Biogeographical patterns of species richness, range size and phylogenetic diversity of ferns along elevational-latitudinal gradients in the tropics and its transition zone
Although elevational-latitudinal patterns of plant and animal diversity have been studied for centuries, factors driving the observed patterns are still controversially discussed. Tropical mountains offer an excellent natural experiment to unravel these drivers of biogeographical patterns and the in...
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|Although elevational-latitudinal patterns of plant and animal diversity have been studied for centuries, factors driving the observed patterns are still controversially discussed. Tropical mountains offer an excellent natural experiment to unravel these drivers of biogeographical patterns and the integration of multiple elevational gradients along an extended latitudinal gradient is an outstanding opportunity to decipher underlying mechanism of large-scale patterns of species richness. In this research the data obtained from Mexico were unified with elevational transects in America spanning 0° to 23°N degrees of latitude, mostly between sea level and mountain tops (200-4000 m a.s.l.). Hereby, the latitudinal gradient covered by this investigation runs from most studied tropics through the transition zone of sub-tropical regions in order to integrate increasingly seasonal and dry climates, which are known to limit the diversity of ferns as humidity-dependent organisms.
How the fern and lycophytes (hereafter ferns) richness and species range size patterns change along elevational-latitudinal gradients is poorly explored, as well as the phylogenetic relationships within species assemblages. The objective of this research was to describe such patterns and understand the broad scale distribution of other important components of biodiversity as the phylogenetic diversity and associated metrics, using a standardized methodology that allows the comparison of elevational transects. In order to assess these patterns, I first intensively sampled an elevational gradient in the Sierra de Juárez, Oaxaca, Mexico, known as the “ferns hunter paradise”, a hotspot of biodiversity in a hyperhumid tropical region. Following the uniform sampling protocol of the working group around Michael Kessler (Zurich) and Jürgen Kluge (Marburg), I sampled at each elevation four to eight plots of 20 x 20 m (400m2) and analyzed changes in fern assemblages along this elevational gradient. The plots were established in natural zonal forest, avoiding special structural features and disturbed areas whenever possible. In each plot, DNA samples were taken and all fern species and their abundances were recorded for terrestrials and epiphytes. The results are presented in a series of chapters starting from the analyses of the gradient in Mexico, followed by the central work of this research, where richness and range size patterns of eight elevational transects in Mexico were analyzed together with climatic correlates, and the assessment of the historical component (phylogenetics) of 11 elevational gradients understanding the processes that governs the species richness. In the following the central questions and results of the three studies will be briefly described.
In Chapter 2 we analyzed in detail one of the most diverse elevational transect in Mexico with 15 elevational steps in Sierra de Juárez Oaxaca, a hotspot of biodiversity, comparing the richness patterns with central and South America. In 121 plots we found 195 fern species, with the highest species numbers recorded at mid-elevations (1,100–2,200 m) and lower richness at both gradient extremes. This hump-shaped pattern was mainly driven by epiphytes, which contributed about 40% to the total species richness. This species group had wider elevational ranges than terrestrial species, which contributed 38% to the total richness (20 % were recorded within both life forms), and were more or less constant at low to mid elevations about 2,500 m, followed by a gradual decline. Overall, richness per plot was low compared to other elevational gradients within the tropics closer to the Equator. This decline of species richness towards the subtropics and temperate regions appears to be rather abrupt within Central America, but additional data are needed to close the remaining gap of knowledge between Costa Rica and Mexico. This work has been published 2018 in the American Fern Journal (Hernández-Rojas et al. 2018).
In the central work of this research (Chapter 3), we assessed the richness and range size patterns of ferns and lycophytes along seven elevational gradients (with in total 658 plots) at different latitudes and searched for predictors of range size from a set of environmental factors. We calculated the latitudinal range using the northern and southern limits of each species and averaged the latitudinal range of all species within assemblages weighted by their abundances. We related climatic factors and the changes with latitude and elevation with range size using linear mixed-effects models. Species richness per plot increased with elevation up to about 1,500-2,000 m, with strong differences in overall species richness between transects and a reduction with increasing latitude. The mean weighted range size of species within assemblages declined with elevation, and increased with latitude. However, we also found marked differences between the Atlantic and Pacific slopes of Mexico, as well as low range size in humid regions. The best models described about 76%–80% of the variability in range size and included the seasonality of both temperature and precipitation, and annual cloud cover. As a conclusion, latitudinal and elevational patterns of range size in fern assemblages are driven by an interplay of factors favoring wide-ranging species (higher latitudes with increasing temperature seasonality and dryer habitat conditions) and those favoring species with restricted ranges (higher elevations with more humid habitat conditions), with additional variation introduced by the specific conditions of individual mountain ranges. Climatically stable, humid habitats apparently provide favorable conditions for small-ranged fern species, and should accordingly be given high priority in regional conservation planning. This work was published 2020 in the Journal of Biogeography (Hernández-Rojas et al. 2020).
Finally, phylogenetic approaches should be seen as an integral component of studies of the causal basis of community structure, which permits the understanding of how communities have evolved through time, and a deeper understanding of biogeographical processes. This historical aspect is included as the final part of this research (Chapter 4) that was aimed to unfold the complex changes in the phylogenetic structure along elevation (200-4000 m.a.s.l.) and latitude (0°-23° N). We compiled a comprehensive data set including eleven elevational transects (more than 900 spp. and 868 plots), closing the gap of information observed in Chapter 2. At this point I should highlight the relevance of this research as a product of the work of many researchers that I have the honor to synthesize to get a complete view of biogeographic and phylogenetic patterns. One of the major tasks before analyzing the data was to compile the phylogenetic information of the more than 900 species, of which only 70 % were already present in the most recent dissolved phylogenetic tree, and the remaining 30 % species had to be added on the basis of putative closest relatives due to exhaustive examination of morphological characteristics and expert knowledge. Our results showed that latitude and elevation per se are weak explanatory variables for phylogenetic diversity, but incorporating climatic variables clearly enhance the explanatory power of the models. We used different phylogenetic measures in order to capture different aspects of evolutionary history of this plant group and were thus able to understand, how species groups have evolved to currently form the observed gradients in Central America. Overall, the phylogenetic diversity decreased with increased latitude and elevation, as could be observed by increasingly clustering of closely related species. Additionally, it is worthwhile to look at taxonomic group of ferns separately for the main lifeforms, as they are not only ecologically different, but also not congruent in their evolutionary history. Epiphytic phylogenetic diversity had a positive relationship with humidity, while the phylogenetic diversity of terrestrial was determined by temperature. The general pattern of phylogenetic diversity is mainly driven for the suppression of the old, basal tropical, terrestrial lineages rather than by recent evolutionary history and better explained by climatic variables. This work was submitted to frontiers of biogeography. In continuation of this work and as an outlook to further studies, I am heading for to continue the research in tropical areas, where the deforestation and disturbance, respectively, and general human activities play a central role, which are impacting tremendously the legacy of millions of years of evolution. In the context of the species richness patterns, it is necessary to fill the gaps of knowledge present in America. In this regard, I will intend to