Consequences of reduced bird densities for seed dispersal
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
The decline in range and density of frugivorous birds worldwide could have consequences for the functioning of ecosystem processes such as seed dispersal. I endeavoured to determine the effects of bird declines on seed dispersal and assess the current status of dispersal in New Zealand. My first aim was to determine whether decreased bird density can reduce competitive interactions between birds, leading to birds concentrating on higher-reward fruit species. I measured fruit removal rates and fruit preferences of birds in Canterbury forest remnants with high (Hay and Prices) and low (Lords and Kaituna) bird densities. Removal rates of Melicytus ramiflorus and Coprosma spp. were lower at low-bird sites, and the size of this effect was greater for low-reward plant species. Coprosma areolata (a low-sugar fruit) had limited dispersal at Kaituna (59% fruit removed by end of 2012 season), compared with 92% removal at Hay, whereas 99-100% of M. ramiflorus and C. robusta (higher-sugar) fruits were removed at both sites. My second aim was to determine whether seed dispersal by introduced possums (Trichosurus vulpecula) can compensate for bird declines. I analysed seeds present in possum and bird faecal samples collected at Kowhai Bush, Kaikoura. Possums dispersed <3% of the total seeds, much less than bellbirds (21%), silvereyes (16%), song thrushes (33%) and blackbirds (28%). Possums also destroyed approximately 15% of seeds found in faeces, reduced the germination of gut-passed C. robusta seed to half of that from bird faeces (30% vs. 60-70%), and did not swallow fruits any larger than those moved by the much smaller birds (c. 7 mm diameter). My third aim was to determine the relationship between percentage of fruits dispersed and distance from parent tree for three large-seeded trees; Beilschmiedia tawa, Elaeocarpus dentatus, and Prumnopitys ferruginea. I fitted dispersal kernels to the observed dispersal distances out to 50 m for both undispersed whole fruits and seeds consumed by a bird, and found that dispersal quantity below the parent tree strongly underestimates total dispersal quantity. The average percentage dispersed overall was 81% for B. tawa, 75% for E. dentatus and 91% for P. ferruginea, and for all species finding only 11-18% clean seeds under the parent tree would correspond to an overall percent dispersed of at least 50% of the whole seed crop. My final aim was to determine the consequences of dispersal failure for recruitment in three plant species; Elaeocarpus hookerianus, Ripogonum scandens, and C. robusta. I compared the fate of dispersed and undispersed seeds in a manipulative field experiment. Exclusion of mammals plus removal of fruit pulp had the greatest effect on survival, while the combined effects of dispersal failure (under parent, high density, whole fruit) and inclusion of mammals decreased the number of live seedlings present at the final count by 75-92%. Overall, most native plants were receiving adequate dispersal and there was little evidence for strong risks to plant regeneration from dispersal failure, despite the reduced bird densities and ongoing negative effects of introduced mammals in New Zealand.