christine Reinsch, Michael Hecker, Jörg Peplies, Frank D. Bockelmann, Ulrich Callies, Gunnar Gerdts, Antje Wichels, Karen H. Wiltshire, Frank Oliver Glöckner, Thomas Schweder, Rudolf Amann (2012): Substrate-Controlled Succession of Marine Bacterioplankton Populations Induced by a Phytoplankton Bloom. Science 336: 608-611. doi: 10.1126 / science.1218344
  15. ^ A. Bracco, A. Provenzale, I. Scheuring (2000): Mesoscale vortices and the paradox of the plankton. Proceedings of the Royal Society London Series B 267: 1795 - 1800. doi: 10.1098 / rspb.2000.1212
  16. ↑ Francesco d'Ovidio, Silvia De Monte, Séverine Alvain, Yves Dandonneau, Marina Lévy (2010): Fluid dynamical niches of phytoplankton types. Proceedings of the National Academy of Sciences USA (PNAS) vol. 107 no. 43: 18366 - 18370. doi: 10.1073 / pnas.1004620107
  17. ↑ cf. z. B. Jef Huisman & Franz J. Weissing (2002): Oscillations and chaos generated by competition for interactively essential resources. Ecological Research 17: 175-181.
  18. ↑ Marten Scheffer, Sergio Rinaldi, Jef Huisman, Franz J. Weissing (2003): Why plankton communities have no equilibrium: solutions to the paradox. Hydrobiologia 491: 9-18.
  19. ↑ Elisa Benincà, Jef Huisman, Reinhard Heerkloss, Klaus D. Jöhn, Pedro Branco, Egbert H. Van Nes, Marten Scheffer, Stephen P. Ellner (2007): Chaos in a long-term experiment with a plankton community. Nature 451: 822-825 doi: 10.1038 / nature06512
  20. ↑ Hubbell, SP, 2001. The Unified Neutral Theory of Biodiversity and Biogeography. Princeton University Press, Princeton, NJ, 375 pp.
  21. ↑ James S. Clark, Mike Dietze, Sukhendu Chakraborty, Pankaj K. Agarwal, Ines Ibanez, Shannon LaDeau, Mike Wolosin (2007): Resolving the biodiversity paradox. Ecology Letters 10: 647-662. doi: 10.1111 / j.1461-0248.2007.01041.x