The invertebrate fauna on broom, Cytisus scoparius, in two native and two exotic habitats

This study quantifies the invertebrate fauna found on broom. Cytisus scopar ius, L. (Link), in two countries where it grows as a native plant (France a nd England) and two countries where it grows as an alien plant (New Zealand and Australia). The data are used to test three hypotheses concerning the predicted differences in invertebrate community structure in native versus exotic habitats (1)Are generalist phytophages dominant in exotic habitats a nd specialist phytophages dominant in native habitats? (2) Are there empty phytophage niches in exotic habitats? (3) As a plant species accumulates ph ytophages, do these in turn accumulate natural enemies? The broom fauna was sampled at five sites in each country by beating five broom bushes per sit e. The sampling efficiency of beating was quantified at one field site and it was shown to collect 87 % of invertebrate abundance, 95 % of invertebrat e biomass and 100 % of phytophagous species found on the branches. Generali st phytophages were dominant on broom in exotic habitats and specialists do minant on broom in the native habitats. Thus, the two countries where broom grows as a native plant had higher numbers of total phytophage species and a higher abundance of specialist phytophages per bush. There was no signif icant difference in the average abundance of generalist phytophage species found per bush in native and alien habitats. Phytophages were assigned to s even feeding niches: suckers, root feeders, external chewers, flower feeder s, seed feeders, miners and pollen feeders. Empty niches were found in the exotic habitats; species exploiting structurally specific parts of the host plant, such as flowers and seeds, were absent in the countries where broom grows as an alien plant. The pattern of niche occupancy was similar betwee n native and exotic habitats when just the generalist phytophages were cons idered. As phytophage abundance and biomass increased, there were concomita nt increases in natural enemy abundance and biomass. Thus, it appears that as plants accumulate phytophages, the phytophages in turn accumulate natura l enemies and a food web develops around the plant. Moreover, in the native countries, the history of association between the natural enemies and thei r prey has been sufficient for specialist predators and parasitoids, feedin g on the specialist phytophages, to have evolved. (C) 2000 Editions scienti fiques et medicales Elsevier SAS.