The reason humans have had such a massive negative impact on species and biodiversity throughout history and in the present day is due to the ever-increasing use of the world’s natural resources by the exponentially expanding human population. Human population has grown from 1 billion in 1850 to 6 billion in 1998 and is predicted to reach 10 billion by the year 2050 (Primack, 2000). The physical environmental changes that have occurred as a result of this massive population growth are what drive the current biodiversity crisis. Humans have transformed, degraded or destroyed roughly half of the word's forests; appropriated roughly half of the world's net primary productivity and most available fresh water; and harvested virtually all of the available productivity of the oceans (McNeill, 2000). Such massive disturbances have altered, degraded and destroyed the landscape on a vast scale, driving species and even whole communities to the point of extinction. Four primary causes can be identified for such human-induced biodiversity loss: habitat destruction, over-hunting, pollution, and invasion of non-native species.
By far the most pervasive form of human-driven environmental change affecting organisms is habitat loss and fragmentation (Hunter, 1996). Habitat loss leads to a number of cascading effects on ecosystems and their components. Certain organisms are killed outright during land conversion. Some individuals, populations, and even entire species (especially those with narrow geographic ranges), will not be capable of surviving after the physical environment has been altered from its natural state. Finally, organisms not directly affected by a given habitat loss, but that are dependent upon affected organisms in the trophic web, will be impacted as well. Overall, the effects of a decrease in population numbers and a loss of genetic variation leads to a subsequent loss of population viability, and ultimately may lead to extinction. In addition to land being destroyed outright, habitats that formerly occupied large areas are often divided into small pieces by roads, fields, towns and other human constructs. This is known as habitat fragmentation and, like habitat destruction, it will threaten the persistence of species and reduce biodiversity in an area. This can be done by limiting species’ potential for dispersal and colonisation; reducing the foraging ability of native animals; or by dividing populations into smaller, more vulnerable communities (Primack, 2000).
Several human activities contribute to habitat destruction and fragmentation, including: logging, conversion of landscapes to agricultural land, grazing in some instances, mining, urban development, roads, infrastructure, and water diversion. These developmental activities can all be considered to be proximate causes for habitat destruction and biodiversity loss due to habitat destruction. The ultimate root causes driving the habitat destruction can be considered to be threefold: an increase in human population levels, an increase in human per capita consumption, and a decrease in the efficiency of use of resources (Wood, 2000).
Many believe the greatest threat to our global biodiversity lies in the Tropical rainforests of the world. It is thought these forests harbour as much as 50% of the world’s biodiversity. Direct observations, reinforced by satellite data, documents that the forests are declining, some say at an alarming rate. There is great uncertainty over the rate of loss, but it has been estimated that up to 140,000km² of rainforest is being lost per year. It is thought that roughly 61% of this destruction results from small-scale cultivation of crops by poor farmers, 21% results from commercial clear-cutting and selective logging operations, 11% is land cleared for cattle ranches and the remaining 7% is land cleared for cash-crop plantations and other human activities such as road building and mining (Primack, 2000).
The relative impact of habitat destruction and fragmentation upon biodiversity can be assessed by the Theory of Island Biogeography proposed by MacArthur and Wilson in 1967. According to this model, the number of species on an island or habitat fragment is dependent upon the rate of immigration and the rate of extinction. It also focuses on the species-area relationship, showing that large areas will have more species than small ones. Thus the model can be used to predict the number and percentage of species that would become extinct if habitats were destroyed. Applying this model to a habitat island, it has been estimated that if 50% of the habitat were destroyed, 10% of species would be eliminated. Thus when 90% of the habitat is destroyed, 50% of species will be lost. Using such evidence, Cowlishaw (1999) predicts that 30% of forest primates in African countries will eventually go extinct due to habitat loss (cited by Primack, 2000). However predictions of extinction rates based on habitat loss are problematic and will vary considerably. This is due to a number of factors: each group of species and each geographical area forms characteristic species-area relationships; differences in estimated rates for causal factors such as deforestation will change predicted extinction rates; it is very hard to provide accurate, foolproof estimates for habitat loss, especially with such large and complex areas as tropical rainforests.
There are, of course, other secondary types of human disturbances which pose additional threats to biodiversity. Over-hunting by humans throughout history has been a significant cause of the extinction of hundreds of species and the endangerment of many more, such as whales and many African large mammals. Most extinctions over past several hundred years are mainly due to over-harvesting for food, fashion, and profit. Commercial hunting, both legal and illegal (poaching), is the principal threat in the present time. As human populations have increased, their use of the environment has escalated, and the rise of technology has allowed for such over-exploitation to be done on a larger, faster scale than ever before. One of the most well known examples of over-hunting of wild animals has been whaling. Whaling activities in the Southern Oceans after 1904 hunted populations of blue, fin and minke whales to dangerously low levels, threatening the species with extinction (McNeill, 2000). In 1986 the International Whaling Commission finally banned all commercial whaling, however illegal poaching still continues to this day.
Pollution from chemical contaminants poses a further threat to species and ecosystems and thus to global biodiversity. While not commonly a cause of extinction, it likely can be for species whose range is extremely small, and threatened by contamination. The most common causes of environmental pollution are pesticides, chemicals and sewage released by industries and human settlements, emissions from factories and automobiles and sediment deposits from eroded hillsides (Primack, 2000). Such pollution can affect water quality, air quality and global climate in a detrimental way which can pose a large threat to biological diversity. One of the most commonly talked about effects of pollution through increased human activity is recent change to global climate in the form of global warming. This is occurring as a result of increased worldwide emissions of greenhouse gases which act as a blanket round the earth’s atmosphere and trap more heat near the earth’s surface. Such a changing global climate may threaten species and ecosystems. The distribution of species (biogeography) is largely determined by climate, as is the distribution of ecosystems and plant vegetation zones (biomes). Global warming may shift these distributions but, for a number of reasons, plants and animals may not be able to adjust and so some species and ecosystems are likely to be eliminated by the climate change (Primack, 2000). Furthermore global warming may bring with it sea level rises which could endanger low lying coastal and delta regions. Ecosystems in these areas, if they are flooded by rising sea levels, will be unlikely to survive such a profound environmental change. Particularly vulnerable are the world’s biologically diverse wetlands and coral reefs, as shown by Bellwood and Hughes (2001) who found that coral bleaching as a result of global warming had a detrimental effect on reef biodiversity in a number of survey sites in the Indian and Pacific Oceans.
A final cause of human-induced extinctions and biodiversity loss is the effects of invasions by non-native species. Throughout history humans have succeeded in rapidly altering patterns of evolution by transporting species across the world. Whereas previously the geographical ranges of species was determined by environmental and climatic barriers to dispersal, since pre-industrial times humans have carried and cultivated plants and animals across these barriers either as a result of European colonisation, agriculture of accidental transport. An invasion of introduced non-native species occurs when they increase in abundance at the expense of native species, thus threatening the existence of the native species and reducing biodiversity in that area. Islands are particularly vulnerable to such a process due to high proportions of endemic species and a general lack of immunity to mainland pests and diseases. One example of species invasion occurred in the African Great Lakes - Victoria, Malawi and Tanganyika – which are famous for their great diversity of endemic species, termed "species flocks", of cichlid fishes. In Lake Victoria, a single, exotic species, the Nile Perch, was purposefully introduced for subsistence and sports fishing. Unfortunately the feeding habits of the Perch has caused the extinction of most of the native species of cichlid fishes. Such a threat to the lakes’ biodiversity is likely to prove very costly in the long-term (McNeill, 2000).
Clearly then, the direct effects of human actions - such as habitat destruction, over-exploitation of living resources, invasion by introduced species and pollution - play a very large role in contributing to current rates of global biodiversity reductions. As well as direct causes however, current losses of biodiversity can also be found to have more indirect causes (Wood, 2000). These have their roots in socio-economic and political factors which govern the way human societies work with, or against the environment. Many economic systems worldwide in the past or at present have failed to set a proper value on the environment in their search for profit, and inappropriate social structures and weaknesses in legal and institutional systems can further compound such issues. Often, growing domestic and international pressures facing countries will result in the degradation of habitat and the over harvesting of species (Wood, 2000). Domestic pressures that arise at local, regional and national scales include population growth, migration, poverty, inequality and increased demand for consumer goods. At the same time, international or global factors, such as changing trade opportunities and growing economic interdependence will also create pressure for resource exploitation. The behaviour of governments and private sectors in the face of these growing pressures – including changes in policies and management efforts – have often been unsatisfactory in offsetting the impact of this pattern of development in which many socio-economic factors drive biodiversity loss.
Wood (2000) effectively uses a number of different case studies to exemplify such indirect human causes of recent reductions in global biodiversity. It is important to stress here that each particular region or area will have its own set of unique factors contributing to biodiversity loss and at each site a complex interplay of driving factors were found. One example can be seen with Cameroon. Here the growing bushmeat and wildlife trade is causing biodiversity loss in both Mount Cameroon and in the south-eastern forests. Habitat loss through intensive deforestation and hunting has decimated wildlife in these areas. Such activities continue unrestrained due to various social and political changes that have occurred in Cameroon. The country has a heavy reliance on primary products which in the face of falling prices for such products meant that timber production had to increase to meet the country’s need for foreign exchange. Structural adjustment measures mean the government has little capacity to enforce laws limiting hunting and wildlife collection; and added to this are problems of corruption and a lack of political will to protect the country’s biodiversity. Clearly then, the direct causes of biodiversity loss in Cameroon such as habitat destruction through deforestation and over-hunting were further aggravated by indirect social, economic and political factors.
Similar issues can be seen with the case study of China’s South-Western forests. Destruction of temperate forest driven by the expansion of agriculture as well as commercial logging is eliminating habitats of the giant panda and the snub-nosed monkey, among other species. However government policies – including policies on population, poverty, land reform and logging – have only aggravated the unsustainable exploitation of resources. Political movements in the past, notably the Great Leap Forward and the Cultural Revolution, promoted unsustainable resource use and reduced the government’s capacity for management. Now local governments rely heavily on logging income, leading to over-harvesting of timber, and the opening up of market following the collapse of communism has only created new incentives for logging by freeing timber prices. Thus China’s historical background, as well as current political and socio-economic factors, has contributed to recent reductions of biodiversity in this region.
In conclusion, it is clear that the major factors contributing to current reductions in global biodiversity are complex and highly integrative. Humanity’s history of expansion in agriculture, logging, fishing, and other natural resource-based activities, as well as increases in pollution can be shown to have directly driven the process of biodiversity loss. At the same time however, these processes have been accompanied by different sets of socio-economic and political factors, often unique to different geographical regions, but which overall have served to compound the recent reductions in global biodiversity.
Bibliography
Primack, R B A Primer of conservation biology (2000)
Perlman, D Biodiversity (1997)
Wood, A The root causes of biodiversity loss (2000)
Bellwood, D R and Hughes, 2001. Regional-scale assembly rules and biodiversity of coral reefs. Science 292, 1532-4
Roberts, R G et al, 2001. New ages for the last Australian megafauna: Continent-wide extinction about 46,000 years ago. Science 292, 1888-92
