Issue link: https://wardsworld.wardsci.com/i/1411487
Endangered Species (continued) fewer than 1% of all species that ever existed are present today. Throughout the history of life, there have been certain periods with high extinction rates. For example, many dinosaur spe- cies, along with many other types of species, became extinct within a short time at the end of the Cretaceous Period. These periods of mass extinction have occurred five times. Currently, there is wide agreement among biologists that human activity has created a sixth mass extinction spasm, called the Anthro- pocene extinction, which continues today. According to some estimates, at least one in eight plant species and one in four mammal species are threatened with extinction. The regions with the highest number of extinctions and en- dangered species are those with the greatest species diversity. Species diversity refers to the abundance of species in a given area. In general, larger areas contain greater species diversity, al- though other factors are also relevant. Species diversity tends to follow geographic patterns: there are more species at lower lati- tudes and altitudes. Tropical rainforests in countries such as Brazil and Indonesia harbor most of the world's species. In addition, many unique species live on islands, particularly tropical islands. Island species often exist under special conditions where they have comparatively few competitors and predators. They are re- stricted to a small range and commonly have small populations. These endemic species are particularly susceptible to extinction because their ranges and population sizes are small. Causes of species loss The main factors that cause species to become endangered (and that have led to the current mass extinction spasm) are habitat destruction, invasive species, pollution, and overexploitation. Habitat destruction Habitat destruction is the single greatest threat to species around the globe. Natural habitat includes the breeding sites, nutrients, physical features, and processes such as periodic flooding or periodic fires that species need to survive. Humans have altered, degraded, and destroyed habitat in many differ- ent ways. Logging, for example, has destroyed forests around the world that are habitat to many species. This activity has a great impact in tropical areas, where species diversity is high- est. Although cut forests often regrow, many species depend on old-growth forests that are more than 200 years old; these forests are destroyed much faster than they can regenerate. The threatened northern spotted owl (Fig. 2), which is native to the Pacific northwest of the United States, is a good example of a species that needs to live in old-growth forests for access to the small mammals that they eat and to appropriate nest sites. Agriculture has also resulted in habitat destruction. For exam- ple, in the United States, tallgrass prairies that once were home to a variety of unique species have been almost entirely con- verted to agriculture. Less than 2% of tallgrass prairies remain. Housing development and human settlement have cleared large areas of natural habitat. The construction of roads (which is associated with log- ging, mining, and housing de- velopments) is a major threat to species because animals are killed by vehicles and because they eventually avoid areas with too many roads and too much human activity. Mining has destroyed habitat because making the desired minerals accessible routinely involves altering the landscape by creating huge pits, removing mountaintops, or stripping off the top layer of large swaths of land. Mining procedures for extracting minerals also create pollution: for example, cyanide is used to extract gold. Water development, especially in arid regions, has fundamentally altered habitat for many species. Dams change the flow and temperature of river waters, and they block the movements of species upstream and downstream. Channelization, in which trees and other bank vegetation are removed, results in higher water temperatures and higher rates of flow along waterways. The reallocation of water for human use (usually agriculture) has dried up vegetation along rivers and left many aquatic spe- cies with insufficient water. Invasive species Invasion by nonnative species is another major threat to spe- cies worldwide. Invasive species are ones that, when intro- duced to a new range or environment, will establish themselves and take over space and nutrients from native species. As trans- plants, these invasive species often live in the absence of their natural predators and competitors, and can therefore grow and multiply almost unchecked. Invasive species are especially problematic for island species, which often do not have de- fenses against the new predators or competitors. For example, the brown tree snake was introduced to Guam and nearby is- lands where snakes were unknown. The snake multiplied vastly and ate so many birds and other animals that most of the bird species on those islands became extinct. In Florida, Burmese pythons (predominantly introduced to the wild unintentionally via the pet trade) are preying on many endangered species, in- cluding the wood stork and Key Largo woodrat, as well as other species (for example, white ibis, limpkins, and round-tailed muskrats) that are being watched carefully by conservationists (Fig. 3). Habitat destruction and invasion of nonnative species can be connected in a positive feedback loop: when habitat is + ward ' s science Fig. 2 The northern spotted owl is a threatened species in the Pacific northwest of the United States. (Credit: John and Karen Hollingsworth/U.S. Fish and Wildlife Service)