Key points

• By leveraging data spanning more than three decades and covering diverse habitats across the Mediterranean, our study provides an unprecedented large-scale assessment of how Mass Mortality Events reshape trait diversity of marine benthic communities over time.
• Our research explicitly quantifies how Mass Mortality Events disrupt key ecological functions by altering trait composition, allowing for a more precise understanding of which functions are most at risk, including habitat formation, structural complexity, and trophic interactions.
• Our findings suggest that as Mass Mortality Events continue to intensify, unique Mediterranean benthic species might be lost. Up to 10.8% of the trait diversity has been affected in the last five years, which could have cascading effects on associated marine communities.

Mass Mortality Events (MMEs) and their ecological consequences

Over the past few decades, MMEs have been recorded with increasing frequency across the Mediterranean Sea, affecting a wide range of benthic species, from corals to sponges and macroalgae. In our recent study, we quantified that severe mortality (mortality rate of colonies or individuals across sites higher than 60%) accounts for over half of the observations over the past 35 years. In addition, MMEs have been observed with increasing frequency, from 88 observations between 1986 and 1990 to 997 observations between 2016 and 2020. These MMEs, primarily driven by extreme temperature anomalies, have led to the decline of key structural species, raising concerns about the resilience of benthic communities. In this regard, we aimed to quantify the functional impact of these MMEs by examining the diversity of traits within affected species.

A functional perspective on biodiversity loss

Traditional approaches to biodiversity conservation often focus on species richness, but understanding how ecosystem functions are altered requires a deeper analysis of functional diversity—the range of traits that species contribute to ecosystem processes. To this end, we compiled a dataset of 389 benthic species across the Mediterranean, including the species impaired by MMEs over the last 35 years and categorized each species according to 10 ecological traits. We identified 228 distinct functional entities (i.e., group of species sharing the same unique combinations of traits). Of these, 55 functional entities have been impacted by MMEs, with the most vulnerable being large, slow-growing, calcifying organisms with tree-like or massive structures. We highlighted that 29 functional entities suffered extreme mortality (mortality rate of colonies / individuals across sites higher than 90%) at least once over the past 35 years.

Shifts in ecosystem functioning

One of the most striking findings of our study is the increasing vulnerability of trait diversity over time. First, we demonstrated that 85% of the observed MMEs were caused by temperature anomalies. Second, we quantified trait vulnerability at 0.6% in the 1990s, rising to 7.1% in the 2010s, and reaching 10.8% in the last five years, highlighting an intensification of MMEs over time. Consequently, the disproportionate loss of specific functional entities, particularly habitat-forming species like corals and gorgonians, could trigger cascading effects on associated communities. If this trend persists, it may reduce the resilience of benthic ecosystems, impairing essential processes such as habitat provision, nutrient cycling, and carbon sequestration.

Spatial heterogeneity in trait vulnerability

We further investigated how trait vulnerability is distributed across the Mediterranean Sea. Our findings revealed that the highest vulnerability was currently observed in the Western Mediterranean, highlighting a rapid and ongoing ecological shift in this region. In contrast, the Eastern and Central Mediterranean appeared to have been impacted less by MMEs. One of our hypotheses is that MMEs have impaired those two regions much earlier, suggesting that these areas have already undergone long-term structural and functional impairments. This spatial pattern indicates that different regions of the Mediterranean are experiencing ecosystem shifts at varying rates, which could have significant implications for conservation strategies.

Towards a less heterogeneous ecosystem

Overall, MMEs are reshaping the functional landscape of Mediterranean benthic ecosystems, with significant implications for biodiversity conservation and ecosystem stability. Our results highlight a worrying trend for Mediterranean benthic ecosystems under global change. The persistent loss of key trait categories suggests increasing vulnerability, potentially leading to reduced trait diversity. These shifts could disrupt essential ecosystem functions, impacting biodiversity and the services these ecosystems provide.

To explore our full findings, read our article in Nature Communications.