Biodiversity has great value to human society in the form of goods and services. Human activities have resulted in a rapid loss of biodiversity, a decline detectable at genetic, population, species, ecosystem, and global scales. Species extinctions can occur when human and natural events reduce effective population sizes and cause inbreeding depression, genetic drift, and demographic stochasticity. Methods exist for estimating the extinction probability of species that take into account their population sizes, genetic variation, life history traits, and ecology.
learning outcomes
You should be able to:
List major factors causing declines in biodiversity.
Explain the concept of effective population size; use examples to explain how small population sizes can lead to population and species extinctions.
Discuss, with examples, why it is difficult to determine the extinction rate of species.
Define biodiversity loss, and list four reasons for its loss.
Biodiversity loss is the loss of diversity at genetic, population, species, ecosystem, and global scales. The four major causes of biodiversity loss are habitat destruction (reduction in habitat quantity) and degradation (reduction in habitat quality); overharvesting of species; species invasions and emerging diseases; and climate change.
Using the extinction vortex concept as your guide, describe how the heath hen became extinct in 1932.
The heath hen’s population sizes were reduced by hunting until only one very small population remained on the island of Martha’s Vineyard, Massachusetts, in 1830. By 1908 only 50 birds remained. Although the population recovered to several thousand birds once a reserve was established for them, human-
Why is it difficult to determine how many species are extinct or threatened with extinction?
There are multiple reasons why it is hard to determine how many species are extinct or threatened with extinction. First, we do not know how many species live on Earth today because many have not been described. Some of those undescribed species may have already become extinct. Second, many described species are small, reclusive, and rare, and thus hard to keep track of. Third, it is difficult to determine whether a species is truly extinct, especially if it is rare. For example, there have been cases in which species were thought to be extinct but were later discovered to still be extant. Fourth, we rarely know all the interactions among species. It is hard to determine if the loss of one species will lead to the loss of others unless we have detailed ecological knowledge of all the species affected.
We’ve said that human activities have played a major role in the loss of biodiversity. In the next section we will describe the types of human activities that most threaten biodiversity.