Friday, September 28, 2012

Corals versus algae (brought to you by overfishing)

Coral reefs, the rainforests of our oceans, are in dramatic decline due to a number of diverse factors that range from global climate change to run off of nutrients and sediment. This is of major concern as coral reefs are home to a significant portion of marine biodiversity and provide many measurable ecological benefits, such as supporting fish stocks that we eat and providing coastal wave breaks. One of the most significant causes behind this global decline of coral reefs has been overfishing.

Large declines in marine herbivore populations (from sea turtles to fish to urchins) result in a decrease in algae consumption, allowing algae growth to spiral out of control. Furthermore, by harvesting large amounts of fish from our oceans, we restrict the ability of marine ecosystems to balance themselves. For instance, with top predators removed from the equation, smaller fish are not kept in check, allowing their population to spike, deplete their food sources, and crash. Multiple scenarios have the same common result: algae growth is not kept in check and the ecosystem continues to falter.

The impact of algae on coral reefs deserves its own post, and I hope to get to that at some point. For the sake of this article, algae can be seen as the coral’s primary competitor for space, light, and the limited nutrients in the nutrient poor tropical waters. When their growth is left unchecked due to the absence of herbivores, they can overgrow corals and smother them. What’s left behind is not a pretty sight: wastelands of slimy algae covering coral rubble.

I should point out that there is such a thing as alternate stable states in nature, where ecosystems will shift between one stable state (dominated by species A, B, & C; in this case a state dominated by corals) to another (dominated by species D, E, & F; in this case, a state dominated by algae). These stable states can work without causing alarm because they can sustain similar amount of biodiversity. However, in the case of the corals versus algae beds (ABC vs. DEF), it is not even a close competition.

While algae can provide a three dimensional structure for fish to swim in, raise their young, and hide from predators, that’s basically all algae is good for in comparison to the ecological services provided by coral reefs. Coral reefs are the second most productive ecosystems in the world behind rainforests, and one of the most diverse. Additionally, coral reefs live up to their nickname the “rainforests of the sea” because like the rainforests, they support a wealth of biodiversity while existing in very nutrient poor conditions. It’s really amazing how much of an influence corals have on their environment. They are the so crucial to a functioning and efficient ecosystem that they are irreplaceable. Furthermore, coral reefs support fisheries and provide shoreline protection, while algae just sits there selfishly, being less productive and slimy.

In the defense of algae (which was the subject of my Master’s degree), it definitely has its place in the marine environment, where it is a significant primary producer and bottom component of the food web. Algae can be harvested for human consumption, and could be part of the solution to solve world hunger. However, algae does not belong in large quantities along coral reefs, period.

Because humans have had such a significant impact on ecosystems across the planet, we need to do something in order to rectify the situation. What’s at stake is the loss of many diverse lineages of life, which took millions upon millions of years to evolve. Additionally, a lack of ecosystem diversity could allow diseases to cause further damage to the environment.

So what can we do? Well, we can do our best to restore the balance of marine ecosystems by developing and applying comprehensive conservation strategy. A good conservation strategy strikes a balance between passive approaches (e.g. creating National Parks and protected areas) and active strategy (restoring ecosystems through the reintroduction of species, physically rebuilding ecosystem structure, etc.). Unfortunately, many marine protected areas (MPAs) have been poorly managed and have not succeeded in preventing harm within their boundaries. While MPAs will likely be central to most future conservation strategies, they need to be managed with more authority with laws being strictly enforced.

A new idea I came across is certainly controversial, but I think there is a lot of merit to it. This idea, put forth by Briggs [1, 2], proposes to introduce exotic fish species to depauperate coral reefs in an attempt to reinvigorate the ecosystem. This solution may certainly work if local stocks are not available to replenish an ecosystem, which is certainly the case in many areas of the western Atlantic [1]. Briggs [1] argues that there is "no reason for individuals or conservation organizations to maintain that marine invasions constitute a threat to biodiversity." Briggs [1] explains that:
Among conservation organizations, there has developed a kind of militaristic attitude toward invading species, i.e. invaders are bad and must be repelled at all costs. This attitude is somewhat understandable in view of the many terrestrial extinctions that have been caused by invaders, particularly on oceanic islands. Yet, even in those cases, invasions have generally resulted in an overall increase in biodiversity [3]. If the primary conservation goal is to preserve biodiversity, then the presence of exotic species does not appear to be detrimental, and often has a positive effect.
Here's a prime example of how the "invasion" of exotic species lead to an overall increase in biodiversity [1]:
Two invasive, herbivorous siganid (Siganidae) species, Siganus rivulatus and Siganus luridus, comprise a third of the total fish biomass along Israeli rocky shores [4]. Before the arrival of the siganids, the algae entered the food chain by the slow process of decomposition. The siganids grazed on the algae, grew rapidly in numbers, and began to provide the major food source for three species of grouper (Serranidae) [5]. The more rapid turnover of algal resources augmented the production of the large carnivorous fishes and changed the community structure to resemble that of more tropical seas.
This idea would work because the introduction of exotic fish would not be displacing any fish currently in the ecosystem, because, well, there aren't any fish present to be displaced. In order to have a functional ecosystem in which to restore a coral reef ecosystem, it is absolutely pertinent that algae growth is kept in check and substrate is available for coral recruitment. While it is plausible algae growth could be kept in check by volunteers using SCUBA, (re)introducing fish species makes much more sense, as it is an active attempt to reestablish the ecosystem. While reintroducing fish species may be the first step in restoring local coral reefs, but that's a topic for another post.

While restoring coral reefs should be approached on a case by case basis as each restoration site will likely need a tailored approach, it is very likely one of the first and most important steps will be restoring fish populations to the reef. I would like to see some studies investigate the possibility of using exotic species to fill unfilled ecological niches, as some reefs do not have the native populations to fill them themselves. This essay was just a short exercise to discuss a few ideas. I will likely revisit this post and discuss things in more detail, while adding more posts and pages to discuss other factors, issues, and ideas.


[1] Briggs JC. 2007. Marine biogeography and ecology: invasions and introductions. J. Biogeogr. 34:193-198.

[2] Briggs JC. 2009. Atlantic coral reefs: the transplantation alternative. Biol. Invasion. 11:1845–1854

[3] Sax, DF, Gaines, SD & Brown, JH. (2002) Species invasions exceed extinctions on islands worldwide: a comparative study of plants and birds. The American Naturalist. 160:766–783.

[4] Goren, M & Galil, BS. (2005) A review of the changes of the fish assemblages of Levantine inland and marine ecosystems following the introduction of non-native fishes. Journal of Applied Ichthyology. 21:1–7.

[5] Aronov, A. (2002) Comparative study of the ecology of three groupers (Epinephelinae, Serranidae) at the shallow rocky habitats of the Israeli Mediterranean coast. MSc Thesis, Tel-Aviv University, Israel.

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