Effects of changes in bird community composition and species abundance on plant reproduction, through pollination and seed dispersal

Abstract

The consequences of global bird declines on ecosystem function are gaining increasing attention. Here we consider the effects of changes in bird community composition and species abundance on reproduction for two endemic plant species in New Zealand. Birds service a relatively high proportion of the New Zealand flora as both pollinators and seed dispersers, and bird losses have been extensive during the 700 years since human colonization. We compare fruit set and seed removal for plants in reserves free of introduced predators, where seven endemic flower and fruit visiting bird species were present and relatively abundant, with plants outside these reserves where endemic bird species were absent or at lower abundance. The study plants, Dysoxylum spectabile and Pittosporum crassifolium, have both flowers and fruit visited by birds. Both species are also dioecious (a characteristic that has not been commonly associated with bird‐pollinated plants) and despite having mixed pollination systems, bird visits are essential for successful seed production. Outside introduced predator‐free reserves, the winter‐flowering D. spectabile was still visited by endemic pollinators and maintained pollination levels, despite a 59% decrease in endemic bird abundance. However, reduced endemic bird abundance was associated with a halving of pollination levels in P. crassifolium, which flowers in spring, as do other native and introduced flowering plants attractive to birds. The effect of altered bird species composition and abundance on seed dispersal was associated with a 74% reduction in seed dispersal for P. crassifolium. Additionally, undispersed seeds of P. crassifolium had lower germination, and higher pre‐dispersal seed predation, compared with dispersed seeds. We stress three important aspects of these results: (1) for some plant species, the same bird species perform both pollination and seed dispersal, (2) dispersal reduces pre‐dispersal seed predation, so that dispersal failure secondarily increases losses to seed predators, and (3) the effects of mutualism loss are not even across all species. Our results highlight that introduced species do not adequately replace endemic mutualists, which may be a particular problem for plant species with both bird‐visited flowers and fruit.

Ecological Impacts of Exotic Species on Native Seed Dispersal Systems: A Systematic Review

“…Similarly, the long-tailed macaque disperses but also destroys native seeds, and facilitates plant invasions [ 25 ], while exotic birds can disperse both native and exotic plants species via epi- and endozoochory [ 27 , 36 ]. Other exotic birds can surrogate dispersal services in depauperated frugivory communities, but their reduced efficiency can lead to low germination rates and increased pre-dispersal predation in undispersed seeds of native plants [ 31 ].…”

“… Source Tittle Location Exotic Taxa Mechanisms and Impacts [ 24 ] Tree recruitment after native frugivore extinction? A field experiment to test the impact of fruit flesh persistence in a tropical oceanic island Africa (Mauritius) Lissachatina immaculata ; Rattus rattus (−|+) Potential herbivory (by L. immaculata ; seed dispersal (by L. immaculata and R. rattus ) [ 31 ] Effects of changes in bird community composition and species abundance on plant reproduction, through pollination and seed dispersal Oceania (New Zealand) Pittosporum crassifolium (−|+) Low germination and high pre-dispersal predation in undispersed seeds; seed dispersal [ 23 ] Exotic guavas are foci of forest regeneration in Kenyan farmland Africa (Kenya) Psidium guajava (+) Forest regeneration, contribution to native seed dispersal and seedling recruitment [ 67 ] Biodiversity impacts of an invasive grass: ant community responses to Cenchrus ciliaris in arid Australia Oceania (Australia) Cenchrus ciliaris , ants (0) Seed removal by ants with unknown effects [ 32 ] Environmental and biological factors affecting the abundance of Prosopis flexuosa saplings in the central-west Monte of Argentina South America (Argentina) Bos primigenius taurus, Equus africanus asinus, E. ferus caballus , E. mulus ...…”

Evaluating the success of functional restoration after reintroduction of a lost avian pollinator

“…Second, hihi may be effective pollinators, but restoring function may not be as simple as reintroducing them to the ecosystem. Elsewhere in New Zealand, controlling introduced predators has increased native pollinator populations without demonstrably improving function (Kelly et al., 2005; Anderson et al., 2021 [but see Iles & Kelly, 2014; Bombaci et al., 2021]); some evidence suggests that pollinator numbers were too low to achieve full functionality (Kelly et al., 2005). It is possible our assumption was incorrect that hihi populations had reached sufficient densities to improve pollination; if so, further management may be needed to promote population growth, and our results could provide a baseline against which to compare future functional improvements.…”

“…Elsewhere in New Zealand, controlling introduced predators has increased native pollinator populations without or absence (−) (differing letters, significant differences across all means; stars, significant differences between conditions [lines] and interactions with hihi presence [arcs]). Region either had a marginally nonsignificant (a) or significant (b−c: Auckland > Wellington) effect (Table 1), so all points, means, and SEs (bars) are from models with region as a covariate demonstrably improving function (Kelly et al, 2005;Anderson et al, 2021[but see Iles & Kelly, 2014Bombaci et al, 2021]); some evidence suggests that pollinator numbers were too low to achieve full functionality (Kelly et al, 2005). It is possible our assumption was incorrect that hihi populations had reached sufficient densities to improve pollination; if so, further management may be needed to promote population growth, and our results could provide a baseline against which to compare future functional improvements.…”

“…Historically, birds were thought to make only incidental contributions to pollination because the majority of native plants have an entomophilous flower syndrome (Clout & Hay, 1989; Godley, 1979). However, more recent studies show that several native birds are important (Anderson, 2003; Anderson et al., 2021; Kelly et al., 2010) or essential to pollination (Anderson et al., 2011). Translocations are frequently used to restore native bird populations, with many motivated at least in part by ecosystem restoration (Parker, 2013), but few actively test whether restoration is achieved.…”