The relative importance of birds and bees in the pollination of Metrosideros excelsa (Myrtaceae)

Abstract

Exclusion experiments were used to assess the effect of different pollinator groups on outcrossing and seed production in Metrosideros excelsa. The main study site was Little Barrier Island, New Zealand where indigenous bird and native solitary bees are the main flower visitors. Our results showed that native birds were more important pollinators of M. excelsa than native bees. Seed production was much higher in open pollination than in two exclusion experiments where either birds were excluded and native bees only had access to flowers, or where all pollinators had been excluded. The number of fertile seeds per capsule was 45% higher after open pollination than in treatments with bee visitation only and 28% higher than in treatments where all flower visitors were excluded. Estimated outcrossing rates were significantly higher (tm = 0.71) for open pollination in the upper canopy (>4 m above-ground level) where bird visitation is presumed to be more frequent than for a treatment with native bee access only (tm = 0.40). Our results also suggest that a large proportion of seeds (66%) arise from autonomous self-pollination when all pollinators are excluded. In four trees of a modified mainland population with predominantly introduced birds and a mixture of introduced and native bees there was no decrease in seed production for the treatment allowing bee access only, indicating that -in contrast to native bees -honeybees may be more efficient pollinators of M. excelsa. Observation of the foraging behaviour of both groups of bees showed that native bees contact the stigma of flowers less frequently than honeybees.This is likely to be a consequence of their smaller body size relative to honeybees.

Invasive rats and recent colonist birds partially compensate for the loss of endemic New Zealand pollinators

“…Previous studies have suggested that M. excelsa and K. excelsa are specialized for bird pollination on the basis of copious nectar production, the size of floral parts [25] and, in the case of M. excelsa, exclusion experiments [39]. However, the continued presence of these species at North Island sites where most native bird pollinators are absent has been suggested as evidence that invertebrates may be adequate pollinators [25].…”

“…The importance of mammal-mediated pollination in New Zealand may have been overlooked because most previous pollination studies either ignored the shorttailed bat or regarded it as an incidental flower visitor [24,25,39], despite reports of nectar-feeding behaviour and pollen in guano [45 -48]. As a consequence, the potential role of introduced mammals in maintaining pollination function for native plant species has not been seriously considered previously [45,46].…”

Biological flora of New Zealand 14:Metrosideros excelsa, pōhutukawa, New Zealand Christmas tree

“…Native birds are more important pollinators of M. excelsa than native bees because of their greater abundance, larger body sizes and foraging patterns which are more efficient at inducing seed set and promoting outcrossing. Similarly, introduced honeybees are more effective pollinators than native bees because their larger body size promotes stigma contact (Schmidt-Adam et al 2009). Nocturnal visitation by New Zealand's endemic geckos (Whitaker 1987) and short-tailed bat (Mystacina tuberculata) has also been reported (Arkins et al 1999;Pattemore & Biological flora of New Zealand 373…”

Severe pollen limitation in populations of the New Zealand shrub Alseuosmia macrophylla (Alseuosmiaceae) can be attributed to the loss of pollinating bird species

“…DEP conducted fieldwork in 2008and 2009 , and SHA conducted fieldwork in 2005and 2009. We thank the tribes Ngati Wai and Kawerau-A-Maki for permission to undertake this research in areas of importance to them.…”

“…Schmidt-Adam et al 2009;Anderson et al 2011;Pattemore & Wilcove 2012). The existence of island refuges that retain populations of bird species now locally extinct on the main North and South Islands affords an opportunity to compare pollination processes before and after pollinator loss (e.g.…”