The Evolutionary Survival of Coral Reefs
A recently released 2020 scientific paper has identified key evolutionary traits among Scleratinian coral species, which have allowed them to survive Cretaceous-Tertiary (K-T) mass extinction event. The researchers then went one step further and compared these traits among modern species and their status on the IUCN Red List of endangered species, and reported that species that possessed these survival-associated traits had relatively more stable populations compared to species that lack one or more of these traits. Given the general decline in coral reefs worldwide, the identification of such traits is important in understanding how species may respond to climate change and the ongoing Anthropocene extinction event.
The End-cretaceous Mass Extinction Event
An extinction event is a widespread and rapid decline in biodiversity. There have been five major extinction events in the past, the most recent of which was the Cretaceous-Paleogene extinction event (66Mya), which saw around 75% of all species on earth going extinct. Most famously, this extinction event is known for the end of all non-avian dinosaurs, but a number of marine animals, such as the ammonites and aquatic reptiles, also went extinct. The extinction is widely accepted to have been caused by the impact of a massive meteorite in Mexico. The impact likely caused earthquakes, tsunamis and a global heat pulse. After the initial heat pulse, the dust particles released into the atmosphere are thought to have caused an “impact winter” or a 6 °C drop in temperature due to the sunlight being blocked out. This was then followed by a heating event that is thought to be due to greenhouse gas release from extensive volcanism. Major stressors to Scleractinian coral species are thought to have been prolonged darkness, major changes in temperature, eutrophication (or nutrient overloading) and ocean acidification.
The Holocene (Anthropocene) extinction event
We are currently living through the sixth mass extinction event, with extinction rates estimated to be 1000 times that of expected background extinction rates. Theoretically, species are always going extinct, but new species are also always emerging, and the rate of extinction is measured against the rate of speciation. Currently, the rate of extinction is very high compared to that of speciation. This is largely due to human activity, such as habitat destruction, deforestation, pollution, ocean acidification, climate change due to greenhouse gases and overutilization of ecological resources. 75% of species are predicted to go extinct if the pressures humans exert on the environment are not mitigated soon.
Key Evolutionary Traits Associated With The Survival Of Mass Extinction Events In Scleratinian Corals
1) Deep water residing (>100 m)
Shallow water species are thought to be particularly sensitive to temperature changes, eutrophication and ocean acidification compared to deeper water species. This is most likely because they are more impacted by such events. During the K-T extinction event, rapid temperature changes and acid rain from the gases released into the atmosphere were more likely to affect species residing closer to the ocean surface, as they would experience more significant changes in temperature and acidity. Water has a high specific heat capacity and therefore doesn’t change temperature quickly, so deep water species are more buffered. The acid rain would also become more diluted in deeper water, again buffering the deeper species.
In the modern era, eutrophication is commonly associated with the shoreline, as excess nutrients largely result from runoff from human activities, such as farming. Excess nutrients cause algal blooms in the photic (sunny) zone of the ocean, where shallow corals which harbor photosynthetic zooxanthellae usually outcompete the algae.
In deeper waters, solitary, non-photosynthetic, non-symbiotic corals have a distinct advantage over symbiotic corals, which require sunlight to grow.
Ocean acidification or an increase in sea surface temperatures can lead to coral bleaching, as corals expel their photosynthetic symbionts when they become stressed. Without their little symbionts, corals lose an important source of nutrients and experience drastically reduced growth rates, or may die altogether. Increased sea surface temperatures are also associated with increased disease and mortality rates in corals. However, corals which have a higher thermal or acidification tolerance are less susceptible to bleaching and are therefore more likely to survive changes in environmental temperature. The high proportion of bleaching resistant corals present after the K-T extinction suggests that there was strong selection for bleaching-resistant corals, and that it is an important factor for surviving mass extinction events. Interestingly, because coral species that survived the K-T extinction event have already experienced selection for bleaching resistance, it is likely that these species may be more likely to survive the impacts of climate change than species which originated after the K-T boundary.
4) Cosmopolitan distributions
The more widely distributed a species is, the more likely it is to survive extinction events, as it can likely tolerate a wider range of environmental conditions than a species which are highly specialized to its immediate environment. Also, having a wide range increases the probability that a species can shift its distribution in response to environmental change.
5) Solitary or existing in small colonies
Being small or existing in small colonies is thought to be beneficial because it allows corals in “sheltered” niches to survive. This trend is also observed in molluscs and other marine invertebrates that smaller bodied animals are more likely to survive mass extinction events (but more susceptible to background extinction). During the K-T extinction event, larger terrestrial animals were also much more heavily impacted than smaller terrestrial animals.
In summary, although coral are likely experiencing different stressors in the Holocene extinction event to those that they faced during the K-T extinction event, there are distinct similarities between the corals that survived the K-T extinction and the species that appear to have more stable populations currently, suggesting that there are certain evolutionary traits which make some species more resistant to extinction. Because of this, we may see a shift in coral reef community assemblages to more solitary, non-symbiotic communities, which lack the reef building ability of other coral species, leading to a decline in the structural complexity which maintains current high levels of biodiversity within reef systems.