Natural ecosystems across the globe are already modified in their structure and composition. This often leads to a decline of biodiversity which can in turn affect important ecological functions. Species richness as a measure of biodiversity ignores almost 89% of the overall diversity and many traits of species show a phylogenetic signal. Therefore, phylodiversity is often a better indicator of functional processes than species richness. To better understand changes in ecosystem functionality it is thus essential to study and compare taxonomic and phylogenetic diversity patterns across environmental gradients such as elevation. In this study I therefore analyse the effects of elevation and soil parameters on the taxonomic and phylogenetic diversity of tree communities in an Andean mountain rainforest in southern Ecuador.
427 tree species were recorded on 54 plots at three elevation levels at 1000, 2000 and 3000 m, with phyloalpha diversities ranging between -1.91 and 2.20. There was no distinct pattern of over- or underdispersed phyloalpha diversities along the gradient. Elevation affected both the alpha taxonomic and phylogenetic tree species diversity: While taxonomic diversity declined along the elevation gradient, phyloalpha diversity showed a humped relationship with highest phylodiversity values at mid-elevations. I assume environmental filtering to reduce the taxonomic diversity at high elevation. Simultaneously, I suppose the occurrence of facilitator species to enable a nevertheless phylogenetic diverse tree community to persist at high elevations. At low elevations my findings indicate neutral or stochastic processes to shape the phyloalpha dispersion. Elevation had the strongest direct effect on the phyloalpha diversities. Moreover, it influenced soil parameters which explained an additional amount of the phyloalpha diversities.
The pattern of phylobeta diversity supported the idea of facilitator species at high elevations and revealed two phylogenetic distantly related tree communities to exist at low and high elevation levels. The ranges of both species sets overlap at mid elevations where phylodiversities are most diverse, corresponding to a mid-domain effect at the phylogenetic scale. Drawing conclusions from phylodiversity to functional traits suggests an ecosystem with a high functional trait space along the gradient with a peak at mid-elevations.
Altogether, the analysis of phylodiversity patterns revealed a much more complex picture of the diversity distribution than taxonomic diversity. Furthermore, phylodiversity permitted to find the evolutionary biotic and abiotic processes which shaped the diversity structure.