The collection of this species for the aquarium trade may lead to overharvest and localised reductions in abundance, especially for populations of naturally rare species (Bruckner and Borneman 2006). However, the wild collection of corals is highly selective and considered low impact in the long-term relative to other activities such as coral mining and dynamite fishing (Green and Shirley 1999, Pratchett et al. 2020).
Other members of the Plerogyra genus have a high relative susceptibility to bleaching. However, the turbid low-light conditions Plerogyra species typically occur in may protect these taxa from strong radiation and subsequent bleaching compared to shallow-water corals in non-turbid environments. In contrast, turbid conditions may create narrow environmental tolerance limits that do not favor acclimation or adaptation to anomalous conditions (McClanahan et al. 2007).
Coral reefs are threatened by human and natural stressors at a range of scales. In general, the greatest large-scale threat to corals is from global climatic change, which is linked to lethal seawater temperature anomalies, along with increased frequency and severity of El Niño Southern Oscillation (ENSO) events and storms, and ocean acidification (Pandolfi et al. 2011, IPCC 2018), each a major threat to reefs in their own right. The most recent, and first, multi-year, global ‘bleaching’ event (spanning hundreds of kilometres or more) was from 2014 to 2017. Globally, 75% of reefs were affected by bleaching-level stress, with more than 50% of affected reef areas impacted at least twice over the period (Hartfield et al. 2018, Hughes et al. 2018, Eakin et al. 2019), and some locations experienced almost complete coral cover loss (Vargas-Ángel et al. 2019). The first global coral bleaching event was in 1997-98, however this had also been preceded by multiple smaller regional and local scale bleaching events since at least 1982 (Goreau et al. 2000). While coral populations can be resilient to coral bleaching and bounce back (e.g., Diaz-Pulido et al. 2009, Pisapia et al. 2016), more frequent bleaching events in the future are expected to prevent full reef recovery and cause local extinctions of some species (van Hooidonk et al. 2016, Sheppard et al. 2020). Heating episodes are also increasing in intensity with the 2014-2017 global bleaching event exposing more than three times as many reefs to bleaching-level heat stress than the 1998 event (Skirving et al. 2019). Almost all coral reefs are very likely to have degraded from their current state by 2100, even if global warming remains below 2oC from pre-industrial levels (Frieler et al. 2012), meaning species composition will differ and diversity and extent will be reduced from present levels (IPCC 2018). There is limited scope for future latitudinal range extension of current reefs towards the poles (Muir et al. 2015), and severe bleaching episodes can also cause positive feedbacks, including impairment of larval recruitment via mortality of adult brood stock (Hughes et al. 2019).
Coral disease has emerged as a serious threat to coral reefs worldwide with increases in numbers of diseases, coral species affected, and geographic extent (Ward et al. 2004, Sutherland et al. 2004, Sokolow et al. 2009). Outbreaks of coral diseases have damaged coral reefs worldwide with the most widespread, virulent, and longest running coral disease outbreak currently occurring on the Florida Reef Tract and throughout the Caribbean. The disease, stony coral tissue loss disease, has been ongoing since 2014 (Precht et al. 2016) and has devastated affected reefs along Florida (Walton et al. 2018, Williams et al. 2021) and throughout the Caribbean (Alvarez-Filip et al. 2019, Kramer et al. 2019). Numerous disease outbreaks have also occurred in the Indo-Pacific (Willis et al. 2004, Aeby et al. 2011; 2016), Indian Ocean (Raj et al. 2016) and Persian Gulf (Howells et al. 2020). Escalating anthropogenic stressors combined with the threats associated with global climate change of increases in coral disease, frequency and duration of coral bleaching and ocean acidification place coral reefs in the Indo-Pacific at high risk of collapse.
Crown-of-thorns starfish (COTS) (Acanthaster planci), found throughout the Indo-Pacific, can undergo massive outbreaks that rapidly devastate reefs on a local and regional level, triggered through human impacts such as enhanced nutrient loads (Pratchett et al. 2014). Populations of COTS have greatly increased since the 1970s and have been known to kill large areas of coral reef habitat, and have contributed to the overall decline and destruction of reefs in the Indo-Pacific region (Pratchett et al. 2017).
Tropical coral reef biomes are also at particular risk from localised human pressures, with 58% of coral reefs <30 minutes from the nearest human settlements (Maire et al. 2016). Localised threats to corals include over-intensive fisheries, coastal development (industry, settlement, tourism, and transportation), changes in native species dynamics (competitors, predators, pathogens and parasites), introduction of invasive species (competitors, predators, pathogens and parasites), destructive fishing (e.g. using dynamite), chemical fishing, pollution from agriculture and industry, domestic pollution, and recreation and tourism activities and global trade (Burke et al. 2012). Some of these threats impact corals directly, such as being physically disturbed and smothered with sediment during a construction project (Erftemeijer et al. 2012), while others operate indirectly via ecosystem processes and linkages between corals and other reef organisms. Macroalgae is major competitor with corals that reduces growth, causes disease, prevents new coral recruitment and can tip the entire ecosystem into a less diverse and less productive ‘algal-dominated’ reef (Hughes 1994, Bellwood et al. 2004). Macroalgal levels are controlled by both bottom-up provision of nutrients (Fabricius 2005), and top-down herbivory by parrotfish (Mumby et al. 2007), hence while the immediate threat to the coral is be the algae, the ultimate threat may be sewage, fertiliser from agriculture or overfishing of herbivorous fish. The complex nature of the coral reef ecosystem means that while the immediate threat may be obvious (e.g., macroalgae, disease, crown-of-thorns outbreak), the ultimate threat is often less clear (Nyström et al. 2008, Anthony et al. 2015).
