Mountain ecosystems are complex, dynamic and exceptionally fragile. We are only beginning to understand the functional ecology of mountain ecosystems, but international research already suggests that changing species communities will be detrimental to the environment, to biodiversity and therefore to a critical part of Earth’s life-support system. Because mountain ecosystems are highly sensitive to global change, they are considered as sentinels of change. Climate change is modifying and will continue to modify the occurrence of extreme events, the amount of precipitations (rain and snow), as well as freeze and thaw cycles, with impacts on the onset of snow melt (and thus length of growing season) and water temperatures, aggravating impacts from inappropriate land use practices. Global change with all the different pressures outlined above causes imbalances in the functioning of mountain ecosystems, which lead to changes in vital biological, biochemical, and chemical processes, critically reducing ecosystem health with repercussions for animal and human health. Because mountain biodiversity is the basis for ecosystem functioning and the provision of ecosystem services, changes in mountains will have far reaching impacts for the human society.
Following the onset of the industrial revolution and the use of fossil energy, humans can indisputably be seen as major geological agents and as global pathogen vectors. Due to the increase in human activities over the last 300 years, human impact has become at least as strong a force as natural processes, marking the geological era of the Anthropocene. Mountains are an essential component of the global life-support system. They are characterized by a rugged, heterogenous landscape with rapidly changing environmental conditions providing myriad ecological niches over relatively small spatial scales. Although montane species are well adapted to life at extremes, they are highly vulnerable to human derived ecosystem threats. Mountains cover a large part of the Earth’s terrestrial surface. They hold an estimated one-third of terrestrial species diversity, and represent half of the 34 global biodiversity hotspots. Nevertheless, even in remote areas human impact is strong, as mountains are part of the global socio-ecological systems which have been shaped by geological forces and by human activities.
Generally, people are unaware of the threats to mountain ecosystems and the services mountains provide to humanity. Mountain ecosystems sequester CO2, clean water and air, regulate climate, provide biomedical resources, and regulate floods. Mountains also provide for the livelihoods of more than half of humanity. All these goods and services are provided by mountain ecosystems through complex processes that are maintained by communities of species interacting with each other and with the abiotic environment. These mountain communities are comprised of prokaryotic and eukaryotic microbes, zooplankton species, woody and non-woody plants, as well as invertebrates and vertebrates. By destructing, rebuilding, changing, and shaping the environment, these species produce organic matter and oxygen as well as bind CO2.
With increasing elevation, montane climates become more extreme, providing habitat for fewer species. System redundancies (i.e. different species with similar functions), available in ecosystems at low altitudes, are increasingly scarce in mountain ecosystems. Such redundancies usually provide stability to ecosystems. The absence of these redundancies renders mountain ecosystem particularly vulnerable to the impacts of global change. However, multiple threats to mountains are arising from climate change alone. Moreover, interactions with socio-cultural, economic and political developments, such as the exploitation of mountains, e.g. for timber, food production, including fish and livestock, tourism, and hydro-electricity, exacerbate these threats, calling for urgent consideration by policy-makers.
Threat 1: Climate Change
Climate change will increase the unpredictability of precipitation patterns. Generally, water availability is expected to decrease in the future due to lower water storage capacities in areas with glacier cover and higher outflows of excess water during periods of extreme precipitation and melting events. These changes will jeopardize the role of mountains as global water towers and the drinking water supply for billions of people.
Threat 2: Acidification and pollution
The scavenging of atmospheric organic and inorganic pollutants is pronounced at high altitudes and can take the form of dry and wet deposition of aerosols to the ground surface. Main categories of atmospheric pollutants transferred to freshwater ecosystems in mountains are Nitrogen and Sulphur oxides, trace elements, organic and synthetic pollutants, including current-use and legacy pesticides, polycyclic aromatic hydrocarbons (PAHs), endocrine disrupting compounds (EDCs), and polychlorinated biphenyls (PCBs) or microplastics. These compounds are introduced into the atmosphere via evaporation, or binding to particles light enough to be carried by wind into mountains.
In mountains, acidification occurs through the atmospheric Nitrogen and Sulphur emissions in synergy with climate change. Acidification will also have consequences on biodiversity, trophic networks, and drive cascade effects throughout mountain ecosystems. In addition to atmospheric transport of micropollutants, local human activities such as mining, logging, agriculture, pastoralism and tourism are the main sources of pollution. Introduction of new pollutants and changes in pollutant mobilization due to climate change may challenge mountain ecosystem health and increase the vulnerability of freshwater species and humans to pathogens, increasing health risks.
Threat 3: vegetation changes
Current mountain vegetation is largely shaped by historical and current land use, with the degree of influence depending mainly on the accessibility of the area. Vegetation and changes occur through a change in land cover, through intensification or extensification of existing land use practices (Niedertscheider et al., 2017). Most of these changes are driven by pastoral activities, such as livestock grazing, which is the major agricultural activity in most mountains. An illegal activity related to pastoralism is slash and burn to avoid expansion of forest areas. The clearing intensity and frequency increases with increasing pastoral pressure. Other land use changes include logging of forest stands, afforestation, vegetation regrowth in abandoned lands. All these changes intervene deeply in the existing ecosystem, altering and threatening underlying processes and associated ecosystem services. An emerging land-use trend is the growing impact of tourism on ecosystems, where damage to vegetation can occur and is playing an increasing role in mountain regions.
Threat 4: Touristic activities
Tourism also drives fish stocking of naturally fishless lakes. Stocking montane lakes with fish for subsistence purposes has been occurring since the Neolithic, through fish translocations from nearby lakes and rivers. However, before 1950 such introductions had limited geographic extent and their impacts were rather local. Since 1950, introductions of fish increased dramatically as a consequence of the increasing popularity of recreational angling, both in large and relatively small lakes as well as in adjacent wetlands. Supported by institutional stocking campaigns and non-authorized translocations by anglers, fish spread rapidly in mountain deep-ponds and lakes of all sizes, as well as in all the colonizable downstream habitats, with a long list of negative impacts: i) decline/elimination of native species (e.g., invertebrates and amphibians; ii) cascading effects in the trophic network, affecting the chemical/microbiological quality of waters, and the ecological linkages with surrounding terrestrial habitats; iii) impacts such as predation, competition, transmission of pathogens and hybridization on native fish inhabiting downstream habitats, and iv) further collateral introductions of fish used as live bait. Hence, fish stocking in mountain lakes is particularly detrimental to water quality and biodiversity, especially as now nearly all these ecosystems are affected, including large lakes, small lakes, ponds, connecting streams and their adjacent mountain wetlands.
Threat 5: Water abstraction
Most valley bottoms have been heavily altered by human activities that impact freshwater systems. These activities include land drainage, dredging, flood protection, water abstraction for hydroelectric powerplants, and inter-basin water transfer, building dams to create reservoirs, and digging new canals for navigation. In mountains, water abstraction has been increased to excessive levels. Hydrological interventions include (hydroelectric) dams, pipelines and derivation channels, agricultural ponds, irrigation and snowmaking reservoirs, quarries, water removal, and flow regime alterations. Water abstraction in concert with climate driven changes in hydrological regimes will lead to a gradual drying up of aquatic mountain ecosystems, likely causing massive water shortages in cities that depend on drinking water from mountains. The desertification of these ecosystems will also be detrimental to their unique mountain biodiversity, leading to an irreversible degradation of these sensitive ecosystems, if no or too little action for their preservation are put in place immediately.
Threat 6: Pathogen introductions
Pathogens, and other microorganisms, can be easily introduced to mountains through pastoralism, tourism or wind drift. However, we remain largely oblivious to how the complexity of the abiotic and biotic environment in mountain ecosystems influences beneficial microbe-species interactions, host-pathogen interactions, and health risks for the human population. In particular, fungi and bacteria with resistant aerosolised spores are capable of long-distance transport of e.g. dust. Global dust dispersion is a natural phenomenon, but global warming and changes in land use practices (e.g. deforestation and overgrazing) have accelerated desertification in many areas, resulting in increased dust dispersion even to remote places. In a mountain context that could mean that resources of clean drinking water will diminish at a much faster rate than currently predicted.
Risks
Negative impacts on mountain biodiversity threatens ecosystem integrity and functioning, and hence also the multiple ecosystem services mountain provide to local communities, populations downstream and local stakeholders, including tourists. The capacity of mountain ecosystems to provide ecosystem services is deteriorating due to biodiversity loss driven by global change. Further, as glaciers retreat and permafrost thaw, the decreased land-surface stability results in increased hazards in the form of landslides and rock fall, increasing risks for wildlife, tourists and livestock. Glacial lake outburst floods may also intensify due to glacier retreat and glacial lake formation, with potentially devastating consequences for populations downstream.
Options
Humanity has a wide range of options in its hand to mitigate human-driven impacts on mountains and to change the current trajectory as humanity is at the nexus of it all. All relevant actors need to coordinate their efforts in extensive collaborations to achieve the necessary conservation measures: in mountain areas with a protection status conservation policy needs reinforcement; for mountain areas without a protection status, evaluation of its status, importance and future perspective need to be used to prioritize (i) protective measures, (ii) re-evaluations of impacts of touristic and pastoral activities, (iii) evaluation of sustainability management of natural resources, and (iv) development of early-warning systems of ecosystem degradation and biodiversity loss. These measures will then be able to inform about trajectories towards detrimental outcomes (pathogen emergence, ecosystem services). As mountain stakeholders are numerous, regional networks and coordination mechanisms must urgently be installed, and a broad communication strategy needs to be developed to raise awareness about the threats to mountains and their complex and far reaching consequences. These different aspects need to be included in comprehensive ecosystem management plans, considering the cumulative and hierarchical context of disturbance regimes to prevent reductions in ecological variability and ecosystem resilience, particularly in areas that are more sensitive to large disturbances, such as mountains.
Only if we maintain a high ecosystem resilience will we be able to maintain ecosystem functioning and ecosystem services. Threats to mountains are numerous and the repercussions to humanity demand conservation and restoration of mountain ecosystems, as they are an essential and highly sensitive part of the global life-support system.