Reefs at Risk Project Purpose
The Reefs at Risk in the Caribbean project brings together the best available knowledge on the Caribbean region’s coral reefs, as a basis for a region-wide analysis using a consistent method. Wide-ranging information is consolidated within a geographic information system (GIS), including data on coral reef locations (maps); pressures on coral reefs (observed threats, pollution, physical impacts); changes in condition; observations of coral bleaching and disease; and information on the management of coral reefs. Once these data are collected and integrated, we review and improve the data sets, although many gaps in the information remain. The project then attempts to fill in some of those gaps through inferential modeling of threats to coral reefs from human activities, including overfishing pressure, coastal development, and pollution and sediment from land-based and marine-based sources. These threat estimates were calibrated using the data available from the compiled sources (Data from CARICOMP, AGRRA, Reef Check, and REEF were used. See a full description in a later section on model calibration and validation) and have been subjected to several reviews by experts. Changing climate, coral bleaching, and coral disease are also significant threats to Caribbean coral reefs, but we were not able to model such threats using currently available data. The Reefs at Risk in the Caribbean report, however, presents current knowledge of the extent of and projections for these threats, within the context of the other pressures facing Caribbean coral reefs.
Threat Analysis Method
The project’s modeling approach involves identifying sources of stress that can be mapped for each threat category. These “stressors” include simple population and infrastructure features, such as population density and location and size of cities, ports, and tourism centers, as well as more complex modeled estimates of riverine inputs. Model rules were developed to build proxy indicators of threat level for these stressors. This process involved the development of distance-based rules by which the threat declines as distance from the stressor increases. For ease of interpretation, these threats are simply divided into “low,” “medium,” and “high” categories. Substantial input from scientists in the region contributed to the selection of the stressors and threat rules (thresholds) developed, while the threat indicators were further calibrated against available information on observed impacts on coral reefs.
Table 1 summarizes the threat analysis method and limitations for each threat category. The following sections provide details of the threat analysis methodology for coastal development, watershed-based sources of sediment and pollution, marine-based threats, and overfishing. Details of the model calibration and validation are at the end of this document.
The Reefs at Risk in the Caribbean project implemented this analysis method at 1-kilometer resolution for the Wider Caribbean region. In order to better evaluate land-based sources of threat transported via small watersheds in the Eastern Caribbean , the watershed-based analysis was implemented at 270-meter resolution for the Easter Caribbean sub-region. The methodology described in this document is scale-independent and can be implemented at finer scales, where more detailed data are available.
Table 1. Reefs at Risk Analysis Method
| Threat | Analysis Approach | Limitations |
|
Coastal Development |
-Threats to reefs evaluated based on distance from cities, ports, airports, and dive tourism centers. Cities and ports stratified by size. -Coastal population density (2000), coastal population growth (1990–2000), and annual tourism growth combined into indicator of “population pressure” treated as an additional stressor. -Thresholds selected for each stressor based on guidance from project collaborators and observations of local damage from coastal development (including sewage discharge). Stressors aggregated into single map layer. -Management effectiveness included as mitigating factor for threats to reefs inside marine protected areas (MPAs). |
-Provides a good indictor of relative threat across the region, but is likely to miss some site-specific threats. -Data sets used are the best available, but limitations regarding accuracy and completeness are inevitable. -In particular, rapid growth of tourism sector makes it difficult to capture the most recent developments.
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Watershed-Based Sources of Sediment and Pollution |
-Watershed-based analysis links land-based sources of threat with point of discharge to the sea. -Analysis of sediment and pollution threat to coral reefs implemented for more than 3,000 watersheds discharging to the Caribbean. -Relative erosion rates estimated across the landscape, based on slope, land cover type, precipitation (during the month of maximum rainfall), and soil type.[a] Erosion rates summarized by watershed (adjusting for watershed size) to estimate resulting sediment delivery at river mouths. -Sediment plume dispersion estimated using a function in which sediment diminishes as distance from the river mouth increases. Estimated sediment plumes calibrated against observed sediment impacts on selected coral reefs.[b] |
-Nutrient delivery to coastal waters probably underestimated due to lack of spatial data on crop cultivation and fertilizer application and resulting use of a proxy (sediment delivery) for indirect estimation. [c] -Sediment and nutrient delivery from flat agricultural lands probably underestimated, because slope is a very influential variable in estimating relative erosion rates.
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Marine-Based Sources of Threat |
-Threats to coral reefs from marine-based sources evaluated based on distance to ports, stratified by size; intensity of cruise ship visitation; and distance to oil and gas infrastructure, processing, and pipelines. |
-Estimates focus on ships in or near port. Threat associated with marine travel lanes probably underestimated due to lack of sufficiently detailed database on Caribbean shipping lanes. |
|
Overfishing |
-Threats to coral reefs evaluated based on coastal population density and shelf area (up to 30 m depth) within 30 km of reef. Analysis calibrated using survey observations of coral reef fish abundance. -Management effectiveness included as mitigating factor for threats to reefs inside marine protected areas (MPAs). -Destructive fishing practices not evaluated, as these are rare in the Caribbean region. |
-Local overfishing pressure captured in proxy indicator (based on human population per unit of coastal shelf area), due to lack of spatially-specific data on numbers of fishers, landing sites, fishing method/effort, or fish catch from reef fisheries. -Indicator reflects fishing within 30 km of shore. Impacts of larger-scale commercial fishing pressure, illegal fishing, or movement of fleets not included in analysis. |
TABLE NOTES:
a. “Relative Erosion Potential” was estimated at WRI using a simplified version of the Revised Universal Soil Loss Equation , United States Department of Agriculture (USDA) Agricultural Research Service (Washington, DC: USDA, 1989).
b. Data from Reef Check surveys and expert opinion from the Reefs at Risk workshop were used to calibrate the estimate of threat from inland sources. Data on percent live coral cover and algal cover from Atlantic and Gulf Rapid Reef Assessment (AGRRA) surveys were used to evaluate results.
c. Although phosphorus is often attached to soil particles, nitrogen is highly soluble and moves more independently of soil particles.
