Local and regional dynamics of a specialist herbivore: overexploitation of a patchily distributed host plant

Abstract

This study reports a rare example where a native herbivorous insect frequently overexploits local populations of its perennial host. Local dynamics of a flightless weevil (Hadramphus spinipennis, Curculionidae) and its host plant (Aciphylla dieffenbachii, Apiaceae) were assessed for one discrete patch. In this main study site local weevil population structure, dynamics and movement were investigated using a capture-recapture study. Local plant dynamics were studied by mapping plant location, size, sex and the phenological stage of each plant. Regional weevil and plant dynamics were studied for six plant patches using line-transect counts to estimate local weevil numbers and repeated counts of the number of flowering adult plants to assess plant numbers. Dispersal was assessed by regularly searching all plant patches for marked weevils that emigrated from the main study site. Prior to extinction, local weevil abundance, survival and recruitment rates increased continuously. At the same time the feeding damage on the plants increased and the area covered by A. dieffenbachii decreased until no plants were left. An increase in weevil abundance was clearly associated with the extinction of the local host plant population. Weevils stayed in their local host plant patch whilst food was available and dispersed only after local extinction of the plant. Over a 4-year period four local population extinctions were observed. This study was too short to allow explicit conclusions to be drawn about the ratio of extinction to colonisation rates for both the weevil and the host plant populations. However, persistence of this locally unstable system appears possible only in a fragmented habitat where asynchrony in local dynamics is maintained.

Intermediate disturbance in experimental landscapes improves persistence of beetle metapopulations

“…Increasing the dispersal ability between patches in landscapes also lowered the metapopulation persistence of the bruchid beetle, Callosobruchus maculatus (Strevens and Bonsall 2010) due to sustained predation from its relatively sedentary parasitoid. Our result also is consistent with the increased extinction and lower regional persistence observed in coxella weevils (Hadramphus spinipennis), which was attributed to higher dispersal rate aided by high patch connectivity and consequent greater synchrony among subpopulations (Schops 2002).…”

Propagule vs. niche limitation: untangling the mechanisms behind plant species’ distributions

“…Moore, S.C. Elmendorf, unpublished data). It is possible that build‐up of inconspicuous herbivores or pathogens might eventually extirpate species from some environmentally suitable sites (Schops 2002) or induce metapopulation fluctuations (Thrall & Antonovics 1995; Holyoak & Lawler 1996).…”

Local populations and metapopulations are both natural and operational categories

“…birth and death). In the real, natural world some species are distributed in metapopulations where regional processes (immigration and emigration) are more important than local dynamics (birth and death), such as the Glanville fritillary Melitatea cinxia on the Åland islands (Hanski 1999), or the weevil Hadramphus spinipennis (Schöps 2002). For such species, extinction of local populations is a normal, frequent process and it is therefore impossible to find a habitat patch in which population changes are largely the results of birth and death processes.…”