Red List of Ecosystems assessment 737
nutrients, a biota dominated by cold-adapted microorgan- isms that inhabit several habitat compartments on (supra- glacial), in (englacial) or below (subglacial) the icy sub- strate, truncated trophic networks and low productivity and diversity (Anesio et al., 2017;Hotalingetal., 2017;Keith et al., 2022). Tropical glaciers differ from others in their expos- ure to daily variations in temperature that far exceed the differ- ences in monthly mean temperatures across the year. Precipitation can occur throughout the year as rainfall or snowfall, although it tends to concentrate during the rainy or wet season, which typically extends for .8 months (Sagredo & Lowell, 2012). Annual precipitation at the highest elevations in theCordillera de Mérida is estimated to be 1,000– 1,200 mm, with 10–17% of the total falling during December–
March, and the mean annual temperature is −0.4 °C at the highest station where records are available in the country (4,766 m), with a ratio of diurnal variation to annual variation of 2.15–2.36 (Pulwarty et al., 1998; Andressen, 2007). The long-term mass balance of the icy substrate of trop-
ical glaciers is therefore probably dominated by interannual fluctuations in precipitation and exposure to solar radiation, but this process can be accelerated by scale and edge effects (Ceballos et al., 2006; Andressen, 2007; Vuille et al., 2018). The Cordillera de Mérida has lost several glaciated areas in the last 150 years (Braun & Bezada, 2013), and the glacier retreat rates increased after 1998, exceeding 7% area loss per year, which is higher than the rates reported for the last 30 years in other glaciers of the tropical Andes (Ramírez et al., 2020). Atmospheric or aeolian deposition (windfall) provide
key nutrients such as carbon, nitrate and ammonium to the micro- and mesobiota of the supraglacial zone (Edwards, 1987;Hotaling et al., 2017), but this is also a source of light-absorbing particles that increase melting processes (Gilardoni et al., 2022). Nutrients and meltwater can be transported through interglacial cracks and crevasses to
reach the subglacial zone (contact zone between rock and ice substrate), where they combine with small particles pro- duced by rock comminution (Hotaling et al., 2017). Greater concentrations of black carbon (sub-micron particle volume increasing from 0.19 to 1.4 μm3/cm3) at high elevations of the Cordillera de Mérida have been linked to biomass burning in Venezuelan savannah, with greater fire activity and high- er concentrations observed following El Niño years (Hamburger et al., 2013). The ice substrate, proglacial waters and glacier forefield
share part of their microbiota because of dispersal through meltingwater (Hotalingetal., 2017) and are here considered compartments of the same ecosystem. Glacier retreat asso- ciated with climate change and subsequent soil development triggers multitrophic changes from simple food webs domi- nated by microbes and heterotrophic organisms to more complex networks as deglaciated areas are colonized by li- chen, mosses and vascular plants, marking the transition to a novel ecosystem (Ficetola et al., 2021; Khedim et al., 2021; Llambí et al., 2021; Rosero et al., 2021; Anthelme et al., 2022). Prospective samples from englacial and subglacial
microbiota were collected from Bolívar and Humboldt peaks (Fig. 1, Plates 1 & 2; Ball et al., 2014; Balcazar et al., 2015; Rondón et al., 2016). These include samples taken prior to the complete disappearance of ice from Bolívar peak, but they do not allow for a full assessment of the role of the microbiota in the ecosystem. However, amore detailed study of themicrobiota in the glacier fore- field at Humboldt peak is underway (B. Huber, pers. comm., 2023). In the forefield area, a diverse array of pi- oneer lichen and bryophyte species (including many new records for Venezuela and several endemic species) ap- pears to facilitate the establishment of pioneer wind- dispersed and wind-pollinated vascular plants during the first 6 decades of primary succession after glacier ice retreat (Llambí et al., 2021).
PLATE 1 View of the glacier at Humboldt Peak in the Cordillera de Mérida, Venezuela, in August 2022. The photograph was taken from the peak, looking west towards the direction of (1) Bolívar Peak and (2)La Concha Peak. Photo: J.A. González.
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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