Drivers of wetland conversion: a global meta-analysis

Abstract

Meta-analysis of case studies has become an important tool for synthesizing case study findings in land change. Meta-analyses of deforestation, urbanization, desertification and change in shifting cultivation systems have been published. This present study adds to this literature, with an analysis of the proximate causes and underlying forces of wetland conversion at a global scale using two complementary approaches of systematic review. Firstly, a meta-analysis of 105 case-study papers describing wetland conversion was performed, showing that different combinations of multiple-factor proximate causes, and underlying forces, drive wetland conversion. Agricultural development has been the main proximate cause of wetland conversion, and economic growth and population density are the most frequently identified underlying forces. Secondly, to add a more quantitative component to the study, a logistic meta-regression analysis was performed to estimate the likelihood of wetland conversion worldwide, using globally-consistent biophysical and socioeconomic location factor maps. Significant factors explaining wetland conversion, in order of importance, are market influence, total wetland area (lower conversion probability), mean annual temperature and cropland or built-up area. The regression analyses results support the outcomes of the meta-analysis of the processes of conversion mentioned in the individual case studies. In other meta-analyses of land change, similar factors (e.g., agricultural development, population growth, market/economic factors) are also identified as important causes of various types of land change (e.g., deforestation, desertification). Meta-analysis helps to identify commonalities across the various local case studies and identify which variables may lead to individual cases to behave differently. The meta-regression provides maps indicating the likelihood of wetland conversion worldwide based on the location factors that have determined historic conversions.

Figure 1. Sites of wetland conversion.

In green wetland areas (including lakes and areas with partial wetland cover) from the Global Lakes and Wetland Database (from Lehner and Döll, [27]).

Figure 2. Proximate and underlying drivers of wetland conversion.

Figure 3. Number of times proximate causes of wetland conversion are documented in the 105 analyzed case-studies.

Figure 4. Most frequent occurring combinations of proximate causes and underlying forces of wetland conversion.

Agricultural development includes pasture expansion. For each proximate cause at least the two most important underlying forces are indicated, and for each underlying force at least two associated proximate causes indicated.

Figure 5. Number of times underlying forces of wetland conversion are documented in the 105 analyzed case-studies.

Figure 6. Probability of conversion of wetland areas and converted and non-converted wetland sites (data set 4).

Grey areas are non-wetland areas. Wetland areas are defined based on the Global Lakes and Wetland Database [27].