Climate conditions play a decisive role in the composition, structure and function of natural systems. High mountain ecosystems - characterised by a richness of endemic species which have a narrow range, effectively low population size, specific habitat requirements and limited dispersal capabilities - are particularly vulnerable to variations in climate conditions. Climate is one of the main regulators of ecosystem function in mountain areas, and even small temperature variations can impact the distribution and abundance of many species.
Climate change can affect the physiology of many species, influencing their productivity and, ultimately, their long term survival. The correlation between climate variables and demographic parameters for high mountain species has been thoroughly demonstrated by the scientific community. The impact of climate change on the physiology and productivity of some species is more evident in the Alpine biogeographic region than in others. Climate is the main ecosystem regulator in this area, and its animal and plant communities are therefore in a delicate balance with climate variables.
Amphibians are among the groups of vertebrates most vulnerable to physiological changes induced by climate change. Their permeable skin, biphasic life cycle and eggs with no shell render them extremely sensitive to small temperature and humidity changes. Their reproductive success, immune function and degree of sensitivity to chemical contaminants have been shown to be directly vulnerable to global warming. One of the most pronounced negative effects on amphibian physiology is caused by changes in the length of the winter hibernation period, a clear example of which is the increased mortality and reduced reproductive capacity of some populations of common toad (Bufo bufo): increasingly mild winters shorten their hibernation period, leading to a general worsening of the physical state of many individuals. In addition, increasingly milder temperatures prevent these amphibians from entering complete hibernation throughout the winter period, meaning they continue to consume their reserves and experience reduced body mass as a result. Loss of body mass has a direct impact on survival capability. Extreme climate events such as droughts and heat waves are also generating negative effects in some amphibian populations. There has been confirmed considerable year-on-year decreases in populations of the Pyrenean newt (Calotriton asper) in nine gullies in the Ordesa y Monte Perdido National Park which are directly related to those years with a greater incidence of extreme climatic events (mainly floods and droughts).
Lastly, invertebrate physiology and behaviour can also be affected by changes in climate conditions. This is the case for the pine processionary (Thaumetopoea pityocampa), the populations of which have increased their reproductive success and distribution over recent years in some areas of the Pyrenees as a result of an increase in minimum temperatures and a reduction in the number of days with rainfall.
One of the most evident effects of climate change on fauna are life cycle changes. Significant events in the lives of animals such as reproduction, laying, migration and hibernation are happening at different times as a result of temperature increases. The biological cycle of several species is changing significantly and climate change seems to be the main cause. Phenological responses to climate change differ from one species to the next and may lead to desynchronisation of some key interactions between species. Desynchronisation can result in considerable alterations to the structure of high mountain communities.
Migratory species and those (both terrestrial and aquatic) whose body temperature depends on the temperature of the environment are particularly vulnerable to these changes. Phenological changes are not only good climate change indicators but are also hugely significant from an ecological perspective. They can affect the ability of different species to compete and therefore the structure of communities, in effect changing how the ecosystem functions as a whole.
In the case of migratory birds, the greater climatic variability of recent years is changing the migratory patterns of some species in the Pyrenees. The main changes recorded include earlier arrival on the European continent in spring (observed in more than 100 species), leaving later in autumn, and general changes in migratory patterns, the latter being particularly pronounced in birds which migrate short distances. Earlier arrival to Europe has been linked to the increase in winter temperatures in Sub-Saharan Africa (where the majority of these species winter), and the later exit in autumn seems to be related to the high temperatures recorded in the migratory destinations. It has been calculated an average advance of the arrival date of around 0.16 days per year since 1959, up to a maximum of 0.27 days per year for some species.
Phenological changes also affect invertebrates. Among insects, studies of Lepidoptera and the main pollinators have showed these to be particularly sensitive to temperature changes. Earlier first-sighting dates have been recorded for the majority of Lepidoptera species studied, seemingly related to the increase in average temperatures.
It is highly likely that the current trend in phenological changes in many animal species will remain the same or even intensify in the future as temperatures and climatic variability increase because of global warming. In addition to modifying the phenological calendars of some species, the combined effect could seriously alter some inter-species interactions, with consequences at all levels of the food chain and impacts on the dynamics of many species and high mountain ecosystems.
One of the most common faunal responses to climate change is displacement towards higher latitudes and/or altitudes in search of areas where climate conditions are still suitable, therefore modifying their original range. Most studies—conducted using different models and with different focuses—concur in predicting a general displacement of current ranges towards the north, shrinking of ranges more to the south, and displacement of mountain species towards higher altitudes. The effects of such displacement could be particularly negative for those species already at the extreme or margin of their current range, as is the case for many mountain species. Overall, variations in current species’ distribution could cause changes in existing mountain communities. These could be characterised generally by increases in certain generalist species, to the detriment of some, more-specialist species. There is consensus that the intensity of these displacements will depend not only on how climate change unfolds and the response capability of each species, but also (and undoubtedly to a greater extent) on the development of other non-climate factors integrated into the concept of global change.
In general terms, it is estimated that the range of European species has been displaced by an average of roughly 17 km towards higher latitudes and/or 11 m towards higher altitudes, per decade. A critical factor affecting mountain fauna is that ascending displacement is often limited by other human-derived factors such as habitat fragmentation or land use changes. These factors can alter connectivity between geographical areas and hinder migrations towards higher altitudes, therefore affecting the capacity of different species to adapt to the new climate conditions. Furthermore, displacements in high mountain areas entail loss of habitat, since species range inevitably reduces with increasing altitude. As a result, some species run the risk of being gradually isolated in the small remaining areas of suitable habitat, thus increasing their vulnerability and even their risk of extinction owing to food scarcity and decreasing genetic variability in their populations.
For the majority of high mountain species, the lower limit of their range is restricted by unsuitable climate conditions, while the upper limit is determined by the availability of certain vegetation, plant species or habitat. Hence, changes in the distribution of Pyrenean species will also depend on the capacity of vegetation to shift their ranges over time: if plant communities are displaced more slowly than the rate at which climate conditions change, the potential new distribution areas of many animals could be considerably smaller than their original ranges.
Moving on to mammals, endemic high mountain species with low dispersal capability seem to be more sensitive to the effects of climate change on their range. One example is the Pyrenean desman (Galemys pyrenaicus). The average summer temperatures and changes in water balance seem to be the main factors which determine the presence and potential distribution of G. pyrenaicus, and both are predicted to vary significantly throughout this century as a result of global warming. If the projections in this model are validated, the effects of climate change could act in synergy with habitat loss to endanger the future of the species.
Some bird species could also see their ranges reduced in the Pyrenees. Although their ability to fly considerably increases their dispersal capability, the reduced range of the plant communities on which many species depend could be decisive for some, such as the Pyrenean rock ptarmigan (Lagopus muta pyrenaica).
The combined action of climate change impacts may desynchronise the life cycles of ecologically interconnected species, such as those in predator/prey and parasite/host relationships. If these species respond differently to variations in climate conditions (different phenological changes), the interactions between them may become desynchronised resulting in alterations of the functioning of the entire ecosystem.
Las respuestas fenológicas y los cambios en la distribución de las especies tienen potenciales repercusiones negativas en las especies de niveles tróficos superiores y en general en aquellas especies que interactúan entre sí a través de sinergias de distinta naturaleza (relaciones interespecíficas). Algunos ejemplos son los casos de los sistemas herbívoro-planta, o depredador-presa, ya que los cambios en la presencia y/o abundancia que afectan a una determinada especie inciden también en las demás especies relacionadas, a través de desfases o desajustes entre los calendarios fenológicos. Estos desfases pueden alterar la capacidad de algunas especies para ejercer funciones clave en el ecosistema, y como consecuencia también el funcionamiento del ecosistema.
Phenological responses and changes to species distribution entail potential negative repercussions on species at higher trophic levels and generally on species which interact with one another via different types of synergies (interspecific relationships ). Some examples are herbivore/plant systems or predator/prey relationships. Changes in presence and/or abundance which affect a certain species also impact other related species by causing the temporal misalignment of their phenological calendars. Said misalignment can affect the capacity of some species to carry out key functions in the ecosystem and how the ecosystem functions.
Although the cold environments characteristic of elevated zones and high latitudes have been considered less vulnerable to biological invasions, global warming could increase the risk of invasions and establishment of non-native species, including in high mountain environments. Climate change could encourage the establishment of new non-native species in the Pyrenees given that new favourable climate conditions and/or less interspecific competition with native species enable their transport and subsequent establishment. Invasive non-native species are mostly generalist and opportunistic and therefore tend to adapt better than most native species to rapid climatic variations. Furthermore, the increase in temperatures displaces climate barriers to higher altitudes, increasing the probability of new invasions (particularly plants) and of the establishment of some species which are already present
Altitudinal displacement of native species’ ranges may favour the expansion and establishment of some non-native species, since these gain access to new climatically suitable niches and low levels of competition for resources owing to the displacement of the species which originally occupied the area
The movement of animals and plants towards areas where the climate conditions are suitable could displace their ranges beyond the limits of current protected areas, increasing their vulnerability to different dangers and stress factors.
Similarly, it is likely that some protected areas which are currently interconnected by natural or artificial corridors will end up cut off to certain species which cannot tolerate the new climate conditions in these corridors. This phenomenon could become particularly evident in protected areas housing endemic species with lower dispersal capability and a highly specialised ecology.
Mountain ecosystems are particularly sensitive and vulnerable to climate change and often constitute the habitat of highly specialist or endemic fauna. They also provide a multitude of highly valuable goods and services to society as a whole. The international scientific community agrees in stressing that the combined action of climate change and habitat loss caused by land use changes will be the main causes of biodiversity loss and species extinctions across the world in the coming decades. Climate change affects biodiversity in mountain regions by causing physiological and phenological changes in plants and animals and changes in the length of the growing season and in population distributions, as well as by increasing the risk of invasion, establishment and spread of non-native species.
In order to understand the responses of Pyrenean species to the effects of climate change and their different adaptation mechanisms, we must increase efforts to promote and incentivise new networks for observing high mountain biodiversity, as well as promoting the medium-to-long term maintenance of existing observation networks. Only by improving our knowledge can we better define adaptation actions to guarantee effective conservation strategies in the long term.
This section provides a non-exhaustive summary of some recommendations for adaptation of the sector in the Pyrenees.