Influence of vegetation structure on spatial patterns of seed deposition by birds

Abstract

The bird‐disseminated species in the seed rain and the soil seed bank were sampled on Mana Island, New Zealand, to assess the importance of communally roosting starlings (Sturnus vulgaris) as seed dispersers. Density and species composition of dispersed seeds were compared between forested sites and grassland, representing potential perch sites and non‐perch sites, respectively, to determine the influence of vegetation structure on deposition patterns of bird‐dispersed seeds. Mean density of bird‐dispersed seeds falling in forest was 3742 ±914 seeds m‐2 compared with 7.7 ± 1.0 seeds m‐2 in grassland. This enormous disparity was reflected in the seed bank, with mean densities of 86 852 ± 16 579 and 883 ± 189 seeds m‐2 for forest and grassland sites, respectively. Species richness in forest ranged from 3 to 23 species 0.25 m‐2 for the seed rain, and from 1 to 19 species 0.002 m‐2 for the seed bank. Relative abundance of most species differed between the seed rain and seed bank. The seed rain was dominated by native woody species (66.9% relative density of identified species); alien species comprised 26.6%. Native woody species were proportionally less important in the seed bank (51.1%) which was instead characterised by an increased proportion of alien species (45.3%), some of which may have persistent seed banks. The potential role of frugivorous birds in facilitating succession from grassland to native forest may be inhibited by a lack of dispersal of native seeds into grassland areas, and by the longevity of seeds of alien species in the soil.