Pollinator service affects quantity but not quality of offspring in a widespread New Zealand endemic tree species

Abstract

Human-mediated introductions of predatory mammals to New Zealand have led to massive bird extinctions and population declines. This reduction in bird abundance could have a variety of negative effects on the plants they pollinate, and this has led to recent management efforts to eliminate predatory mammals in some areas. In this study, we utilized variation in management efforts to determine the effects of pollinator service on progeny quantity and quality of an endemic New Zealand tree species, Fuchsia excorticata. Due to its gynodioecious breeding system (separate hermaphroditic and female plants), we were able to estimate effective pollination by scoring pollen deposition on females as a proxy for pollinator service, which we used to examine the effect of pollinator service on both the quantity of fruits produced and the quality of fruits, as measured by the selfing rate of hermaphrodites. We found that increased pollinator service increased progeny quantity, with a stronger effect in females, but had no effect on the quality via selfing rates. The combined effects of pollen limitation and loss of seeds due to inbreeding depression led to hermaphrodites having an estimated 24–80% reduction in the number of offspring reaching reproductive age, even in the highly managed populations. Our results suggest management efforts can have substantial impacts on progeny quantity, but progeny quality is likely influenced by more difficult to manage population or species-specific traits such as tree size, plant sex ratio, and pollinator behavior.