Frame-based modelling as a method of simulating rangeland production systems in the long term

Abstract

Rangelands are complex systems with many interacting components. Modelling such systems presents two particular and interrelated problems. Firstly, the processes involved operate on greatly differing time scales: long-term changes in rangelands typically depend on much shorter-term processes such as rainfall, vegetation growth and livestock growth. Secondly, while complex mechanistic models of single animals or small herds have been developed (also involving differing time scales, from minutes to months), extending such models to an entire multi-species rangeland is generally an intractable problem. Recently, a new modelling paradigm has been introduced which facilitates the building of ‘economical’ systems models by using the concept of a frame. Frames are chosen to represent distinct states of the system (e.g. grassland with scattered mature trees, as opposed to dense bush cover). Independent models are constructed for each frame; these models simulate the key processes identified within that frame (and may themselves be simplifications of more complex models). Rules are established for switching frames. In this paper we describe a frame-based model of a typical Southern African rangeland supporting cattle and goats. Simple quantitative models for each frame are developed from the output of a complex mechanistic model, depending on rainfall, stocking density and animal condition. Results from the overall frames model show how long-term responses of the system (in terms of production and vegetation state) to various management strategies (such as livestock sales) may be predicted, and in this way allows comparison of different management strategies.