Metapopulation dynamics and behaviour of the endangered weevil, Hadramphus spinipennis in relation to its host plant Aciphylla dieffenbachii on the Chatham Islands, New Zealand

Abstract

The endangered monophagous coxella weevil, Hadramphus spinipennis (Coleoptera:
Curculionidae) is confined to two outlying islands of the Chatham Island group. This study was carried out on Mangere Island. The native forest, which used to cover Mangere, has been burned and replaced by grassland. A metapopulation of weevils inhabits 11 distinct host plant (A. dieffenbachii, Apiaceae) patches. Adult weevils feed on the foliage and flowers of A. dieffenbachii. Eggs are laid from September until May. Larvae hatch after two to three weeks and feed on the root parenchyma. Development from egg to adult takes between six months and a year. Pupation takes place in the soil next to the roots. Adults are flightless. The weevil responds to host plant specific olfactory cues.
Mangere was visited six times between summer 1994 and summer 1997. To assess weevil-host plant interaction and to investigate if the weevil causes local extinction of its host plant, the population dynamics of H. spinipennis and its host plant were investigated in one discrete patch. Population estimates for the weevils were obtained with a capture-recapture study using Pollock's robust design. Location, phenology and size of A. dieffenbachii plants were recorded once a year. Weevil numbers more than quadrupled over three consecutive summers and survival and recruitment rates increased. Plant numbers halved over the same period. By the fourth summer the plant population had collapsed and no weevils were found. Overexploitation, particularly via root feeding, caused host plant extinction.
Weevil intra-patch movement was studied to determine the mobility of the weevils at varying food levels and the extent to which they leave a host plant patch. Over 90% of the weevils stayed within 0-6 m of where they were caught the previous day. Intra-patch movement followed a random walk model. High survival between capture occasions (during one visit) suggested that the weevils had a low tendency to leave a patch.
An annual census was used to estimate weevil and host plant abundance for six patches and to assess if local weevil populations are regulated by density-dependence. The total weevil population almost tripled over three consecutive summers while plant numbers increased by 11%. No density-dependent regulation of local weevil populations was detected. After a weevil density of between 18 and 33 per plant was exceeded, three patches collapsed.
During each visit all patches were searched for marked weevils to assess inter-patch movement before and after the collapse of a local patch. Inter-patch movement was never detected before the collapse of a patch, but subsequently weevils were found in all neighbouring patches. The weevils' low tendency to disperse in the presence of food, yet high dispersal ability in its absence, can be explained by the natural habitat. Weevils and host plants were originally confined to rugged coastal areas, where the risk of inter-patch dispersal is likely to be greater than the disadvantage of intra-specific competition. After the collapse of a local patch, however, high dispersal power is needed to reach a new one. The rate of change in weevil numbers was not spatially correlated. Therefore, stability of the weevil metapopulation is probably maintained by spatial heterogeneity acting in a 'shifting mosaic' mode.... [Show full abstract]