Native biodiversity collapse in the eastern Mediterranean

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.

Keywords: Lessepsian invasion; Mediterranean Sea; Mollusca; biodiversity collapse; novel ecosystem.

Figure 1.

(a) Distribution of ratios between current and historical richness of the native (blue) and non-indigenous (red) molluscan components on the Mediterranean Israeli shelf (n: observed number of species). The mesophotic samples do not have a significant non-indigenous species (NIS) component. Current native species richness is markedly low in the shallow subtidal, and non-indigenous richness always higher than native richness in the same habitat. (b) Difference between literature and sample maximum size of the native (blue) and non-indigenous molluscan components (red) on the Mediterranean Israeli shelf (n: number of measured species). Negative values mean that the maximum size we recorded is smaller than that in the literature, the opposite for positive values. In contrast to the intertidal, where the six species here analysed constitute 75% of the diversity, the mesophotic samples had only 2 (5%) species with sufficient sample size and were excluded from the analysis. The figure shows that in the shallow subtidal, approximately 60% of the native species do not reach half the maximum size, a proxy for size at first reproduction. By contrast, all intertidal native species were larger than this threshold. Additionally, most NIS were also larger than the size at first reproduction, marking a distinct reproductive potential compared with native species. (Online version in colour.)

Figure 2.

Yearly seawater temperature range (averaged over 2016–2019) in the Western (Malaga) and Eastern (Tel Aviv) Mediterranean, and in the Gulf of Suez, northern Red Sea. The surface temperature in Tel Aviv is indistinguishable from that in the Gulf of Suez (Kolmogorov–Smirnov D = 0.25, p = 0.869; Wilcoxon W = 68, p = 0.843) and much higher than in the mesophotic zone (K–S D = 0.67, p = 0.008; W = 30, p = 0.015). The latter is indistinguishable from the median temperature in Malaga, Western Mediterranean (K–S D = 0.42, p = 0.256; W = 66, p = 0.755), although characterized by a much more restricted seasonal amplitude. (Online version in colour.)