Phylogenetic measures reveal eco‐evolutionary drivers of biodiversity along a depth gradient

Abstract

Energy and environmental stability are positively correlated with species richness along broad-scale spatial gradients in terrestrial ecosystems, so their relative importance in generating and preserving diversity cannot be readily disentangled. This study seeks to exploit the negative correlation between energy and stability along the oceanic depth gradient to better understand their relative contribution in shaping broadscale biodiversity patterns. We develop a conceptual framework by simulating speciation and extinction along energy and stability gradients to generate expected patterns of biodiversity for a suite of complementary phylogenetic diversity metrics. Using a time-calibrated molecular phylogeny for New Zealand marine ray-finned fishes and a replicated community ecological sampling design, we then modelled these metrics along largescale depth and latitude gradients. Our results indicate that energy-rich shallow waters may be an engine of diversity for percomorphs, but also suggest that recent speciation occurs in ancient fish lineages in the deep sea, hence questioning the role of energy as a key driver of speciation. Despite potentially facing high extinction early in their evolution, ancient phylogenetic lineages specialized for the deep-sea were likely preserved by environmental stability during the Cenozoic. Furthermore, intermediate depths might be a 'museum' (or zone of overlap) for distinct lineages that occur predominantly in either shallow or deep-sea waters. These intermediate depths (500-900 m) may form a 'phylogenetic diversity bank', perhaps providing a refuge during ancient (Mesozoic) extreme anoxic events affecting the deep sea and more recent (Pliocene-Pleistocene) climatic events occurring in shallow ecosystems. Finally, the phylogenetic structures observed in fish communities at intermediate depths suggest other processes might restrict the co-occurrence of closely related species. Overall, by combining a conceptual framework with models of empirical phylogenetic diversity patterns, our study paves the way for understanding the determinants of biodiversity across the largest habitat on earth.

Are phylogenies resolved at the genus level appropriate for studies on phylogenetic structure of species assemblages?

“… Faith's (1992) phylogenetic diversity (PD) and Webb et al.’s (2008) mean pairwise distance (MPD) and mean nearest taxon distance (MNTD) are among the most commonly used metrics of phylogenetic diversity ( Carvallo et al., 2014 ; Eme et al., 2020 ). They focus on different depths of evolutionary history.…”

Functional beta diversity of New Zealand fishes: Characterising morphological turnover along depth and latitude gradients, with derivation of functional bioregions

“…Knowledge of functional distinctiveness, turnover and bioregions can be used along with taxonomic and phylogenetic measures of diversity (e.g. Eme et al 2020) to help designate areas of marine protection in order to protect regional biodiversity in a broader and more ecologically relevant manner (Stuart-Smith et al 2013). In addition, boundaries corresponding to transition zones between functional bioregions may provide ideal places to monitor over time in order to detect major biogeographic shifts in fish communities in response to anthropogenic change.…”

Evolutionary drivers of the hump-shaped latitudinal gradient of benthic polychaete species richness along the Southeastern Pacific coast

“…In addition, the study of phylogenetic diversity may shed light into the role of evolutionary processes shaping patterns of species richness (Davies & Buckley, 2011;Fritz & Rahbek, 2012;Eme et al, 2020). For instance, a species-rich area composed by many closely related species (i.e., low phylogenetic diversity) may imply elevated in situ speciation rates of some lineages.…”

“…On the contrary, a species-rich area composed by many distantly related species (i.e., high phylogenetic diversity) may be associated to immigration influenced by filtering effect acting upon a trait phylogenetically over dispersed. There are many a-phylogenetic diversity indices (Winter, Devictor & Schweiger, 2013), that can be broadly classified into three categories that measures amount of richness, divergence and regularity (Tucker et al, 2017); therefore, the combined used of indices reflecting each one of these three aspects may be more informative of the role of evolutionary processes shaping species richness (Davies & Buckley, 2011;Fritz & Rahbek, 2012;Eme et al, 2020). Despite the fact that these phylogenetic diversity indices are typically based on calibrated molecular phylogenies, the taxonomic structure may be still be used as a coarse proxy of phylogenetic relationships of species in groups lacking robust and complete molecular phylogenies (Soul & Friedman, 2015).…”