Temporal latitudinal-gradient dynamics and tropical instability of deep-sea species diversity

Abstract

A benthic microfaunal record from the equatorial Atlantic Ocean over the past four glacial-interglacial cycles was investigated to understand temporal dynamics of deep-sea latitudinal species diversity gradients (LSDGs). The results demonstrate unexpected instability and high amplitude fluctuations of species diversity in the tropical deep ocean that are correlated with orbital-scale oscillations in global climate: Species diversity is low during glacial and high during interglacial periods. This implies that climate severely influences deep-sea diversity, even at tropical latitudes, and that deep-sea LSDGs, while generally present for the last 36 million years, were weakened or absent during glacial periods. Temporally dynamic LSDGs and unstable tropical diversity require reconsideration of current ecological hypotheses about the generation and maintenance of biodiversity as they apply to the deep sea, and underscore the potential vulnerability and conservation importance of tropical deep-sea ecosystems.deep-sea Ostracoda ͉ global climate change ͉ latitudinal species diversity gradients ͉ macroecology ͉ Quaternary paleoceanography L atitudinal species diversity gradients (LSDGs), the patterns in which tropical regions contain more species than high latitudes, are one of the most basic ecological patterns on the earth (1). In the modern ocean, deep-sea bivalves, gastropods, isopods, cumaceans, and foraminifera all show strong LSDGs (2-6), and studies of benthic foraminifera assemblages indicate that the deep-sea gradients were established Ϸ36 million years ago (7). The environmental stability hypothesis holds that stability in tropical (1,8), and deep-sea (9, 10) environments might enhance species diversity, but there is now evidence for highly fluctuating high-latitude deep-sea diversity during Quaternary climatic cycles (11)(12)(13)(14)(15)(16). Surprisingly little attention has been given to understanding low-latitude deepsea diversity and the temporal dynamics of the LSDGs. Although pollen records suggest a persistent latitudinal diversity gradient existed in terrestrial ecosystems over the last 13,000 years (17, 18), we know of no studies of species-level temporal dynamics of LSDGs based on fossil assemblages from marine environments, despite the sensitivity of marine ecosystems to climatic change (12,13,(19)(20)(21)(22)(23)(24).The Ostracoda (Crustacea) are an important component of the deep-sea benthos (25)(26)(27), and the only commonly fossilized metazoan group in deep-sea sediments (12,13,28). Their various habitats and ecological preferences represent a wide range of deep-sea benthic niches, and their fossil record is considered representative of the benthic community (12,13,28). Furthermore, large (Ϸ130 m) glacial-interglacial sea-level changes (29), which drastically altered shallow-marine environments, had negligible effects on deep-sea habitats (e.g., Ͼ1,000 m water depth). Here, we examine low-latitude Quaternary records of deep-sea ostracods and temporal changes in LSDGs in the North Atlantic Oc...

Major impacts of climate change on deep-sea benthic ecosystems

“…North Atlantic fossil records during the last de-glaciation showed abrupt changes in deep-sea biodiversity associated with a rapidly changing climate (specifically deep-water circulation and temperature) over decadal to centennial time-scales (Yasuhara et al, , 2014. Longer time-scale paleo-ecological studies have also shown systematic changes in faunal structure and biodiversity related to glacial-interglacial climate cycles over the last three million years, and more specifically to climate-driven changes in bottom temperature or particulate organic carbon (POC) flux, depending on the ocean and taxonomic group (Cronin et al, 1996;Cronin and Raymo, 1997;Yasuhara et al, 2009Yasuhara et al, , 2012a. Paleoecology (Yasuhara et al, 2015) has thus revealed the dynamic and sensitive nature of deep-sea ecosystem structure and biodiversity across a wide range of time scales in response to changing climatic conditions.…”

Climatic forcing of Quaternary deep-sea benthic communities in the North Pacific Ocean

“…Despite these advances in documenting pattern, the underlying drivers that structure diversity and faunal patterns are still controversial (Stuart et al 2003;Carney 2005;Rex et al 2005;Yasuhara and Cronin 2008;Corliss et al 2009;Yasuhara et al 2009b;Tittensor et al 2011). Uncertainty remains in part because these faunal patterns likely have complex causes, reflecting not only species' present-day tolerances for environmental factors (e.g., temperature, productivity) but also spatial and bathymetric differences in speciation, extinction, and dispersal (Rex et al 2005;Jablonski et al 2006), acting within a biogeographic template (McClain et al 2009b;McClain and Barry 2010).…”

“…Faunal composition also shows strong depth zonation (Carney 2005), and species diversity usually increases to a mid-bathyal maximum before decreasing again (Rex 1973(Rex , 1981Rex et al 2005;Rex and Etter 2010). Perhaps most surprisingly, many taxa show latitudinal gradients of decreasing alpha diversity from the tropics to the poles, although the gradient may be stronger in the Northern Hemisphere (Rex et al 1993(Rex et al , 2000(Rex et al , 2005Culver and Buzas 2000;Stuart et al 2003;Corliss et al 2009;Yasuhara et al 2009b;Rex and Etter 2010).…”

“…Recent micropaleontological studies have demonstrated a link between glacial-interglacial Milankovitch cycles and deep-sea species diversity during the late Quaternary and mid-Pliocene, generally with a glacial-low and interglacial-high species diversity pattern (Cronin and Raymo 1997;Cronin et al 1999;Yasuhara and Cronin 2008;Yasuhara et al 2009b for ostracods; Ohkushi et al 2000;Hunt et al 2005 for foraminifera). However, almost all evidence for this link between Plio-Pleistocene climate cycles and deep-sea diversity comes from the North Atlantic Ocean, and it is not certain if such phenomena are truly global.…”

Late Cenozoic onset of the latitudinal diversity gradient of North American mammals

“…6-9). For example, some proposed mechanisms to explain the modern LDG invoke particular climatic conditions that exist today, and the fossil record can be used to evaluate these mechanisms under different climatic conditions in the past (6,(10)(11)(12)(13)(14)(15).…”