A rule-based ecological model for the management of an estuarine lake

Abstract

It is often difficult to build conventional dynamic models for an ecological system because the relationships between the abiotic and biotic components of the system are understood only in a rough, qualitative rather than a detailed, quantitative sense. This paper shows how a rule-based model can be formulated and used in this type of situation. The purpose of the model is to assist managers of a large, shallow coastal lake connected to the sea by a narrow estuary. Fluctuations in the salinity of the lake have a marked effect on the biotic components and managers are interested in the likely effects of alternative strategies for ameliorating the salinity. The paper shows how available information lends itself to a representation in terms of rules that indicate how important biotic components change (on a crude scale of 1 to 5) depending on the prior state of the system and current water conditions. The model was incomplete at the time of writing, but at least one important (and unexpected) result has already emerged: the abundance of underwater plant biomass is sensitive to the rate of change of salinity rather than the salinity level per se. Several consequences of this type of modeling are noted: it draws on the experience of both scientists and non-scientists, provides a consistent, logical basis for discussion, improves communication between field biologists and managers, lends itself to an adaptive approach, and can provide assessments of the quality of each simulation. It is suggested that this approach is pertinent whenever the effects of abiotic events dominate mutual interactions between the biotic components of a system.