Biodiversity is distributed unevenly because environmental conditions, evolutionary history, geographical isolation and human pressures differ between locations. Regions where favourable conditions have supported extensive speciation and endemism may become biodiversity hotspots, especially when their habitats are severely threatened.
This content belongs to IB ESS HL, Subtopic 3.1: Biodiversity and evolution. The uneven global distribution of biodiversity is HL-only content.
Why Does Biodiversity Vary Between Regions?
Species richness is the number of species present, while endemism describes species found naturally in only one geographical area. Biodiversity generally increases where conditions permit high productivity, long-term survival and the formation of new species.
| Factor | Mechanism affecting biodiversity |
|---|---|
| Climate | Warm temperatures, high rainfall and abundant sunlight increase primary productivity, supporting more trophic levels and ecological niches. |
| Environmental stability | Long periods without major glaciation or disruption allow species to survive, specialize and diversify. |
| Habitat complexity | Mountains, forests and varied soils create many microclimates and niches, reducing direct competition. |
| Geographical isolation | Islands, mountain ranges and fragmented habitats restrict gene flow, promoting reproductive isolation and speciation. |
| Evolutionary history | Regions differ in how long species have had to evolve and in their exposure to extinction events. |
| Human activity | Deforestation, overexploitation, pollution, climate change and invasive alien species reduce biodiversity unevenly. |
The Tropical Andes contain varied altitudes and isolated habitats, producing exceptional richness and endemism. Madagascar's long isolation has similarly produced many endemic species, but habitat destruction places them at high risk.
A common misconception is that every species-rich region is automatically a biodiversity hotspot. A hotspot is distinguished not only by exceptional biodiversity, particularly endemism, but also by substantial threat from habitat loss. Its designation therefore influences conservation priorities and can create environmental justice tensions if protection restricts local or Indigenous communities' access to land and resources.
Exam Technique
For an explain question, connect each factor to a mechanism such as productivity, niche diversity, isolation or speciation. Use a named example and distinguish clearly between high species richness and hotspot status.
