Latitudinal diversity gradient

Imagine making plant lists in regions near the equator and farther toward the poles. Many groups have more species near the equator, though oceanic-island plants show important exceptions. What histories produce the broad pattern and its exceptions? A region gains species when new ones form or arrive, and loses them when populations disappear; today's count does not tell us which process dominated.[1] [2]

A tropical region can have a rich plant list while an oceanic island has far fewer species than a model predicts for a mainland region at that latitude. Which histories produce the broad rise in richness toward the equator, and which produce exceptions? Regional counts reflect new species forming, species arriving, extinction and local disappearance; these processes must be distinguished over time. One proposed island mechanism is establishment failure: a plant may arrive without an essential partner in a mutually beneficial relationship, such as a pollinating animal or root-associated fungus. The 2024 study linked partner-dependent plants to the island deficit relative to its mainland model. This was not an experiment removing partners, and a reanalysis and author response dispute the robustness of that attribution.[1] [2] [3] [4]

Several processes can create similar patterns. Their importance differs between groups and environments.[1]

Multiple rate-changing mechanisms can generate similar gradients. Evidence for mutualism effects on island plants does not settle the explanation for every taxon or environment.[1] [2] [3] [4]

A species map contains a history of how life spread and diversified. Island plants also raise a practical biological distinction: reaching suitable weather need not be enough for a plant that depends on an absent pollinator or root partner.[1] [2] [3]

A species can disappear from one region without going extinct, and a region can gain species without any new species evolving there. Distinguishing those changes connects local biodiversity to the broader history of how lineages originated and spread.[1]