Scope of the assessment
This study, or Pilot Analysis of Global Ecosystems (PAGE), examines grassland ecosystems of the world using a large collection of spatial and temporal data. We analyze datasets primarily at the global level, presenting quantitative indicators and qualitative information on the condition of the world’s grasslands. Grassland condition is defined in terms of the current and future capacity of these ecosystems to provide goods and services important to humans.
Grassland extent, change, and human modification
PAGE analysts define grasslands as terrestrial ecosystems dominated by herbaceous and shrub vegetation and maintained by fire, grazing, drought and/or freezing temperatures. This definition includes vegetation covers with an abundance of non-woody plants and thus lumps together some savannas, woodlands, shrublands, and tundra, as well as more conventional grasslands.
Our comprehensive view of grasslands allows us to make use of a variety of global datasets and to avoid somewhat arbitrary distinctions among different land cover types.
We examine the spatial extent of grasslands and modifications that have altered their extent, structure, and composition over time. Modifications include human-induced changes such as cultivation, urbanization, desertification, fire, livestock grazing, fragmentation, and introduction of invasive species.
Grassland goods and services
This analysis focuses on a selected set of grassland goods and services. Our choice was determined partly in consultation with grassland experts worldwide and partly by availability of data.
Our goal was to use global datasets, preferably in electronic form, available spatially and with time-series. Where global data were not available, we used regional, national, and sometimes sub-national studies.
The data and indicators presented in this report address the condition of the following goods and services provided by grasslands:
- Food, forage, and livestock;
- Carbon storage; and
- Tourism and recreation.
Each good or service is discussed in terms of its current status, trends over time, and modifications that have changed its condition. The good or service also is discussed in terms of the type of data required to expand our knowledge about the ecosystem’s ability to provide the service. When quantitative indicators are available, we explore the potential to use them to evaluate the condition of grasslands. In other cases we present qualitative measures of condition, sometimes based entirely on expert opinion.
This study attempts to locate and draw together global, spatially represented databases on grassland ecosystems. It is not an exhaustive review of literature available on grassland types. Nor is it complete in its search for spatial datasets related to grassland ecosystems. Some important goods and services provided by grasslands also have not been covered. For example, woodfuel, often collected from shrublands or savannas, is not discussed in this report (but see the PAGE analysis on forest ecosystems), nor are the important services that grasslands provide in terms of water and nutrient cycling.
Rather, we present an examination of many of the global datasets most readily accessible, and of quantitative and qualitative indicators that can be used as starting points for a more comprehensive, international effort to evaluate the condition of grassland ecosystems worldwide.
PAGE researchers have found that global-scale analysis of grassland condition is difficult not only because of lack of sufficient data but also because of the variability in definitions of grasslands, inconsistency in scales of reported data, out-of-date information, and data based on expert opinion rather than scientific measurements.
Despite these difficulties, the indicators examined in this pilot analysis show unambiguous declines in the extent of grasslands, especially in the temperate zone. Areas of grassland before major modification by humans are now cultivated or urbanized, especially in North America and Europe.
The indicators also suggest that although the major goods and services provided by grasslands are in good to fair condition, the capacity for grassland ecosystems to continue to provide these goods and services is declining. Indicators of soil condition show that more than half of the grassland area analyzed under PAGE has some degree of soil degradation; over 5 percent of these grasslands are strongly to extremely degraded.
Measures for detecting changes in net primary productivity and rain-use efficiency show declines in some grassland areas. Indicators of grassland biodiversity show marked declines in grassland birds of North America, with negative effects from fragmentation and non-native species suggested for this region and others.
Although the carbon storage potential for grasslands is large, degraded areas store less carbon and there is heavy burning of some grassland areas, especially the African savannas. Tourism and recreational activities in grasslands appear to make important economic contributions to some countries, with revenues generally increasing. Overuse and declines in wildlife populations, however, suggest possible declines in the capacity to continue to provide these services.
Global scale analysis of grassland condition is further complicated by our limited ability to detect responses of grassland ecosystems to degradation. On the global scale, we rarely detect degradation involving changes in the age structure of plant populations or in the ability of species to reproduce. We might detect a decrease in plant productivity and cover with current satellite data and biomass measures. We can with certainty detect a complete loss of vegetation and evidence of soil erosion through a combination of satellite data, data from meteorological stations, and relationships modeled with NPP and RUE measures. At this stage, however, it may be too late to manage for complete recovery of the degraded ecosystem.
This pilot analysis reinforces the importance of establishing indicators that can be used to detect declines in grassland condition with sufficient time to implement changes in management strategies before degradation becomes irreversible.
Recommendations for future grassland assessments
PAGE researchers make several recommendations for future grassland ecosystem assessments. The most important recommendation is to recognize that a global assessment based on systematic measurement would be a large step forward in the field of ecosystem evaluation and monitoring.
GLASOD is praised because it collates and generalizes available datasets on the condition of the world’s soils. GLASOD is unsatisfactory for global appraisal, however, because it is not built on systematically collected data and thus cannot be used to monitor changes in condition.
Another important recommendation is to closely monitor changes of primary concern in land use of grasslands, including conversion of grassland to cropland, and degradation of grasslands in dry areas.
Specific recommendations for future grassland ecosystem assessments include the following:
- Use higher-resolution satellite data to delineate grassland ecosystems.
- Verify classifications of grasslands through field reconnaissance along selected transects of global land cover maps.
- Expand efforts to present time-series data on vegetation condition indicators such as net primary productivity and rain-use efficiency.
- Expand data collection efforts to produce maps of management systems showing extensive and intensive, or static and mobile grazing patterns.
- Use case studies on resilience to identify links between goods and services and changes in ecosystems, and to differentiate between permanent losses and potential recovery.
- Expand systematic data collection on biodiversity.
- Further research the role of carbon in grassland ecosystems, and the potential for both grassland vegetation and soil under different management systems to store carbon.
- Systematically collect data on human use of and revenues collected from grassland parks, reserves, and recreation areas.