Insect Species Richness of the British Rosaceae: The Importance of Host Range, Plant Architecture, Age of Establishment, Taxonomic Isolation and Species-Area Relationships

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Abstract
(1) The number of species of Lepidoptera, Hymenoptera, Homoptera and Diptera feeding on British Rosaceae generally increases with the size of the geographic range of the host species, and with the complexity of the host''s morphology. (2) Within the Rosaceae, trees support more insect species than shrubs which in turn support more insect species than herbs. In addition, trees have more insect species within each insect guild and taxonomic grouping associated with them than shrubs, which in turn have more than herbs. (3) Although shrubs and herbs begin to accumulate insect species at a lower level of abundance than trees, trees accrue species at almost twice the rate of herbs and shrubs. The species-accrual rate for different guilds of insects (chewing, sucking and gall-forming) and for the four taxa of insects considered (Lepidoptera, Hymeoptera, Diptera and Homoptera) is greatest on trees except in the case of gall-forming insects and Diptera, where the rate is similar for trees, shrubs and herbs. (4) The older the plant''s history of establishment, based on the number of Quaternary records, the more insect species that are associated with it. Also, more recent less widesperead plants have a greater polyphagous insect fauna than older more widespread plants. (5) Various analyses enabled 83%, 83% and 45% of the variation in insect species richness between trees, shrubs and herbs to be accounted for, respectively. The low variation accounted for in the case of the herbs may be the result of the poor quality of insect records for this group of plants. (6) Within given genera of plants, increases in geographical range and plant complexity are associated with an increase in the number of insects feeding upon them. When each genus within the Rosaceae was considered as a separate entity a similar pattern was seen. (7) Of the six plant genera with at least three members (Prunus, Sorbus, Rubus, Rosa, Cotoneaster, and Potentilla), Prunus has the most insect species associated with it and Potentilla and Cotoneaster the least, on a per member basis. (8) The influence of plant chemistry and apparency on the rate of accrual of insect parasites is discussed.