Fruiting failure and limited recruitment in remnant populations of the hawkmoth-pollinated tree Oxyanthus pyriformis subsp. pyriformis (Rubiaceae)

• Published in: Biological conservation 2004-11, Vol.120 (1), p.31-39
• Main Authors: Johnson, Steven D., Neal, Paul R., Peter, Craig I., Edwards, Trevor J.
• Format: Article
• Language: English
• Subjects: Animal, plant and microbial ecology; Applied ecology; Biological and medical sciences; Breeding system; Conservation; Conservation, protection and management of environment and wildlife; Forest ecology; Fundamental and applied biological sciences. Psychology; Parks, reserves, wildlife conservation. Endangered species: population survey and restocking; Pollen-limitation; Pollination; Reintroductions; Rubiaceae; South Africa; Sphingidae; Sporophytic self-incompatibility
• ISSN: 0006-3207; 1873-2917
• DOI: 10.1016/j.biocon.2004.01.028

Plants with highly specialized pollination systems may be particularly vulnerable to the effects of habitat fragmentation. The sub-canopy tree Oxyanthus pyriformis subsp. pyriformis has deep tubular flowers (c. 90 mm in length) that are specialized for pollination by long-tongued hawkmoths. Existing natural populations of this taxon appear to be confined to two suburban habitat fragments in the city of Durban, South Africa. Breeding system experiments showed that the trees are self-incompatible and thus completely reliant on pollen vectors for sexual reproduction. Hawkmoths were rarely observed to visit flowers, despite intensive monitoring over four seasons from 1997 to 2003. Fruit set as determined from examination of more than 150,000 flowers at the two sites was extremely low (0.04% and 0.07% of flowers), but increased to more than 70% in flowers that were hand-pollinated with pollen from another tree. This >1000-fold increase in the likelihood of fruit production following pollen-supplementation indicates that trees experience severe pollen-limitation of fruit set. Natural fruit set is so low that only a few dozen fruits were recorded in each population during two years of monitoring. This is of concern as O. pyriformis trees are dependent solely on seeds for recruitment. Demographic surveys showed that remaining natural populations have fewer saplings than adult trees and only two seedlings for every adult tree. The long-term viability of reintroduced populations of O. pyriformis is also in doubt, as we observed an almost complete lack of recruitment in a population that was established in a seemingly suitable forest site 20 years ago. Regular hand-pollination and, in some instances, further planting of saplings, may be the only way to safeguard these populations.