Tropical rabbitfish and the deforestation of a warming temperate sea

Abstract

Summary 1.A striking example of climate-mediated range shifts in marine systems is the intrusion of tropical species into temperate areas world-wide, but we know very little about the ecological consequences of these range expansions. 2. In the Mediterranean Sea, the range expansion of tropical rabbitfishes that first entered the basin via the Suez Canal provides a good example of how tropical herbivorous fish can impact the structure of rocky bottoms in temperate seas. Two species of rabbitfishes have now become a dominant component of total fish biomass in the southernmost part of the eastern Mediterranean. Experimental evidence shows these species can profoundly transform benthic communities, turning algal forests into 'barrens', but the specific mechanisms that facilitate this shift have not been established. 3. We surveyed~1000 km of coastline in the eastern Mediterranean and identified two clearly distinct areas, a warmer group of regions with abundant tropical rabbitfish and a colder group of regions where these consumers were absent/ extremely rare. In regions with abundant rabbitfish, canopy algae were 65% less abundant, and there was a 60% reduction of overall benthic biomass (algae and invertebrates) and a 40% decrease in total species richness. 4. Video-recorded feeding experiments showed that the extensive barrens characteristic of regions with abundant rabbitfish were not due to greater rates of herbivory by these tropical consumers, but rather by functional differences among the herbivores. Temperate herbivorous fish displayed the greatest macroalgae consumption rates overall, but they fed exclusively on established adult macroalgae. In contrast, in regions with abundant rabbitfishes, these consumers fed complementarily on both established macroalgae and on the epilithic algal matrix, which typically contains macroalgal recruits. 5. Synthesis. Range-shifting tropical rabbitfish can severely reduce the biomass and biodiversity of temperate reefs at a scale of hundreds of kilometres. A shift from macroalgal dominance to barrens is mediated by the addition of functionally diverse herbivores that characterize tropical reefs. This work highlights the importance of assessing the functional traits of range-shifting species to determine potential mechanisms of impact on ecological communities.

Long-term empirical evidence of ocean warming leading to tropicalization of fish communities, increased herbivory, and loss of kelp

“…We propose that a reduction in resilience, combined with increased consumption rates of adult kelp and growing populations of grazers targeting lower abundances of kelp recruits, led to the observed phase shift in benthic community structure. Although such synergistic effects of warming remain to be confirmed, these findings are consistent with a growing body of literature that shows how the full suppression of large seaweeds on reefs is mediated by increases in herbivore functional diversity and feeding complementarity among herbivores (18,21,27).…”

“…A recent review identified 80 species of tropical and subtropical herbivorous fishes expanding their ranges into temperate reefs in association with polewardflowing boundary currents around the globe (16). The impacts of these warmwater herbivores can be much greater than those of native species, as documented in the eastern Mediterranean (17,18).…”

Setting an ecological baseline prior to the bottom-up establishment of a marine protected area in Santorini island, Aegean Sea

“…Given, however, the strong alterations in ecosystem structure, the establishment of the MPA will have to be accompanied by early restoration actions targeting the decrease (and if possible removal) of invasive species. Therefore, priority should be given to management measures for the regulation of the two herbivore fishes that have led to severe deforestation of the infralittoral rocky shores of Santorini Island, as well as other locations in the southern Aegean Sea (Bianchi et al, 2014;Vergés et al, 2014;authors' unpublished data).…”

The tropicalization of temperate marine ecosystems: climate-mediated changes in herbivory and community phase shifts

“…However, if canopy-forming macroalgae are replaced by corals, then biodiversity may be retained or increase [76]. In the eastern Mediterranean, a shift away from macroalgae has led to a loss of over 60% of benthic biomass and species richness [43]. The ecosystem services provided by a new suite of species will change, and management practices will need to adapt to shifts in resource use by humans [77].…”