Optimising the management of complex dynamic ecosystems. An ecological-economic modelling approach

Abstract

Keywords: ecological-economic modelling; ecosystem services; resource use; efficient; sustainability; wetlands, rangelands.Ecosystems supply a wide range of goods and services to mankind. This includes, for example, timber supplied by forests, and animal feed supplied by rangeland systems. In addition, ecosystems supply a range of essential life support services, such as the regulation of climatic and biochemical processes, and they have a non-use value related to their importance for the conservation of global biodioversity. In response to a continuously growing global population as well as progressively expanding economic activities, human pressure on ecosystems has been steadily increasing. Currently, substantial areas have been degraded by pollution, overharvesting of resources, and/or ill-planned land-use conversion. Hence, in view of the importance of ecosystems for mankind, there is a need to ensure optimal management of the current stock of ecosystems.Two key elements in optimal ecosystem management are economic efficiency and sustainability. Economic efficient management can be interpreted as management that maximises the present value of the net present and future benefits derived from ecosystem services. Sustainable management is, in this thesis, interpreted as management that maintains the capacity of ecosystems to provide future generationswiththe amount and type of ecosystem services at a level at least equal to the current capacity. Both the efficiency and the sustainability of ecosystem managementhasbeen studied intensively in the past four decades, in particular in the fields of ecology and environmental and ecological economics.The traditional models that analysed the responses of ecosystems to human management assumed that ecosystems respond in a gradual, reversible manner to stress. However, increasingly, it is now recognised that many ecosystem show much more complex dynamics, such as irreversible responses to stress, multiple states separated by thresholds, and stochastic behaviour. Such complex dynamics have been observed in, for instance, marine fish stocks, freshwater lakes, woodlands, rangelands, coral reefs and coastal estuaries. These dynamics have important implications for ecosystem management; they determine the response of the ecosystem to human pressures and rehabilitation measures. In view of these emerging insights in ecosystem dynamics, there is a need to further analyse the implications of complex dynamics for efficient and sustainable ecosystem management.This thesis examines how complex dynamics can be included in ecological-economic modelling, and how efficient and/or sustainable management can be effected for ecosystems with complex dynamics. In chapter 2 of the thesis, a framework is developed that can be used to analyse the economic efficiency and sustainability of management options for ecosystems subject to complex dynamics. The framework includes three main types of interactions between the economic system and ecosystems, and three types of ecosystem management options. Application of the framework requires the modelling of the causal chain 'ecosystem management' - 'ecosystem state' - 'supply of ecosystem services'. The efficiency of ecosystem management options can be derived from a comparison of the costs of the management options and the benefits resulting from changes in the supply of ecosystem services. The sustainability can be assessed on the basis of the long-term dynamics of the ecosystem. The approach is based upon dynamic systems modelling, using sets of connected (differential) equations. In order to reach an enhanced degree of ecological realism in the models, the models contain different ecosystem components as well as the interactions between these components. The thesis contains three case studies of ecosystems subject to different complex dynamics: (i) a hypothetical forest ecosystem; (ii) the De Wieden wetland (theNetherlands); and (iii) the Ferlo semi-arid rangeland (Senegal).(i) The forest ecosystem. In chapter 3, the implications of irreversible ecosystem responses for the efficient and sustainable management of a hypothetical forest ecosystem are examined. The forest supplies two services, wood and erosion control, and contains two components: forest cover and topsoil. The management variable is the rotation period applied to harvest wood. The paper first presents a general model, based upon the Faustmann model that has been widely used to examine the efficiency of forest management. Subsequently, it demonstrates the implications of pursuing efficient and sustainable forest management for a forest ecosystem in case of a reversible respectively an irreversible response to intensive harvesting. The paper shows how pursuing efficiency or sustainability can lead to the selection of very different management strategies. The possibilities to combine efficiency and sustainability considerations in management are influenced by the (ir-)reversibilityof the ecosystem's response to harvesting. (ii) The De Wieden wetland. Chapters 4 and 5 focus on the De Wieden wetland. Chapter 4 examines the four main ecosystem services supplied by the De Wieden wetland, and the spatial scales at which they are supplied to stakeholders.These services are reed cutting, fisheries, recreation, and the habitat service. Using different valuation methods, an economic value of the four services is provided, and it is examined how much of this value accrues to stakeholders at the municipal, provincial, national and international scale. The chapter shows that the four services generate a combined annual value of around 830 euro/ha/year. This is high compared to the value generated by surrounding agricultural land, which can be estimated at around 300 to 400 euro/ha/year. The analysis also shows that stakeholders at different spatial scales can have very different interests in ecosystem services. In De Wieden, reed cutting and fisheries are only important at the municipal scale, recreation is most relevant at the municipal and provincial scale, and the habitat service is important at, in particular, the national and international level. Consequently, stakeholders at different scales may have very different interests in the management of the ecosystem, which needs to be considered when valuation of ecosystem services is applied to support the formulation or implementation of ecosystem management plans. In chapter 5, the costs and benefits of eutrophication control in the four main lakes of De Wieden are analysed. These shallow lakes have two steady states separated by a threshold. The two states are a clear water state and a turbid water state; the switch from one state to the next is sudden once a certain threshold in nutrient concentrations in the lakes is passed. Currently, the lakes are in a turbid water phase, as a consequence of the high nutrient loading in, in particular, the 1960s and 1970s, and the local authorities are considering rehabilitation of the lakes. In chapter 5, an ecological-economic model is developed that describes the response of the lakes to reduced nutrient loading, without and with biomanipulation. The model is used to determine the economic optimal level of eutrophication control - including the costs of the eutrophication control measures and the benefits resulting from an increased supply of ecosystem services following a change to clear water in the lakes. The model shows that an approach with biomanipulation is substantially more cost-effective than trying to reach clear water through reductions in nutrient loading alone. A clear water state can be reached most cost-effectively by reducing the yearly inflow of total-P with 2 ton per year in combination with biomanipulation. The incremental benefits of an increased supply of the recreation and habitat service need to be valued at at least 210,000 euro per year in order to justify, from an economic perspective, rehabilitation of the lakes.(iii) The Ferlo semi-arid rangeland.The Ferlo inNorthern Senegalis studied in chapters 6 and 7. In chapter 6, it is examined how grazing and rainfall variability influence the productivity of the herbaceous layer in the rangeland. Data are based upon a ten years grazing experiment conducted in the Widou-Thiengoly catchment in the Ferlo,Northern Senegal. This experiment examined the impacts of a high (0.15-0.20 tropical livestock units per ha) and a medium (0.10 tropical livestock units per ha) grazing pressure - with the high grazing pressure corresponding to the current grazing pressure in the Ferlo. The paper shows that species composition, biomass production and rain-use efficiency differ markedly for the two grazing regimes - and that this impact is concentrated in years with low rainfall. In dry years, both biomass production and rain-use efficiency are significantly reduced in the plots subject to a high grazing pressure. Consequently, the impact of high grazing pressures on the productivity of the Ferlo is hardly noticed during years with normal or above normal rainfall, but the rangeland's productivity is strongly affected during a drought. The findings have important implications for the management of rangelands; the experiments indicate that high grazing pressures may increase the vulnerability of rangeland ecosystems and local people to droughts.In chapter 7, an ecological-economic model of the Ferlo is constructed that includes two drivers for ecosystem change: grazing pressure and stochastic rainfall variability. The impacts of grazing pressure are modelled conform the analyses conducted in chapter 6. The chapter presents an ecological-economic model that captures the impacts of grazing, as well as a case study of the Ferlo rangeland. The theoretical model shows that accounting for the impact of grazing on the herbaceous layer leads to a lower optimal stocking rate compared to a model that does not take this feedback into account. On the basis of the model, the long-term optimal livestock stocking rate for the Ferlo is calculated. The case study for the Ferlo also accounts for fluctuations in livestock prices; livestock prices strongly decrease during a drought when many pastoralists want to sell their livestock because of animal feed shortages. With the constructed ecological-economic model, it is shown that the economic optimal long-term stocking rate for the Ferlo is around half the current stocking rate. Reduction of the stocking rate to 0.09 tropical livestock unit per hectare would increase the per hectare income of the pastoralists from the current 237 CFA/ha/year to around 460 CFA/ha/year. Government actions should be aimed at reducing the livestock density in the area rather than promoting further expansion of the herd sizes.The thesis demonstrates the importance of considering complex dynamics in cost-benefit and sustainability analysis of ecosystem management options. Complex dynamics determine how the state of the ecosystem, and its capacity to supply ecosystem services, changes following the implementation of management measures. Application of the developed ecological-economic modelling approach can lead to concrete recommendations to support the formulation of economic efficient and/or sustainable ecosystem management strategies. In the De Wieden wetland, the benefits of eutrophication control measures largely depend upon their capacity to cause a switch from the current turbid water state to a clear water state. The developed ecological-economic modelling approach allows comparison of the costs and the benefits of these measures. In the Ferlo rangeland, the dynamics of the herbaceous layer depend upon the stochastic rainfall conditions in combination with the impact of grazing. With the ecological-economic modelling approach, the optimal livestock stocking density can be calculated. Based upon these results, it is anticipated that the framework and the modelling approach can also be applied to support the formulation of optimal management strategies for other ecosystems subject to complex dynamics