In order to fully appreciate the importance of a balanced ecosystem such as a coral reef, we must first understand its trophic structure; the organisms that make up each trophic level, and the functions of each level in the maintenance of a healthy reef.
Coral reefs, just like any other ecosystem on our planet, rely on a variety of biotic and abiotic factors to keep them healthy and functional. Without stable temperature, pH, light/dark cycles, water flow, salinity, and chemical composition of sea water, coral reefs could not exist, but without a stable trophic cascade, coral reefs could not survive. Trophic structure in any environment refers to the different levels of the food chain and illustrates the transfer of energy from one level to the next in the form of a pyramid; energy is always lost as it travels “up” the food chain.
Figure 1: Coral reef trophic pyramid, showing its trophic levels. On each level, several important groups of species are shown (Diagram by Alex Rose).
There are three categories of organisms in every ecosystem: producers, consumers, and decomposers. Producers, often referred to as primary producers, consist of organisms that are capable of making their own food and are consequently always photosynthetic (and in some cases chemosynthetic). Consumers are placed higher on the trophic pyramid than producers, and they can be herbivorous, omnivorous or carnivorous.
Primary consumers are herbivorous, whereas secondary consumers prey on herbivores and tertiary consumers eat other carnivores. Decomposers are responsible for breaking down dead and decaying plant and animal matter into components that are once again usable for growth by producers. None of these three categories of organisms can exist without the others in order to complete the cycle of production, consumption, and decomposition.
"There are three categories of organisms in every ecosystem: producers, consumers, and decomposers".
This article will concentrate on breaking down the creatures of a typical coral reef ecosystem into these three groups and considering their importance to the stability and success of a reef.
Figure 2: The astonishing diversity of phytoplankton is visible only under a microscope. One trait all phytoplankton share, however, is chlorophyll—the green pigment that converts energy from the sun into food (Images copyright Smithsonian Environmental Research Center).
Producers
Primary producers, or autotrophs, make up the base of all food chains. Autotrophs are capable of synthesizing complex organic compounds such as
glucose from a combination of simple inorganic molecules and light energy in a process known as photosynthesis. Other much less common autotrophs (some bacteria) derive their energy from the oxidation of inorganic compounds such as hydrogen sulfide, ferrous iron, and ammonium and are referred to as chemoautotrophs. A good example of marine chemoautotrophs are the bacteria that inhabit deep sea hydrothermal vents; they are the primary producers in this hostile environment and are able to convert heat, methane, and sulfur into energy through a process called chemosynthesis and can survive in water temperatures of 750 °F (400 °C) and a pH of 2.8.
These extremophile bacteria are very important in certain deep sea ecosystems, but are certainly not the main primary producers in the ocean. Most primary production occurs within the first 70 meters (230 feet) of water, an area referred to as the euphotic zone. Primary productivity is measured in grams of carbon produced per square meter of ocean surface per year (g C/m2/yr) and total ocean productivity is estimated to be between 75 and 150 g C/m2/yr. Some common autotrophs in a coral reef ecosystem are phytoplankton, coralline algae, filamentous turf algae, the symbiotic zooxanthellae in corals, and many species of seaweed. Phytoplankton is one of the most important primary producers in the world and includes a wide variety of organisms such as: diatoms which are the most productive type of phytoplankton and have tests (exoskeletons) made of silica, dinoflagellates and silicoflagellates which move by way of flagella, coccolithophores which have tests made of calcium carbonate, cyanobacteria, and other extremely small phytoplankton species referred to as nanoplankton (2.0-20 µm) and picoplankton (0.2-2.0 µm).
Figure 3: The green sea turtle, Chelonia mydas, is a true herbivore and therefore a primary consumer (photograph: Hans Leijnse).
Consumers
- primary consumers
Phytoplankton and other single-celled primary producers are eaten by primary consumers. Primary consumers are herbivorous and they encompass a wide
range of marine creatures including zooplankton, invertebrate larvae, benthic grazers (e.g. bivalves, gastropods, tunicates, sponges, polychaete and feather duster worms), some corals, sea urchins, some crabs, green sea turtles, and herbivorous fish. Zooplankton are undoubtedly the most abundant primary consumers in the water column; copepods, amphipods, ciliates, and tintinnids are all common types of zooplankton. Due to the large community of primary consumers on coral reefs, phytoplankton levels in coral reef waters can be 15-65% lower than in adjacent open ocean waters. Benthic grazers and some coral species feed by filtering phytoplankton out of the water while other vertebrate and invertebrate grazers eat algae and seaweed; many species of parrotfish, surgeonfish and blennies have a diet that consists entirely of coralline, filamentous, and calciferous algae.
- secondary consumers
The animals in this trophic level feed on primary consumers and are consequently carnivorous. Secondary consumers in a reef ecosystem can be divided into four main groups: (1) plankton feeders, (2) corallivores - organisms that feed on coral tissue, (3)
feeders on other benthic invertebrates, and (4) piscivores - fish eaters.
Plankton feeders can be small sessile invertebrates like barnacles, corals like sun polyps (Tubastrea sp.) and gorgonians, small damselfish or 15-ton whale sharks.
Figure 4: A beautiful purple gorgonian, having polyps which actively feed on zooplankton. This makes these corals carnivores, or secondary consumers (photograph: Hans Leijnse).
Corallivores can be sub-divided into polyp eaters, coral scrapers, mucus feeders, and coral tissue generalists. Many species of butterfly fish, file fish, and damselfish specialize in eating coral polyps; their elongated rostrums make it easy for them to pick individual polyps off a large coral animal. Some common coral scrapers are specific species of triggerfish, parrotfish, blennies, puffers, and butterfly fish. Some animals that feed on coral mucus are coral guard crabs and shrimps, and ornate butterfly fish (Chaetodon ornatissimus). Coral tissue generalists are the most destructive of the corallivores and include the Crown-of-thorns sea star (Acanthaster planci), parasitic nudibranchs (e.g. Phistella sp.), parasitic snails (e.g. Epitomium and Drupella sp., the latter often found to be feeding on Stylophora pistillata in the Red Sea), and acoel polyclad flatworms.
Benthic invertebrates like mollusks, gastropods, worms, and crustaceans are eaten by many kinds of fish (e.g. goatfish, wrasses, triggerfish, etc.) and other larger invertebrates; cone shells specialize on specific invertebrates, Tritons (Charonia tritonis) eat Crown-of-thorns sea stars, cephalopods primarily eat shelled invertebrates, and arthropods (e.g. lobsters, mantis shrimp,.etc.) eat a wide range of benthic invertebrates.
Piscivores are carnivores that eat fish; many species of fish are piscivores as well as some mollusks and arthropods.
Figure 5: This snail actively feeds on algae, and is a primary consumer. Species like these are consumed by secondary consumers such as various fish species (photograph: Hans Leijnse).
- tertiary (top) consumers
Tertiary consumers are large reef fish at the top of the food chain that eat many smaller fish. Some examples of top consumers in a coral reef ecosystem are sharks, barracudas, and moray eels. Marine mammals such as dolphins and seals, and sea birds, if present, are considered tertiary consumers, too.
Decomposers (and Detrivores)
Decomposers serve an extremely important function in all ecosystems; they break down dead biological matter and waste products and convert them into usable energy while returning important materials to the environment. The main decomposers in coral reefs are bacteria; these bacteria play an integral part in the nitrogen cycle whereby ammonia (NH4) is converted into nitrite (NO2) by bacteria in the genus Nitrosomonas, after which nitrite is then converted into nitrate (NO3) by bacteria in the genus Nitrobacter. The ultimate result is that levels of toxic wastes are kept very low and that waste products are converted into components that are available to producers in a readily-usable form. Decomposers are particularly important in coral reef environments considering the heavy bio-load. Detrivores, or scavengers, play a similar role in recycling dead or waste material; sea cucumbers and some species of snails, crabs and bristle worms consume dead organisms and/or decaying plant and animal matter (detritus).
"Decomposers serve an extremely important function in all ecosystems; they break down dead biological matter and waste products and convert them into usable energy while returning important materials to the environment."
Concluding remarks
Coral reefs are complex ecosystems that require a balanced trophic structure to function properly and efficiently. Imbalances can occur in this intricate trophic cascade from the top down or the bottom up. For an example of bottom-up effects, nutrient-rich agricultural run-off can cause a massive increase in primary productivity (e.g. algal blooms), the effects of which often cannot be buffered by consumers fast enough to prevent a coral reef ecosystem from collapsing. A good example is the uprise of the Crown-of-thorns population on the Great Barrier Reef several years ago, which was
linked to high algal blooms by some researchers. High phytoplankton levels may have allowed more Crown-of-thorns' larvae to survive, which had devastating effects on some reefs.
As for a top-down effect, the over fishing of a keystone tertiary consumer like some reef sharks can allow some fish populations to grow so rapidly that their food sources cannot be replenished fast enough to support their growing numbers, resulting in an eventual population collapse. Another example is the overharvesting of another important predator, the Triton snail, which may have contributed to the Crown-of-thorns uprise.
The magnitude of these potential consequences reinforces the fact that we need to be more aware of how our actions can affect the environment in both negative and hopefully positive ways.
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