Dispersal and Migration

Explaining the persistence of rare tree species in highly diverse forests is essential to understanding currently observed patterns of biodiversity and predicting how those patterns might change in response to climate change. Although long-distance dispersal (LDD) of seeds is thought to maintain rare species by providing a constant source of potential immigrants (through a high migration rate m), the role of LDD in maintaining forest biodiversity has not been directly tested. The scales that determine successful migration make direct estimation of m difficult. We applied new computational tools to long-term and spatially extensive demographic data and found that when directly estimated, m is orders of magnitude smaller than posited in previous models that estimate migration from species abundance distributions.

The rarity of migration due to LDD indicates that for long-lived forest trees, reaching a potential site through dispersal is not as important as successfully competing for it. Observed levels of forest biodiversity can only result  from differences in species’ competitive abilities or release from competition by disturbance. We also found that without mechanisms that reduce competition with resident individuals, threatened species are unlikely to shift ranges with changing climate boundaries. This research demonstrates the importance of niche differences and disturbance in releasing rare forest species from the numerical dominance of common local species.