Changing Biodiversity

Biodiversity is not fixed. It changes over time as a result of natural events and, today, mainly because of human activities. Current rates of extinction are thought to be many times higher than natural background rates.Human activities that reduce biodiversity1. Habitat loss and fragmentation: clearing forests, draining wetlands and building roads and towns. This is the biggest single cause of biodiversity loss. Small, separated fragments of habitat support small populations, which lose genetic diversity and are more likely to die out.2. Agriculture: large monocultures, removal of hedgerows, and pesticides and herbicides reduce the number of niches and kill non-target species.3. Pollution: fertiliser run-off causes eutrophication; other pollutants poison organisms.4. Over-exploitation: overfishing, hunting and the illegal wildlife trade.5. Introduced (invasive) species: predators, competitors and diseases brought to new areas, which native species have no defences against.6. Climate change: species unable to move or adapt quickly enough to changing temperature and rainfall decline.Natural changesBiodiversity also changes naturally. During succession, bare ground is colonised by pioneer species and gradually develops into a more complex community, so species diversity usually increases. Events such as volcanic eruptions, floods and fires reduce biodiversity locally for a time.To monitor change, the same area is sampled in the same way at intervals, and the index of diversity is calculated each time. A falling value of D indicates declining biodiversity. Human activity can also increase biodiversity, for example by restoring habitats, replanting hedgerows or creating ponds and nature reserves.