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

D Eme,E M V Myers,M J Anderson,L Liggins,C D Roberts

doi:10.1111/ecog.04836

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 large‐scale 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.

Highlights

Despite harbouring some of the strongest environmental gradients on earth, little is known regarding mechanisms shaping biodiversity along the ocean’s depth gradient (Ramirez-Llodra et al 2010)
We considered four sets of models (Supplementary material Appendix 2 Table A2): 1) a two-factor ANOVA design with depth and latitude as factors; 2) an ANCOVA design with latitude as a factor and depth as a covariate; 3) an ANCOVA design with depth as a factor and latitude as a covariate; and 4) a regression model with depth and latitude as continuous predictors including quadratic terms for both and their interactions
There are some discrepancies between the observed and expected relationships (Fig. 1) for some phylogenetic diversity metrics, overall our results suggest that energy-rich shallow waters are an engine (i.e. ‘cradle’) of diversity for percomorphs, and that recent speciation for ancient lineages occurring in the deep sea is not driven by energy

Summary

Introduction

Despite harbouring some of the strongest environmental gradients on earth, little is known regarding mechanisms shaping biodiversity along the ocean’s depth gradient (Ramirez-Llodra et al 2010). Along other broadscale gradients (e.g. latitude, elevation), biodiversity tends to increase with increasing energy, temperature and environmental stability (e.g. at low latitude) (Mittelbach et al 2007, Jetz and Fine 2012, Fine 2015). Temperature and environmental stability are, positively correlated along latitudinal and elevational gradients (Fine 2015, McClain and Schlacher 2015, Valentine and Jablonski 2015). While energy and temperature both decrease with increasing depth, environmental stability increases (Fig. 1a) (Ramirez-Llodra et al.2010, Fine 2015, Valentine and Jablonski 2015). Studying patterns of biodiversity along the depth gradient offers an opportunity to disentangle the often-confounded drivers of energy/temperature and stability on biodiversity (Fine 2015, Valentine and Jablonski 2015)

Methods

Results

Discussion

Conclusion