Potential disruption of seed dispersal in the absence of a native Kauai thrush

Abstract

Hawaii has experienced a catastrophic decline in frugivorous native birds coupled with the introduction of non-native species. Puaiohi (Myadestes palmeri), a critically endangered thrush, is the sole extant native songbird capable of dispersing fleshy fruited plants in the rainforest of Kauai island, Hawaii. As this species has declined to occupy a small proportion of its original range, a suite of largely omnivorous non-native birds have been introduced to this region, including the common and widespread Japanese White-eye (Zosterops japonicus). This reshuffling of the bird community could have long-term implications for plant community composition if introduced birds incompletely replace the ecological role of native species. The objective of this study was to evaluate the potential consequences of the local extirpation of Puaiohi for seed dispersal. Specifically, we compared the diet of Puaiohi and Japanese White-eye, vegetation characteristics, and seed rain at sites with and without Puaiohi in the Na Pali-Kona Forest Reserve on the island of Kauai. We found high overlap in the composition of seeds consumed by the two bird species, but differences in the characteristics of seeds consumed; Japanese White-eye appeared more likely to consume smaller seeded species compared with Puaiohi. Sites with Puaiohi received substantially higher seed rain during the study period, despite no significant differences in overall fruit abundance. Our results suggest that non-native birds are unlikely to completely replace the seed dispersal services provided by Puaiohi. If Puaohi continue to be rare and range restricted, we predict a shift in plant community composition through an increase in non-native and small-seeded plants, and possible dispersal failure of other native species. Our findings lend further support to efforts to conserve Puaiohi across its current and former range, and to consider introductions to other suitable areas to ensure the persistence not only of the species and but also its functional role in Hawaii’s montane ecosystems.

Functional traits of avian frugivores have shifted following species extinction and introduction in the Hawaiian Islands

“…Recent studies of seed dispersal in Hawaii found non‐native birds frequently consume fruit with small seeds (Chimera & Drake, 2010; Culliney et al., 2012; Pejchar, 2015; Vizentin‐Bugoni et al., 2019) and in studies using seed rain traps, the maximum seed width of animal‐dispersed seeds was 2.23–8.1 mm, depending on the study (Appendix S3: Tables S1–S3; Chimera & Drake, 2010; Foster & Robinson, 2007; Hruska, 2019; Kaushik, Pejchar, & Crampton, 2018; Pejchar, 2015). While more work is needed to evaluate the roles of each disperser in the modern assemblage, from what is known, 8.1 mm in width appears to be the largest seed that is dispersed (into seed rain traps) by modern frugivores.…”

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

“…Hence, the introduction of a new frugivore differing in size from the native/s one/s might involve a different spatial distribution of seed sizes, which in turn might have implications for the plants, both at the population and community level (Kaushik et al. ).…”

“…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).…”

“…In the particular case of animal-mediated seed dispersal, larger frugivores are known to ingest larger seeds in greater number, and transport them to larger distances (e.g., Wotton and Kelly 2012;Pérez-Méndez et al 2016). Hence, the introduction of a new frugivore differing in size from the native/s one/s might involve a different spatial distribution of seed sizes, which in turn might have implications for the plants, both at the population and community level (Kaushik et al 2018).…”

Invasive birds: global trends and impacts

“…Hawaii, for example, has more invasive land bird species (58 species) than native species, and the implications of removing these must be carefully considered, as some fulfil important ecological niches of extinct native species (Kaplan, 2007). However, invasive birds disperse both native and invasive plants in Hawaii and do not fulfil the complete ecological role of larger native frugivores (which are becoming increasingly rare) due to their smaller gape sizes Kaushik et al, 2018).…”