Birds and small mammals in kanuka (Kunzea ericoides) and gorse (Ulex europaeus) scrub and the resulting seed rain and seedling dynamics

Native kanuka (Kunzea ericoides) and adventive gorse (Ulex europaeus) stands aged 10-14 years, and not grazed by domestic stock, were studied near Nelson, New Zealand. The aim was to determine their use by introduced small mammals, and native and adventive birds, and the effects of these animals on seed rain and seedling dynamics as factors influencing vegetation succession. Seed traps were established where they could catch only bird-dispersed or wind-blown seed, and seedling emergence and growth were monitored. Bird abundance was estimated by five-minute bird counts, and small mammal abundance by trapping. The summed frequencies of all birds, and those likely to disperse seeds, were similar in kanuka and gorse. The endemic native bird species, bellbirds (Anthornis melanura) which are omnivorous, brown creepers (Mohoua novaeseelandiae) and grey warblers (Gerygone igata) which are insectivorous, were more frequent in kanuka than in gorse, while fantails (Rhipidura fuliginosa) were equally frequent in both stands. Non-endemic silvereyes (Zosterops lateralis) which are omnivorous were the most abundant seed-dispersing species, and they were significantly more frequent in gorse, as were adventive California quail (Callipepla californica) which are granivorous. Other small adventive granivores and omnivores were either more frequent in kanuka or gorse, or equally common in both stands. Ship rats (Rattus rattus) and possums (Trichosurus vulpecula) were in low numbers throughout. Mice (Mus domesticus) were more frequent in the gorse, probably because of the shelter offered by the dry gorse litter, and food supply, e.g. gorse seed. More seeds of native, fleshy-fruited shrubs fell in the kanuka, largely those of Coprosma spp. and Cyathodes juniperina, which grow in the kanuka. Seed species richness was similar in kanuka and gorse. In both cases, the seed rain appeared more influenced by the local seed source than by the different bird communities. In both kanuka and gorse, the relationship between seed rain and seedling numbers was close only for the most common fleshy-fruited species. Seedling emergence and survival was greater in gorse because of openings in the canopy, and the lower density of the introduced hares and rabbits. Overall, the different morphology and structure of the adventive gorse probably have the greatest influence in seedling dynamics, and ultimately on vegetation succession.