2.1 Introduction to Biodiversity

All three levels matter. A region full of different habitats tends to hold many species, and each species does better if its populations are genetically varied.

• Genetic diversity: the variety of genes within one population or species. It's why individuals of the same species look and respond differently.
• Species diversity: the number and mix of different species in an area.
• Habitat (ecosystem) diversity: the variety of habitats in a region, like forests, wetlands, meadows and streams. More habitats usually means more species, because each habitat supports different organisms.

Species diversity has two parts. Species richness is simply the number of different species in an area. Species evenness describes how equally individuals are spread among those species.

Two forests could each have 10 tree species (same richness). If one forest has about the same number of each species, it's very even. If the other is 95% oak with a few of each other species, it's very uneven. The even forest is considered more diverse. A community is most diverse when it has both high richness and high evenness.

Richness is often used to judge ecosystem health. A drop in the number of species living in an area can be an early sign of pollution, habitat loss or another problem.

When the environment changes, through a new disease, a drought or a warmer climate, some individuals are likely to carry traits that help them survive. Those survivors reproduce and the population carries on. A population with little genetic variety may have no individuals with the right traits, so the whole population can be wiped out. Low genetic diversity also leads to inbreeding, which increases harmful inherited conditions.

The same idea applies to whole ecosystems. A more diverse ecosystem is more resilient, meaning it can recover more easily after a disturbance like a fire, drought or disease. If one species is lost, others that fill similar roles can take over its job.

A bottleneck happens when a population shrinks sharply, for example from hunting, disease, habitat loss or a natural disaster. Only a small, random set of individuals survives, so most of the original genetic variety is gone. Even if the population grows back to large numbers, the new population descends from just those few survivors and stays genetically similar.

Northern elephant seals were hunted until possibly fewer than 100 remained in the 1890s. With protection, they now number well over 100,000, but they have very little genetic diversity. Cheetahs also show signs of an ancient bottleneck, which leaves them more vulnerable to diseases.

Habitat loss is the biggest threat to biodiversity today. When forests, wetlands or grasslands are cleared or broken into small pieces, the species that depend on them lose food and shelter. Specialists, which need very particular conditions, usually decline first. Generalists hold on longer, but they decline too as habitat loss continues. Species that need large territories, like big predators, also suffer, because small patches can't hold enough of them. Smaller habitat patches also hold smaller populations, which are more likely to go through bottlenecks.