Over the past 60 years, many amphibian species have experienced significant population declines and some species have become ext
inct. Scientists suspected that local human activities such as the destruction of wetlands, regional pollution, and deforestation were the main reasons for these losses. However, research over the past 20 years reveals significant amphibian population declines in protected areas of the world, such as nature preserves and parks. These global declines suggest widespread problems including increased ultraviolet radiation, acid rain, and disease. In Switzerland, for example, 14 of the 20 native amphibian species are threatened with extinction. Some biologists urge the collection of the few remaining individuals of some of the most threatened amphibian species, to preserve them if they become extinct in the wild. If such captive breeding programs could produce thousands of individuals from just a few of the remaining survivors, the species will still be threatened because of ______.
Genetic drift refers to the change in the allele frequencies in a population due to chance (random) events. The bottleneck effect, also known as population bottleneck, can be defined as a severe case of genetic drift caused by a drastic reduction in the population size. The bottleneck effect is well known to cause the loss of important genetic variation since alleles that were present in the original population are lost, thereby limiting its genetic diversity. One of the most common causes that may lead to a population bottleneck in a natural population is environmental disasters (e.g., earthquakes, floods, etc).
The alteration of a river's flow and sediment transport downstream of a dam often causes the greatest sustained environmental impacts. Life in and around a river evolves and is conditioned on the timing and quantities of river flow. A dam also holds back sediments that would naturally replenish downstream ecosystems.
