No safe havens - even the most remote coral reefs are unexpectedly at risk from humans

New study from University of Helsinki published today in Nature Communications shows that while distant fish populations on reefs encounter less human-caused disturbance, their associated fish populations are much more susceptible to global environmental changes due to strong ecological dependencies. The study suggests that no place on Earth, no matter how remote, is a safe haven.

The researchers from University of Helsinki and other institutions combined a massive dataset of fish distribution and traits for more than 9000 fish species. They mapped the interactions among corals, fish prey and fish predators in all reef localities worldwide. The analysis confirmed that coral loss might detrimentally affect, on average, around 40% of fish species in each coral reef area. They further modelled how fish communities are affected through coral loss and showed that the coral reefs furthest from the human influence were most susceptible for environmental change.

Human impacts to biodiversity are often distance dependent

“Further away from human influence, the threats to species are usually lower.  We can notice it ourselves when we move away from big cities and enjoy the countryside”, professor Giovanni Strona from the University of Helsinki, the lead author of the study, explains.

Science also strongly supports the positive effect of remoteness: for example, fishing pressure decreases with distance from big cities. Because of this effect, remote localities are generally considered as safe havens and potential reservoirs for biodiversity.

The new study shows that this simplification misses an important consideration, namely how sensitive remote communities are. Ecological communities are not just lists of individual species, but the species also interact with each other. Thus, the decline or disappearance of some species might have detrimental--often fatal--effects on the species that depend on them. For example, a parasite cannot survive without hosts, as much as a predator will starve without prey or a plant will not reproduce without pollinators.

“Such events where a species disappears following the loss of other species it depends on, known as co-extinctions, are now recognized to be a primary driver of the ongoing global biodiversity crisis. The potential risk stemming from ecological dependencies is a major concern for all ecological systems”, Strona says.

Earlier this summer, an international research team, led by Strona showed that almost half of tropical fish diversity is threatened by mass events of coral mortalities due to a widespread dependency of fish on corals (https://royalsocietypublishing.org/doi/full/10.1098/rspb.2021.0274). The team, now including Mar Cabeza from the Global Change and Conservation lab at the University of Helsinki,  has now gone further, identifying a general macro-ecological mechanism that calls for a reconsideration of global conservation strategies.

Remote communities are more fragile

The researchers chose fish and coral reefs as the study system, because they are a very endangered ecosystem with good data coverage. They combined data on all reef localities worldwide. Using artificial intelligence techniques, the researchers mapped the network of interactions among corals, fish prey and fish predators considering over 9000 fish species. Combining this with distances to cities they found that the dependency between fish and corals becomes stronger with distance from humans. That is, remote reefs are those where coral mortality affects the largest proportion of fish species.

No safe havens on Earth

To understand how the two effects may play out: the protection conferred by the distance from humans or the threat experienced by more dependable communities, they devised a novel risk assessment framework, that could be applied  to any ecosystem worldwide. This framework combines local impacts, such as overfishing and pollution, global impacts, such as climate change, and the risk deriving from ecological interactions within the community.

The application of the framework to coral reefs and their associated fish communities revealed that taking into account ecological dependencies counteracts the positive effects of remoteness. The risk posed by humans is thus quite uniform throughout the world.

"This reveals a stark picture. The hotspots of risks for fish communities from local human impacts and global change are almost perfectly complementary with the hotspots of risk from fish coral dependencies. This produces a global map of risk for fish communities where no place is safe, regardless of distance from humans" says Mar Cabeza.

"The relevance of these findings might extend far beyond reef fish, depicting a world where remote localities, rather than safe havens for biodiversity, might be, instead, areas of critical vulnerability", Giovanni Strona suggests.

References:

Strona G, Beck PSA, Cabeza M, Fattorini S, Guilhaumon F, Micheli F, Montano S, Ovaskainen O, Planes S, Veech JA, Parravicini V (2021) Ecological dependencies make remote reef fish communities most vulnerable to coral loss. Nature Communications, https://doi.org/10.1038/s41467-021-27440-z

Strona G, Lafferty KD, Fattorini S, Beck PS, Guilhaumon F, Arrigoni R, Montano S, Seveso D, Galli P, Planes S, Parravicini V (2021) Global tropical reef fish richness could decline by around half if corals are lost. Proceedings of the Royal Society B, 288(1953):20210274. https://doi.org/10.1098/rspb.2021.0274