Endangered Species Day

01 · Biology of decline

Why species become endangered

The IUCN identifies five primary drivers of biodiversity loss, often summarised as HIPPO: Habitat loss, Invasive species, Pollution, Population growth (human), and Overexploitation. Climate change is increasingly treated as a sixth driver acting across all five.

The biological mechanism underlying most of these drivers is population fragmentation. When a habitat is fragmented into isolated patches, populations become too small to sustain genetic diversity. Inbreeding increases — increasing the frequency of homozygous genotypes (including recessive deleterious alleles — see the Allele glossary entry). Demographic stochasticity (random variation in births and deaths) can then tip a small population into extinction even without further habitat loss — the extinction vortex.

The minimum viable population (MVP) — the smallest population size that has a 95 % chance of persisting for 100 years — is typically estimated at 500–5 000 individuals, depending on generation time, reproductive rate, and environmental variability. Below MVP, genetic erosion accelerates.

02 · Conservation science

Genetics as a conservation tool

Modern conservation biology is increasingly genomic. Applications include:

• Population structure assessment — microsatellite and SNP panels identify genetically distinct subpopulations (management units) that should be conserved separately to preserve local adaptation.
• Kinship and inbreeding coefficients — whole-genome sequencing of captive breeding programme individuals guides pairings to maximise heterozygosity.
• Environmental DNA (eDNA) — water or soil samples contain shed DNA from organisms living there. PCR amplification and sequencing detects species without trapping or observation.
• CRISPR-assisted rescue — still experimental, but researchers are exploring reintroducing lost alleles (e.g. cold tolerance genes) from museum specimens back into extant populations. See the CRISPR base editing article.

For the underlying genetics — Hardy-Weinberg equilibrium, inbreeding coefficients, selection against recessive homozygotes — see the Genetics and Inheritance guide.

03 · Evolutionary perspective

Extinction in the context of evolution

From an evolutionary standpoint, extinction is normal: more than 99 % of species that have ever existed are now extinct. Background extinction rates — the baseline rate in the fossil record between mass extinctions — are approximately 0.1–1 species per million species per year. Current extinction rates are estimated at 100–1000 times this background rate, placing us in a sixth mass extinction event comparable in speed to the end-Cretaceous event 66 million years ago.

Natural selection can only preserve a species if: (1) there is sufficient genetic variation for beneficial traits to be selected, and (2) the selection pressure changes gradually enough for selection to keep pace. Both conditions fail when populations are small (low variation) and the environment changes rapidly (climate change, habitat transformation). See the Natural selection guide and the Evolution guide for the underlying mechanisms.

04 · How to mark it

Engaging with the science in school

• Use the IUCN Red List to find the conservation status of a local or favourite species. Identify which HIPPO driver(s) are responsible.
• Model the extinction vortex using a simple population genetics simulation — track how allele frequencies shift as population size drops below MVP.
• Extract eDNA from a water sample (pond, stream) using a simple filtration kit; compare to DNA barcoding databases to identify species present.
• Research a successful conservation story (Arabian oryx, white-tailed eagle, Californian condor) and identify the genetics and management strategies that enabled recovery.