Abstract
Mutualistic relationships between plants and their pollinators have played a major role in the evolution of biodiversity. While the vulnerability of these relationships to environmental change is a major concern, studies often lack a framework for predicting impacts from emerging threats (e.g. biological invasions). The objective of this study was to determine the reliance of Platanthera ciliaris (orange-fringed orchid) on Papilio palamedes (Palamedes swallowtail butterfly) for pollination and the relative availability of alternative pollinators. Recent declines of P. palamedes larval host plants due to laurel wilt disease (LWD) could endanger P. ciliaris populations that rely heavily on this butterfly for pollination. We monitored pollinator visitation and fruit set and measured nectar spur lengths of P. ciliaris flowers and proboscis lengths of its floral visitors in Jackson County, MS, USA. Papilio palamedes was the primary visitor with minimal visitation by Phoebis sennae (cloudless sulfur butterfly). Lengths of P. ciliaris nectar spurs were similar to proboscis lengths of both pollinator species. Fruit set was moderate with access to pollinators (55 ± 10.8 %), yet failed (0 %) when pollinators were excluded. Visitation increased with inflorescence size, but there was no such pattern in fruit set, indicating that fruit set was not limited by pollinator visitation within the range of visitation rates we observed. Our results are supported by historical data that suggest P. palamedes and P. sennae are important pollinators of P. ciliaris. Although P. sennae may provide supplemental pollination service, this is likely constrained by habitat preferences that do not always overlap with those of P. cilaris. Observed declines of P. palamedes due to LWD could severely limit the reproductive success and persistence of P. ciliaris and similar orchid species populations. This empirical-based prediction is among the first to document exotic forest pests and pathogens as an indirect threat to plant-pollinator interactions.
Highlights
It is estimated that 87.5 % of all flowering plants are pollinated by animals (Ollerton et al 2011)
Papilio palamedes represented 92 % of our observations while it accounted for only 63 % of visits (2-year average) in the surveys conducted by Robertson and Wyatt (1990a); despite inter-annual variation in the total number of individuals they observed, the proportion of visits made by P. palamedes and P. sennae was consistent between years (Robertson and Wyatt 1990a)
If nectar spur lengths are optimal for ensuring pollination, it remains unclear why these results suggest a lower rate of fruit set than what has been observed in other populations (Robertson and Wyatt 1990a)
Summary
It is estimated that 87.5 % of all flowering plants are pollinated by animals (Ollerton et al 2011). Bateman’s principle of sexual selection suggests that the reproductive output of female plants (seed set and maturation) is limited by resource availability rather than access to mates (pollen receipt) (Bateman 1948; Janzen 1977; Wilson et al 1994), reviews of empirical data indicate that reproductive. Chupp et al — Variability and vulnerability of orchid pollination success is commonly (and often severely) limited by pollen/pollinator availability (Burd 1994; Ashman et al 2004). It is clear that interactions between plants and their pollinators have played a major role in the evolution of biodiversity. Human activities have posed serious threats to the maintenance of these relationships
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