Understanding patterns of reef fish diversity on the Great Barrier Reef

Preserving biodiversity is an essential component of most conservation and environmental management strategies but what actually is biodiversity and why is it important? Put simply, biodiversity is the incredible variety of life that surrounds us and the most fundamentally important reason for its preservation is that critical reductions in biodiversity can lead to the degradation of ecosystems and reduced quality of life for all inhabitants, including humans. Loss of biodiversity can have this impact because the actions or presence of all organisms will have varying consequences for those around them. Think of clear felling of ancient rain forest trees and the consequences for all the mammals, birds and insects that relied on those trees for food and a home. However, the effects of removing or overharvesting species within ecosystems are often nowhere near as obvious as in the previous example, so a precautionary approach of preserving biodiversity is seen to be one of the best actions to maintain healthy ecosystems.

Coral reef ecosystems are one of the most diverse on the planet but are still vulnerable to unwanted reductions in biodiversity due to a range of natural disturbances and the actions of humans. Well known examples on coral reefs have resulted from over-fishing of large herbivorous fishes that eat seaweeds. This has triggered persistent changes from vibrant coral communities to dull seaweed communities in varied tropical regions (Hughes 1994, Cheal et al 2010).

Havanna GBR IMG 6990

Of over one hundred reefs surveyed regularly by the AIMS LTMP, only one, Havannah Island Reef in the Palm Islands, just north of Townsville has experienced a persistant shift to a macro-algae dominated benthic community. Further research is being undertaken to understand what factors are preventing recovery of a coral dominated community.

by AIMS - LTMP

Given that coral reefs support the most diverse fish communities on the planet and their roles in the food chain are so varied, from huge predatory cods to small plankton feeding damselfishes, it is likely that reef fishes are generally important in maintaining a healthy ecological balance on coral reefs. For this reason it is important to understand patterns of reef fish diversity within coral reef ecosystems and the environmental drivers of these patterns so as to best manage potential threats.

The Great Barrier Reef (GBR) is the world’s largest coral reef ecosystem, stretching >2300km form north to south, making it logistically difficult to quantify broad-scale patterns in reef fish diversity. Fortunately, the Australian Institute of Marine Science initiated an annual long term monitoring program (AIMS LTMP) for shallow water reef fishes in 1992 that covered large areas of the GBR. This program has provided unique information on fish communities throughout the GBR, ideal for understanding patterns of fish diversity and the factors that may drive them.

A GBR wide study of the diversity of fish using the multinominal diversity model (MDM) showed that the diversity of shallow water reef fishes varied throughout the GBR. An analysis of the long term data of the AIMS LTMP fish surveys showed the lowest diversity values were recorded inshore and highest values offshore, and diversity tended to be higher in the northern-central regions and lower in the southern regions. Importantly, although fish diversity did fluctuate throughout the GBR between 1992 and 2013 there was no evidence of loss of fish species. While this is encouraging, given that many fish species rely on corals for shelter and food, if coral cover continues to decline on the GBR (see Death et al. 2012) then the current maintenance of fish diversity may be threatened.

Coral supports high fish diversity on GBR reefs

A school of the surgeon fish, Naso tuberosus and damselfish at Jenkins Reef, 2013. If  you look closely there are also coral trout, butterfly fish and a fox-face rabbitfish.

by AIMS - LTMP

Detailed studies of subsets of the GBR shallow water reef fish communities using AIMS LTMP data have also highlighted strong cross shelf patterns in diversity; species richness of large herbivorous fishes (Cheal et al. 2012), territorial damselfishes (Emslie et al. 2012) and butterflyfishes (Emslie et al. 2010) tended to be lower inshore and higher offshore. This is a pervasive pattern for many groups of organisms on the GBR and is likely to reflect the very different environmental processes that influence inshore waters (coastal influences) compared to offshore waters (oceanic influences).

Indeed Cheal et al. (2013) found that increasing herbivorous fish diversity as you head further offshore on the GBR was highly associated with increases in a simple measure of water clarity. It appears that many herbivorous fish species prefer not to inhabit water of low clarity, possibly because reduced penetration of sunlight in dirtier water reduces the productivity of their algal food and/or sediment settling on those algae lessens its palatability. This is a concern because if inshore GBR water clarity decreases further due to increased runoff of sediments, nutrients and pollutants from ongoing human activities on adjacent lands, the diversity of associated herbivorous fish communities may also decrease, so increasing the likelihood of overwhelming seaweed growth. This could result in more seaweed dominated inshore reefs with few corals; an undesirable outcome because weedy reefs have lower ecological, social and economic value. This example shows how knowledge of spatial patterns of fish diversity along with an understanding of species environmental associations and ecology can provide important information relevant to the management of reef ecosystems and in this case, associated catchments.

Cheal, A. J., M. A. MacNeil, E. Cripps, M. J. Emslie, M. Jonker, B. Schaffelke, and H. Sweatman. 2010. Coral-macroalgal phase shifts or reef resilience: links with diversity and functional roles of herbivorous fishes on the Great Barrier Reef. Coral Reefs 29:1005–1015.

Cheal, A. J., M. Emslie, I. Miller, and H. Sweatman. 2012. The distribution of herbivorous fishes on the Great Barrier Reef. Marine Biology 159:1143–1154.

Cheal, A. J., M. Emslie, M. A. MacNeil, I. Miller, and H. Sweatman. 2013. Spatial variation in the functional characteristics of herbivorous fish communities and the resilience of coral reefs. Ecological Applications 23:174-188.

De'ath, G., K. E. Fabricius, H. Sweatman, and M. Puotinen. 2012. The 27–year decline of coral cover on the Great Barrier Reef and its causes. Proceedings of the National Academy of Sciences USA 109:17995-17999.

Emslie, M. J., M. S. Pratchett, A. J. Cheal, and K. Osborne. 2010. Great Barrier Reef butterflyfish community structure: the role of shelf position and community type. Coral Reefs 29:705-715.

Emslie, M. J., M. Logan, D. M. Ceccarelli, A. J. Cheal, A. S. Hoey, I. Miller, and H. P. A. Sweatman. 2012. Regional-scale variation in the distribution and abundnace if farming damselfishes on Australia’s Great Barrier Reef. Marine Biology 159:1293-1304.

Hughes, T. P. 1994. Catastrophes, phase shifts and large-scale degradation of a Caribbean coral reef. Science 265:1547–1551.