Insects on isolated colonies of common rock-rose Helianthemum chamaecistus

ABSTRACT.

• 1 A study was made of the insect species present on twenty-eight colonies of H.chamaecistus Mill, in 1978 ranging in size (cover) from 0.4 m² to about 6500 m² and separated from nearest large neighbours by distances up to 6.5 km. Twenty-three insect species were collected of which nine or ten were more or less monophagous.
• 2 There was a clear relationship between species richness and size of host colony. Sites with less than 30 m² of rock-rose, about 67% of those in the study area, were likely to have less than half of the monophagous species. Distance to neighbouring colonies did not contribute significantly to species richness.
• 3 The consistency of results from year to year was examined at five sites re-sampled in 1979. The 1978 data were also used to compare‘expected’and observed faunas at five new sites sampled in 1979 based upon the species-area relationship and the relative frequency of species at known sites.
• 4 Insect dispersal was examined using a suction trap placed near the largest rock-rose site. Colonization of new sites was studied by experimental outplanting of pot-grown plants.
• 5 These direct observations, and the monitoring of fourteen sites for 5 years, suggested that colonization and interchange of faunas were rare events. The experimental introduction of one to five species at four sites showed in several cases that the sites were suitable for the species, and that introduction could significantly enhance a site's complement.