Marine Extinction - four

白鱀豚

John C. Briggs in 2001.

In this article, I argue that the species changes at the K-T boundary were neither sudden nor catastrophic. They were most likely caused by a regression of seal level that led to a decrease in primary production.

The catastrophic extinction under way today as the result of human destruction of the biosphere is far faster and greater than the events of 65 million years ago.

J. C. Briggs ten years later.

Marine extinctions and conservation

Abstract

In contrast to the large number of terrestrial extinctions that have taken place over the past 12,000 years, there have apparently been very few marine extinctions.
But these small losses should not be reason for complacency. During the past 50 years, government supported, commercial fishing has resulted in the collapse of about a thousand populations that once supplied most of the world’s seafood.

For the collapsed species, now existing as small remnants of their former population sizes, the future is bleak.
They suffer from loss of genetic diversity, inbreeding depression, and depensation. Because marine species were eliminated by historic climatic changes, continued global warming is likely to result in the extinction of small populations that already have a precarious existence. They may be considered evidence of an extinction debt that must be paid as the climate change becomes more severe.

For some of the remnant species, extinction can be avoided if there is a rapid management conversion to the use of more marine protected areas (MPAs) and extensive ocean zoning where fishing is prohibited.

From the Introduction

Ceballos et al. (2010), in their article on the sixth extinction crisis, observed that extinctions caused by human activities are thousands of times greater than the background rate.
Indeed, many other articles have also concluded that we are in the midst of the world’s sixth, great mass extinction. Most such articles, in discussing the rate of global extinction, are actually referring to the terrestrial (and freshwater) habitat and not the marine waters that cover 71% of the earth’s surface.

What about the marine environment?
Does the alarming rate of land extinctions also extend to marine species?

From Species at Risk

Although extinctions have been few, this should not detract from the fact that uncontrolled exploitation (overfishing) has drastically reduced most populations of large marine animals from their sizes prior to human exploitation (historic baselines).
Hutchings and Reynolds (2004) examined data from 230 marine fish populations and found a median reduction of 83% in breeding population size from known historic levels. Lotze and Worm (2009) similarly investigated records of 256 exploited populations and found an 89% decline. In the latter study, the groups most severely affected were
(1) diadromous fishes (salmon and other species that migrate to and from freshwater),
(2) ground fishes (large demersal species such as cod and halibut),
(3) reef fishes,
(4) deep sea fishes, and
(5) sea turtles.
All of these groups had suffered declines of 90% or more from their pre-exploitation abundance.

As noted, most of the major global fisheries are severely stressed, but in some areas, the situation is demonstrably worse.
For example, in the western North Atlantic, the populations of almost all the large-sized, predatory fishes have collapsed (Essington et al. 2006). Included are the ground fishes such as the cod, flounder, haddock, and halibut as well as pelagic species such as blue marlin, sailfish, bluefin tuna, and albacore.

All of the fishery crises, demonstrated by a thousand or more collapsed populations that are presently at risk, have occurred with the past 50 years.

Overfishing still continues, often due to scientific advice being ignored by politicians in charge of setting the fishery regulations. But the main drivers are generally identified as an oversupply of fishing vessels and, perhaps more important, government subsidies to fisheries estimated at about $30 billion per year (Pauly 2009).

From Extinction Later?

Of all the reef denizens, the fishes are by far the best known, and, in places such as the Florida Keys or Hawaii, any extinctions occurring during the past 50 years would have been noticed.
Worldwide, only two marine fishes are known to have become extinct (Dulvy et al. 2008): the Galapagos damselfish, Azurina eupalama, last recorded in 1982, and the Mauritius green wrasse, Anampses viridis, missing since 1839. (But please re-read this!)
The New Zealand grayling, Prototroctes oxyrinchus, is also extinct, but it resided primarily in freshwater. Each of the extinct marine species had been found only around a small, oceanic island and was probably represented by a small population. Species with small geographic ranges and low population densities are likely to have an enhanced extinction risk (Purvis et al. 2000).

Aside from the fishes, a few other marine species are known to have become extinct during the past 12,000 years: four mammals, eight birds, four mollusks, and one algae (Dulvyet al. 2008).

When these losses are considered against the total marine species diversity of at least one million, the rate of species extinction has been exceedingly low.

From Conservation

The conservation movement needs to focus on the small populations that are currently at risk.

In the marine habitat, the greatest risk is borne by about a thousand small populations produced by overfishing over the past 50 years.
They have demonstrated a remarkable tenacity in being able to hang on, despite handicaps such as the loss of genetic diversity, depensation (Allee effect), and inbreeding depression.Although some populations that have collapsed may have lost the capability to rebound, those that still consist of many individuals, thousands rather than hundreds, may be able to recover.

There are two conservation alternatives for fishery science and management, one is to continue business as usual and the other is to convert to an ecosystem-based management program (Pauly 2009).
The transformation will require extensive use of ocean zoning, spatial closures, and no-take marine protected areas (MPAs). MPAs presently cover only 0.7% of the world’s oceans, but international agreements have called for at least 10%. If we wish to re-establish functional ecosystems where uncontrolled exploitation has obliterated them, these changes must take place more rapidly than is currently envisioned.
MPAs that are appropriately supervised would benefit most species, while broad no-fishing zones would help the wide-ranging species.

The current pace of global warming indicates that another global extinction is on the horizon.
This means that our small marine populations, remnants of once larger ones, are at risk because, among other handicaps, they may no longer possess the genetic diversity necessary to cope with environmental change, i.e., they represent an extinction debt that has been built up during the past 50 years.

We cannot afford to sit back and lose several hundred of the large fish species that provided the bulk of our seafood only 50 years ago.
In addition, there is now a myriad of smaller fish and invertebrate species that have also been overfished and need to be restored to the food web.

Conclusions

Marine species extinctions are spasmodic, occurring primarily in response to drastic environmental changes.
Over the long run, most extinctions have taken place in the temperate zones, probably because these areas have been subjected to more climatic changes than the tropics. But there is also contemporary evidence that extinction patterns have originated in the high diversity center of the tropics, even though the final elimination of the species concerned may eventually take place in peripheral areas.

After a quiescent period of 1.5 million years, global warming has taken the world to the edge of another precipitous extinction episode.
In the marine environment, there are a thousand species or more that overfishing has reduced to small remnants of their former population sizes. These species are already at risk because of the handicaps of small populations and that risk will greatly increase with the environmental change that lies ahead. As things now stand, they represent an extinction debt that almost certainly will be paid as warming proceeds.

But many of them can still be saved if management of marine fisheries is shifted to an ecological basis.
MPAs must be at the core of a new management philosophy, but at present, they cover only 0.7% of the world’s oceans when they should cover at least 10%. But MPAs cannot solve the entire problem because some species, especially those that are wide-ranging, will need to be protected by broad ocean zones where fishing is prohibited.

Amen to that, brother!