NATL ACAD. SCI.
a, Bird species in the lowland regions of South America, where habitat types are more widespread, are more wide-ranging than those on the western edge of the continent, where the Andes create quick changes in elevation and habitat type. Colours indicate median range-size in units of 1° latitude × 1° longitude cells. b, The composition of bird communities changes asymmetrically as one looks outwards from a location in the Amazon basin (here 1–2° S, 69–70° W), mimicking the underlying distribution of riverside habitat. Colours indicate the number of species in common with the focal location whose coordinates are listed. Neutral theory may be consistent with a but Graves and Rahbek are correct that it cannot predict the ecological importance of habitat evident in b. The theory may still be relevant at smaller scales, however, and species differences in habitat requirements can evolve under dispersal-assembly on a heterogeneous landscape. (Figures reproduced from ref. 2.)