47.1A: Loss of Biodiversity

Learning Objectives

• Explain the biodiversity crisis

The Biodiversity Crisis

Traditionally, ecologists have measured biodiversity, a general term for the variety of species present in the biosphere, by taking into account both the number of species and their commonness. Biodiversity can be estimated at a number of levels of organization of living things. These estimation indexes, which came from information theory, are most useful as a first step in quantifying biodiversity between and within ecosystems, yet they are less useful when the main concern among conservation biologists is simply the loss of biodiversity. However, biologists recognize that measures of biodiversity, in terms of species diversity, may help focus efforts to preserve the biologically or technologically important elements of biodiversity.

Cichlids in Lake Victoria

The Lake Victoria cichlids provide an example through which we can begin to understand biodiversity. The biologists studying cichlids in the 1980s discovered hundreds of cichlid species representing a variety of specializations to particular habitat types and specific feeding strategies: eating plankton floating in the water, scraping and then eating algae from rocks, eating insect larvae from the bottom, and eating the eggs of other species of cichlid. The cichlids of Lake Victoria are the product of an adaptive radiation. An adaptive radiation is a rapid (less than three million years in the case of the Lake Victoria cichlids) branching through speciation of a phylogenetic tree into many closely-related species; typically, the species “radiate” into different habitats and niches. The Galápagos finches are an example of a modest adaptive radiation with 15 species. The cichlids of Lake Victoria are an example of a spectacular adaptive radiation that includes about 500 species.

At the time biologists were making this discovery, some species began to quickly disappear. A culprit in these declines was a species of large fish that was introduced to Lake Victoria by fisheries to feed the people living around the lake. The Nile perch was introduced in 1963, but was not a problem until the 1980s when its population began to surge by consuming cichlids, driving species after species to the point of extinction (the disappearance of a species). In fact, there were several factors that played a role in the extinction of perhaps 200 cichlid species in Lake Victoria. These factors included not only the Nile perch, but also the declining lake water quality due to agriculture and land clearing on the shores of Lake Victoria, and increased fishing pressure. Scientists had not even cataloged all of the species present, so many were lost that they were never named. The diversity is now a shadow of what it once was.

Figure \(\PageIndex{1}\): Lake Victoria and biodiversity loss: Lake Victoria in Africa, shown in this satellite image, was the site of one of the most extraordinary evolutionary findings on the planet, as well as a casualty of devastating biodiversity loss.

Causes of Biodiversity

The cichlids of Lake Victoria are a thumbnail sketch of contemporary rapid species loss caused by human activity occurring all over earth. Extinction, a natural process of macroevolution, occurs at the rate of about one out of 1 million species becoming extinct per year. The fossil record reveals that there have been five periods of mass extinction in history with much higher rates of species loss. The rate of species loss today is comparable to those periods of mass extinction. However, there is a major difference between the previous mass extinctions and the current extinction we are experiencing: human activity. Specifically, three human activities have a major impact: destruction of habitat, introduction of exotic invasive species, and over-harvesting. Predictions of species loss within the next century, a tiny amount of time on geological timescales, range from 10 percent to 50 percent. The five previous extinctions on this scale were caused by cataclysmic events that changed the course of the history of life in each instance. Earth is now in one of those times.

Key Points

• Biodiversity is the variety of species present in the biosphere; the main goal of conservationists is to preserve biodiversity.
• An example of biodiversity loss was the extinction of over 200 species of cichlids in Lake Victoria; this was caused by the introduction of the Nile Perch as well as increased agriculture and fishing.
• Unlike the five previous mass extinctions, the current one is a result of detrimental human activities.
• Current human-caused biodiversity loss is the result activities such as habitat destruction, the introduction of exotic invasive species, and the over-harvesting of species.

Key Terms

• biodiversity: the diversity (number and variety of species) of plant and animal life within a region
• adaptive radiation: the diversification of species into separate forms that each adapt to occupy a specific environmental niche
• invasive species: any species that has been introduced to an environment where it is not native and has since become a nuisance through rapid spread and increase in numbers, often to the detriment of native species
• extinction: disappearance of a species from earth; local extinction is the disappearance of a species from a region