Causes of extinction have prehistorically been dominated by natural earth processes such as geological transformation of the Earth's crust and major climatic oscillations, as well as species interactions; however, since the ascent of modern man during the Holocene, the causes of extinction have been dominated by the activities of humans. Rates of species extinction have increased rapidly since the early Holocene epoch, chiefly due to activities of humans; further acceleration of extinction rates began approximately 1600 AD, with the onset of accelerated human population growth and expanded scope of agriculture. Natural causes of extinction are regarded as being an irrelevantly small fraction of present extinction events, but are important to understand for historical and academic context. Darwin was the first to fully articulate the concepts of speciation and extinction as applied to natural succession, although he never used the terms evolution or extinction. (Darwin. 1859) The primary cause of human-induced extinction events is simply human overpopulation of planet Earth. The most important causal anthropogenic activities are habitat destruction, overexploitation, pollution and the introduction of alien species to an environment. Habitat destruction elements include agricultural land conversion, deforestation, overgrazing and urbanization; within these activities the process of habitat fragmentation is a sometimes hidden cause of major biodiversity loss. Overexploitation consists of intensive mineral and other geological resource extraction, overharvesting of wild flora and fauna (mainly for human food), hunting or fishing threatened fauna and killing of threatened fauna for herbal or cultural extracts. Pollution impacts include buildup of toxic atmospheric substances, discharge of water pollutants into natural water reserves, chemical contamination of soils and noise pollution. Introduction of alien species is usually an unintended activity where seeds, stowaway fauna aboard ships and other viably reproducing biota are transported by man to a new environment which has insufficient resident predators (or predators unfamiliar with, and therefore naive to the new prey) to control the invading taxon, or exotic predators inadvertently (or intentionally) introduced to a new region, where the native fauna are often unable to recognise the invading organism as a threat.
Aerial photograph of dry forest scrub in southern Zambia,
fragmented by agricultural land conversion. 2008
See main article: Habitat fragmentation
Habitat destruction is the greatest contributor to the extinction of many species; moreover, impacts to biota from habitat fragmentation is a critical mechanism of driving species to extinction. This destruction is ongoing in both terrestrial and aquatic biomes, with approximately 80% of all extinctions being attributed to human caused habitat destruction. Terrestrially the destruction includes forests, deserts and grasslands, but the latter may account for the greatest losses due to the attractiveness to humans of agricultural and urban conversion. In fact, it is agricultural rather than urban loss that is the greater cause for species extinctions. (Ehrlich and Ehrlich. 1981) Most of the habitat reduction is due to conversion of natural habitat to agricultural or logging uses, chiefly driven by the expanding human population. The effects of habitat destruction are especially prevalent in areas of the world with a formerly rich biodiversity that are being converted into land to be utilized commercially or agriculturally. For example, rainforests are being destroyed chiefly to expand agricultural production to feed an undernourished human population; such massive deforestation destroys and fragments the habitat for innumerable species. Such agricultural land conversion is amplified because many previously productive tracts such as the North China Plain have suffered great reductions in crop yield due to unsustainable agricultural practices. In terms of the marine environment, water pollution and trawling (a form of fishing in which a net weighted by anchors is dragged across the ocean floor) have contributed significantly to habitat destruction; marine ocean pollution consists not only of introduced chemicals, but also thermal pollution and excess turbidity, the latter of which leads to sedimentation and destruction to coral reefs among other marine biotic impacts. .
The theory of habitat destruction area holds that the number of species lost in an island type environment varies with the fourth root of the area of land disturbed. As E.O. Wilson points out that islands which have experienced near total habitat destruction follow this law faithfully; namely the number of reptile and amphibian species lost as a function of habitat loss include the following Caribbean examples: the small island of Saba (five square miles and ten species lost); Montserrat 33 square miles and 25 species lost; Puerto Rico (3435 square miles and 40 species lost); and Cuba (44,164 square miles and 100 species lost). (Wilson. 2005)
Modern biotic extracts store in Hong Kong, China.
Certain of these products represent trade in organisms
that are endangered species. Source: C. Michael Hogan
Since prehistoric times, humans have used the earth’s resources to enrich their own lives. However, there is a point when the resources are being overexploited, and this exploitation begins to threaten the existence of other species. Overexploitation presents itself in many forms: exhausting a species as a supply of food or hunting a species for trophies, clothing, medicine or souvenir. In the aquatic biomes, overfishing is a woldwide manifestation of over-exploitation. In the case of terrestrial ecosystems, overgrazing and intensive cropping systems are the chief elements of over-exploitation. Hunting for trophy or medicinal extracts comprises a smaller biomass destruction, but is specifically targeted at some of the most threatened fauna of the planet. These practices are generally overtly mercenary, rather than being motivated by subsistence or hunger, as most ot the farming exploitation. For example, tigers have been an integral part of traditional Chinese medicine for over 1000 years and as such, they have been hunted to the brink of extinction as a product of the lucrative trade in tiger body parts.
Pollution is the introduction of potentially harmful chemical or physical constituents into the environment, which substances substantially harm individual species metabolisms, or which strongly and rapidly alter a stable historic ecosystem composition. This introduction usually enters the atmosphere, soil or natural water systems of the Earth. Chemical pollutants may interfere with metabolic functions, causing functional impairment or death of organisms. Reduction in species numbers anywhere within a given food chain, of course, have ramifications to other members of the ecosystem. Pollution is often a contributing factor along with habitat degradation in extinction processes. Widespread air pollutants are sulfur dioxide, carbon monoxide, and oxides of nitrogen. Water and soil pollutants of concern are heavy metals and a large category of pesticide and herbicide compounds. Pollutants uniquely associated with surface waters are thermal sources (generated from power plants and other industrial facilities) and silt (arising from intensive agriculture and urban runoff. Widespread, but less pernicious pollutants, are noise pollution and light pollution.
There are cases, particularly involving nutrient introductions of phosphate or nitrate compounds, where water pollutant additions may promote the metabolism of a selective number of component species (i.g. algae blooms), but impair or alter the fundamental dynamics of the ecosystem as a whole. Thus in the case of human accelerated lake eutropication, massive short term gains in bioproductivity of algae is derived at the expense of fish and amphibian species as well as benthic and lower water column biota, which species experience light level reduction and other chemical imbalances induced by algal blooms.
Introduction of alien species
The Signal crayfish has been introduced into
vast areas of Europe and Asia, where it has decimated
many native species. Source: C.Michael Hogan
See main article: Alien species
The introduction of alien species to a new environment can have major dissociative effects to an entire ecosystem and be a key driver in species extinctions. Moreover, the introduction of a foreign species to a new habitat can cause a number of distinct and pronounced adverse ecological impacts. For example, the introduced species could be a predator of certain of the resident species. In particular, the introduction of rats, weasels and other mammalian predators (Hogan. 2009) to most of New Zealand has been the instrument of decimation for the kiwi species and numerous other birds, who evolved in the absence of terrestrial mammalian predators, and thus who are unarmed with defensive strategies or adaptations. The predation of the local species may diminish prey populations and potentially drive certain local populations of prey below their minimum viable population size Similarly, the introduced species could be a competitor with the existing species within the given habitat. Limited resources are always a constraint, and when a new taxon arrives, certain limited resources may become prominent constraints to ecosystem function.In certain cases faunal species have been introduced to increase agricultural or fishery productivity.
The Signal Crayfish, Pacifastacus leniusculus, an aquatic feeding machine has been unleashed in western Europe and many parts of Asia, under the guise of increasing the crayfish harvest beyond that available from native crayfish biomass productivity. The experiment was successful from a food production perspective; however, the outcome has been an ecological disaster from the standpoint of numerous prey species, whose populations have been severely degraded by the presence of the aggressive Signal Crayfish. (Nystrom. 1999)
Disease and parasitism
The phenomena of disease and parasitism often weaken organisms and interfere with metabolic function. As a result, faunal species can suffer from the ability to find food, find and attract a mate, seek shelter, migrate, or engage in fertile outcome in the breeding process. The deficiencies in these basic functions may lead to premature deaths of considerable numbers of the population or in the reduced fecundity and birth rates. Correspondingly bacterial infections, viral introductions and infection by higher level parasites may be a contributory cause to species extinction.
In the case of flora, diseases and parasitism can also interfere with metabolic processes, although there are countless examples of symbioses, which manifest morphological changes to the plant, but do not significantly alter plant development. In cases where metabolic functions are altered severely, the plant height may be compromised, prohibiting many individuals to attain heights to gather sufficient light. Correspondingly there may be interference with leaf, stem or root development, such that the roles of these structures are insufficient to allow the plant to reach maturity and optimum seed development. In other cases the disease or parasite may compromise the reproductive and seed dispersal vigor, such that the overt population dynamics are affected.
Often, disease and parasitism are synergistic causes of extinction, as in the case of the Hawaiian Islands, where human-caused habitat destruction, habitat fragmentation, human introduction of alien species and human introduction of diseases have combined to produce a pernicious cocktail for eradication of most of the indigenous species of these islands.
Small geyser pool at Yellowstone NP, USA
showing a cryptic habitat inhabited by extremophile
micro-organisms. Source: C. Michael Hogan
A special problem arises in the case of cryptic habitats, defined as portions of an ecosystem that are largely hidden from normal investigative view of scientists, or which have components that are not readily discoverable with state of the art research techniques. In these cases substantial habitat degradation and associated biodiversity loss may have occurred before researchers have intercepted the ecological data substantiating the loss. Cryptic habitats represent a very large reservoir of the Earth's biota, since many of these are micro-habitats such as reef cavities and burrows, where vast numbers of species inhabit very small enclosed and virtually hidden spaces. (Flugel. 2004) Deep ocean environments represent other massive extents of the planet, which environments are not only cryptic habitats, but also effectively an unexplored domain. Arthropods and micro-organisms represent a vast number of the species within cryptic habitats, since many of these species require special tools of observation or capture techniques (Jamieson and Campbell. 1998) to invade the realm of their environment.; these aspects of the worlds ecosystems are quite noteworthy, since arthropods as a class make up the majority of faunal biomass of the Earth. Furthermore, these minute species are over-represented in the organisms which are currently undescribed. Hence, these are the very species that are currently passing into extinction prior to their recording. (Wilson. 2005)
- Charles Darwin. 1859. The Origin of Species. reprinted by Doubleday Books, Garden City, New Jersey, USA
- Paul R. Ehrlich and Anne H. Ehrlich. 1981. Extinction: The causes and consequences of the disappearance of species. Random House, New York, New York, USA
- E.O. Wilson. 2005. The Future of Life. Alfred A. Knopf. New York, New York, USA
- Laura F. Landweber and Andrew P. Dobson (eds.) 1999. Genetics and the Extinction of Species. Princeton University Press. Princeton, New Jersey, USA
- C. Michael Hogan. 2009. GlobalTwitcher.com ed. N. Stromberg New Zealand Robin: Petroica australis
- P.Nystrom. 1999. Ecological impact of introduced and native crayfish on freshwater communities: European perspectives. In Gherardi, F. and Holdich, D.M. (eds.) Crustacean Issues 11: Crayfish in Europe as Alien Species (How to make the best of a bad situation?) A.A. Balkema, Rotterdam, Netherlands: 63-85
- Erik Flugel. 2004. Microfacies of carbonate rocks: analysis, interpretation and application. Springer Pubishers. 976 pages
- G.S. Jamieson and Alan Campbell. 1998. Proceedings of the North Pacific Symposium on Invertebrate Stock Assessment. National Research Council Canada. 462 pages