Native biodiversity collapse in the eastern Mediterranean.

Martin Zuschin,Quan Hua,Elina Turapova,Zara Guifarro,Paolo G. Albano,Gil Rilov,Marija Bošnjak,Darrell S. Kaufman,Beata Dunne,Jan Steger

doi:10.1098/rspb.2020.2469

Abstract

Global warming causes the poleward shift of the trailing edges of marine ectotherm species distributions. In the semi-enclosed Mediterranean Sea, continental masses and oceanographic barriers do not allow natural connectivity with thermophilic species pools: as trailing edges retreat, a net diversity loss occurs. We quantify this loss on the Israeli shelf, among the warmest areas in the Mediterranean, by comparing current native molluscan richness with the historical one obtained from surficial death assemblages. We recorded only 12% and 5% of historically present native species on shallow subtidal soft and hard substrates, respectively. This is the largest climate-driven regional-scale diversity loss in the oceans documented to date. By contrast, assemblages in the intertidal, more tolerant to climatic extremes, and in the cooler mesophotic zone show approximately 50% of the historical native richness. Importantly, approximately 60% of the recorded shallow subtidal native species do not reach reproductive size, making the shallow shelf a demographic sink. We predict that, as climate warms, this native biodiversity collapse will intensify and expand geographically, counteracted only by Indo-Pacific species entering from the Suez Canal. These assemblages, shaped by climate warming and biological invasions, give rise to a ‘novel ecosystem’ whose restoration to historical baselines is not achievable.

Highlights

The unprecedented speed of global warming recorded in the last few decades and the projections for the near future are an increasing threat to marine biodiversity [1,2]
Ranges are expected to expand at the leading edge and contract at the trailing edge [5]. Along continental margins, such changes in species distributions cause ecosystem reconfigurations that can be as drastic as a transition from kelp forest to persistent seaweed turfs, as shown for Western Australia [6]
A critical situation occurs when such contractions happen in semienclosed basins like the Mediterranean Sea where land masses and oceanographic barriers constrain the arrival of southern species from contiguous biogeographical provinces as would have occurred along open continental margins

Summary

Introduction

The unprecedented speed of global warming recorded in the last few decades and the projections for the near future are an increasing threat to marine biodiversity [1,2]. 2000 km of the coastline) has a summer maximum temperature of just 22.3°C [11] This solid thermic barrier isolates the Mediterranean Sea from the geographically adjacent West African tropical species pool, which repeatedly served as donor of the thermophilic biota that entered the basin during the warmest interglacials of the Pleistocene [12]. We here quantify historic and current native and nonindigenous species richness along the approximately 200-km-long Israeli shelf from the intertidal to mesophotic depths. Due to the particular substrate, intertidal samples did not contain a death assemblage In this case, we compared the living assemblage with a checklist based on the literature and experts’ advice (electronic supplementary material, table S2). At each subtidal sampling station, we quantified the age distribution of shells comprising the death assemblages by radiocarbon dating 9–15 valves. All analyses were conducted in the R statistical environment [28]

Results

Discussion

22. Kowalewski M et al 2003 Quantitative fidelity of

Findings

75. Hobbs RJ et al 2006 Novel ecosystems: theoretical