Keystone Species: Competition for Honeydew Among Exotic and Indigenous Species

Abstract

New Zealand is one of the most highly invaded areas on earth, with a large number of aliens. Some invaders were deliberately introduced or arrived as stowaways.The small size,and the dispersal and reproductive modes of many invertebrates make them ideal candidates for stowing away in transported goods. There are an estimated 2,200 exotic invertebrate species established in New Zealand (Ministry for the Environment 1997), and increasing global trade results in new species arriving continually (e.g.,Harris and Berry 2001; Hoare 2001; Lester 2005). Despite this, few exotic invertebrates appear to have had a significant impact on New Zealand’s indigenous biota (Barlow and Goldson 2002). Exotic social wasps are an exception. Two species of Vespula have established in New Zealand, namely German wasps (Vespula germanica Fabr.,Hymenoptera:Vespidae),and common wasps (V. vulgaris L.). Invasive species can have a range of impacts on the receiving ecosystem, but what happens when the invasive species affects an indigenous keystone species? A keystone species is one whose effect is disproportionately large rel- ative to its biomass (Power et al.1996).Keystone species are not always of high trophic status, and they can exert effects through a range of mechanisms (Power et al. 1996). Honeydew-producing, endemic scale insects (Ultra- coelostoma assimile and U. brittini Morales,Homoptera; Margarodidae) that infest some of New Zealand’s beech (Nothofagus) forests (Belton 1978; Morales et al. 1988) may be keystone species because of the strength of their effect on the beech forest community.Hereafter, these forests are referred to as honeydew beech forests. Although a range of animal species have invaded New Zealand, common wasps are arguably the worst of the invaders in honeydew beech forests in terms of environmental impact. In years of high abundance, wasps consume more than 90% of the honeydew for about 4months of the year (Moller et al.1991a), and compete with indigenous species dependent on this resource. Honeydew provides such a large sugar resource that high densities of wasps are supported,with major flow-on effects throughout the community (Beggs 2001). In order to evaluate the impact of invasive wasps in honeydew beech forest, it is necessary to understand the primary interactions between native biota in prehuman forests, before the arrival of introduced species. This chapter first reconstructs the prehuman honeydew beech forest food web, and then evalu- ates the impact of invasive wasps, particularly via competition for honeydew. Finally, it identifies knowledge gaps that must be filled if we are to fully understand the ecosystem consequences of invasive species on a native key- stone species.