3.4.9 Competition

Competition is the interaction between organisms for limited resources, such as space, food, light and mates, and is one of the key processes influencing community structure on coral reefs.902 The complexity of competitive interactions on coral reefs may also contribute to their high species diversity.902,903 In reef habitats, competition is perhaps most evident between macroalgae, coral, and other sessile invertebrates which vie for physical space to colonise hard substrates.903,904,905 

In healthy reefs, corals dominate over macroalgae, but this dynamic can be upset by environmental changes, such as elevated sea temperatures,906 increased nutrients, and declines in herbivorous species.907,908 Among reef-building corals, species-specific responses to climate change 909 and local stressors 910 can increase the competitive advantage of some corals over others,902 ultimately leading to changes in the structure and function of coral assemblages.339 In some inshore areas of the Reef, competition with macroalgae is currently hindering recovery of coral communities (Sections 2.3.5 and 2.4.3).190

Competitive dominance of corals over macroalgae can be disrupted by disturbances

On coral reefs,907,911 and more generally,912 cumulative stressors and increasing disturbance frequency are likely be increasingly favouring fast-growing and opportunistic species at the expense of the often slower-growing species capable of competitive dominance in mature ecological communities. This process results in simplified ecosystems with reduced biodiversity and low ecological complexity.913,914 The extent to which such shifts may be occurring in the Region is not well understood. 

Competition for resources is also observed among mobile species, such as reef fishes,915 particularly those with similar diet and habitat requirements. Resource competition can manifest as aggressive interactions between species 905 and lead to species-specific patterns in space use.916 Loss of reef habitat may exacerbate competitive interactions in coral associated invertebrate 323 and fish 915,917 communities. Impacts can be influenced by the degree of habitat specialisation 917,918 and position in the competition hierarchy 915 of different species.

Increasing sea temperatures, nutrient inputs and changes in species composition and habitat availability may be altering the balance of competitive outcomes in the Region. For example, macroalgae continue to outcompete corals in some inshore areas. Cumulative stressors and increasing disturbance frequency may be increasingly favouring fast-growing and opportunistic species. 

Clear, shallow-water landscape showing the juxtaposition of brown fleshy macrogalgae growing on sandy bottom on the left with coral reef matrix on the right. The corals are mostly plating/foliose pinky brown pachyseris colonies, some reaching up to the water surface, with some pink pocilloporids in the background.
Macroalgae and coral compete for space in inshore habitats.  © Matt Curnock 2021
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