Human Impacts on Biodiversity

Species have always gone extinct. The background extinction rate is the slow, steady pace of loss that happened before humans dominated the planet, roughly 1 species per million species per year. Biologists compare today's rate against that baseline because the number alone means nothing without a reference point. When the current rate comes out tens or hundreds of times higher, that comparison is the evidence that something unusual is happening, and that humans are the cause.

Several drivers cause this acceleration, and habitat destruction tops every ranked chart of causes, followed by invasive species, pollution, human population growth, and overharvesting. Habitat loss does more than shrink living space. When a continuous forest gets sliced into disconnected patches by roads or farms, the damage goes beyond the acres literally removed. Each patch gains more edge relative to its interior, exposing species to wind, light, predators, and invaders they never faced in deep forest. Patches also become islands cut off from each other, blocking the gene flow and movement that keep populations healthy.

This connects directly to island biogeography, which describes how isolated islands support fewer species as they get smaller and farther from a source of colonizers. A fragmented mainland behaves the same way: each patch acts like its own island, and smaller, more isolated patches lose species faster, a process called relaxation. That model lets biologists predict extinction from fragmentation data alone, tying habitat loss, biodiversity decline, and the oceans and climate topics of this unit into one planet-scale story of human-driven change.