Threats To Global Biodiversity

The worst thing that can happen during the 1980s is not energy depletion, economic collapse, limited nuclear war, or conquest by a totalitarian government. As terrible as these catastrophes would be for us, they can be repaired within a few generations. The one process ongoing in the 1980s that will take millions of years to correct is the loss of genetic and species diversity by the destruction of natural habitats. This is the folly that our descendents are least likely to forgive us.

- E.O. Wilson, 1985
01/04/2006 Format for printing

We wish to learn:

  • What is biodiversity?
  • What are the threats to biodiversity?
  • How can we estimate rates of species loss?
  • What can we do about this issue?
Jump to: [What is Biodiversity?] [What are the Threats?] [Rates of Species Loss[Summary][Suggested Readings]
1. What is Biodiversity?
What is biological diversity ? In its narrowest sense this term refers to the number of species on the planet, and it also is used more broadly as an umbrella term.

Biological diversity refers to the variety and variability among living organisms and the ecological complexes in which they occur. Diversity can be defined as the number of different items and their relative frequency. For biological diversity, these items are organized at many levels, ranging from complete ecosystems to the chemical structures that are the molecular basis of heredity. Thus, the term encompasses different ecosystems, species, genes, and their relative abundance (Office of Technology Assessment, 1987). Or to paraphrase: number and variety of species, ecological systems, and the genetic variability they contain.

The simplest representation is:

Group Number of Described Species
Bacteria and blue-green algae  4,760 
Fungi  46,983
Algae 26,900
Bryophytes (mosses and liverworts)  17,000 
Gymnosperms (conifers)  750
Angiosperms (flowering plants)  250,000 
Protozoans  30,800 
Sponges  5,000 
Corals and Jellyfish  9,000 
Roundworms and earthworms  24,000
Crustaceans  38,000
Insects  751,000 
Other Arthropods and minor invertebrates  132,461
Mollusks 50,000 
Starfish  6,100 
Fishes (teleosts)  19,056 
Amphibians  4,184
Reptiles 6,300 
Birds  9,198 
Mammals  4,170



Three pie charts (number of species of all kinds, number of animal species, number of plant species) show the same information diagramatically.

These numbers reflect a huge amount of scientific study. In order for a species to be included on the above list, it was examined by a specialist, who carefully compared its features to those of all similar species. Then, once satisfied that the species in hand was different from all known forms, the specialist published a thorough description including drawings, counts of hairs along forelegs, length of measurable parts such as limbs and antennae, photographs, etc. The new species has then been described, and is added to our list of "known" species.

Remarkably, our estimates of the number of unknown species greatly exceed our count of the number of known species. Most experts estimate the world's species diversity at 10 to 30 million, but that is very approximate. Only 1.4 million species are "known to science" -- meaning that they have been classified by a specialist. The estimates of 10 to 30 million species are based on expert opinion of how many species are yet to be formally identified. One study of insects in the forest canopy found 5 out of 6 to be new species. Even vertebrates are not completely known -- it is estimated that nearly half of the freshwater fishes of South America are undescribed. New finds are made continuously in the tropics, and exploration of deep-sea hydrothermal vents recently led to the discovery not just of new species, but of new life forms at the family level (20 families or sub-families). When you consider that virtually every species has its own parasite, and how many groups such as nematodes and bacteria have yet to be well-studied, it is apparent that the estimates of 10 to 30 million are not out of line.

The global distribution of biodiversity -- its geography -- is interesting in its own right, and relevant to conservation. Biological diversity is greatest near the equator, and declines towards higher latitudes . Tropical rain forests are especially known for their exceptional diversity. Some locations known as "hotspots" harbour an unusually rich local diversity, perhaps because conditions favour evolutionary diversification.

2. What are the Threats to Biodiversity?

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 my is 1-2 species per year, and 3-4 families per my. The average duration of a species is 2-10 million years (based on last 200 million years). There have also been occasional episodes of mass extinction, when many taxa representing a wide array of lifeforms have gone extinct in the same blink of geological time. [see last Fall's lecture on the Emergence Of Complex Life]

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.

What are these human actions? There are many ways to conceive of these - let's consider two.

Source: World Conservation Monitoring Centre, "Global Biodiversity" Chapman & Hall, London, 1992).

First, we can attribute the loss of species and ecosystems to the accelerating transformation of the earth by a growing human population (GCII). As the human population passes the six billion mark , we have transformed, degraded or destroyed roughly half of the word's forests (GCII). We appropriate roughly half of the world's net primary productivity for human use (GCII). We appropriate most available fresh water (GCII), and we harvest virtually all of the available productivity of the oceans (GCII). It is little wonder that species are disappearing and ecosystems are being destroyed.

Second, we can examine six specific types of human actions that threaten species and ecosystems - the "sinister sextet"

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

Passenger Pigeons
Commercial hunting, both legal and illegal (poaching), is the principal threat. Snowy egret, passenger pigeon, heath hen are USA examples. At $16,000 per pound, and $40,000 to $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 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 $5 billion, with perhaps 1/4 to 1/3 of it illegal.

Sport or recreational hunting causes no endangerment of species where it is well regulated, and may help to bring back a species from the edge of extinction. Many wildlife managers view sport hunting as the principal basis for protection of wildlife.

While over-hunting, particularly illegal poaching, remains a serious threat to certain species, for the future, it is less important than other factors mentioned next.

Habitat loss/degradation/fragmentation is an important cause of known extinctions. As deforestation proceeds in tropical forests, this promises to become THE cause of mass extinctions caused by human activity.

All species have specific food and habitat needs. The more specific these needs and localized the habitat, the greater the vulnerability of species to loss of habitat to agricultural land, livestock, roads and cities. In the future, the only species that survive are likely to be those whose habitats are highly protected, or whose habitat corresponds to the degraded state associated with human activity (human commensals).

Habitat damage, especially the conversion of forested land to agriculture (and, often, subsequent abandonment as marginal land), has a long human history. It began in China about 4,000 years ago, was largely completed in Europe by about 400 years ago, and swept across USA over the past 200 years or so. Viewed in this historical context, we are now mopping up the last forests of Pacific Northwest.

In the new world tropics, lowland, seasonal, deciduous forest began to disappear after 1500 with Spanish and Portuguese colonization of the New World. These were the forested regions most easily converted to agriculture, and with a more welcoming climate. The more forbidding, tropical humid forests came under attack mainly in 20th C, under the combined influences of population growth, inequitable land and income distribution, and development policies that targeted rain forests as the new frontier to colonize.

Tropical forests are so important because they harbor at least 50%, and perhaps more, of world's biodiversity. Direct observations, reinforced by satellite data, documents that these forests are declining. The original extent of tropical rain forests was 15 million km2. Now there remains about 7.5-8 million km2, so half is gone. The current rate of loss is estimated at near 2% annually (100,000 km2 destroyed, another 100,000 km2 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% of their original extent by late 21st C. 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 mai ntain 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.

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% of 30 documented extinctions of species and sub-species of amphibians and reptiles, 93% of 176 species and sub-species of land and freshwater birds, but only 27% 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 ( 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 inter-dependent, or when the affected species is a "keystone" species, meaning that it has strong connections to many other species (GCI).

The seeds of the tree Calvaria major, now found exclusively on the island of Mauritius, must pass through the abrasive gut of a large animal in order to germinate. Their tough seed coats are protection against digestion, but also a kind of living coffin, for the seed can not germinate unless abraded. None of the animals currently on Mauritius have that ability. The dodo (a 25 kg pigeon), hunted to extinction in the late 17th century, probably was the key to recruitment in this species. Some seeds, abraded, roughened, and excreted by dodos, germinated and grew. Today, no seeds germinate, and only a few very old trees now survive. The blackfooted ferret was once very abundant in the western prairies. It preyed upon prairie dogs and used their burrows to nest in. Poisoning of prairie dogs has greatly reduced their abundance, and the blackfooted ferret is now the rarest mammal in North America

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. 

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) [GCI]. 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. Agricultural production likely will show regional variation in gains and losses, depending upon crop and climate.

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

3. How Can We Estimate Rates of Species Loss?

The Number of species living on islands increases or decreases with the area of the island. The diversity of reptiles and amphibians in the West Indies is depicted here. A reduction of 90 percent in area from one island to the next results in a 50 percent loss of species.

Estimates of current and future extinction rates are based on well-documented relationships between the number of species in a region and habitat area, and on reasonably well-known rates of habitat loss. We must also employ some ratio to approximate the total number of species (described and undescribed), from the number of described species.

The relationship between species (S) and area (A) is described by the equation:

S = c A z

where z is the slope of the log-linear relationship, and c is a constant which described the height of the line. Based on censuses of species on islands, the number of species found on an island increases log-linearly with island area. Conversely, as island (or habitat area) is reduced, so is the number of species that will be found there. The slope (z) usually varies between 0.15 to 0.35. When combined with current rates of loss of tropical forest (this calculation uses 1.8% per year), these values of the slope translate into species extinction rates of roughly 0.5% annually. Extrapolated to the year 2020, roughly 20% of remaining species will disappear. Simply using the most conservative values of the slope, and assuming the true biodiversity of tropical forests is roughly 10 million species, the projected rate of loss of species is 27,000 per year, and three during this hour.

4. Summary

  • Biodiversity refers to the number and variety of species, of ecosystems, and of the genetic variation contained within species.
  • Roughly 1.4 million species are known to science, but because many species are undescribed, an estimated 10-30 million species likely exists at present.
  • Biodiversity is threatened by the sum of all human activities. It is useful to group threats into the categories of over-hunting, habitat destruction, invasion of non-native species, domino effects, pollution, and climate change.
  • Habitat loss presents the single greatest threat to world biodiversity, and the magnitude of this threat can be approximated from species-area curves and rates of habitat loss. The spread of non-native species threatens many local species with extinction, and pushes the world's biota toward a more homogeneous and widely distributed sub-set of survivors. Climate change threatens to force species and ecosystems to migrate toward higher latitudes, with no guarantee of suitable habitat or access routes. These three factors thus are of special concern.

Suggested Readings

  • Wilson, E.O. 1992. The Diversity of Life. W.W. Norton & Co., New York

  • Quammen, David. 1997. The Song of the Dodo: Island Biogeography in an Age of Extinctions. Simon & Schuster, New York.


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