8 Epilogue

NORBERT W ALZ

The different continuous culture techniques described in this volume offer - in various steps of complexity and technical investments - a world of new aspects both for experimental and on theoretical ecology. This concerns such broad fields as population and community ecology, ecosystem theory, ecotoxicology, and aquaculture. For the application of continuous cultures to these fields, many preparations and prerequisites have been necessary. For example, the math­ematically well-formulated chemostat theory worked out for microorganisms had to be modified for metazoans (Chap. 3.3).

Semicontinuous cultures have been developed (Chap. 2.1) as a method well suited to give excellent support to work on questions of the growth characteristics of populations (Chap. 3.1). Another possibility involves study­ing competition between populations within the community by means of re­source partitioning (Sect. 6.2.5) or by "shift-up" and "shift-down" experiments (Chap. 6.1). Another application is in ecotoxicology (Chap. 7.3). This method is in the best sense very simple and at a low technical level, so it may be applied in many laboratories. The characteristic dilution of the system is done mechani­cally by a regular discard of a fixed part of the culture volume supplemented with a new feed suspension. The manpower requirement, on the other hand, seems to be fairly high. In particular, the working hours needed could restrain this method. Fed-batch cultures (Sect. 2.1.2 may be particularly adapted to experiments on zooplankton population dynamics. The first method has been utilized with rotifers, but nothing speaks against the successful application of both lines to cladocerans and protozoans.

In contrast, the turbidostat is the technically highest developed continuous culture device (Chap. 2.3). Its operation field includes studying the maximum performances of physiological and population dynamic parameters and select­ing the fittest popUlations (Chap. 3.4). Maximum performances (e.g., rmax) are affected the most by toxins. This technique, therefore, should be well suited for the assessment of ecotoxicity (Sect. 7.3.8) as a system response. Hitherto, this kind of turbidostat has not been used in aquaculture, for which it should be a very interesting method.

The effort to build up and operate chemostats is shared between both parts. This technique is nearly as time consuming as semicontinuous cultures, while

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the methodical investment may be a little lower than that for turbidostats (Chap. 2.2). Because of its more constant dilution rate, it supplies more evident results. Because of its substrate limitation, it imitates best the situation in the community and in the ecosystem. The chemostat, therefore, is mostly adapted as a model ecosystem both in its single and two-stage version. This system has been successfully used to study the carbon metabolism and energy balance of different rotifer species (Part 4). This method was applied (among others) to approach experimentally questions of numerical and functional responses of populations, of food quality effects, of competition, of the role of the "microbial loop" (Chap. 6.2), and of community regulation mechanisms (Chaps. 3.2-3.4). A very promising employment is in aquaculture (Chap. 7.4).

The different continuous culture methods have their own benefits and special employments. Above these mostly technically defined applications of continuous cultures, there are more theoretically founded arguments. These refer to two different points which should not be separated:

1. Dilution principle in the model ecosystem 2. Experimental testability of hypotheses

Continuous cultures are model ecosystems or mesocosms most likely adapted to the system of the plankton community because the structure of the plankton community is reflected truly in continuous cultures. Since there is an energy flow along the food chain and simultaneously a real information feedback in continuous cultures from predators to prey [e.g., from the consumer to its food and from the dilution rate, which imitates the action of a predator (Fig. 7.2.1), to the consumer], this system allows us to study the interaction between trophic levels and, therefore, the "emergent properties" of the system (Preface, Chap. 7.2).

By these methods, therefore, many problems of the plankton community may be assessed, especially problems of competition for food resources or of com­petition under simultaneous predation. With continuous cultures, the principles, of many predator-prey problems may be studied experimentally. Competition can be studied under different regimes of dilution, Le., of the feeding rate of the "predator," and at different levels offood input. This is a fundamental difference to other mesocosms (e.g., Kersting 1985).

So far, these model ecosystems are very simple and are based on the ecological knowledge of a few species only. In the future, more models with increasing complexity have to used. The energetic and kinetic relationships of more and especially of "key" species, which are important in the food web of the community, have to be studied. The dilution rate as a model "predator" selects for individuals with the highest growth rates. This is a natural situation in highly flooded lakes and rivers only. Indeed, in future experiments the hydraulic dilution must be supplemented by the feeding action of planktivorous fish selecting according to other rules. On the other hand, experiments with hydraulic dilution may reveal the most fundamental-but perhaps too abstract - action of a predator and of the reaction of the community.

In all continuous cultures, the dilution rate is fixed for each stage. But which dilution rate do natural communities have? And which dilution rate do

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more complicated model systems have, especially if more trophic levels are usually present, e.g., if additional fish are introduced? How are the "dilution rates" of the different trophic levels coupled? Is it right that if there is a top-down control, then the feeding rate of the organisms of the top level must give the rhythm for the turnover of the whole community like the dilution rate in simpler models? But how do the turnover rates of the other trophic levels adapt to this overall turnover? This question seems to be one of the main problems of the upscaling of the conceptions arising from simple continuous culture models to natural communities (Chap. 7.2).

The regulation model which is developed on the basis of continuous cultures and is in accordance with observations must not have any a priori expectations of equilibrium (see Introduction). To be certain, the model is based on continuous cultures in which the system finally achieves a steady state or equilibrium. But with the same structure and with those parameters worked out for steady states, this model can also describe transient states (Chap. 5.1). Equilibria are the result of regulation processes. Models with time lags may not show such steady states.

Regulation models include the food resources explicitly. The population development and its density depend dynamically on these resources, which are controlled by feedback reactions. Consistently, the regulation model dominates over the widespread logistic and Lotka-Volterra models. Those models are not adapted to regulation processes because they do not explicitly include the resources, which are regulated, too. As a consequence, any discussion of the density dependence of populations should include the decisive resources. In future, the resource conception should replace the formulation of density­dependent models. Additionally, for this reason it makes no sense to speak of "density-independent" factors. Of course, further factors are effective which are not regulated if they come from the outside of the system, for example, the temperature.

Having accepted that Lotka-Volterra models are no longer useful for predictions in population dynamics, there are a lot of theoretical consequences for all the fields of ecology concerned, because many fundamental conceptions are based on these models, especially competition models, predator-prey models, and models for community dynamics.

Another topic which contradicts the regulation principle is the identification of a carrying capacity, an important term in the logistic and in Lotka-Volterra models. In the regulation model, the carrying capacity is not a predefined parameter but the result of nutrient or food input, i.e., dependent on steering factors and on boundary limits (see, for example, Chap. 3.3). A second step is the consumption of the supplied resources by the community, which is a regulated process; for example, the growth efficiency is not constant. In contrast, in the logistic and Lotka-Volterra models the carrying capacity has to be explicitly declared as a specific value.

This is also true for the so-called competition coefficients in the Lotka­Volterra competition models which are known only a posteriori and have no predictable value. This consequently invalidates the well-known r/K concept.

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With the rmaJK. life history strategy model (Fig. 5.1.6; Chap. 6.3) or with the Tilman model (Chap. 6.2), however, it seems possible to predict the outcome of a competitive situation based on the energetic and kinetic data of the compet­ing species. These models are an extension of the regulation principle.Co­existence is possible by food partitioning (Sect. 6.2.5) and by time lags in the population dynamics (Sect. 5.1.6).

Continuous cultures refer obviously to the pelagial, in which the plankton density is relatively simple to measure. In this community, the food resources may easily be assessed, too. Other populations and communities are thought to be food limited, too, although the exact proof is not easily furnished. Nevertheless, the conclusions drawn from the regulation model-based on continuous cultures as model ecosystems - should indeed be transferred to other communities. As a consequence, they should replace orthodox density­dependent concepts which do not explicitly include the resources. Some of these ideas have been developed in Sect. 7.2.6.

In the past few years this regulation principle, the feedback reaction between the different trophic levels, has been used practically to reduce the eutrophication effects in water management. In many cases, the structure of the plankton community of small lakes could be successfully changed in this way so that the community could fulfill a prominent function in a sanitation concept. This can be done by so-called biomanipulation, food chain manipulation, or top-down control (e.g., Shapiro 1980; Benndorf 1987). The structure and the dynamics of the community are the result of boundary limits. In order to reach a maximum transparency (Secchi depth) of a lake, the large cladocerans, the most effective filtrators, have to be promoted by a reduction of zooplanktivorous fish. This is managed by intensive fish catches or by the introduction of piscivorous fish.

The questions are, how sufficient is this top-down effect and how stable is this community, as otherwise these measures have to be repeated regularly. For the system analysis, the deciding point is the permanent change of the steering factors. Only with an ongoing reduction of nutrients, one external factor having mostly dictated the model ecosystem and the regulation model, could a continuing stability of the community within the desired limits be found (Benndorf 1987). From this standpoint, the action of top predators seems to be an internal factor. It depends on the model itself whether this factor is to be understood as external or internal. As a consequence, without nutrient reduction this community regulates itselfto a stable point outside the desired limits. Under these conditions, manipulation has to be considered as "work" to be done against this regulation. It follows that a fundamental knowledge of the regula­tion characteristics has to be involved to apply the manipulation measures successfully.

A very interesting question deciding the usefulness of any predictive model is the occurrence of a chaotic behavior in it. As for any deterministic nonlinear model, it is known from Lotka-Volterra models that chaotic development depends on the values of some specific parameters (May 1981, Sect. 7.1.2). Even the

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simulation of the regulation model in Chap. 5.1 demonstrates a chaotic course at higher time lags, depending on the boundary limits of the system. The effect of these conditions, however, is very different in both models. Lotka-Volterra models tend to -show cyclic or "strange" attractors (chaotic behavior) at higher algal growth rates and this only if the model has three interacting levels (Scheffer 1991). On the other hand, the regulation model shows a better resistance to cyclic oscillations and chaotic behavior in simulations with higher food inputs and higher dilution rates (Sect. 5.1.5). The stability of the models is lost under exactly opposite conditions.

This means that the conditions necessary to create chaos are dependent on the structure of the model. For this reason, it is too early to draw conclusions between model ecosystems and the real world. Scheffer (1991) found chaotic behavior in the mesocosms of Kersting (1985), but in this model system, no predators are present. In continuous cultures, a stabilization by higher dilution rates, i.e., by predators, seems to be possible. In the above-mentioned sense, top level organisms are able to structure the whole community. A very forward conclusion from the regulation model founded on continuous cultures is that communities generally tend to become chaotic but are stabilized by predators or, in the case of highly flooded lakes, by the dilution rate. The existence of chaos observed in computer simulation models for model ecosystems and in the field remains controversial and should be investigated further in the real world. For this purpose, continuous cultures seem to be ideal test systems.