Frontiers of Biogeography

Mountains are excellent systems for studying species responses to changing conditions because climatic conditions and related productivity measures change rapidly over short distances (Körner 2007).  Mountains also often have high biodiversity and high levels of endemism (Hoorn et al. 2018, Rahbek et al. 2019), and are typically relatively less disturbed by people compared to flat lowlands. Although new insights about biodiversity patterns associated with mountains and elevation gradients have been achieved (Rahbek et al. 2019) the underpinning mechanistic causes of these biodiversity patterns are still open for debate (cf. Jablonski et al. 2006). This is the underpinning motivation for this special issue on 'Elevational Gradients and Mountain Biodiversity'.

Alexander von Humboldt and Aimé Bonpland in their Essay on the Geography of Plants discuss what was known in 1807 about the elevational limits of vascular plants in the Andes, North America, and the European Alps and suggest what factors might influence these upper elevational limits. Here, in light of current knowledge and techniques, I consider which species are thought to be the highest vascular plants in twenty mountain areas and two polar regions on Earth. I review how one can try to compare elevational limits in different parts of Earth. I discuss recent advances in high-elevation plant ecology that would surely have fascinated von Humboldt such as the special snow-roots in some snow-bed plants and the coldest place on Earth where a vascular plant is growing. I briefly outline an ignored von Humboldt legacy, Mendelssohn’s Humboldt Cantata. In conclusion I summarise the foundations and legacies that von Humboldt created for global high-elevation ecology and biogeography.

The Apuan Alps are one of the most peculiar mountain chain in the Mediterranean, being very close to the coastline and reaching an elevation of almost 2000 m. Based on published flora, we investigated the distribution of plant species richness along the whole elevational gradient of this chain considering: (i) all species, (ii) endemic versus alien species; and (iii) functional groups of species based on Raunkiær life forms (RLF). Generalized Linear Models (GLMs) were used to analyse richness patterns along the elevational gradient, and elevational richness models versus the area of the elevational belts were fitted to test the effect of surface area. Our results showed decreasing species richness with increasing elevation. In contrast, endemic species richness increased along the elevational gradient. Alien species were mainly distributed at low elevations, but this result should be taken with caution since we used historical data. Species life forms were not equally distributed along the elevation gradient: chamaephytes and hemicryptophytes were the richest groups at high elevations, while therophytes showed highest species richness at low elevations. Our findings suggest that in the Apuan Alps there is a major elevational gradient in species composition that could reflect plant evolutionary history. Furthermore, we highlight the key role of published floras as a relevant source of biodiversity data.

Spatial variation in plant species diversity is well-documented but an overarching first-principles theory for diversity variation is lacking. Chemical energy expressed as Net Primary Production (NPP) is related to a monotonic increase in species richness at a macroscale and supports one of the leading energy-productivity hypotheses, the More individuals Hypothesis. Alternatively, water-energy dynamics (WED) hypothesizes enhanced species richness when water is freely available and energy supply is optimal. This theoretical model emphasises the amount and duration of photosynthesis across the year and therefore we include the length of the growing season and its interaction with precipitation. This seasonal-WED model assumes that biotemperature and available water represent the photosynthetically active period for the plants and hence, is directly related to NPP, especially in temperate and alpine regions. This study aims to evaluate the above-mentioned theoretical models using interpolated elevational species richness of woody and herbaceous flowering plants of the entire Himalayan range based on data compiled from databases. Generalized linear models (GLM) and generalized linear mixed models (GLMM) were used to analyse species richness (elevational gamma diversity) in the six geopolitical sectors of the Himalaya. NPP, annual precipitation, potential evapotranspiration (derived by the Holdridge formula), and length of growing season were treated as the explanatory variables and the models were evaluated using the Akaike Information Criterion (AIC) and explained deviance. Both precipitation plus potential evapotranspiration (PET), and NPP explain plant species richness in the Himalaya. The seasonal-WED model explains the species richness trends of both plant life-forms in all sectors of the Himalayan range better than the NPP-model. Despite the linear precipitation term failing to precisely capture the amount of water available to plants, the seasonal-WED model, which is based on the thermodynamical transition between water phases, is reasonably good and can forecast peaks in species richness under different climate and primary production conditions.

Understanding the pattern of biodiversity along environmental gradients helps in identifying diversity hotspot areas that can be prioritized for conservation. While the elevational distribution of several taxa has been studied, responses of the sub-groups within a taxon to elevation and its associated factors are not properly understood. Here we study species richness and butterfly density along an elevation gradient in Sikkim, Eastern Himalaya, India and explore the underlying causes of the patterns. We sampled butterflies using a fixed-width point count method in 16 elevational bands (150–200 m intervals), between a range of 300 and 3300 m a.s.l. We categorized butterflies into various sub-groups based on family, range size, biogeographic affinity, and host-plant specialization. We recorded 3603 individuals and 253 species of butterflies after the completion of 1860 point counts. Overall, species richness in the majority of the sub-groups (except for Riodinidae and Palearctic species) declines with elevation, as does the density of almost all the sub-groups. From a selection of environmental factors, annual actual evapotranspiration has the strongest effect on the species richness pattern of butterflies as well as on the density of the overall butterfly community, especially the Lycaenidae family. The richness and density of butterfly groups display varied responses to the richness and density of trees and shrubs. The conducive climatic conditions and diverse habitats in the lower valleys of the Eastern Himalaya support a high diversity of butterflies (with majority of small range species) and thus warrants conservation attention.

Much attention has been directed to understanding species richness patterns, but adding an evolutionary perspective allows us to also consider the historical processes determining current diversity patterns. We analyzed phylogenetic patterns of fern species assemblages in 868 plots along a wide range of elevational (0-4000 m) and latitudinal (0°-23°N) gradients in the Neotropics to allow a deeper understanding of evolutionary processes underlying current patterns of diversity and community assembly. Overall, we found that phylogenetic mean pairwise distance (sMPD) and mean nearest taxon distance (sMNTD) decreased with increasing latitude and elevation, but that these geographical factors per se were weak explanatory variables. Incorporating environmental variables strongly enhanced the power of the predictive model, indicating that fern assemblages are phylogenetically more diverse under wet and warm to cool conditions at low latitudes and elevations. Further, whereas epiphytic fern assemblages were strongly influenced by climatic factors, this was not the case for terrestrial ones, suggesting that edaphic conditions and vegetation structure may have a stronger influence on the evolution and diversification of terrestrial ferns. We conclude that fern assemblages are strongly influenced by phylogenetic niche conservatism and environmental filtering. This has also been found for angiosperms, but the direction of the environment-phylogenetic relationship is often opposed in the two groups, suggesting that the older age of many fern lineages includes historical signals that are not evident in the more recent angiosperm radiation.

Positive plant–plant interactions are thought to drive vegetation patterns in harsh environments, such as semi-arid areas. According to the stress-gradient hypothesis (SGH), the role of positive interactions between species (facilitation) is expected to increase with harshness, predicting associated variation in species composition along environmental gradients. However, the relation between stress and facilitation along environmental gradients is debated. Furthermore, differentiating facilitative interactions from other underlying mechanisms, such as microtopographic heterogeneity, is not trivial. We analysed the spatial co-occurrence relationships of vascular plant species that form patchy vegetation in arid lapilli fields (tephra) from recent volcanic eruptions on La Palma, Canary Islands. Assuming a harshness gradient negatively correlated with elevation because the lower elevations are more arid and water availability is considered the most limiting resource, and that an outcome of facilitation is plants co-occurring in the same patch, from the SGH we expected a greater degree of co-occurrence at lower elevation. We tested this at both the species and the individual plant level. We analysed the species composition of 1277 shrubby vegetation patches at 64 different sampling points, ranging from the coast to around 700 m a.s.l. Patch morphology and microtopographic heterogeneity variables were also measured, to account for their potential effects on the species composition of patches. We used generalized linear models and generalized mixed-effects models to analyse species richness, number of individuals in patches and percentage of patches with positive co-occurrences, and a pairwise co-occurrence analysis combined with a graphical network analysis to reveal positive links between 13 of the species. We found that the percentage of patches with positive co-occurrences increased at higher elevations, in contrast to the predictions of the SGH, but in accordance with a refined stress-gradient hypothesis for arid sites, in which characteristics of the interacting species are incorporated.

Insect communities vary seasonally with changing climatic conditions and related changes in resource availability, strength of competition, or pressure by natural antagonists. But seasonal dynamics, particularly in tropical mountain ecosystems, are not well understood. We monitored cavity-nesting Hymenoptera communities on Mt. Kilimanjaro, Tanzania, to analyse temporal patterns of nest-building activity, ecological rates, and life-history traits in relation to seasonal climatic variation and elevation. We installed trap nests on 25 study sites in natural and disturbed habitat types covering the colline (<1,300 m) and submontane zones (≥1,300 m a.s.l). We analysed patterns of seasonality in the cavity-nesting ecology of Hymenoptera at three different trophic levels –bees, caterpillar-hunting wasps and spider-hunting wasps– over a complete annual period, covering two rainy and two dry seasons. Nest-building activity showed strong seasonal trends in all three investigated trophic levels and peaked at the end of the short rainy season at low elevations. Nest-building activity was considerably higher and seasonal trends were better synchronised between the different trophic levels in the colline zone at low elevations. We also detected seasonal patterns for parasitism and natural mortality rates, sex ratio, and development time, which varied with trophic level and between elevation levels. Climate and flower abundance were important predictors for seasonal patterns in nest-building activity, ecological rates and life-history traits. These results reveal that seasonal trends in nest-building activity of lowland Hymenoptera seem to be linked to changes in climate and resource availability that reflect the seasonal patterns in plant growth and flowering documented in lowland savanna ecosystems. Higher resource availability also increased the sex ratio in bees towards the more costly females and enhanced their survival rates. These spatiotemporal links between climate, resources, ecological rates, and life-history traits indicate high sensitivity of plant-host-antagonist interactions to environmental changes.

An impression of the recent volcanic history of the Canary Island La Palma is given by the southern tip of the island, which is covered with a mosaic of vegetation patches and bare soil of black lapilli (tephra). See the article from Eibes and colleagues, in this issue, on how frequently different plant species in these patches co-occur along an arid elevational gradient. Photo by Pia M. Eibes.