A co-modelling process of social and natural dynamics on the isle of Ouessant: Sheep, turf and bikes
Introduction
Nowadays modern agriculture should be regarded as a major anthropogenic threat to biodiversity and social structure, comparable to global climate change in its ability to affect vast areas (Fuller et al., 1995, Siriwardena et al., 1998, Maes and Dyck, 2001, Krebs et al., 1999, Tilman et al., 2001). The current major trends in European agriculture are characterised, on the one hand, by an intensification process that shows in, for example, transformation of dry grassland into fast-growing crops through irrigation, or increase of fertiliser and pesticide application; and on the other hand, by a shift to more environmentally friendly practices. However, on the European scale of change, agriculture is declining as an indirect result of intensification because marginal lands become less profitable to farm and traditional more labour intensive agricultural practices are displaced by more cost-effective practices. While there is agreement about the negative impact of agricultural intensification on biodiversity, there is however no consensus on how shrub encroachment affects biodiversity. When intensive agricultural land is abandoned, it can become a source of biodiversity (Grossi et al., 1995), whereas when shrubs occupy low-intensity land with high biodiversity, it is often perceived as a loss of biodiversity (Diáz et al., 1997, MacDonald et al., 2000, Franco and Sutherland, 2004). This is especially the case in isolated ecosystems such as small islands where agriculture becomes inefficient and is replaced by a tourism-based economy (Di Castri and Balaji, 2002). At the same time, tourism can also impact biodiversity.
These synergistic impacts on biodiversity are poorly studied and require a detailed integrative analysis, involving innovative interdisciplinary work and use of new dynamic tools (Etienne et al., 2003, Levrel et al., 2009a).
This study was carried out within the framework of a project on “Organisation of access to resources and biodiversity applied to the French Biosphere Reserves (BR)”, funded by the French Institute for Biodiversity. It aimed at modelling interactions between natural and social dynamics within the theoretical framework of multi-agent modelling. In this project, the stakeholders involved with natural areas of heritage or environmental interest put forward their concern for possible changes in environmental dynamics. The main goals were to understand the future environmental dynamics and to represent the main actors' view on natural resources and their dynamics in the context of their own objectives and according to their own, individual criteria. The management strategies established for each type of stakeholders were then formalised and the impact of these strategies on biodiversity was measured at various temporal and spatial scales. Within the framework of the project, the Companion Modelling approach (ComMod, 2006) was used for the study of four French Biosphere Reserves (Vosges du Nord, Lubéron, Ventoux and Mer d'Iroise). In this paper, we present the model developed for the isle of Ouessant, to give an example of an application of this integrative methodology and the analysis that it helped to perform.
Section snippets
Study area, context, driving force and data
Ouessant is a small island (1541 ha) located 20 km west off the western coast of Brittany, France (48°28′N, 5°5′W). The isle is currently in a period of rapid socio-ecological change. Social change is characterised by decrease in the island's native population and a great increase in the number of tourists.
Background
In 2001, a first spatial modelling approach using GIS led to the drafting of environmental evolution scenarios according to sheep grazing patterns in 1992 (Gourmelon et al., 2001, Gourmelon, 2003). The scenarios were based on three parameters: environmental typology (vegetation) and sheep distribution in 1992, pasture concentration in proximity of dwellings, and the threshold of 2–3 sheep/ha accepted as the stocking rate required to maintain the environment as it was. On this basis, four
Scenarios of potential social changes
In order to focus on a limited number of scenarios, we chose the most probable trends for each dynamics identified as well as specific scenarios linked to specific stakeholder requests.
Results and discussion
Following the implementation of this scenario, approximately thirty simulations were launched using the year 2002 as a reference state (initial parameters are defined in Table 3) and a time horizon of 15 years, in order to measure the variability of observed results. Fig. 7 shows the results of the analysis of simulation variability using a mean stabilising method. Indeed, because of the random proportion in the model, a certain number of simulations had to be carried out in order to consider
Conclusion
The originality of the Ouessant agent-based model results largely from the co-construction process applied and its capacity to bring together lay information and scientific knowledge from different disciplines and varied viewpoints. The co-constructed conceptual model was obviously a simplified representation of the Ouessant socio-ecological system but the stakeholders involved in its conception, validation or use described it as relevant in terms of its key variables and their interrelations (
Acknowledgements
This study was carried out within the framework of a project on “Organisation of access to resources and biodiversity applied to the French Biosphere Reserves”, funded by the French Institute for Biodiversity.
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