Introduced blackbirds and song thrushes: useful substitutes for lost mid-sized native frugivores, or weed vectors?

Abstract

Abstract:The New Zealand avifauna has declined from human impacts, which might leave some larger-seeded native plants vulnerable to dispersal failure. We studied fruit dispersal in a lowland secondary forest near Kaikoura, where the only remaining native frugivores are relatively small (silvereye Zosterops lateralis, and bellbird Anthornis melanura). We tested whether two larger exotic frugivores (blackbird Turdus merula and song thrush T. philomelos) dispersed native plants with seeds too large for the two smaller native frugivores. Diet breadth was measured by identifying seeds in the faeces of 221 mist-netted birds, and by observations of birds foraging. We then compared the plant species dispersed to the range of locally available fruits. All four bird species had varied diets (6-9 plant species per bird species) that differed significantly, although Coprosma robusta was always the most-eaten fruit. As predicted, the maximum fruit size eaten was larger for exotic birds (11.3 mm diameter) than natives (7.4-7.7 mm diameter), but all birds ate mainly smaller fruits. However, 7/21 fruiting plant species were not seen to be dispersed by any species, and the chance of being undispersed was independent of fruit size. Blackbirds and song thrushes jointly dispersed all four woody weeds with fruits >7.5 mm diameter, but neither of the two similar-sized native plants. Although the two species of exotic birds dispersed some native plants, our study suggests that their net effect is negative through facilitating the spread of invasive weeds. Studies evaluating the contribution of exotic frugivores to novel plant communities need to distinguish potential effects (what the frugivores might be capable of doing) from actual effects (what the frugivores are observed doing).

Disturbance maintains native and exotic plant species richness in invaded grassy woodlands

“…), but if they spread exotic seeds through faeces or destroy desirable native species, the overall effect may be detrimental (MacFarlane et al. ).…”

“…Future research could assess whether generalist seed predators can be attracted to restoration sites to reduce exotic plant species. If granivores destroy seeds or seedlings of exotic plants, this may be of local conservation benefit (Pearson et al 2014), but if they spread exotic seeds through faeces or destroy desirable native species, the overall effect may be detrimental (MacFarlane et al 2016).…”

Conflicting selection onCneorum tricoccon(Rutaceae) seed size caused by native and alien seed dispersers

“…In some cases, alien species act as beneficial partners of native species, and to some extent can be functionally adequate replacements of native counterparts (Kawakami et al 2009;Burns 2012;García et al 2014;Pejchar 2015). In fact, in some cases, these alien species may be the only available partners acting as "rescuers" of mutualistic interactions and the derived ecosystem functions (Foster and Robinson 2007;Muñoz-Gallego et al 2019), despite having an overall negative effect on the native community by, for instance, enhancing further invasions (e.g., Kawakami et al 2009;Pejchar 2015;Wotton and McAlpine 2015;MacFarlane et al 2016;Kaushik et al 2018).…”

“Liaisons dangereuses”: The invasive red‐vented bulbul (Pycnonotus cafer), a disperser of exotic plant species in New Caledonia

“…This is encapsulated in Simberloff and Von Holle's () “invasional meltdown” hypothesis that postulates that mutualistic interactions between invaders can facilitate secondary invasions (Green et al., ). This phenomenon has been demonstrated widely in island territories (Bourgeois, Suehs, Vidal, & Médail, ; Davis, O'Dowd, Mac Nally, & Green, ; Traveset & Richardson, ), with positive interactions having been reported between introduced plants and birds (MacFarlane, Kelly, & Briskie, ; Traveset & Richardson, ).…”