Dramatic biodiversity changes threaten Mediterranean coastal dune habitats

Marta Gaia Sperandii discusses her recently published Journal of Ecology article: Getting the measure of the biodiversity crisis in Mediterranean coastal habitats

Find out more about taxonomic and functional trends they observed in coastal dune systems in Central Italy and the importance of their findings for conserving dune ecosystems.

Coastal dunes are certainly considered among the most peculiar and fascinating environments on earth. They’re home to highly specialized flora and fauna, provide us with ecosystems services such as protection from storms and water purification, and hold an immense recreational value. Nevertheless, they’re currently listed among the most endangered ecosystems worldwide; with erosion, urbanization and mass tourism acting as major threats.

*Figure 1. A coastal dune system on the Tyrrhenian Sea. Photo: Marta Gaia Sperandii*

As coastal dunes originate through, and are maintained by, a close interaction between wind, sand and a handful of plants that are exclusive to these fragile environments, assessing their vegetation dynamics over time is key to monitoring their conservation status and their ability to provide the above-mentioned ecosystem services. However, research efforts in this direction are scarce. Studies accounting jointly for various aspects of community structure, multiple facets of diversity and individual focal species are especially rare.

In our paper, we provide an EU-habitat based, comprehensive overview of taxonomic and functional changes in Mediterranean coastal dunes and assess how much of this change was greater than would be expected by chance. To do so, we performed a so-called resurveying study. We relocated 334 historical vegetation plots that had been surveyed 10-15 years ago in coastal dune systems located in Central Italy and performed new vegetation relevés. As there had been different types and intensities of disturbances occurring along the coastal zonation, we expected changes to vary among habitats.

In the first place, we detected an impressive habitat loss, with almost 25% of the historical plots being found, at the time of the resurvey, devoid of vegetation or submerged by the sea. In plots that could be resurveyed, our analyses revealed major taxonomic and functional changes. Depending on the habitat, we recorded a taxonomic dissimilarity of 67-100%, with significant differences among habitats and mostly driven by species loss. Functional dissimilarity, on the other hand, turned out to be low to medium, rather homogeneous among habitats, and mostly driven by functional gain. The general trend, from a functional perspective, was that of a decrease in mean community height, and an increase in both specific leaf area and proportion of annual species.

Things got more and more interesting when we tested, by running ad-hoc built null models, to what extent observed changes were greater than that which could be reasonably expected under random circumstances. Whereas taxonomic trends, and especially species loss, were found to be non-random in almost all habitats, functional changes were mostly in line with random expectations, except for the two key habitats of embryo and shifting dunes. Our species-level analysis confirmed the community-level trends, revealing important losses in occurrence and frequency of key focal species. The most important sand-binding species in European coastal dune systems, Ammophila arenaria subsp. australis, dropped in occurrence by 80%.

*Figure 2. A plot found devoid of vegetation at the time of the resurvey. Photo: Marta Gaia Sperandii*

Overall, our results suggest that, in a very short time-span, Mediterranean coastal dunes of Central Italy underwent alarming habitat loss and remarkable, mostly non-random, taxonomic changes that were only partially buffered at the functional level. Confirming our initial hypothesis, we found that the extent of change differed considerably along the coastal zonation, peaking in habitats closer to the sea (upper beach, embryo and shifting dunes). Because these habitats (especially shifting dunes) not only provide shelter for retrodunal vegetation but, most importantly, guarantee high-valued ecosystem services, their degradation is likely to have cascading effects on the entire coastal dune ecosystem and on the wide range of services it provides. By alerting us about the dramatic conservation status of representative Mediterranean coastal dune ecosystems, our evidence calls for an investigation about the causes of changes and urgent measures to protect these important ecosystems.