‘Invasional meltdown’: evidence for unexpected consequences and cumulative impacts of multispecies invasions

Abstract

Empirical support for 'invasional meltdown', where the presence of one invading species facilitates another and compounds negative impacts on indigenous species, is equivocal with few convincing studies. In Ireland, the bank vole was introduced 80 years ago and now occupies a third of the island. The greater white-toothed shrew arrived more recently within the invasive range of the bank vole. We surveyed the abundance of both invasive species and two indigenous species, the wood mouse and pygmy shrew, throughout their respective ranges. The negative effects of invasive on indigenous species were strong and cumulative bringing about species replacement. The greater white-toothed shrew, the second invader, had a positive and synergistic effect on the abundance of the bank vole, the first invader, but a negative and compounding effect on the abundance of the wood mouse and occurrence of the pygmy shrew. The gradual replacement of the wood mouse by the bank vole decreased with distance from the point of the bank vole's introduction whilst no pygmy shrews were captured where both invasive species were present. Such interactions may not be unique to invasions but characteristic of all multispecies communities. Small mammals are central in terrestrial food webs and compositional changes to this community in Ireland are likely to reverberate throughout the ecosystem. Vegetation composition and structure, invertebrate communities and the productivity of avian and mammalian predators are likely to be affected. Control of these invasive species may only be effected through landscape and habitat management.

Range expansion and comparative habitat use of insular, congeneric lagomorphs: invasive European hares Lepus europaeus and endemic Irish hares Lepus timidus hibernicus

“…Invasive species have significant negative impacts on native species and ecosystems worldwide (Parker et al 1999;Sakai et al 2001;Montgomery et al 2012). Potential issues include interspecific competition for resources (Hamel et al 2013) and hybridization with related native species (Huxel 1999).…”

Diet diversity of the Common Buzzard (Buteo buteo) in a vole-less environment

“…Evidence from diet studies where Bank Voles and Greater White-toothed Shrews are naturally available to Buzzards suggests it is unlikely that these invasive species will be incorporated into the diet of Buzzards in significant numbers (Selås 2001, Reif et al 2001Sergio et al 2002, Zuberogoitia et al 2006Selås et al 2007). However, under invasive conditions the small mammals are not limited in such a way as would be experienced by natural populations (Montgomery et al 2012) and encounter rates between invasive species and Buzzards may be significantly higher in Ireland. A provisional investigation of Buzzard diet where they overlap with Bank Vole in Ireland identified Bank Vole in 8.7% of prey items in pellets (n = 104), and 1.2% of prey remains of Buzzards during the breeding season (n = 85) (T. Nagle unpubl.…”

“…This is a similar overall frequency in dietary composition as Wood Mouse in the present study, and provides an early indication that this prey species will be readily incorporated into Buzzard diet as its range expands to overlap increasingly with that of Buzzard. The invading populations may potentially replace native prey in the order of importance in Buzzard diet, as they replace them in the environment (Montgomery et al 2012). If Buzzards were to take a substantial proportion of the populations of invasive species, there may be a competitive effect on the Kestrel and Barn Owl, and the potential release of corvids from constraints of direct predation, at the expense of primary prey, i.e.…”

“…However, the southern third of the island has been subject to the accidental introduction of two invasive small mammals, the Bank Vole Myodes glareolus (Claassens & O'Gorman 1965) and Greater Whitetoothed Shrew Crocidura russula (Tosh et al 2008). Where their ranges overlap, the negative effects of these invasive mammals on indigenous small mammals are cumulative and ultimately bring about species replacement (Montgomery et al 2012). The importance of these introduced prey species to predators in Ireland is not yet known, but small mammal specialists, the Barn Owl Tyto alba and Eurasian Kestrel Falco tinnunculus, depredate these invasive species heavily within the region of invasion (J. Lusby pers.…”

Invasive rodents, an overlooked threat for skinks in a tropical island hotspot of biodiversity

“…Cumulative impacts have been suggested for sympatric introduced rodents (Montgomery et al 2012). Here, our observations reveal that the rodent pressure is not homogeneous across the different skink species.…”