Paradox of the plankton

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

Water is a homogeneous medium. In it live algae species that have very similar with each other needs to abiotic resources they need in addition to light a handful of nutrients, mainly nitrogen, phosphorus, and a number of metal ions, most in extremely low concentrations. Nutrient concentrations are so low that it is assumed that a strong competition of the algal species with each other in most places and at most times. The in ecological theory entrenched competition exclusion principle predicts that several species, which differ to the same resources needs, can not coexist in the same habitat should; the stronger competitor would displace the inferior to extinction. However, very many, sometimes hundreds, planktonic algae species in the same water body live.

This apparent paradox can be explained that the basic assumptions of the scientific models that were used to describe it is incorrect or incomplete. The plankton paradox is therefore an important test case for the ecological theory, in particular the modeling of competition in general and the theory of the ecological niche. Accordingly, it is also used as a test case for the declaration of biodiversity in other habitats. Has introduced the term plankton paradox of American Limnologist and ecologist George Evelyn Hutchinson in 1961. Regarding the possible explanation of the paradox are several theories that could be partly confirmed by laboratory tests, field experiments or observations in special cases. These can be divided into several categories. Some theories predict that the apparent homogeneity and simplicity of the system is an illusion, in reality existed here more niche dimensions that allow species to coexist. Other theories based on the fact that the competitive exclusion principle applies only in the case of equilibrium. The system never comes to equilibrium (either from external or from intrinsic reasons), may also be a superior in the case of equilibrium type not displace their competitors.

Resources and their relationships

An influential theory in plant ecology, " Resource -ratio theory " considers the possible relations when two (or more) species to two (or more) competing resources. Accordingly, two kinds, the two compete for resources, then stably coexist with each other when each of the types of each one of the resources is limiting. Each of the types reduces by their growth from the resource levels in the habitat until a minimum resource level is reached, which just hands her to survive. The kind that requires less of the resource wins. Of two types and two resources, a range of values ​​exist (according to the requirements of the kinds and the concentrations in the habitat ), wherein each of the species is prevented by the other resource because the extent to lower the level that they auskonkurriert the other type. A substantial prediction of the theory is that through this mechanism, can never exceed the number of species that can reach a stable equilibrium, the number of the limiting resource. In simple laboratory experiments have shown that algal species can behave the predictions of the theory according. As in natural water bodies, the number of resources that can limit the growth, because their supply is limited only relatively low, according to this mechanism, only very few species are coexisting with each other directly explainable. However, the species can be so against each other eingenischt along a resource gradient, that each one of them is competitive over lay each in a narrow range of values. This increases in a ( spatially or temporally ) heterogeneous habitat to the number of potentially coexisting species, although there are 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 through trade-off effects balance in some way.

Such ecological niches on the basis of a single factor has been convincingly demonstrated, for example, for the factor light in planktonic algae. In marine habitats coexist tiny photoautotrophic cyanobacteria (formerly known as " blue-green algae " ) of the genus Synechococcus with different light-absorbing pigments, each operating in different wavelength ranges of the most effective. Depending on water depth, turbidity and ingredients is always superior to another type. Contrary to appearances, this is not a homogeneous water medium. By excited vibrations of radiation has the water molecule defined absorption maxima between free spectral ranges can be delimited from one another, which can act as niche spaces.

Interactions with other species

The allocation of resources among several species can be modified by interactions with other species in other ways than by competition itself. This case is no different from a resource allocation by competing in principle. Here is the supervening types of niche space between the competitors divided differently, so that the in simple, isolated, case superior competitor, this superiority can not actually play. This provides the system with more degrees of freedom, in which it can be differentiated. It is especially important modification of the competitive relationships by antagonistic species, such as predators ( predators ) and pathogens ( pathogens). Within the limnology has already been known for decades that predators of phytoplankton, especially zooplankton, algae density can reduce the extent that previously heavily clouded by algae waters are clear again. The consumption rate of algae by zooplankton can many orders of magnitude above the gorging of land plants are by their herbivores. In contrast to land plants, many of phytophagous zooplankton are relatively non-specific in their choice of food, so that eg with higher number of species of algae that of the zooplankton does not increase. But it is important to differentiate according to the size. This can compete well with different types of small zooplankton density, depending on the large phytoplankton. A long underestimated role also play planktonic bacteria and, in particular, viruses, their density and biomass may exceed that of the algae several times. Plays a role under certain circumstances, the presence of the ( relatively few) toxic algae species.

Spatial and temporal heterogeneity: succession and compartmentalization

An important role in the coexistence of species of algae, it does obvious that the environmental conditions are not constant in a body of water. They change both predictable over the year as well as unpredictable and chaotic weather phenomena. A competitive superior under a specific combination of factors kind normally need to be prepared that the conditions that promote, not permanent. You will probably be replaced by conditions that promote a different kind. The changes during the year in the order are comparable with changes in the course of centuries or millennia for durable organisms such as forest trees for planktonic species with short generation times. The lawful and predictable sequence of communities within a biotope is known in ecology as succession. Such successions, which follow styles in the course of successive waves, are typical of almost all types of waters. During each phase of one type or a few species has a maximum, by the presence of species of preceding or following phases in lower density the number of species increases greatly. A similar mechanism has now been demonstrated for marine bacterial communities. G. E. Hutchinson himself was the one who first made aware of the fact that the high number of species could be especially associated with it here, that the period of environmental variation and the generation time of the organisms in the same order of magnitude. Both short and long phases would make the exclusion of competitors likely.

In addition to the timing, it seems, surprisingly, to give something like a spatial compartmentalization of water bodies. Stable vortices and fronts, longer time to weeks, remain stable and isolate different water bodies against each other. Such water bodies with different phytoplankton communities were in the sea (where they are believed to be of particular importance ) also detected directly by remote sensing methods. Although in each area by low environmental differences (or simply by chance ) only one or a few species are dominant, is finally entering the mixing into being a high diversity.

Systems without equilibrium: Chaos

In addition to the previously listed models which have in common that they are ultimately based on deterministic predictions, there are serious indications that in systems of many kinds and numerous resources may no equilibrium state exists, which could be achieved by even such a long time. It possibly is ( deterministic) chaotic systems. Their properties have been studied by Jef Huisman and Franz J. Weis Sing in a series of works. Chaotic systems are characterized by the fact that even small changes in the initial parameters completely different system states the effect that make the system ultimately unpredictable. Chaos does not occur in fewer species interactions on ( in Lotka -Volterra models after four ways, and only in a small parameter range ) and is therefore overlooked in too simplistic models. Systems are subject to a chaotic dynamics, exclusion of competition is not expected ( or only after extremely long periods of time ). It is problematic, of course, actually using stochastic environmental fluctuations to distinguish chaotic systems of "only" a very complicated also. However, there are indications that natural plankton communities might actually behave chaotically. Some laboratory miniature ecosystems ( " microcosms " ) with numerous types ran some more than ten years under constant environmental conditions, without being able to come to an equilibrium state.

Systems without competition: Neutral Theory

Some models attempt to explain the number of species in that they simply deny the fact that the system is structured by competition advantage. Is the competition irrelevant as ordering principle or absent, their absence also requires no special explanation more. This so-called "neutral theory " has been worked out mainly by the American ecologist Stephen P.Hubbell. The neutral theory is introduced either as an actual explanation or it serves only as a null model to describe, as a hypothetical ecosystem without competition would be like. According to the neutral theory, all types are mutually equivalent. Accordingly, each of them may simply be more or less often by chance. Ultimately, any kind will die by a random fluctuation after long (possibly very) periods. The neutral theory can easily explain the coexistence of many species in each habitat. However, many of their other predictions are not observed in real plankton companies. Both types of change and the dominance relationships are, according to most researchers do not properly described by them

Conclusions

The plankton paradox remains an active and fruitful field of research. At the moment it looks like that there is not an explanation for its existence, but many who are based in each part and to certain situations "right." Although there are not so unified theory, it is now explicable in principle how many species can coexist with similar claims in a habitat. However, the numerous explanations are not yet in their relative importance to each other to assess. Therefore, although many researchers, and to have each solved by good arguments, take claim for themselves, the problem that exist for each system to have many solutions side by side, without would be that clear what is relevant in each case in this case.