Prehistoric Man lived essentially in balance with nature, as both predator and prey in the food web.

Since then the human population has grown dramatically due to industrialisation and mechanised farming practices creating more food, and improvements in health and nutrition increasing life expectancy.

The effects of overpopulation on the Earth's environment, however, are causing extinction of species at an alarming rate. These many effects include:

Hunting and over-fishing Mechanised "predation techniques" (guns, nets, and trapping) decimate animal populations directly, and indirectly affect other organisms in the food chain. Indiscriminate netting also kills undersize fish (reducing future breeding), as well as wasting non-commercial species important in the food chain.
Mining Habitat destruction through land clearing and road building destroys nesting areas and food sources. Chemical pollution from ore refining processes kills many species.

Farming and logging
Clearing of land, and farming practices, impact on the environment in many ways.

• Extensive land clearing destroys natural habitats and affects global climate (tropical rainforest destruction is altering global rainfall due to reduced transpiration).
  
  
• Planting of introduced single species (either crops or plantation trees) decreases
  biodiversity and increases food for pests.
  
  
• Over-grazing causes erosion and loss of topsoil.
  
  
• Indiscriminate pesticide spraying effects natural pollination. Chemicals can accumulate in higher order consumers (see Eat and be eaten: food chains) e.g. DDT passed through food chain builds up to lethal levels in birds ultimately effecting strength of egg shells and survival of young.
  
  
• Land clearing and over-irrigation cause salinity and rise in water tables e.g. loss of
  orchards in the Goulburn Valley.
  
  
• Controlled flooding affects natural breeding cycles in wetland birds.
  

Fertilisers washed into streams cause algal blooms which in turn deplete oxygen supplies in streams. Land mismanagement has increased desertification, claiming 70,000 sq.km per year throughout the world.

Urbanisation

• Damaging levels of air and water pollution result from high density urban developments.
  
  
• Burning fossil fuels (cars and factories) increases carbon dioxide levels resulting in
  global warming from the The greenhouse effect.
  
  
• Sulphur dioxide exhausts cause acid rain, killing some species and causing corrosion.
  
  
• Higher fluorocarbon levels damage the ozone layer, increasing UV radiation resulting in
  higher incidence of skin cancers.
  
  
• Smog aggravates human respiratory problems.
  
  
• Use of river water for industrial cooling of machines in factories increases temperature of the rivers, thereby killing aquatic life from decreased oxygen availability.
  
  
• Clearing of surrounding bushland to accommodate the "suburban sprawl" destroys natural habitats.
  
  
• Domestic pets compete with natural organisms, and sometimes turn "feral", killing many native species.

Sewage treatment and rubbish removal can cause major environmental problems, but recycling can reduce the need for landfill areas and save resources especially for non-biodegradable materials. Nuclear wastes are particularly damaging to living things.

Recreational activities

• Off-road driving causes soil erosion, destruction of native flora and frightening of fauna.
  
  
• Increased access to "wilderness" areas by 4 WD vehicles threatens isolated fish breeding areas and fauna "havens".
  
  
• Aquatic motor sports cause pollution through oil spills.
  
  
• Campers can damage animals' hollows through firewood collection, and bushfires can be started by lack of care. Wastes products can pollute streams.
  
  
• Seeds of noxious weeds are spread by boots or tyres.

Burning activities
Slash-burning forms of agriculture on a large scale can cause considerable smoke, reduced visibility, and alter local weather patterns.

Introduced species

• Plants and animals introduced into a different area often lack natural predators and soon become very prolific and jeopardise survival of native species (see Introduced species: "Not in my backyard").
  
  
• Ease of global travel has necessitated quarantine restrictions to prevent accidental or deliberate introduction of foreign species. Some marine species have been introduced in the bilge water of ships.

No-one is an island... our actions have serious wide-ranging repercussions.