What a hotspot is
Biodiversity is not spread evenly across the planet. Some regions hold far more species per unit of land than others, and some of those species live nowhere else. The term “biodiversity hotspots” was popularized by the ecologist Norman Myers in the late 1980s and later adopted and refined by a conservation organization. In the common definition, a hotspot is a region that meets two conditions: it contains a large number of endemic plant species (species found only there), and it has already lost most of its original natural vegetation. The usual thresholds are at least 1,500 endemic vascular plant species and at least 70 percent of the original habitat gone.
The count of recognized hotspots has changed a little as the list has been revised, but it is typically described as a few dozen regions. Together they cover a small share of the planet’s land surface yet are often said to hold a very large share of its plant and vertebrate species. Exact percentages depend on the source and the date, so it is safest to treat them as rough.
The key point for this guide is that most hotspots are defined largely by their forests. Many are tropical or subtropical forest regions, and others combine forest with scrub, grassland or mountain habitat.
Why forests hold so many species
Forests give living things a great deal to work with. A single forest can contain a canopy, a mid-story, shrubs, a ground layer, leaf litter, soil, standing dead trees and fallen logs. Each layer offers different light, moisture, food and shelter, and each supports different specialists. The structure of a tropical forest in particular, with its stacked rainforest layers, is one reason it can hold so many animals, plants and fungi in a small area.
Several other factors tend to push species numbers up:
- Stable, warm and wet climates. In many tropical regions, conditions have changed relatively little over long periods, which gives species time to specialize and accumulate. The details of why tropical rainforests are so diverse are still debated, and no single explanation is accepted as complete.
- Many different trees. Each tree species can host its own insects, fungi and epiphytes, so high tree diversity (often discussed as tree species richness) tends to support high diversity in other groups.
- Close relationships between species. Plants depend on animals for pollination and seed dispersal, and many animals depend on particular plants for food. These links encourage specialization.
Why hotspots contain so many endemics
Endemic species are found where populations have been separated from each other long enough to evolve in different directions. Forests produce this separation in several ways.
Islands. Water isolates populations. Forested islands often hold species that exist nowhere else, and their small ranges make them especially vulnerable.
Mountains. Slopes create bands of different temperature and moisture over short distances. Isolated mountain forests, including cloud forests, can act like islands in the sky, with populations separated by drier lowlands.
Old, patchy refuges. In some regions, forests survived past climate shifts in scattered pockets while surrounding areas dried or cooled. Species that persisted in those refuges can be found only there today.
Varied soils and terrain. Different rock types and soils within a small area can favor different plants, and the animals that depend on them follow.
Why hotspots are priorities
Two features make a region a priority at the same time: it contains many species that cannot be found elsewhere, and its habitat is under pressure. A species with a small range can be pushed toward extinction by clearing a relatively small area, whereas a widespread species can often persist elsewhere. When a place has both a large number of such species and extensive habitat loss, each additional patch of forest cleared can remove a disproportionate share of the world’s total for those species.
The main pressures on hotspot forests commonly include clearing for agriculture, logging, mining, road building, fire and the spread of invasive species. These pressures often interact. A road opens a forest to settlers, logging and hunters; the resulting patches become isolated, and small populations in isolated patches are more exposed to chance events. The wider causes of forest clearing in the tropics are covered in the discussion of tropical deforestation, and the way clearing splits habitat is explored in the cluster’s articles on forest fragmentation.
Climate change adds another layer. Species with narrow ranges, especially those in mountain forests, may have few places to move to as conditions change, which is a major reason researchers worry about forest biodiversity loss in the coming decades. How well species can follow shifting species ranges depends heavily on whether connected habitat remains.
What the hotspot label leaves out
The hotspot approach is a useful way to direct limited conservation money, but it has limits that are worth keeping in mind.
- It is built mostly around plants and a few animal groups, so it can miss regions that are rich in other groups, such as insects or fungi, or in fresh water life.
- It counts endemism and habitat loss, not the number of people who live there or depend on the forest. Conservation that ignores local communities often fails, and many hotspot forests are homes and livelihoods for people.
- Intact forests with fewer endemic species, such as large boreal or temperate regions, may not qualify as hotspots but still matter greatly for climate, water and wide-ranging animals.
- A boundary on a map does not describe conditions inside it. A hotspot contains healthy forest, degraded forest, farmland and towns.
What helps
Approaches that are commonly used in hotspot forests include creating and managing protected areas, supporting community management of forests, restoring degraded land, reducing the pressure from illegal logging, and paying for the services intact forests provide through ecosystem payments. Any one of these can help; none works everywhere, and results depend on local enforcement, funding and the interests of the people involved. Outcomes are uncertain, and success is usually measured over decades rather than a few years.
Forests are central to the hotspot story because they combine structure, age, stable conditions and isolation, the same ingredients that produce many species and many unique ones. They are also where the combined threats of clearing, fragmentation and warming are most visible, which is why the idea continues to shape conservation priorities.