Mutualisms with the wreckage of an avifauna: the status of bird pollination and fruit-dispersal in New Zealand

Worldwide declines in bird numbers have recently renewed interest in how well bird–plant mutualisms are functioning. In New Zealand, it has been argued that bird pollination was relatively unimportant and bird pollination failure was unlikely to threaten any New Zealand plants, whereas dispersal mutualisms were widespread and in some cases potentially at risk because of reliance on a single large frugivore, the kereru (Hemiphaga novaeseelandiae). Work since 1989, however, has changed that assessment. Smaller individual fruits of most plant species can be dispersed by mid-sized birds such as tui (Prosthemadera novaezelandiae) because both fruits and birds vary in size within a species. Only one species (Beilschmiedia tarairi) has no individual fruits small enough for this to occur. Germination of 19 fleshy-fruited species, including most species with fruits >8 mm diameter, does not depend on birds removing the fruit pulp. The few studies of fruit removal rates mostly (7 out of 10) show good dispersal quantity. So dispersal is less at risk than once thought. In contrast, there is now evidence for widespread pollen limitation in species with ornithophilous flowers. Tests on 10 of the 29 known native ornithophilous-flowered species found that in 8 cases seed production was reduced by at least one-third, and the pollen limitation indices overall were significantly higher than the global average. Birds also frequently visit flowers of many other smaller-flowered native species, and excluding birds significantly reduced seed set in the three species tested. So pollination is more at risk than once thought. Finally, analyses of both species numbers and total woody basal area show that dependence on bird pollination is unexpectedly high. Birds have been recorded visiting the flowers of 85 native species, representing 5% of the total seed-plant flora (compared with 12% of those with fleshy fruit) and 30% of the tree flora (compared with 59% with fleshy fruit). A higher percentage of New Zealand forest basal area has bird-visited flowers (37% of basal area nationally) than fleshy fruit (31%). Thus, bird pollination is more important in New Zealand than was realised, partly because birds visit many flowers that do not have classic “ornithophilous” flower morphology.