Human Impacts on Ecosystems
Why Humans Change Ecosystems
No ecosystem on Earth is untouched by people. Eight billion humans need food, energy and water, and meeting those needs reshapes ecosystems at every scale. This course asks you to study one tropical biome (most commonly the rainforest) and one contrasting biome — we will use the savanna here. The pressures look different in each, but the underlying story is the same: deforestation, intensive farming and resource extraction shorten food webs, leak nutrients out of soils and reduce biodiversity.
The Amazon has lost around 17% of its original forest cover since 1970, mostly to cattle ranching, soya farming, logging and road building. Brazil alone has cleared more than 80 million hectares — an area larger than France.
Food: Farming Tropical Rainforests
Most rainforest deforestation is driven by food. In the Brazilian Amazon, more than 70% of cleared land becomes monoculture — cattle pasture or soya fields. Removing the canopy halts the local water cycle: trees evapotranspire huge volumes of water, and without them rainfall in the southern Amazon has fallen by around 25% since 1970. Soils degrade quickly because heavy rain hits bare ground rather than leaves: at peak clearance years, the Amazon has been losing soil at rates above 50 t/ha/yr. Once degraded, the land needs more fertiliser, which then runs off into rivers and triggers algal blooms downstream.
Energy: HEP and Wind in Sensitive Ecosystems
Generating electricity also reshapes ecosystems. The Belo Monte hydroelectric dam in Brazil, completed in 2019, drowned 668 km² of forest and indigenous land with enough electricity for about 60 million people. Reservoirs trap sediment, alter river temperatures and emit methane from rotting submerged biomass. In the UK, energy production focuses on offshore wind and tidal power. The London Array (off Kent) is one of the world’s largest offshore wind farms; while it produces clean energy, it also creates noise during construction that disturbs harbour porpoises and seabirds. Every energy choice has an ecosystem cost — the question is which costs we accept and where.
The proposed Swansea Bay tidal lagoon would have produced enough electricity for 155,000 homes for 120 years — but it required a 9.5 km sea wall that would have blocked migration routes for sea-trout and salmon. The UK government rejected the plan in 2018 because it was poor value for money, though ecologists had raised concerns too. Onshore wind farms in mid-Wales generate clean energy on hilltops but raise concerns about peat damage when access roads are dug, releasing carbon stored in upland soils. Energy planners weigh climate benefits against habitat costs in each scheme.
Water: Cycles, Demand and Pollution
Demand for water transforms whole ecosystems. The Aral Sea in Central Asia shrank to 10% of its 1960 size because Soviet engineers diverted its source rivers for cotton irrigation; salinity rose, fish died, and the local fishing economy collapsed. In England, drained peatlands, abstraction from chalk streams and run-off from farms have left only 14% of rivers in ‘good’ ecological status (Environment Agency, 2020). Tropical ecosystems suffer too: clearing rainforest reduces evapotranspiration, weakening regional rainfall and feeding back into food production.
A Contrasting Biome: The African Savanna
Tropical rainforest is one biome — the African savanna offers a contrasting case. Savanna ecosystems in Kenya and Tanzania have been changed by overgrazing, expansion of agriculture and poaching. Wildebeest numbers in the Serengeti remain healthy at around 1.3 million, but elephant populations across Africa have fallen from 12 million in 1900 to around 415,000 today. Drought and rising temperatures linked to climate change reduce grass growth, shrinking carrying capacity. Soil erosion accelerates as cattle overgraze the dry-season pastures, leaving bare ground that bakes in the sun. The same chain of consequences as in the Amazon — just on grass instead of forest.
When habitats shrink, populations get split into smaller, isolated groups (sometimes called habitat fragmentation). Small populations breed in fewer numbers and lose genetic variety, making them more vulnerable to disease and extreme weather. The Amazon’s pink river dolphin and the savanna’s African wild dog are both classed as endangered partly for this reason. Connecting fragments through wildlife corridors — or hedgerows in the UK — is one of the cheapest ways to slow biodiversity loss without restoring whole habitats.
Monoculture farming changes ecosystem processes in three ways: it cuts evapotranspiration so regional rainfall falls; it exposes soil to erosion by heavy rain; and it removes plant variety, leaving fewer species in the food web.