Building mountain biodiversity: geological and evolutionary processes

Key roles of mountains for biodiversity

Over long periods, topographic, geological, and geophysical conditions modify the rates and properties of four key processes that determine the distribution and diversity of life on Earth: speciation, dispersal, persistence, and extinction ( Table 1). The emergence of the Andes, for example, has influenced the diversification and evolution of plants in South America in at least four different ways: (i) by creating a region of new habitats for species at high altitude; (ii) as a dispersal barrier for lowland organisms, dividing populations in the east and west of the mountain range, as well as in the interior of the valleys and peaks; (iii) as a north-south corridor for species dispersal; and (iv) as a modifier of environmental, hydrological and mineralogical conditions in the rest of the continent, through mountain effects on the climate system and as a source of mineral components released by erosion and continuous weathering. (12, 13).

The impact of mountain regions on biodiversity extends well beyond their topographical boundaries, often affecting entire continental biotas. For example, although the Andean region is in itself the most biodiverse region on the planet, the Andean Orogeny is also considered a key factor in the accumulation of biodiversity throughout the Americas. South (13). In the last 10 million or so years or so, the Andean Orogeny has changed the regional topography several times, forcing the Amazonian watershed to change course. These changes have altered gene flow in the Amazonian lowlands, affecting terrestrial and aquatic biogeography (14). Mountain regions can also play a role as sources of new evolutionary lineages that will later colonize lowland areas. Phylogenetic and biogeographic reconstructions reveal an Andean origin for many Amazonian species, including plants (12), amphibians (15) and the tanagers (16).

The influence of specific mountain ranges on the biodiversity of larger regions and whole continents depends on their geographic location, spatial orientation, local biotic context, and history (2). Thus, the European Alps, oriented east-west, have been recognized as refuges for cold adapted species but do not provide an insufficient habitat connectivity to allow the persistence of many late Neogene lineages in the northwest from Europe through the Pleistocene (17). In contrast, the north-south oriented Rocky Mountains facilitated latitudinal adjustments, providing dispersal corridors in fluctuating climates and reinforcing the persistence of North American populations and species during glacial and interglacial climatic cycles. Pleistocene (18). These processes are often cited to explain why the arboreal flora of Western Europe is also impoverished relative to the arboreal flora of North America (18).

The influence of mountains probably differs between taxonomic groups. Speciation in plants, for example, often reflects adaptations to soil geochemistry and mineralogy (19). In birds, speciation may be more sensitive to the rupture of ranges of species following narrow elevation bands. These include changes in the tree line and in the cloud forest belt (20) and the fragmentation of geographic areas by major rivers (21). For invertebrates, many speciation events are likely to follow plant specialization related to the production of specific metabolites.

Evolutionary processes in the mountains

Mountains are hotbeds of speciation, influenced by geological and climatic dynamics over time. Mountains can form during compression and stretching of the lithosphere. Orogeny (mountain construction) typically includes tectonic stacking of lithosphere domains of varying age, origin, and composition, including a remodeled ocean floor, intrusive magmatic bodies, and volcanic structures. area. The mountains are therefore heterogeneous regions lithologically and topographically. Evolutionary radiation from species is often associated with active uplift phases, suggesting that orogenic processes play a driving role in diversification (Fig. 1A) (14, 22, 23), mainly through recurrent formation, connectivity and loss of habitat in the mountains (Fig. 1A) (Fig.24). Orogenic dynamics, including surface uplift and formation of intermountain ponds and resulting erosion, alter watersheds, river flows, and nutrient fluxes. These processes alter soil composition and nutrient levels, resulting in the adaptation of plants and their associated biota to new types of habitat. The formation of mountains also affects the local climate by creating areas of rain or persistent fog, with a strong filtering effect on the communities of species (2).

Over the past 2.6 million years, Quaternary climatic cyclicity has resulted in dynamic changes in habitat connectivity that have stimulated speciation in some groups (Figure 1B) (Figure 1B).25). These changes are related to the Milankovitch eccentricity cycle, with a periodicity of about 100,000 years, possibly amplified by the obliquity cycle of about 41,000 years, and are believed to induce Cyclical, climate – dictated habitat changes that result in temporally rapid "water pumps". (ten, 25, 26). Vegetation belts moved upward during hot and humid interglacials, resulting in population fragmentation and genetic divergence. When temperatures again fell during glacial episodes, plant belts moved downward, forcing secondary contact of populations, resulting in founding effects, disruptive selection and displacement of characters, creating conditions classically associated with allopatric speciation. In a simulation model of beach dynamics in South America, Rangel et al. recently found support for these predictions, with the Andes acting as an episodic pump for the species (Fig. 1B) (5).

At large spatial and temporal scales, these processes can yield very different species distributions, some from old isolated lineages and some from recent radiation events. The relative contribution of these two groups to mountain diversity varies considerably among mountain regions.[ComparezlaFig2avec([CompareFig2with([comparezlaFig2avec([compareFig2with(3), Figure 3]. Badgley et al. specific and verifiable predictions based on three (non-exclusive) models for the occurrence of radiation in topographically complex landscapes: (i) speciation by active tectonic dynamics, (ii) speciation is consistently high in the mosaic topographic complex habitats, and (iii) Climate-related immigration stimulates speciation (4). In an empirical study of North American rodents, Badgley and his colleagues found some support for the first and third scenarios (4). In a recent global analysis, Antonelli et al. have also found a substantial effect of mountain topography on species diversity, although the effects of erosion and erosive potential are relatively small, which are also determinative influences in geologically dynamic landscapes (2).

Mountains: cradles, museums or graves of diversity?

Stebbins (27) was asked whether the diversity of species in the tropics was so high because the tropics are cradles (areas of particularly rapid origin) or museums (areas of persistence especially long term). Since then, other metaphors have been added (Table 1), including the notion of burial to describe geographical areas with particularly high extinction rates. The identification of graves from contemporary distribution data or fossils remains difficult to achieve. However, their existence, location, and timing have been predicted by process-based simulation models (28) the dynamics of geographical areas over time, driven by simulated paleoclimates (5).

For the most diverse tropical mountains, it seems that the answer to Stebbins' question is that mountains are both cradles and museums (Fig. 2C) (29). For example, the Andes not only house several clusters of recently highly diversified species with strong phylogenetic relationships, but also many ancient relict lines aggregated in centers of endemism (30). The combination of cradle and museum effects seems essential to the emergence of the Andes as the most diverse region of the Earth (3).

While the Andes have a large number of both divergent and recently derived species, the mountainous regions of Southeast Asia are mainly occupied by recently derived species (Figure 2). A plausible explanation for these distinct regional patterns is that tropical mountain ranges with very high peaks and more rugged terrain, such as the Andes and Southeast Asia, harbor a high altitude biota characterized by a small number of lineages adapted to colder environments. These few lineages can become very rich in species at the regional level thanks to a rapid and local diversification (31). In the Andes, this process may have occurred more frequently than in Southeast Asia. On the other hand, the Afromontane regions and the mountainous region of the South American tropical rainforest show a greater predominance of early divergent species.

In a simulation of the dynamics of the temporal distribution of South American biota, the cradles derived from the Andean founders (each simulation started with an initial seed species) were concentrated along the Andean slopes, while the tombs tended to be at a lower altitude in the Upper Amazon Basin (5). In contrast, biota derived from the founders of Atlantic Forest had a much greater spatial coincidence between cradles and graves. These results are consistent with the view that topographic complexity and climatic gradients at elevation promote beach fragmentation and serve as cradles, while serving as museums providing climate refuges against extinction (5, 26, 32). The simulations also revealed that the spatial positions of cradles, museums and tombs can be dynamic and change shape, size and intensity over time and in response to Quaternary climate cycles (5). These temporal and spatial dynamics imply that inferring cradles or museums on the basis of current distributions of existing young and old lineages can be misleading.

Geological heterogeneity and biodiversity

The potential importance of the geology of the mountain, including the mineralogical composition of the substrates, has been highlighted in recent work (1, 2, 4). Mountainous substrates generally differ substantially from those in the surrounding lowlands, which are often dominated by eroded mountain-derived materials deposited in valleys and plains. Mineral composition and soil nutrient levels affect plant physiology, vegetation composition, primary productivity, and therefore species diversity. In addition, mountain regions with high levels of geological heterogeneity are likely to support higher levels of spatial renewal of species and local endemic forms, particularly among plants. However, empirical studies linking the diversity of edaphic conditions with plant species diversity are rare, and little is known about how edaphic heterogeneity affects diversity at higher trophic levels.

In a recent global analysis, which also confirmed the classical correlation of species diversity with topographic relief and climate (3, 33, 34), Antonelli et al. Found correlations between the species richness of mammals, birds and amphibians, with short- and long-term erosion rates and a measure of soil diversity (2). Although the association is generally weaker than the correlation of wealth with climate variables, soil heterogeneity was still a significant predictor of wealth in several biogeographic regions of the world. Antonelli et al. Suggested that soil heterogeneity underpinned small scale habitat renewal, creating new habitats and ecological opportunities, increasing local and regional species richness. In Figure 3, we explore in greater detail the relationship between species diversity and bedrock heterogeneity, as measured by the number of main bedrock categories represented in each mountain region.

Geological heterogeneity, thus simplified, does not explain much of the variation of the total species richness and does not take into account the great richness of the species with small distribution which characterizes the mountains with the low latitudes.[ComparezlaFig3Aavec([CompareFig3Awith([comparezlaFig3Aavec([compareFig3Awith(3), Figure 3]. This disconnection may imply that the way in which rock classes result in ecologically relevant soil properties is much more complex than that described by our simple classification. The presence or absence of mafic and ultramafic rocks is a special case of ecological importance (Fig. 3C). Soils from ultramafic rocks have a well-described effect on plant adaptations and diversity. Their unusual geochemistry, with a high magnesium content and low availability of phosphorus (35), requires specific adaptations and slows growth rates of plants. Serpentine soil vegetation, which forms on ultramafic bedrock, is highly specialized and generally woody, with high levels of phenols and lignin in the leaves, resulting in side effects on nutrient cycling in litter decomposition. leaves. The serpentine soils are powerful selective filters for plants, excluding many groups but favoring the radiation of clades tolerant to these soils (36). Ultimately, habitat heterogeneity, fragmentation, and specialized food adaptations of herbivores can spur speciation cascades in these habitats for all major groups of organisms, including vertebrates. .

The mapping of the global occurrence of large mafic and ultramafic mafic areas contiguous in mountain regions (Fig. 3B) reveals that all hyperdiverse mountain regions are rich in such rocks. Areas of interbedded mafic and ultramafic rocks are often part of ophiolites, fragments of oceanic lithosphere reinstated and set up in continental orogens in plate collisions. The location of ophiolites is therefore an integral part of the orogenic processes that form the mountain ranges of the Cordillera. Ultramafic and mafic rocks, mostly associated with ophiolites, are at the base of> 5% of the dark red zone of FIG. 3B.

Almost all the most biologically diverse mountain regions have three common features: a great diversity of rocks, the presence of an improved oceanic lithosphere and a geographical location in the humid tropics (Fig. 3C). On the other hand, low rock and no ultramafic mountain ranges, even in the mesic tropics, tend to be relatively poor in species. Mountain regions without components of the oceanic lithosphere show little relationship between biodiversity and rock diversity, regardless of climate zone. Understanding the geochemical contrast between the components of the continental and oceanic lithosphere can therefore be a key element in understanding how bedrock geology could influence the production and maintenance of species richness.

The high turnover rate of distinct habitat patches created on geologically heterogeneous surfaces, even in the same local climate, could contribute substantially to the process of population division and differentiation that characterizes the dynamics of mountain speciation. . Plant characteristics on soils derived from ultramafic rocks may also pose problems for herbivorous animals. Soil geochemistry affects the metabolism of plants and can increase the production of secondary metabolites, the plants of these soils investing a lot in chemical and physical defenses against herbivores. Such adaptation challenges to plant-dependent fauna can cause a higher rate of adaptive divergence among consumers during the phases of population isolation and thus precipitate extremely high rates of diversification at the local level, as seen in nectarivores (such as hummingbirds) and frugivores (such as New World). sparrows and tanagers).

Final perspective

The idea that geology and biology intertwine is self-evident in von Humboldt's ideas Cosmos, expressed as his "unity of nature" (37). Later, in 1880, Wallace deduced the recurrence of glaciations in the history of the Earth from the distribution of related animal species on Indo-Pacific islands (38). While much research has been stimulated by this early work, the lack of robust data and analytical frameworks has long hampered efforts to fully integrate biological and geological processes into rigorous statistical models of diversity and evolution. mountain species (2).

New methods in geomorphology, including stable isotope altimetry, thermochronology and advances in digital multispectral imaging (39) – pave the way for precise reconstructions of geological dynamics, thus creating a solid foundation for testing evolutionary theories on the origin and maintenance of mountain diversity over time. Combined with genomic sequencing, these approaches can infer the timing, and perhaps even localization (using old environmental DNA), changes in the actual size of populations and bottlenecks. strangulation genetic. The next generation of geologically and evolutionarily explicit models could radically alter our understanding of biotic evolution and resolve the historically controversial debate about the extent to which ecological and evolutionary processes, historical contingency or simply stochasticity and time shape diversity. distribution of life on Earth.

The idea that the heterogeneity of the properties of geological substrates can directly influence evolution is still mainly based on indirect evidence, supported by statistical schemes with weak or mixed correlations with diversity (see Fig. 3) (1, 2, 4). Quantifying biologically relevant specific geological variables and distinguishing topographic effects from geochemical effects remains a challenge for the establishment of causality. Alternative, process-based explanations should be sought in interactions between individual mountain regions and other components of the Earth system, particularly the atmosphere and oceans. An emerging hypothesis is that mountains are hubs of innovation to such an extent that the Earth's biodiversity would have been completely different in the absence of high mountain regions. Par exemple, les montagnes sous les tropiques offrent-elles des conditions environnementales exceptionnelles qui encouragent la fixation des mutations et provoquent des changements adaptatifs localisés chez les plantes, entraînant à leur tour des cascades de spéciation (la spéciation d’un groupe menant à la spéciation dans d’autres groupes)? Flenley a suggéré que les taux d'ultraviolet B plus élevés sur les sommets des montagnes tropicales pourraient directement affecter l'ADN, entraînant un taux de mutation élevé et conduisant à une innovation évolutive (40). Selon cette hypothèse, les périodes chaudes de climat qui poussent les espèces vers le haut, telles que les interglaciaires, devraient être suivies d'une augmentation de la spéciation.

Selon un consensus croissant, les modèles qui incorporent explicitement les dynamiques géologiques et écologiques doivent prendre comme point de départ la vision holistique selon laquelle tous ces processus, agissant à différentes échelles temporelles et spatiales, façonnent les schémas contemporains de la biodiversité. Le défi qui se profile consiste à incorporer ces informations dans un modèle unifié qui génère des prévisions qui peuvent être testées avec des données indépendantes.