3.1.6.1 - Biodiversity and ecosystem sustainability

Measuring biodiversity and its trends

Biodiversity includes variation within species, between species and between ecosystems. Genetic variation helps a population respond to disease or environmental change. Species diversity concerns both richness, the number of species, and evenness, how evenly individuals are distributed. A meadow with ten species dominated almost entirely by one grass differs from a meadow with ten similarly abundant species. Ecosystem diversity includes the variety of habitats and their ecological relationships across a landscape.

Compare like with like when analysing trends. More species recorded after increased survey effort may reflect improved detection. A falling abundance index measures population change, not necessarily extinction. A threatened-species estimate concerns future risk; it is not a count of species already lost. IPBES's 2019 global assessment estimated that around one million animal and plant species faced extinction risk. It also reported a fall of at least 20% in average native-species abundance across most major terrestrial biomes, with much of the decline since 1900. This is a broad abundance trend, not a 20% loss of all species. Its global synthesis does not mean every local site declined equally.

Local monitoring can reveal contrasting trajectories. A 2021 study of former arable land at Wicken Fen, Cambridgeshire, compared vegetation inside and outside grazing exclosures during nine sampled years between 2007 and 2017. Ungrazed annual mean species richness declined, while grazed vegetation was richer. These findings concern sampled recovering fields and grazing treatments, not all species in the entire reserve. Changes in canopy height and litter help explain why a global trend and a local management response may differ.

Original illustrative monitoring data give the same pond plant count, 40 species in 2010, 32 in 2020 and 28 in 2025, using the same survey protocol. Loss was eight species per decade, or 0.8 species per year, in each interval. The percentage decline was 20% in the first interval and 12.5% in the second, because each uses a different starting count. Constant absolute loss does not imply a constant percentage rate. The data alone cannot identify the cause.

Why biodiversity declines at different rates

Habitat conversion removes food, breeding sites and shelter. Fragmentation divides remaining habitat into smaller patches, increases edges and can obstruct migration and recolonisation. Overexploitation removes organisms faster than populations replace them. Pollution changes water or soil chemistry; nutrient enrichment may favour a few competitive plants. Introduced predators, competitors or pathogens can harm native species, while climate change shifts suitable temperatures and seasonal food availability.

These drivers interact. Clearance can leave small populations vulnerable to disease and prevent them moving as temperatures rise. An abrupt development may cause rapid local losses; extinction can also be delayed after fragmentation because surviving adults persist without successful recruitment. Rates therefore depend on the time window, indicator and baseline. A short period of apparent stability does not establish long-term sustainability.

To compare decline rates, identify the geographical boundary, observation period, unit and denominator. A 10% change in an abundance index cannot be directly subtracted from a 10% change in habitat area. Monitor abundance, reproductive success, habitat extent and connectivity together to test a causal explanation. A correlation between road building and species loss needs evidence of clearance, disturbance or access for exploitation before attributing the loss to the road.