striking example of climate-mediated range shifts in marine systems is the
intrusion of tropical species into temperate areas worldwide, but we know very
little about the ecological consequences of these range expansions.
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.
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, there was a 60%
reduction of overall benthic biomass (algae and invertebrates) and a 40%
decrease in total species richness.
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 characterise tropical reefs. This work highlights the importance of assessing the functional traits of
range-shifting species to determine potential mechanisms of impact on