Trends in the detections of a large frugivore (Hemiphaga novaeseelandiae) and fleshy-fruited seed dispersal over three decades

The kereru (Hemiphaga novaeseelandiae) is a large fruit pigeon that in New Zealand is an important seed disperser for native plant species. However, little is known about recent changes in kereru densities and how these changes might affect seed dispersal services. We used long-term kereru counts and seedfall trap data from Pelorus in Marlborough to measure trends in bird abundance and seed dispersal. Using monthly kereru counts from 1983–1989 and 2002–2006, we found that counts significantly decreased between the two decades. Most of this decline was driven by changes in the seasonal abundance of kereru: a pronounced late-winter/spring peak in numbers in the 1980s had almost vanished by the 2000s. The late-winter/spring increase in kereru in the 1980s was probably driven by kereru moving into the area to feed on lowland foliage. Therefore, the reduction of late-winter/spring kereru in the 2000s could be driven by either a change in regional movement patterns or an authentic decline in the kereru population. Seedfall data for six fleshy-fruited trees (tawa Beilschmiedia tawa, miro Prumnopitys ferruginea, matai Prumnopitys taxifolia, hinau Elaeocarpus dentatus, rimu Dacrydium cupressinum, and kahikatea Dacrycarpus dacrydioides) from 1986–1990 and 2004–2010 allowed estimation of the percentage of each fruit crop handled by frugivores (an index of dispersal quantity). We found that the percentage of seeds handled by frugivores was higher in the 2000s than in the 1980s for tawa, and lower for matai. Seed handling rates were unchanged between the two decades for miro, hinau, rimu, and kahikatea. Over this time period there was no overall worsening in dispersal quantity between the two decades, probably because kereru numbers did not change significantly during the autumn fruiting season, and because other birds could be important dispersers for smaller-seeded species.