The Earth's biota is organized into biogeographic realms with distinct communities, separated by barriers to dispersal. These realms reflect historical processes: continental drift, climate change, and lineage origins and extinctions. Species richness varies predictably with latitude, elevation, and connectivity. Understanding these patterns explains current distributions and predicts responses to future environmental change.
From your work on island biogeography, you already understand how isolation and area shape species richness on islands. Biogeographic realms extend that same logic to the entire planet: the continents themselves are like enormous islands, separated by oceans, mountain ranges, and deserts that act as barriers to dispersal. The result is that different regions of the world harbor fundamentally different sets of species, not because of current climate differences alone, but because of deep history — which lineages happened to be where when barriers formed or disappeared.
The Earth is traditionally divided into major biogeographic realms — the Nearctic (North America), Neotropical (Central and South America), Palearctic (Europe and northern Asia), Afrotropical (sub-Saharan Africa), Indomalayan (South and Southeast Asia), and Australasian (Australia, New Guinea, and nearby islands), among others. Each realm has a characteristic biota shaped by millions of years of evolution in relative isolation. Australia's marsupials are the classic example: when the continent separated from Gondwana and drifted northward, its mammals evolved independently, filling ecological roles that placental mammals occupy on other continents. Kangaroos fill the grazer niche, thylacines (now extinct) filled the predator niche, and wombats fill the burrowing herbivore niche — all marsupials, not because marsupials are inherently better for these roles, but because that is the lineage that happened to be present when Australia became isolated.
Within and across realms, species richness follows predictable gradients. The latitudinal diversity gradient — more species near the equator, fewer toward the poles — is one of the most robust patterns in ecology. Tropical regions have higher energy input, more stable climates that allow specialization, and longer evolutionary histories without glacial disruption. Elevational gradients mirror this: species richness typically peaks at mid-elevations and declines toward summits, where area decreases and conditions become harsh. Connectivity also matters — regions connected by land bridges or island chains show more species exchange than those separated by open ocean, which is why the flora and fauna of North and South America mixed dramatically after the Isthmus of Panama formed about three million years ago (the Great American Biotic Interchange).
These patterns are not merely descriptive — they have direct predictive power. Species found only in one realm (endemics) are concentrated in areas with long isolation histories, such as Madagascar, New Zealand, and oceanic islands. As climate changes and barriers shift, biogeographic theory predicts which species will be able to track suitable habitat and which will be stranded. Understanding why species are where they are — the interplay of plate tectonics, climate history, dispersal ability, and evolutionary time — is essential for predicting how biodiversity will respond to the environmental changes already underway.