Acropora Microclados

This species is particularly susceptible to bleaching, disease, predation and other threats. This species is recorded as having a medium susceptibility to bleaching in both the Maldives and Great Barrier Reef (Muir et al. 2021).

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).

In general, the major threat to Acropora corals is global climate change, in particular, temperature extremes leading to bleaching induced mortality, and an increased susceptibility to disease (Hoegh-Guldberg et al. 2007, Hughes et al. 2017; 2018; 2019). Bleaching can lead to mortality and a reduction in both coral cover and effective population sizes. It also disrupts coral reproduction. Regional coral extinction events following thermally anomalous events are increasingly reported (Sheppard et al. 2020, Richards et al. 2021, Muir et al. 2021). In addition, climate change is predicted to lead to an increased severity of ENSO events and storm intensity, and longer-term changes in ocean chemistry impacting calcification, along with an increase in the severity of flood and fire events impacting catchments.

This species has a corymbose plate growth form and is susceptible to predation by crown-of-thorns seastar. Crown-of-thorns (COTS) (Acanthaster spp.) are found throughout the Pacific and Indian Oceans and the Red Sea. Crown-of-thorns are voracious predators of reef-building corals, with a preference for branching and tabular corals such as Acropora species. Populations of the crown-of-thorns starfish have greatly increased since the 1970s and have been known to consume large areas of coral reef habitat. Increased breakouts of COTS has become a major threat to some species, and have contributed to the overall decline and reef destruction in the Indo-Pacific region. The effects of such an outbreak include the reduction of abundance and surface cover of living coral, reduction of species diversity and composition, and overall reduction in habitat area. Crown-of-thorn outbreaks are particularly concerning in coral communities that are recovering from disturbances such as coral bleaching as feeding on remnant survivors and juveniles can further inhibit community recovery (Haywood et al. 2019).

Coral disease has emerged as a serious threat to coral reefs worldwide and a major cause of reef deterioration (Weil et al. 2006). The numbers of diseases and coral species affected, as well as the distribution of diseases have all increased dramatically within the last two decades (Porter et al. 2001, Green and Bruckner 2000, Sutherland et al. 2004, Weil 2004). Coral disease epizootics have resulted in significant losses of coral cover and were implicated in the dramatic decline of acroporids in the Florida Keys (Aronson and Precht 2001, Porter et al. 2001, Patterson et al. 2002). In the Indo-Pacific, disease is also on the rise with disease outbreaks recently reported from the Great Barrier Reef (Willis et al. 2004, Haapkyla et al. 2013), Indonesia (Haapkyla et al. 2007, Subhan et al. 2020), Thailand (Lamb et al. 2014), Marshall Islands (Jacobson 2006), Micronesia (Myers and Raymundo 2009), American Samoa (Work and Rameyer 2005), and the northwestern Hawaiian Islands (Aeby et al. 2006), the Cocos (Keeling) Islands (Preston and Richards 2021), the Maldives (Montano et al. 2015), and the Persian Gulf (Aeby et al. 2020). Increased coral disease levels on the GBR were correlated with increased ocean temperatures (Boyett et al. 2007, Howells et al. 2020) supporting the prediction that disease levels will be increasing with higher sea surface temperatures. As environmental conditions continue to change, it is predicted that conditions on temperate reefs will become favourable for coral diseases and thermodependent bacteria (Bally and Garrabou 2007, Brodnicke et al. 2019) and the geographical range of tropical coral diseases will extend (Vergés et al. 2019).

Localized threats to corals include fisheries, human development (industry, settlement, tourism, and transportation), changes in native species dynamics (competitors, predators, pathogens and parasites), invasive species (competitors, predators, pathogens and parasites), dynamite fishing, chemical fishing, pollution from agriculture and industry, domestic pollution, sedimentation, and human recreation and tourism activities. The severity of these combined threats to the global population of each individual species is not known.