Experimental Manipulation of Model Ecosystems

Abstract

Ecosystems have been described as the “functional units of the landscape” (Odum, 1971), where organisms interact with their physical-chemical environment and with other organisms. For limnologists, lakes, ponds, streams, and rivers and their associated drainage basins represent the ecosystems of interest. However, these ecosystems are large and complex, and thus it is difficult to decipher the various interactions that occur within them.

Natural ecosystems are exposed continuously to changing environmental conditions, of both internal and external types. These conditions can be changed either by natural (for example, seasonally) or by unnatural means (e.g., through human intervention). Whatever the cause, the effects of any single change may be difficult to separate from those resulting from all of the other changes that are occurring simultaneously. A changing natural ecosystem can be likened to an experiment in which the investigator is trying to control and, at the same time, understand many known and unknown variables. For the creative scientist, this situation can present a real challenge. Nevertheless, the understanding of causation in the ecosystem rarely can be delineated without the benefit of some carefully designed and rigorously controlled experimental work.

The cost in both money and time of doing experimental work on whole ecosystems often is prohibitive. Moreover, while such studies have been done [see, for example, Hasler and Johnson (1954), Likens et al. (1970), and Schindler (1974)], we usually do not have the privilege of being able to alter or to disturb seriously an entire ecosystem for experimental purposes. A common solution is to recreate, in the laboratory, microecosystems or microcosms [see, for example, Warington (1851) and Beyers (1963)]. An ecosystem brought into the laboratory can mimic the natural ecosystem in some respects but will differ in others: a microcosm is a simplified ecosystem with discrete boundaries. The scales of events in both time and space are abbreviated. Succession to a new steady state takes place in weeks, rather than in years. Microcosms generally have fewer species than do natural ecosystems and have, in consequence, simpler communities of organisms. Some characteristics of microcosms make them valuable objects of study. Microcosms are expendable, and the experimenter has control over the environmental boundary conditions to a degree impossible to achieve in the field. Also, it generally is assumed that the investigator can establish reproducible or replicable units, thereby allowing statistical evaluation of the data obtained from experimental treatments and controls for each manipulation.

In this exercise, two different approaches, the chemostat and the microcosm, will be described for the study of ecosystems in the laboratory. Both of these approaches require several weeks for stabilization, manipulation, and evaluation. Thus students should select one of these approaches and should understand that appreciable time must be allotted for independent study of these systems. The chemostat requires somewhat more sophisticated equipment and procedures.