Tuesday, March 11, 2014

Overfishing Part 2: The Effects on Species Diversity

In my previous installment, I discussed the impacts of overfishing on an organism’s genetic diversity. When a species is overexploited, its potential to evolve in response  to long-term environmental changes is compromised. As a result, its genetic characteristics may begin to change, and in some cases the population is driven low enough to reduce genetic variability altogether. It is when the latter scenario begins to manifest itself that species diversity begins to suffer as well.

Species richness refers to the number of species per area, and when combined with the pattern of their distribution under normal stresses, we are able to assess species diversity.

Olympia Oysters on the Berkley Marina shoreline in Berkley, CA
Photo: Paul Chinn, The San Francisco Chronicle.
At first glance, overfishing may appear to only encompass overexploitation of certain species. However, in some cases fishing-related activities can actually add species to an existing ecosystem and throw off its natural balance. One popular example of this phenomenon has been observed in the introduction of certain shellfish varieties to marine environments for the purpose of commercial cultivation. In the early 1900s, British Columbia’s shellfish fishery, based on the native Olympia oyster, began to decline. To help boost their production, they introduced the Japanese oyster (Ocenebra japonica). But to their dismay, competition from this non-native oyster-predator actually began exacerbate the decline of the native Olympia oyster. While the native oyster is still extant, scientists no longer consider it ecologically functional, so B.C.’s shellfish industry is now based on the non-native introduced species. 

An important component of ecosystem homeostasis is the preservation of predator, prey, and competitor relationships. If overfishing or its byproducts depress a population to such low levels that a species no longer fulfills its role as predator and/or prey, it can essentially be considered functionally extinct. The cessation of that species has left a hole in the ecosystem’s homeostasis and as a result, predation may become relaxed, or other species may step up to fill their shoes as a dominant figure. This can derail the naturally evolved relationships among species in a community.

 Blackbar Triggerfish (Rhinecanthus aculeatus)
One example of this disruption was observed off the coasts of Haiti, the U.S. Virgin Islands, and Hainan Island in China. Overfishing of wrasse and triggerfish populations, which feed on sea urchins, caused an unchecked acceleration of sea urchin reproduction rates. Because sea urchins are herbivores that feed on algae as their primary food source, their increased numbers over-grazed the areas’ seagrass beds to the point of annihilation. 

Sea creatures are not the only organisms affected by overfishing. As humans, and top predators on this planet, we are witnessing firsthand the decline of species richness when biological communities are changed. When a popular and commercially viable species is overexploited, people are forced to fish for other less marketable species, or even driven out of the fishing business altogether. Species that were once considered “trash” fish have taken the role of their previous overexploited predators, causing fisheries to consider redirecting their fleets. However, fishermen worry that these “trash” fish may not come with sufficient markets for human consumption.

Stay tuned for part three of my series, where I will take a look at ecosystem diversity and wrap up with ways you can help lessen the stresses on our oceans. 

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