Limits to the abundance of rare species: an experimental test with a tree aphid

1. The birch (Betula)-feeding aphid, Monaphis antennata, is always found at low densities on individual hosts and has low local abundance, but another birch-feeding aphid, Euceraphis betulae, is often found at high densities on individual hosts and has high local abundance. 2. This work attempts to establish whether the interaction between M. anten nata and its host or the behaviour of individuals limits its densities. 3. Both species were reared on saplings, and population sizes were monitore d for 6 weeks. Two levels of host quality were used and feeding space was k ept constant throughout the experiment. Adults were prevented from leaving the saplings by clipping their wings, and predators were excluded. 4. On plants of similar host quality, both species achieved similar populat ion sizes. 5. It is concluded that resource availability or the interactions between i ndividuals are unlikely to be important causes of rarity.