Human settlements make coral reefs react unpredictably to climate change

NOT COMFORTABLE AROUND PEOPLE: Coral reefs support half a billion people yet they are severely compromised by human activity.

To plan ahead with appropriate conservation measures, scientists have developed models to understand how reefs respond to increasingly common climate change-related events such as unusual weather patterns or destructive storms.

But given that more than half the world’s coral reefs are located in close proximity to human settlements, it is important to examine how different densities of human populations affect reef health and responses to climate change.

The risks of human proximity

Research has shown that coral reefs can be compromised close to human populations - for example macroalgae can overtake corals as the dominant organisms - but it is also important to understand the interplay between these ‘local impacts’ and climate change.

In a paper published in Frontiers in Marine Science, former centre researcher Amanda Ford, now at the University of the South Pacific, shows that being close to larger human populations changes the relationships between coral reefs and their biophysical environment.

The study was published together with SRC colleagues Jean-Baptiste Jouffray, Albert Norstrom and an international team of researchers.

They call for this to be taken into account when modelling the future of these tropical ecosystems in the face of climate change, as reefs closer to larger populations seem to become less predictable.

Density of humans has an impact

Increasing sewage input, agricultural run-off, and sedimentation are all associated with higher human densities, reducing water quality and affecting communities at the bottom of the ocean.

Added to this, fishing removes species which play an important role in regulating coral reef communities.

Using data from the Pacific Community (SPC) on underwater surveys of 62 reefs in 17 different Pacific Island countries and overseas territories, the authors ran models with various combinations of environmental predictors to test how well they could predict reef structure, for example the cover of coral and different algal groups.

They found that whilst physical predictors (e.g. depth, latitude, storm exposure) were strong at explaining reef composition at reefs close to small human settlements, their ability to predict reef composition at reefs close to relatively denser populations was severely compromised.

At these more locally-impacted reefs, local features related to human activities (e.g. distance to closest market, biomass of herbivorous fish) emerged as better predictors, suggesting humans had become the dominant driver of reef structure.

The study indicates that species homogenisation may be occurring in reefs that are more exposed to local human impacts, as species are selectively removed by human-associated stressors and only less stress-sensitive species are left in the mix.

Ultimately, these changes lead to reefs becoming ‘decoupled’ from their biophysical environment, and reacting fundamentally differently and unpredictably compared to reefs less influenced by humans.

“Reefs close to human populations will require context-specific management to ensure their future sustainability and associated critical ecosystem services,” conclude the authors.

Video: Amanda Ford explains why the central insights from this study