Using Seasonal Succession in Lake Plankton to Illustrate Concepts of Community Structure & Species Diversity

Abstract

[ILLUSTRATION OMITTED] Biological communities are of interest to people for a wide range of reasons. For example, people may want to know the reason for blooms of noxious algae in the lake where they possess recreational property, or whether the creek that borders the backyard of their house can support fish, or they may simply appreciate the of organisms found on a patch of forest where they hike. Ecologists and environmental scientists frequently need to describe biological communities as part of research programs that attempt to explain how the natural world operates or to determine the effects of manmade or natural perturbations. Direct cause and effect is difficult to show. However, careful documentation of the structure of the and how it is changing can provide a wealth of evidence that, in turn, can yield a persuasive argument. There are a number of parameters that can be used to describe the biological structure of communities, for example: the taxonomic identity of organisms present, their trophic role, the number of different taxa present, and the relative abundance of each taxon. The latter two parameters are commonly distilled down to a single metric called species diversity, sometimes referred to as community diversity (Krebs, 2001; Smith & Smith, 2001). The biological structure of different communities can be highly variable. For example, planktonic communities found in extremely saline environments may support only a few species that can physiologically withstand the high salinity, but the abundance of individuals of each species can be very high. Stream communities can be very diverse, but, unlike terrestrial systems, their trophic structures are largely based on detritivores feeding on leaf litter that has fallen into the stream, as opposed to autotrophic activity within the stream. In terrestrial communities, the growth forms of the plant species present can change the physical structure of the community. For example, long-lived trees in forests that persist through the seasons have a more profound effect on structure than short-lived herbaceous species in grasslands that die back in winter. Communities change overtime and, for many people, the usefulness of parameters is best appreciated in this context rather than by comparing geographically distinct populations. Communities can change as part of natural long-term succession, in response to short-term disturbance events, or due to cyclical seasonal fluctuations. For educational purposes, seasonal changes are suitable because they can be studied over a short timespan. Seasonal succession in plankton communities is easier to study than in terrestrial communities. Analyses of terrestrial communities require data collection over a large spatial scale which can entail a considerable amount of time in the field, possibly under inclement weather conditions. In contrast, the microscopic scale of plankton communities allows for easy presepeation and transport of samples to the lab for data collection. Planktonic taxa can be more difficult for students to identify than macroscopic plants and animals, but fortunately, many parameters of structure require merely that students discriminate between species and don't require actual identification. This article presents a simple technique for easy viewing of lake plankton communities, a simple dichotomous key that will ease students into the identification process, and a suggestion for an exercise to demonstrate changes in structure as a result of seasonal succession. I run this exercise as a lab for first-year university students in conjunction with lectures on ecology. For the sake of completeness, this article also presents some background information about lake plankton and measurements of species diversity. The exercise can likely be modified for different levels of high school, perhaps as a more qualitative assessment. …