HO120: Behaviour and feeding ecology of Caribbean reef herbivores
Max Bodmer, Operation Wallacea
Scleractinian (hard) corals are the ecosystem architects of coral reefs. Their sensitivity to a wide-range
of different environmental variables means that the global distribution of reefs is highly limited. On a
local-scale, coral growth is also restricted as a result of their requirements for high light intensity,
meaning that they can only grow at depths <60m. These constraints mean that coral reefs occupy just
0.2% of the Earth’s surface, but, in spite of this they are the single most diverse and productive marine
ecosystem on the planet, providing a permanent home to ca. 25% of all marine species. Restrictions
on coral growth create an incredibly space limited environment, and the huge diversity of reefdwelling organisms must compete for living space – no competition is more intense than that which
occurs between hard corals and macroalgae.
On a natural, healthy coral reef, slow-growing hard corals will dominate over fast-growing macroalgae.
This is beneficial to biodiversity and ecosystem function as the 3D skeletons laid down by corals
provide structure and complexity, which, in turn, creates living space that helps to relax competition
on the reef. The majority of macroalgae do not produce calcium carbonate skeletons and therefore
have simple, 2D growth forms. If macroalgae become dominant over hard corals structural complexity
is lost, which leads to an intensification of space competition and a reduction of biodiversity.
Historically, coral domination has been maintained by the nutrient-poor (oligotrophic) nature of
seawater surrounding reef systems, coupled with large herbivorous fish and invertebrate populations
keeping macroalgal growth in check. Oligotrophy means that macroalgae are unable to fulfil their
nutrient requirements and therefore their growth is curtailed. Corals however, are able to continue
growing in the absence of nutrients because their zooxanthellae (symbiotic microscopic algae living
within the tissues of corals) are provided with everything they need by the metabolic processes of
their coral host. Unfortunately increases in industrial and agricultural runoff, containing high
concentrations of nitrates and phosphates, have provided macroalgae with the nutrients needed for
growth. In the right nutritional environment, many macroalgae can exceed the annual growth rate of
even the very fastest growing corals in a single week and can very rapidly become the dominant
benthic component.
The problems associated with nutrification are exacerbated by the losses of key reef herbivores that
have occurred over the last four decades as a result of disease and overharvesting. Herbivores are an
essential component of healthy coral reef ecosystems because they remove large volumes of
macroalgae and bias the outcome of space competition in favour of hard corals. The threats of
increased nutrient-load and removal of key herbivores have operated synergistically to stimulate the
occurrence of widespread macroalgal phase-shifts throughout the Caribbean – a phenomenon
whereby reefs move from a hard coral dominated stable state to a macroalgae dominated stable state.
Students enrolled on this project will use a variety of in water and videography methodologies to
assess the feeding preferences of different reef herbivores. This project is very broad and students are
encouraged to think of their own research questions that they think would be interesting and relevant
to coral reef ecology and conservation. They may choose to focus on the effects of various
environmental variables on herbivore preferences and behaviour, or, to take a more experimental
approach and use artificial algal plates to explore differences in foraging behaviour driven by different
types and densities of macroalgae. If required, all students can also have access to Operation
Wallacea’s long-term fish population monitoring dataset. A potentially interesting avenue of
exploration would be to combine the long-term ecological data with the behavioural data in order to
try to predict how changes in fish populations over time might manifest themselves in terms of
changes to the structure of the macroalgal community.
Reading List
Alvarez-Filip, L., Dulvy, N.K., Gill, J.A., Cote, I.M., Watkinson, A.R. (2009) Flattening of Caribbean coral
reefs: region-wide declines in architectural complexity. Proceedings of the Royal Society B,
276: 3019-3015.
Bellwood, D.R., Hughes, T.P., Folke, C., Nystrom, M. (2004) Confronting the coral reef crisis. Nature,
429(6994): 827-833.
Bruno, J.F., Sweatman, H., Precht, W.F., Selig, E.R., Schutte, V.G.W. (2009) Assessing evidence of
phase shifts from coral to macroalgal dominance on coral reefs. Ecological Society of
America, 90(6): 1478-1484.
Carpenter, R.C., Edmunds, P.J. (2006) Local and regional scale recovery of Diadema promotes
recruitment of Scleractinian corals. Ecology Letters, 9:271-280.
Connolly S.R., Muko S. (2003) Space competition, size-dependent competition, and the coexistence
of clonal growth forms. Ecology, 84(11): 2979-2988.
Fabricius, K.E. (2005) Effects of terrestrial runoff on the ecology of corals and coral reefs: review and
synthesis. Marine Pollution Bulletin, 50: 125-146.
Gardner, T.A., Cote, I.M. Gill, J.A., Grant, A., Watkinson, A.R. (2003) Long-term region-wide declines
in Caribbean corals. Science, 301(5635): 958-960.
Hoey, A.S., Bellwood, D.R. (2011) Suppression of herbivory by macroalgal density: a critical feedback
on coral reefs? Ecology Letters, 14: 267-273.
Jackson, J.B. Kirby, M.X., Berger, W.H., Bjorndal, K.A., Botsford, L.W., Bourque, B.J., Warner, R.R.
(2001) Historical overfishing and the recent collapse of coastal ecosystems. Science,
293(5530): 629-637.
McCook L.J., Jompa J., Diaz-Pulido G. (2001) Competition between corals and algae on coral reefs: a
review of evidence and mechanisms. Coral Reefs, 19: 400-417.
Rinkevich B., Loya Y. (1985) Intraspecific competition in a reef coral: effects on growth and
reproduction. Oecologia, 66(1): 100-105.
Tanner J.E. (1997) Interspecific competition reduces fitness in scleractinian corals. Journal of
Experimental Marine Biology and Ecology, 214(1-2): 19-34.