Plant hybridization: the role of human disturbance and biological invasion
Anderson & Stebbins (1954, Evolution, 8, 378–388) posited that human activities promote species hybridizations by creating opportunities for hybridization and new habitats for hybrids to persist through disturbances (i.e. the ‘disturbance hypothesis’). While the first part of this hypothesis appears to be well supported, the second part has not been corroborated with empirical evidence, probably because of the lack of appropriate data. In this study, I (1) document the richness and distribution of hybrid plants in the United States; (2) examine the relationships between hybrids of different origins and between hybrid plants and native or exotic plants; and (3) examine possible mechanisms for these relationships and test the disturbance hypothesis.
The United States.
The richness and distribution of plant hybrids was examined at the county level according to origin, that is, formed between native–native species (N × N), native–exotic species (N × E) and exotic–exotic species (E × E), using data from the Biota of North America Program.
The three hybrid types (N × N, N × E and E × E) were positively related to each other and showed stronger positive relationship with exotic richness than with native richness. They also exhibited similar spatial patterns, with richness hotspots concentrated in the north-east United States and Great Lakes region. However, the richness of hybrids of exotic origin (E × E and N × E) was not related to county area, as often observed for native species; instead, it showed strong positive relationships with human population density. Thus, the overall patterns of hybrid richness and distribution support the ‘disturbance hypothesis’.
The results are generally consistent with the disturbance hypothesis. The relationship between the number of hybrids of exotic origin and overall exotic richness provided stronger evidence for human-induced than for naturally caused hybridization, although other possible explanations may also exist.