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

Abstract

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.

Introduction of mammalian seed predators and the loss of an endemic flightless bird impair seed dispersal of the New Zealand tree Elaeocarpus dentatus

“…Fruits were classed as either passed through a frugivore (fruit skin removed but no visible chew damage), preyed on by native parrots (endocarp cleaved in half, destroying the seed inside), chewed by possums (exocarp and mesocarp removed with chew marks), or intact whole fruits found under parent trees (both ripe and unripe). Fruits that had passed through a frugivore were distinguished by their slippery texture, with some mesocarp remaining on the seed (Carpenter et al., ). Further seed trap data were obtained from the Department of Conservation's national seed rain monitoring network, which gave fruit handling indices from an additional mainland site at Pelorus Bridge (Marlborough), about 35 km west of Essons Valley.…”

“…Pelorus Bridge used elevated conical seed traps with a catching area of 0.28 m 2 ; see Carpenter et al. () for a description. Kereru, bellbirds, and tui occurred in low numbers at this site (Carpenter et al., ; Robertson et al., ).…”

“…() for a description. Kereru, bellbirds, and tui occurred in low numbers at this site (Carpenter et al., ; Robertson et al., ). Rodents and possums were also present.…”

“…Fruits were classed as either passed through a frugivore (fruit skin removed but no visible chew damage), preyed on by native parrots (endocarp cleaved in half, destroying the seed inside), chewed by possums (exocarp and mesocarp removed with chew marks), or intact whole fruits found under parent trees (both ripe and unripe). Fruits that had passed through a frugivore were distinguished by their slippery texture, with some mesocarp remaining on the seed (Carpenter et al, 2017). (2017) for a description.…”

“…One species that meets these criteria is hinau (Elaeocarpus dentatus: Elaeocarpaceae), an endemic forest tree with large, shiny dark brown fruits that feature a very thick endocarp and drop to the ground when ripe (Lord, 2002). Hinau currently appears to have very low seed dispersal rates from the canopy F I G U R E 1 Still capture from trail camera footage, showing a weka Gallirallus australis approaching a depot of hinau Elaeocarpus dentatus fruits on Blumine Island (10%-28% of seeds captured beneath parent trees have passed through a bird; Carpenter, Kelly, Clout, Karl, & Ladley, 2017), but it is unclear whether these low rates are due to low local numbers of volant frugivores, or because its seeds are adapted for ground removal by flightless frugivores. The only extant frugivores recorded consuming hinau fruits are volant kereru (Hemiphaga novaeseelandiae) and kokako (Callaeas wilsoni), and flightless weka and brown kiwi (Apteryx mantelli) (Clout & Hay, 1989;Kelly et al, 2010), three of which (kokako, brown kiwi, and weka) are severely range restricted.…”