The cascading effects of submerged macrophyte collapse on geese at Poyang Lake, China

Abstract

Abstract Wetland habitat degradation has become more common in recent decades, leading to an increasing occurrence of food shortages for wild animals. Although the direct effects of food shortages on consumers have been well studied, the cascading effects on interspecific interactions remain poorly understood. Targeted research is required to improve our understanding of the impacts of food shortage on animals and to facilitate the development of conservation plans. Here, we used DNA metabarcoding and field observations of goose foraging behaviour to assess the cascading effects of collapse of tubers of Vallisneria spp., the dominant submerged macrophytes at Poyang Lake, China, on the diet of the swan goose (Anser cygnoides) and on the interspecific competition among goose species, the swan goose, the bean goose (Anser fabalis), and the greater white‐fronted goose (Anser albifrons). The metabarcoding analyses were carried out on faecal samples collected in winter 2016/17 and 2018/19, and behavioural observations were conducted in winter 2021/22. Our study elucidated the dietary compositions of the three goose species in greater detail than previously known. When Vallisneria tubers became rare, swan geese switched their foraging focus to Carex spp. and other grassland plants. When tuber abundance rebounded, swan geese resumed tuber foraging, suggesting the superiority of tubers as food for swan geese. Swan geese foraging on Carex, the major foods of bean geese and greater white‐fronted geese, potentially increased interspecific competition among goose species. Swan geese increased their pecking rates while bean geese increased time spent foraging apparently to cope with interspecific competition. Given the severe impacts of tuber collapses on waterbirds and the important role of Poyang Lake in waterbird protection, we advocate restoring submerged macrophytes to provide high quality foods for tuber‐foraging waterbirds. Our study emphasises that, in addition to directly influencing consumers, food collapses may also influence other species through changes in interspecific interactions. Our study also illustrates the importance of behavioural plasticity to cope with interspecific competition and to respond to environment change.