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Why do continents usually have more species than remote islands?

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Diagram showing energy flow in a food chain from a tree to a caterpillar, then to a bird, and finally to microbes, with red arrows illustrating energy transfer and heat loss at each stage. — community ecology Energy transfer and heat loss along a food chain.

Continents generally support more species than remote islands primarily due to differences in accessibility, historical connectivity, and the theory of island biogeography.12

The disparity in species richness is driven by several key factors:

Colonization Barriers: Remote islands are difficult for many organisms to reach. While plants and some animals may arrive via wind or water, species that cannot fly or travel long distances over open ocean are significantly less likely to colonize isolated islands.1
Historical Connectivity: Many continental areas have been connected to larger landmasses in the past, allowing for the natural migration and establishment of diverse flora and fauna. In contrast, many islands are of recent origin or have remained isolated, providing less time and opportunity for species to arrive and establish themselves.1
Equilibrium Dynamics: According to the theory of island biogeography, the number of species on an island is determined by a dynamic equilibrium between immigration and extinction rates. This equilibrium is proportional to the island's size and inversely proportional to its distance from a mainland source.2

Because continents offer larger areas and fewer geographic barriers to dispersal, they maintain higher levels of species richness compared to isolated, remote islands.12