Coral Reef Food Web
The coral reef food web - like those of all highly diverse biological communities - is exceedingly complex.
Hence, attempting to describe all of the myriad linkages in any coral reef food web is well beyond the scope of this website (or of current science).
Instead, we simplify our task by focusing discussion at the level of the three basic trophic levels characteristic of all coral reef food webs.
This approach reduces the complexities of feeding relationships in coral reef communities to a far more manageable level.
Trophic Levels of the Coral Reef Food Web
Most complex food webs - including that of the coral reef - can be seen as consisting of 3-4 basic "trophic levels". Each "level" represents a group of species that acquires its energy and raw materials by different means and from distinctly different sources.
Typically, either three or four such levels are distinguished:
- Producers (plants)
- Primary Consumers (herbivores)
- Secondary Consumers (carnivores)
In general, for coral reef food webs it is somewhat simpler to consider only these three basic trophic levels. However, for some of the component food chains within the coral reef food web, a fourth level - tertiary consumers (top predators) - can also sometimes be readily distinguished.
Producers: Plants and Other Photosynthesizers
Three major groups of photosynthetic organisms contribute to net primary production (creation of "new" organic compounds) within coral reef ecosystems:
The general biology and ecology of plants and "algae" are discussed elsewhere, on our page dedicated to coral reef plants.
Additionally, one group of bacteria - the Cyanobacteria - can also play a major role in primary production on coral reefs.
These relatively simple unicellular life forms are also commonly called "blue-green algae", but this name is misleading since these prokaryotes are the most distant of relatives to those eukaryotic organisms collectively called called "algae".
Cyanobacteria are aquatic and photosynthetic, and occur in a wide variety of benthic habitats within coral reef ecosystems. They grow on reef surfaces, plant and algal surfaces, and even on the sand grains between reefs.
In addition to their role as autotrophs, Cyanobacteria also help build the reef structure, fix nitrogen, serve as a food source for other reef species, and are (some species) agents of coral disease.
The full magnitude and nature and of the importance of Cyanobacteria to coral reef ecology is only beginning to be unraveled, but it is clear that these tiny creatures are a major piece of the puzzle that we face in understanding and addressing human impacts on coral reefs.
Primary Consumers: The Herbivores
A wide variety of herbivorous animals reside on coral reefs, including invertebrates (such as mollusks and echinoderms) as well as fishes. The most important of the herbivorous coral reef fishes are the parrotfishes, surgeonfishes, rabbitfishes, rudderfishes and damselfishes.
Almost all populations of some of the largest and most important of the other vertebrate plant feeders - sea turtles, manatees, and dugongs - have been severely decimated by human impacts over the past three centuries. These losses have likely altered the pre-disturbance coral reef food webs in substantial ways, but deciphering exactly what those impacts have been is highly speculative.
More information on the feeding habits of these creatures is provided on our webpage dedicated to coral reef animals.
Secondary Consumers: The Carnivores
Animals that feed as carnivores in the coral reef biome are extremely numerous and abundant.
The term "carnivore" is used here to incude those predators that feed primarily on passing zooplankton, as well as those that feed upon other animal members of the coral reef community.
The largest carnivores that dwell on coral reefs are the piscivores - those fishes that feed heavily upon herbivorous and planktivorous fishes. These include (most notably) barracuda, sharks, large seabasses (groupers, hinds), trumpetfishes, and large snappers.
Plankton: Contributions From the Open Sea
Producers are not the only input of "new" food for coral reef food webs. A quite different neighboring marine biome - the open sea - also provides coral reef dwellers with substantial amounts of newly acquired energy and nutrients.
By far, the vast bulk of plant life in the open sea is composed of microscopic planktonic algae (phytoplankton) that comprise the base of the entire marine food chain.
This wealth of plant life in turns supports quantities of tiny drifting animals (zooplankton) which feed upon the drifting plants. Some of these are ultimately swept across coral reefs.
Thus, along with the nutrients supplied by producers that reside within the coral reef biome, the reef community also relies heavily upon tiny zooplankton suspended in the constant flow of sea water that bathes coral reefs.
It is through capture and ingestion of these creatures by myriad coral reef invertebrate animals (including corals themselves) and plankton-feeding reef fishes that some of the organic production of the open ocean is transferred to coral reef food webs.
The overall energetic contribution of zooplankton to coral reef communities is highly variable in both time and space, and is difficult to measure. Nonetheless, reef planktivores have been shown to be highly efficient, removing as many as 60-70% of the drifting animals from the passing water.
Summary: The Coral Reef Food Web
Coral reef communities are extremely efficient at acquiring, retaining and recycling nutrients received from multiple sources. The key elements in the coral reef recycling process are (1) the symbiotic relationship between hard corals and their zooxanthellae, and (2) the rapid and effective exchange of nutrients and energy between different coral reef habitats.
It is this highly efficient "recycling program" - along with the organic contributions from primary producers within the ecosystem as well as from the open sea - that allows for the sustenance and growth of coral reefs in the midst of waters so stripped of dissolved nutrients that they are often referred to as biological deserts.