Species density of foliage-dwelling spiders in field margins: a simple, fuzzy rule-based model

Field margins are grassy strips or banks between arable fields or meadows t hat are not cultivated or ploughed and contain only single shrubs or trees. They not only support beneficial arthropods, e.g. predators of crop pests, but can also be of potential value to species of concern, making them a cr itical target for conservation of biodiversity in agricultural landscapes. Most ecological data and most knowledge of ecological relationships are imp recise, uncertain or ambiguous. An overemphasis on the precision of mathema tical models, thus, does not necessarily translate into a greater represent ation of reality. In contrast, fuzzy expert systems permit the representati on and processing of 'soft' ecological knowledge in terms of natural langua ge. This is done by sets of IF-THEN rules that relate the variables (expres sed in the form of fuzzy sets such as 'rather cold', 'comparably few specie s', etc.) among each other. Data on 96 field margins in southern Germany an d their foliage-dwelling spider assemblages, which had been collected from May to September 1994, were used for fuzzy modelling of the effects of marg in width, margin density, frequency of mechanical disturbance and vegetatio n-architectural complexity on species density of spiders. Habitat factors w ere ordered according to their hierarchical effectiveness: margin width and disturbance determine habitat persistence, habitat persistence and margin density determine colonisation potential, and colonisation potential and he rbaceous-plant cover determine species density. Forty-five rules are necess ary to relate these factors. The predictive power of the fuzzy model was su rprisingly high. The mean average error between predicted and observed numb er of species for test data, which was not used for model development, was less than 1.4 species (observed values ranged between 3 and 25). A multiple -regression model showed a mean average error of 3.17 species. We conclude that fuzzy models are suitable for representing the soft knowledge of field -margin/spider-assemblage relationships. The model permits time- and manpow er-saving validation on larger scales by qualitative rather than quantitati ve data collection and by the inclusion of remote sensing data. We believe that fuzzy models could provide a valuable means for formulating measures n ecessary for conservation of biodiversity in agricultural landscapes. (C) 2 000 Elsevier Science B.V. All rights reserved.