Pest Pressure: the concept of Gillett 1962
"Here is the answer to the mystery. Pest pressure is the inevitable, ubiquitous factor in evolution which makes for an apparently pointless multiplicity of species in all areas in which it has time to operate.."
Gillett 1962 publication on pest pressures
Gillett, J. B. (1962). Pest pressure, an underestimated factor in evolution. Systematics Association Publication, 4, pp.37-46. - for pdf click here.
"At Dandu, North Kenya, in 1952 it was observed that the handsome shrub Adenium somalense [ Adenium obesum ] (Apocynaceae), which has large rose-coloured flowers, although apparently well adapted by its succulence and poisonous sap to prevailing conditions of aridity and heavy grazing, was nowhere abundant, though it occurred over large areas as widely scattered individuals, or in small groups. This seemed puzzling since if some undetected factor was making it scarce, why had this not led to its disappearance? Eventually fruits of this plant were found, but when dried out to obtain the seeds almost all proved to be infested by the ovule-eating larvae of the fly Dacus brevistylus. Here was an answer to the puzzle: had the Adenium been numerous the flies would also have been more numerous and would have found the plant more easily, so that it would have been able to produce even fewer seeds than when it was scarce, and thus it would have again become scarce. Dacus brevistylus might proclaim the ancient Roman maxim 'Debellare superbos, et subjectis parcere'; spare the scarce, but beat down the dominant species."