Threats to biodiversity

Two ways people harm biodiversity: Biological diversity is being damaged by human activity in two ways. Directly: people take or kill the species itself. Indirectly: people change the places and conditions it lives in.

Remember it as: HIPPO: Habitat loss, Invasive species, Pollution, Population growth, Overharvesting. Then add climate change.

Real example: only about 1,000 mountain gorillas live in the Virunga Mountains of Rwanda, Uganda and DR Congo. Farms have pushed up the slopes (habitat loss), snares set for antelope trap gorillas (poaching), and human diseases such as colds spread to them.

Name the threat: 'Humans' or 'human interference' is too vague. Name the threat and what it does: 'poaching for ivory removes breeding adults'.

Ask one question: is the species itself being taken or killed on purpose? If yes, the threat is direct. If the harm is a side effect of something else people do, it is indirect.

Real example: African elephants face both. Poachers killed about 100,000 for ivory in 2010-2012 alone (direct), while farms, roads and towns spread into the land they roam (indirect).

Habitat loss is indirect: Clearing a forest can kill animals, but they are not the target, so habitat loss counts as an indirect threat. 'Hunting' is direct; say 'illegal hunting' or 'poaching' when it is against the law.

Taking too much, too quickly: Overharvesting is a direct threat: people remove a living resource faster than it can replace itself.

Remember it as: Take more than it makes, and the stock shrinks.

Real example: Atlantic cod off Newfoundland, Canada. Catches peaked in the 1960s; by 1992 the stock had fallen by about 99%. Fishing was banned, about 40,000 people lost their jobs, and the cod has still not fully recovered. On land, old-growth forests are logged faster than they can regrow.

Link it to biodiversity: Say what the loss does: fewer individuals, lower genetic diversity, a changed food web and lower resilience. Name your example: 'the Atlantic cod collapse of 1992'.

Poaching and the illegal pet trade are direct threats driven by money.

Real example: rhinos in South Africa: 13 were poached in 2007 and 1,215 in 2014, for horn sold as medicine. African grey parrots were trapped in their thousands for the pet trade; Ghana lost 90-99% of its parrots after 1992, and forests lost birds that spread seeds.

Say 'illegal': 'Hunting' alone may be legal and sustainable. For poaching, say it is illegal and what the animal is killed for.

Big idea: Habitat loss removes the space and resources species need to survive. When habitats shrink, biodiversity usually falls.

Habitat destruction is like bulldozing nature's homes: rainforest cut for cattle, wetlands drained for buildings and roads, grassland ploughed for crops.

Real example: the Amazon rainforest, where about a fifth of the forest has been cleared since 1970, mostly for cattle ranching and soya. Jaguars, birds and insects lose habitat, food webs are broken and the forest's resilience falls.

Name it early, with a place: Write the land use, the place and the effect: 'clearing the Amazon for cattle ranching removes habitat, reduces species richness and lowers the ecosystem's resilience'.

Habitat fragmentation is not the same as destruction. The habitat still exists, but organisms are stuck on separate 'islands'.

Real example: jaguars in South and Central America. Roads and farms have split their forests, so isolated groups cannot meet to breed and inbreeding rises. Wildlife corridors, strips of habitat that reconnect patches, are one fix.

Destruction or fragmentation?: Destruction removes the habitat; fragmentation breaks it into pieces that still exist. And a small population is at risk because of its genes, mates and rarity, not simply because it 'does not reproduce fast'.

Climate change is an indirect threat: people burn fossil fuels for other reasons, and the warming changes the conditions species are adapted to.

Real example: the Great Barrier Reef, Australia, bleached on a mass scale in 1998, 2002, 2016, 2017, 2020, 2022 and 2024. The 2016-2017 heatwaves killed about half of the shallow-water corals in the northern reef.

Change, then effect: Name the change and its effect on the species: 'warmer water makes corals expel their algae, so they bleach and die'.

Two more indirect threats: pollution changes the conditions species live in, and invasive alien species arrive with people and take over.

Real example: in the UK in the 1950s and 1960s, the pesticide DDT built up in peregrine falcons, which laid thin-shelled eggs that broke; numbers recovered after DDT was banned. On Midway Atoll, Laysan albatross chicks die with stomachs full of plastic. And on Guam, the brown tree snake, which arrived in cargo after 1945, wiped out most of the island's forest birds.

'Pollution' alone is too vague: Name the pollutant and where it comes from: 'urban sewage', 'agricultural run-off', 'oil spills from ships'. Invasive species have their own page next.

The same threats hit some groups much harder than others. Data questions ask why one group, such as amphibians or corals, is more threatened, or declining faster, than the rest.

Real example: Panama's golden frog, a national symbol, was last seen in the wild in 2009, after the chytrid fungus spread through its mountain streams.

Tie each reason to the group: 'Amphibians have permeable skin, so pollutants in the water pass straight into their bodies' scores; 'pollution kills animals' does not. For corals, name the pollution source.

Paper 1 gives a fact file, a table and a graph about a real place. Five short skills come up again and again.

Real example: Vietnam's forest fell from 43% of the land in 1943 to 27% in 1990 through war, logging and new farms, then rose to 42% after a national tree-planting programme began in 1998. On Guam, 10 of the 12 native forest bird species were lost after the brown tree snake arrived: 10 / 12 x 100 = 83.33%.

Traps: Say 'negative relationship', not 'inversely proportional'. To reduce a threat, say what the rule does: 'ban diving' or 'create a marine park' alone is too vague.