Legacy of avian-dominated plant–herbivore systems in New Zealand

Avian herbivores dominated New Zealand’s pre-settlement terrestrial ecosystems to an unparalleled extent, in the absence of a terrestrial mammal fauna. Approximately 50% (88 taxa) of terrestrial bird species consumed plant foliage, shoots, buds and flowers to some degree, but fewer than half these species were major herbivores. Moa (Dinornithiformes) represent the greatest autochthonous radiation of avian herbivores in New Zealand. They were the largest browsers and grazers within both forest and scrubland ecosystems. Diverse waterfowl (Anatidae) and rail (Rallidae) faunas occupied forests, wetlands and grasslands. Parrots (Psittacidae) and wattlebirds (Callaeidae) occupied a range of woody vegetation types, feeding on fruits/seeds and foliage/ fruits/nectar, respectively. Other important herbivores were the kereru (Columbidae), stitchbird (Notiomystidae) and two honeyeaters (Meliphagidae). Cryptic colouration, nocturnal foraging and fossil evidence suggest that avian populations were strongly constrained by predation. With the absence of migratory avian herbivores, plant structural, constitutive defences prevailed, with the unusual ‘wire syndrome’ representing an adaptation to limit plant offtake by major terrestrial avian browsers. Inducible plant defences are rare, perhaps reflecting longstanding nutrient-limitations in New Zealand ecosystems. Evidence from coprolites suggests moa were important dispersers of now rare, annual, disturbance-tolerant herb species, and their grazing may have maintained diverse prostrate herbs in different vegetation types. The impact of moa on forest structure and composition remains speculative, but many broadleaved woody species would likely have experienced markedly reduced niches in pre-settlement time. Several distinctive avian-mediated vegetation types are proposed: dryland woodlands, diverse turf swards, coastal herb-rich low-forest-scrubland, and conifer-rich forests. Since human settlement (c. 750 yrs ago), c. 50% of endemic avian herbivore species or c. 40% overall have become extinct, including all moa, 60% of waterfowl and 33% of rail species. Numerically, avian herbivore introductions (c. 24 taxa) since European settlement have compensated for extinctions (c. 27 taxa), but the naturalised birds are mostly small, seed-eating species restricted to human-modified landscapes. Several naturalised species (e.g. Canada goose, Branta canadensis; brown quail, Coturnix ypsilophorus) may provide modes and levels of herbivory comparable with extinct species. The original avian and current introduced mammal herbivore regimes were separated by several centuries when New Zealand lacked megaherbivores. This ‘herbivory hiatus’ complicates comparisons between pre-settlement and current herbivore systems in New Zealand. However, predation, animal mobility, feeding mode, nutrient transfer patterns and soil impacts were different under an avian regime compared with current mammalian herbivore systems. Levels of ecological surrogacy between avifauna and introduced mammals are less evident. Ungulates generally appear to have impacts qualitatively different from those of the extinct moa. Because of New Zealand’s peculiar evolutionary history, avian herbivores will generally favour the persistence of indigenous vegetation, while mammalian herbivores continue to induce population declines of select plant species, change vegetation regeneration patterns, and generally favour the spread and consolidation of introduced plant species with which they share an evolutionary history.