Introduction to Biodiversity

A community with many species and balanced numbers can absorb shocks that would wipe out a simpler one. If a drought, disease, or pest hits, a field with ten plant species has a good chance that some of them tolerate the stress, so the field keeps functioning. A field with one species has no backup. This is the core logic behind biodiversity as a survival strategy: variety spreads risk.

Ecologists measure diversity with two ingredients: species richness, the raw count of different species present, and species evenness, how equally individuals are distributed among those species. A forest can have high richness but low evenness if one tree species dominates and ten others are rare. Simpson's Diversity Index combines both into one number: D = 1 minus the sum of each species' proportion squared. Squaring proportions punishes dominance heavily, so a community run by one species scores low even if it technically has many species on the list. Values closer to 1 mean higher diversity; values closer to 0 mean one or two species dominate.

This sets up the rest of the unit. Genetic diversity within a species and species diversity across a community both feed the ecosystem services that follow, and low diversity is exactly why monocultures and island populations are fragile, topics covered next.