Plant recording across two centuries reveals dramatic changes in species diversity of a Mediterranean archipelago

Abstract

Although islands are model systems for investigating assembly of biological communities, long-term changes in archipelago communities are not well understood because of the lack of reliable data. By using a vast amount of floristic data we assembled a dataset of the plant species occurring on 16 islands of the Tuscan Archipelago, Italy, across two periods, 1830–1950 and 1951–2015. We collected 10,892 occurrence records for 1,831 species. We found major changes in the island plant assemblages between the two periods, with native flora significantly decreasing (−10.7%) and alien flora doubling (+132.1%) in richness. The species–area relationships demonstrated the scale-dependence of the observed changes for native and alien species. The observed floristic changes were dependent on island area, with smaller islands displaying high variability in richness and compositional changes and larger islands having more stable species assemblages. The richness of species associated with open landscapes, that had been maintained for centuries by traditional practices, markedly reduced while the number of woody species, associated with afforestation processes and invasion by alien woody plants, significantly incresed. These results demonstrate the great power of floristic studies, often available in grey literature, for understanding long-term biotic changes in insular ecosystems.

Biotic homogenization of oceanic islands depends on taxon, spatial scale and the quantification approach

“…Although only a small percentage of introduced species ever become invasive and may alter ecosystem functions, biotic homogenization will increase the pressure on native biota and the risk for conservation problems. Climate change and the intensification of human disturbances will probably accelerate both extinctions of rare insular species, including those related to extinction debts (Triantis et al , Otto et al ), and introductions of generalist alien species, which might increase biotic homogenization also at a local scale (Chiarucci et al ).…”

“…In fact, the number of historical extinctions and introduced species are far more abundant in island than in continental systems (Rejmanek and Randall , Whittaker and Fernández‐Palacios ). However, within island systems, the introduction rate of alien species usually surpasses the extinction rate of native ones, although extinctions might be delayed due to extinction debts (Whittaker and Fernández‐Palacios , Triantis et al , Chiarucci et al , Otto et al ). Additionally, islands are discrete units with clear natural biogeographic barriers, which make them convenient systems to analyze natural processes, such as biotic homogenization (Whittaker and Fernández‐Palacios ).…”

Requirements of plant species are linked to area and determine species pool and richness on small islands

“…In contrast, the species pool size constantly predicted around 3.2 species – even for the smallest islands. Indeed, Morrison (, ) showed that some small empty islands are physically capable of sustaining some taxa in the long term, but often lack species due to low immigration rates and temporal variation in species richness (see also Chiarucci et al., ). Also in the studied system, some unsampled small rocks (<2 m 2 ) occasionally harbored small trees or shrubs.…”

“…The binary species pool requires a defined occurrence threshold to determine when the survival of a species is assumed to be sufficient. Especially on small islands, many species are only temporarily present (Chiarucci et al., ; Morrison, ), or considerably rare due to small population sizes (Triantis et al., ). This results in low overall occurrence probabilities, making it difficult to define appropriate thresholds for including or excluding species in binary species pools.…”

Hieracium racemosum subsp. amideii (Asteraceae), a new hawkweed taxon from Montecristo island (Tuscan archipelago, Italy)

“…Hence, in the most recent papers reporting data on the Montecristo flora (Paoli & Romagnoli 1976;Chiarucci et al 2017) only H. sabaudum is mentioned.…”