Red List of Ecosystems assessment 739


FIG. 2 Conceptual ecosystem model for the tropical glacier ecosystem of the Cordillera de Mérida, Venezuela. White ovals and boxes: those with a dashed outline represent the main characteristic biota and biotic processes, those with a solid outline the main elements of the abiotic environment and abiotic processes, respectively. Dark grey boxes: threatening processes. Black lines: relationships between elements (arrow: increases; circle: reduces). Light grey boxes: indicator variables used in the assessment and indirectly linked to ecosystem processes or components by grey dashed lines. Some elements and processes are excluded for clarity of visualization.


over a 50-year period (1948–1998). Given the evidence suggest- ing accelerating rates of decline, we compared the results of using different timeframes for calculation of proportional rates of decline. We used two spatial measures of exposure to threats (criter-


ion B): the extent of occurrence was calculated as the area of the convex hull around the glacier outlines from the RGI 6.0 database (RandolphGlacier InventoryConsortium, 2017), and the area of occupancywas calculated as theminimum number of 10 × 10 km cells that include all of these occurrences. For criterion C1 we used a time series of freezing level


height in metres calculated from climate reanalysis data for 1948–2011 (Braun & Bezada, 2013) and fitted a local polyno- mial regression (loess with gaussian distribution, span = 0.75 and degree = 2, equivalent number of parameters = 4.35)to smooth the temporal trend. We calculated relative severity of degradation (RS) using the formula: RS =OD/MD, where OD (observed decline) is the difference between the initial and final values andMD(maximumdecline) is the dif- ference between the initial value and the collapse threshold (Keith et al., 2013). We used the initial and final values of the smoothed trend in freezing level height and two collapse thresholds of 4,920 and 4,970 m, which span the range of plausible estimates. We took estimates of historical changes in equilibrium-line altitude from Polissar et al. (2006), and these are based on reconstructed palaeo-glacier topography near MucuñuquePeak(locatedinnearbySierra deSanto Domingo; Fig. 1) and cumulative elevation profiles ofmodern


glaciers at Bolívar Peak. For the calculation of relative severity of degradation we used change in equilibrium-line altitude as a directmeasurement of observed decline and themaximum possible change as maximum decline (sub-criterion C3). We also used local results of correlative and mechanistic


global models to project trends in climatic suitability (sub- criterion C2a) and the probability of persistence of tropical glaciers into the future (criterion E). These models incorpo- rated different global circulation models and scenarios of projected greenhouse gas emissions based on alternative socio-economic developments and climate policies (shared socio-economic pathways; Karger et al., 2017). The correlative model of environmental suitability


focused exclusively on tropical regions and compares the bioclimatic conditions of high-elevation areas with and without glaciers using the Gradient Boosting Machine algorithm (Ferrer-Paris & Keith, 2024). We selected occur- rence records using stratified random sampling from glacier outlines in tropical areas (from the tropical Andes, Mexico, Africa and Asia) and non-glaciated areas with a minimum of 3,500melevation and amaximum distance of 25km from the nearest glacier outlines.Wewithheld occurrence records from the Cordillera deMérida for final evaluation of predic- tion performance of the model; we divided all other records into calibration (80%) and test partitions (20%) and used them for tuning of the model parameters (number of trees, interaction depth, shrinkage and minimum number of observations per node) and final model fitting using


Oryx, 2024, 58(6), 735–745 © The Author(s), 2024. Published by Cambridge University Press on behalf of Fauna & Flora International doi:10.1017/S0030605323001771


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