Abstract
Plant–plant interspecific competition via pollinators occurs when the flowering seasons of two or more plant species overlap and the pollinator fauna is shared. Negative sexual interactions between species (reproductive interference) through improper heterospecific pollen transfer have recently been reported between native and invasive species demonstrating pollination-driven competition. We focused on two native Impatiens species (I. noli-tangere and I. textori) found in Japan and examined whether pollinator-mediated plant competition occurs between them. We demonstrate that I. noli-tangere and I. textori share the same pollination niche (i.e., flowering season, pollinator fauna, and position of pollen on the pollinator's body). In addition, heterospecific pollen grains were deposited on most stigmas of both I. noli-tangere and I. textori flowers that were situated within 2 m of flowers of the other species resulting in depressed fruit set. Further, by hand-pollination experiments, we show that when as few as 10% of the pollen grains are heterospecific, fruit set is decreased to less than half in both species. These results show that intensive pollinator-mediated competition occurs between I. noli-tangere and I. textori. This study suggests that intensive pollinator-mediated competition occurs in the wild even when interacting species are both native and not invasive.
Highlights
Animal-pollinated plants provide flower visitors with rewards such as nectar, and pollinating animals in turn facilitate plant reproduction by dispersing pollen to conspecific plants
We address two questions: (1) Do I. noli-tangere and I. textori share their pollination niche? and (2) Is female fitness of both species decreased by interspecific competition (Does deposition of heterospecific pollen on stigmas reduce fruit set in both species)?
Our observations demonstrate that I. noli-tangere and I. textori share important pollination niche features (Figs 1 and 2)
Summary
Animal-pollinated plants provide flower visitors with rewards such as nectar, and pollinating animals in turn facilitate plant reproduction by dispersing pollen to conspecific plants. When two or more species whose flowering seasons overlap and share pollinators, two kinds of plant– plant interaction may occur, facilitation and competition, via the shared pollinators (Mitchell et al 2009). A number of studies have examined pollinator-mediated plant competition (Waser 1978; Fishman and Wyatt 1999; Mitchell et al 2009; Takakura and Fujii 2010; Muchhala and Thomson 2012; Ye et al 2014), studies of plants with specialist pollinators have long neglected it as a possible selective agent of such specialization (Waser et al 1996; Johnson and Steiner 2000; Gomez and Zamora 2006; Sargent and Ackerly 2008).
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