Loss of Marine Biodiversity

Life in the ocean, where evolution began, comprises more major taxonomic groups (phyla), which represent separate evolutionary paths, than does life on land. Of the approximately 55 phyla, approximately 80% include species that are marine while about 50% include species found on land. It is therefore certain that in the ocean there are more species that are very different from each other, although it is not yet known which realm has the most species.

The Problem

• Because so much of the ocean is only accessible with expensive technology and/or remote instrumentation, uncovering the extent of marine biodiversity has been and continues to be a slow and difficult undertaking. Much of ocean life remains a mystery and there are an unknown number of species yet to be discovered. Consequently, scientific estimates of the number of species in the ocean vary greatly, ranging from many thousands to several tens of millions.
• It is also difficult -- and in many cases impossible -- to determine the status of most species in the ocean. So little is known of many species' distribution or range that it cannot be determined whether they are plentiful or naturally rare or whether populations are stable or changing, and if they are threatened or endangered. Marine species that are relatively easily monitored are those restricted to near-shore habitats, especially if they are sedentary or attached (e.g. seagrasses and corals) and those that spend time at the sea surface or on land (e.g. marine mammals and seabirds).
• Because there is little evidence to the contrary, there has been a common impression that marine species and ecosystems are generally in good shape. However, as more is learned, that impression is turning out to be wildly misconceived. We do not have a clear idea of the full extent of the loss of marine biodiversity over the past 500 years as the level of biodiversity at that time is unknown.
• When populations of a species become depleted, the genetic variation is reduced, which compromises the species' ability to adapt to new environmental changes and stresses. Furthermore, due to interdependencies among species, the demise of one can lead to the decrease or demise of others. Ecosystems become impoverished when species disappear or remain only in insignificant populations. The traditional biological roles of depleted species become seriously compromised, and threaten the integrity and stability of the ecosystem as a whole. Impoverished systems may not contain the species and genetic diversity necessary to enable them to survive major environmental changes and stresses, such as global climate change.
• There is increasing evidence that numerous marine species are, in fact, restricted to relatively small areas, which makes them more vulnerable to depletion or extinction. It was once assumed that this phenomenon must be rare in the sea, since most species swim or rely upon the dispersal of reproductive cells and larvae by moving waters and there are few barriers to their dissemination. Thus, it was reasoned that all marine species should be widespread. In fact many are, but it is now also known that many are not.

The Causes

• The major causes of biological impoverishment and loss of marine biodiversity are: fishing and bycatch; hunting mammals, birds, turtles; toxic chemicals and nutrient pollution; habitat destruction; the human-assisted transport and release of species to environments where they did not previously exist; and possibly, the increased ultra-violet radiation due to ozone layer depletion. Global climate change is predicted to have a major impact in the future.
• Many marine species depend upon broad dissemination during motile life stages, and short-lived species in particular must be replenished by means of this dispersal (a process called recruitment). If dispersal routes or migration are interrupted by lethal environmental conditions, populations and ranges of affected species may be reduced as a consequence.
• Nutrient and toxic chemical pollution are invariably associated widh a reduction in biodiversity. Species that can adapt to or thrive under conditions stressful to most living organisms can dominate the biological community, thus changing the entire nature and function of the ecosystem. This may lead to an even greater loss of species from the system.
• The pressures of fishing have given rise to a new category of species depletion: commercial extinction. Fish and shellfish populations are depleted to the point that it is no longer economically feasible to fish for them. While not extinct, these species are certainly no longer playing their traditional roles in their ecosystems, and some, such as white abalone off the coast of California, have been pushed to the brink of exrinction. Fishing operations, such as trawling and dragging destroy bottom habitats and deplete species populations, and repetition of such activities delays or prevents recovery.
• Coastal habitats, such as estuaries and wetlands, are subject to a number of physical alterations that deplete native species populations. Residential development, tourism, aquaculture, industrial development, and dams all have huge impacts. The rapid increase of coastal human populations exacerbates the situation.

The Context

• Protecting marine biodiversity has not been a regulatory priority for the US. The endangered species approach to biodiversity conservation cannot be expected to effectively protect biodiversity in the ocean, because the status of species often can't be assessed. However, marine protected areas, which protect habitat as well as species, are gaining favor. New areas and regulations are being developed.
• While the human benefits of proposed activities in the marine environment are readily evaluated, the threats - and therefore costs - to the environment are generally unknown or underestimated. Furthermore, living marine resources are given significant value in the market, but are not attributed value in the natural environment. Consequently, perceived benefits invariably outweigh perceived costs, when it comes to exploiting marine resources and environments.

Further Reading

Anderson, P.K.  Marine mammals in the next one hundred years: Twilight for a Pleistocene megafauna?  Journal of Mammalogy 82(3): 623-629, 2001.

Bryden, M.M., O'Connor, S., and Jones, R.  Archaeological evidence for the extinction of a breeding population of elephant seals in Tasmania in prehistoric times.  International Journal of Osteoarchaeology 9(6): 430-437, 1999.

Burney, D.A. and Flannery, T.F.  Fifty millennia of catastrophic extinctions after human contact.  Trends in Ecology and Evolution 20(7): 395-401, 2005.

Carlton, J.T., Geller, J.B., Reaka-Kudla, M.L., and Norse, E.A.  Historical extinctions in the sea.  Annual Review of Ecology and Systematics 30: 515-538, 1999.

Dulvy, N.K., Sadovy, Y., and Reynolds, J.D.  Extinction vulnerability in marine populations.  Fish and Fisheries 4(1): 25-64, 2003.

Edgar, G.J., Samson, C.R., and Barrett, N.S.  Species extinction in the marine environment: Tasmania as a regional example of overlooked losses in biodiversity.  Conservation Biology 19(4): 1294-1300, 2005.

Ferreira, C.E.L., Gasparini, J.L., Carvalho, A., and Floeter, S.R.  A recently extinct parrotfish species from Brazil.  Coral Reefs 24(1): 128, 2005.

Jackson, J.B.C.  Reefs since Columbus.  Coral Reefs 16(Suppl.): S23-S32, 1997.

Lotze, H.K. and Milewski, I.  Two centuries of multiple human impacts and successive changes in a North Atlantic food web.  Ecological Applications 14(5): 1428-1447, 2004.

Lotze, H.K.  Radical changes in the Wadden Sea fauna and flora over the last 2,000 years.  Helgoland Marine Research 59(1): 71-83, 2005.

Lotze, H.K. et al.   Depletion, degradation, and recovery potential of estuaries and coastal seas.  Science 312(5781): 1806-1809, 2006.

Mckinney, R.L.  On predicting biotic homogenization: species-area patterns in marine biota.  Global Ecology and Biogeography Letters 7(4): 297-301, 1998.

Munday, P.L.  Habitat loss, resource specialization, and extinction on coral reefs.  Global Change Biology 10(10): 1642-1647, 2004.

Musick, J. A. et al.  Marine, estuarine, and diadromous fish stocks at risk of extinction in North America (exclusive of Pacific salmonids).  Fisheries 25(11): 6-30, 2000.

Myers, R.A. and Ottensmeyer, C.A.  2005.  Extinction risk in marine species.   Pages 58-79 in: Marine Conservation Biology: The Science of Maintaining the Sea's Biodiversity (E. Norse and L. Crowder, eds.).  Island Press, Washington, DC.

Roberts, C.M.  Our shifting perspectives on the oceans.  Oryx 37(2): 166-177, 2003.

Sadovy, Y. and Cheung, W.L.  Near extinction of a highly fecund fish: the one that nearly got away.  Fish and Fisheries 4(1): 86-99, 2003.

Thrush, S.F. and Dayton, P.K.  Disturbance to marine benthic habitats by trawling and dredging: Implications for marine biodiversity.  Annual Review of Ecology and Systematics 33: 449-473, 2002.

Timm, R.M., Salazar, R.M., and Peterson, A.T.  Historical distribution of the extinct tropical seal, Monachus tropicalis (Carnivora: Phocidae).  Conservation Biology 11(2): 549-551, 1997.