Classification is the process of organizing things into groups based on similarities. Carolus Linnaeus developed the modern system of classification using a hierarchy of domains, kingdoms, phyla, classes, orders, families, genera, and species. Biodiversity refers to the variety of living organisms and includes genetic diversity within species, diversity between species, and diversity of ecosystems. Threats to biodiversity include habitat destruction, invasive species, pollution, overpopulation, and over-exploitation of resources.