Biologists are not completely sure how many different species live on the Earth. Estimates of how many species exist on the Earth range from low of 2 million to high of about 100 million. To date, about 2.1 million species have been classified, primarily in the habitats of the middle latitudes. Most of the unclassified species on this planet are invertebrates. This group of organisms includes insects, spiders, mollusks, sponges, flatworms, starfish, urchins, earthworms, and crustaceans. These species are often difficult to find and identify because of their small size and the fact that they live in habitats that are difficult to explore. In the tropical rain forest, the cataloging of species has been quite limited because of this later reason. Scientists estimate that this single biome may contain 50 to 90% of the Earth's biodiversity.
Many species have gone extinct over the Earth's geologic history. The primary reason for these extinctions is environmental change or biological competition. Since the beginning of the Industrial Revolution, a large number of biologically classified species have gone extinct due to the actions of humans. This includes 83 species of mammals, 113 species of birds, 23 species of amphibians and reptiles, 23 species of fish, about 100 species of invertebrates, and over 350 species of plants. Scientists can only estimate the number of unclassified species that have gone extinct. Using various methods of extrapolation, biologists estimate that in 1991 between 4000 to 50,000 unclassified species became extinct, mainly in the tropics, due to our activities. This rate of extinction is some 1,000 to 10,000 times greater than the natural rate of species extinction (2 - 10 species per year) prior to the appearance of human beings. The continued extinction of species on this planet by human activities is one of the greatest environmental problems facing humankind.
Several times during the Earth's history there have been periods of mass extinctions, when many species became extinct in a relatively short time period (a few million years is a relatively short time when compared to the age of the Earth). Scientists are unsure of the causes of both background extinction and mass extinction. Possible explanations for mass extinctions include climate changes or catastrophes such as the Earth being hit by a meteor. Since the beginning of time, five or six mass extinctions have occurred that eliminated between 35% and 96% of all species on Earth (Table 9h-1 and Figure 9h-1). Further, it is believed that of all species that ever inhabited the Earth over 99% of them are now extinct.
Table 9h-1: Major extinction events during the Phanerozoic.
| Date of the Extinction Event |
Percent |
Species Affected |
| 65 Million Years Ago (Cretaceous) |
85% |
Dinosaurs, plants (except ferns and seed bearing plants), marine vertebrates and invertebrates. Most mammals, birds, turtles, crocodiles, lizards, snakes, and amphibians were unaffected. |
| 213 Million Years Ago (Triassic) |
44% |
Marine vertebrates and invertebrates. |
| 248 Million Years Ago (Permian) |
75-95% |
Marine vertebrates and invertebrates. |
| 380 Million Years Ago (Devonian) |
70% |
Marine invertebrates. |
| 450-440 Million Years Ago (Ordovician) |
50% |
Marine invertebrates. |
| Figure 9h-1: Fossilized clams belonging to the group known as the cryptodonts. Cryptodonts went extinct with the dinosaurs about 65 million years ago. |
Assessment of the number of different organisms that live on this planet is plagued with difficulties. First and foremost, biologists lack a precise definition of what exactly defines a species. The concept of a species often refers to a population of physically similar individuals that can successfully mate between each other, but cannot produce fertile offspring with other organisms. However, many species are composed of a number of distinct populations that can interbreed even though they display physiological and anatomical differences. Scientists developed the notion of biodiversity to overcome some of the difficulties of species concept. To accomplish this task, biodiversity describes the diversity of life at the following three biological levels:
- Genetic Level or Genetic Diversity - Genetic diversity refers to the total number of genetic characteristics expressed and recessed in all of the individuals that comprise a particular species
- Species Level or Species Diversity - Species diversity is the number of different species of living things living in an area. As mentioned above, a species is a group of plants or animals that are similar and able to breed and produce viable offspring under natural conditions.
- Ecosystem Level or Ecosystem Diversity - Ecosystem diversity is the variation of habitats, community types, and abiotic environments present in a given area. An ecosystem consists of all living and non-living things in a given area that interact with one another.
The biodiversity found on Earth today is the product of 3.5 billion years of evolution. In fact, the Earth supports more biodiversity today than in any other period in history. However, much of this biodiversity is now facing the threat of extinction because of the actions of humans.