When there is a threat to biodiversity, what would quickly come to mind is extinction. Extinction is a natural event and, from a geological perspective, routine. We now know that most species that have ever lived have gone extinct.

The average rate over the past 200 million years is 1-2 species per million species present per year. The average duration of a species is 1-10 million years (based on the last 200 million years).

There have also been several episodes of mass extinction, when many taxa representing a wide array of life forms have gone extinct in the same blink of geological time.

In the modern era, due to human actions, species and ecosystems are threatened with destruction to an extent rarely seen in Earth history. Probably only during the handful of mass extinction events have so many species been threatened, in so short a time.

First, we can attribute the loss of species and ecosystems to the accelerating transformation of the Earth by a growing human population. As the human population passes the 6 billion mark, we have transformed, degraded or destroyed roughly half of the world’s forests.

We appropriate roughly half of the world’s net primary productivity for human use. We appropriate most available fresh water, and we harvest virtually all of the available productivity of the oceans. It is little wonder that species are disappearing and ecosystems are being destroyed.

Factors that Affect the Extinction of Biodiversity

The following are some of the factors that affect the extinction of biodiversity;

1. Over-hunting

Over-hunting has been a significant cause of the extinction of hundreds of species and the endangerment of many more, such as whales and many African large mammals.

Most extinctions over the past several hundred years are mainly due to over-harvesting for food, fashion, and profit.

Commercial hunting, both legal and illegal (poaching), is the principal threat. The snowy egret, passenger pigeon, and heath hen are US examples.

At US $16,000 per pound, and US $40,000 to US $100,000 per horn, it is little wonder that some rhino species are down to only a few thousand individuals, with only a slim hope of survival in the wild.

The recent expansion of road networks into previously remote tropical forests enables the bush meat trade, resulting in what some conservationist describe as “empty forests” as more and more wild animals are shot for food.

The pet and decorative plant trade falls within this commercial hunting category, and includes a mix of legal and illegal activities. The annual trade is estimated to be at least US $5 billion, with perhaps one-quarter to one-third of it illegal.

Sport or recreational hunting causes no endangerment of species where it is well regulated, and may help to destroyed, another 100,000 square km degraded).

While there is uncertainty regarding the rate of loss, and what it will be in future, the likelihood is that tropical forests will be reduced to 10-25 percent of their original extent by late twenty-first century.

Habitat fragmentation is a further aspect of habitat loss that often goes unrecognized. The forest, meadow, or other habitat that remains generally is in small, isolated bits rather than in large, intact units.

Each is a tiny island that can at best maintain a very small population. Environmental fluctuations, disease, and other chance factors make such small isolates highly vulnerable to extinction.

Any species that requires a large home range, such as a grizzly bear, will not survive if the area is too small. Finally, we know that small land units are strongly affected by their surroundings, in terms of climate, dispersing species, etc. As a consequence, the ecology of a small isolate may differ from that of a similar ecosystem on a larger scale.

For the future, habitat loss, degradation, and fragmentation combined is the single most important factor in the projected extinction crisis.

2. Invasion of Non-Native Species

Invasion of non-native species is an important and often overlooked cause of extinctions. The African Great Lakes–Victoria, Malawi and Tanganyika–are famous for their great diversity of endemic species, termed “species flocks,” of cichlid fishes.

In Lake Victoria, a single, exotic species, the Nile Perch, has become established and may cause the extinction of most of the native species, by simply eating them all. It was a purposeful introduction for subsistence and sports fishing, and a great disaster.

Of all documented extinctions since 1600, introduced species appear to have played a role in at least half.

The clue is the disproportionate number of species lost from islands: some 93 percent of 30 documented extinctions of species and sub-species of amphibians and reptiles, 93 percent of 176 species and sub-species of land and freshwater birds, but only 27 percent of 114 species and subspecies of mammals.

Why are island species so vulnerable, and why is this evidence of the role of non- indigenous species? Islands are laboratories for evolution.

3. Domino Effects

Domino effects occur when the removal of one species (an extinction event) or the addition of one species (an invasion event) affects the entire biological system.

Domino effects are especially likely when two or more species are highly interdependent, or when the affected species is a “keystone” species, meaning that it has strong connections to many other species.

A keystone species is one whose influence on others is disproportionately great. A seminal study of marine invertebrates in the rocky intertidal region of Washington State found that the top predator, a starfish, facilitated the coexistence of many other invertebrates by selectively consuming mussels, which otherwise would crowd out other organisms.

Thus a keystone species is one whose presence or absence both directly and indirectly influences other species through food web connectivity. Contrary to what some may think, not all species are “keystones”, and it requires careful experimental studies to identify keystone species.

4. Pollution

Pollution from chemical contaminants certainly poses a further threat to species and ecosystems.

While not commonly a cause of extinction, it likely can be for species whose range is extremely small, and threatened by contamination.

Several species of desert pupfish, occurring in small isolated pools in the US Southwest, are examples.

5. Climate change

A changing global climate threatens species and ecosystems. The distribution of species (biogeography) is largely determined by climate, as is the distribution of ecosystems and plant vegetation zones (biomes).

Climate change may simply shift these distributions but, for a number of reasons, plants and animals may not be able to adjust.

The pace of climate change almost certainly will be more rapid than most plants are able to migrate. The presence of roads, cities, and other barriers associated with human presence may provide no opportunity for distributional shifts.

Parks and nature reserves are fixed locations. The climate that characterizes present-day Yellowstone Park will shift several hundred miles northward. The park itself is a fixed location.

For these reasons, some species and ecosystems are likely to be eliminated by climate change. Mountaintop species are especially vulnerable.

The plants and animals found on high mountains of the American West include many remnants of a Pleistocene fauna that long ago was displaced toward the arctic, or upslope.

With further warming, many of these mountaintop species likely will be eliminated.

A changing climate will have many other effects. The southern extent of the Everglades, today the site of the most ambitious and expensive restoration project ever undertaken, may be underwater, along with significant areas of human habitation.

Agricultural production likely will show regional variation in gains and losses, depending upon crops and climate.

Some coral reefs will expand, and others will contract or die off. Ecological changes due to an altered climate are difficult to forecast, but expected to be serious.

As a consequence of these multiple forces, many scientists fear that by end of next century, perhaps 25 percent of existing species will be lost.

In summary, when there is a threat to biodiversity, what would quickly come to mind is extinction. Extinction is said to occur when species and ecosystems are threatened with destruction, to an extent rarely seen in Earth history.

Extinction is a natural event and, from a geological perspective, routine. We now know that most species that have ever lived have gone extinct.

Extinction of biodiversity is facilitated by many factors; Over-hunting has been a significant cause of the extinction of hundreds of species and the endangerment of many more, such as whales and many African large mammals; Habitat loss, degradation, and fragmentation are important causes of known extinctions; Invasion of non-native species is an important and often overlooked cause of extinctions.

The African Great Lakes–Victoria, Malawi and Tanganyika–are famous for their great diversity of endemic species, termed “species flocks,” of cichlid fishes. Invasion of non-native species is an important and often overlooked cause of extinctions.

Domino effects occur when the removal of one species (an extinction event) or the addition of one species (an invasion event) affects the entire biological system. Pollution from chemical contaminants certainly poses a further threat to species and ecosystems.

Biodiversity and Species Extinction