Bad neighbors? Niche overlap and asymmetric competition between native and Lessepsian limpets in the Eastern Mediterranean rocky intertidal

Ecological baselines in the Eastern Mediterranean Sea shifted long before the availability of observational time series

Native biodiversity loss and invasions by nonindigenous species (NIS) have massively altered ecosystems worldwide, but trajectories of taxonomic and functional reorganization remain poorly understood due to the scarcity of long-term data. Where ecological time series are available, their temporal coverage is often shorter than the history of anthropogenic changes, posing the risk of drawing misleading conclusions on systems' current states and future development. Focusing on the Eastern Mediterranean Sea, a region affected by massive biological invasions and the largest climate change-driven collapse of native marine biodiversity ever documented, we followed the taxonomic and functional evolution of an emerging "novel ecosystem", using a unique dataset on shelled mollusks sampled in 2005-2022 on the Israeli shelf. To quantify the alteration of observed assemblages relative to historical times, we also analyzed decades- to centuries-old ecological baselines reconstructed from radiometrically dated death assemblages, time-averaged accumulations of shells on the seafloor that constitute natural archives of past community states. Against expectations, we found no major loss of native biodiversity in the past two decades, suggesting that its collapse had occurred even earlier than 2005. Instead, assemblage taxonomic and functional richness increased, reflecting the diversification of NIS whose trait structure was, and has remained, different from the native one. The comparison with the death assemblage, however, revealed that modern assemblages are taxonomically and functionally much impoverished compared to historical communities. This implies that NIS did not compensate for the functional loss of native taxa, and that even the most complete observational dataset available for the region represents a shifted baseline that does not reflect the actual magnitude of anthropogenic changes. While highlighting the great value of observational time series, our results call for the integration of multiple information sources on past ecosystem states to better understand patterns of biodiversity loss in the Anthropocene.

The ecological and evolutionary consequences of tropicalisation

Tropicalisation is a marine phenomenon arising from contemporary climate change, and is characterised by the range expansion of tropical/subtropical species and the retraction of temperate species. Tropicalisation occurs globally and can be detected in both tropical/temperate transition zones and temperate regions. The ecological consequences of tropicalisation range from single-species impacts (e.g., altered behaviour) to whole ecosystem changes (e.g., phase shifts in intertidal and subtidal habitats). Our understanding of the evolutionary consequences of tropicalisation is limited, but emerging evidence suggests that tropicalisation could induce phenotypic change as well as shifts in the genotypic composition of both expanding and retracting species. Given the rapid rate of contemporary climate change, research on tropicalisation focusing on shifts in ecosystem functioning, biodiversity change, and socioeconomic impacts is urgently needed.

The molluscan assemblage of a pristine Posidonia oceanica meadow in the eastern Mediterranean

The seagrass Posidonia oceanica forms extensive meadows in the Mediterranean Sea. Studies on their associated highly diverse invertebrate assemblages are limited to the western Mediterranean. The eastern Mediterranean, however, is a basin undergoing rapid change due to the synergistic effects of climate warming, biological invasions and other human stressors that are driving native biodiversity to regional-scale collapses. We here surveyed the shelled molluscan assemblage of a Posidonia oceanica meadow in Plakias, south-western Crete, the first such study in the eastern Mediterranean Sea. This area has increased its yearly mean temperature by 1 °C in the last 20 years and is under heavy pressure by Lessepsian species. We sampled across a 5- to 20-m depth gradient, in two seasons to capture intra-annual variation and the leaf and rhizome strata separately. Against our expectations, the molluscan assemblage proved to be highly diverse, with species richness, dominant species and trophic guilds comparable to healthy western Mediterranean ones, and with a negligible non-indigenous component. The diversity of the native community (following the biotic resistance hypothesis) and oxygen supersaturation in the meadow may cause greater resistance to biological invasions and warming, respectively, suggesting that Posidonia oceanica meadows may act as a precious refugium for native biodiversity in the fast changing eastern Mediterranean Sea.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12526-022-01292-2.