Introduction

The drivers of plant species distribution and abundance have been studied under different aspects. Three main drivers are usually suggested to shape plant species distribution and abundance: abiotic factors, dispersal and biotic interactions (Soberon, 2007; Boulangeat, Gravel, & Thuiller, 2012). Abiotic factors, such as soil moisture, temperature, and nutrient availability, influence species distribution and abundance in relation to a species’ fundamental niche (Chase & Leibold, 2003). Limited dispersal ability can prevent a species from reaching suitable habitats, even if these are available. Conversely, excellent dispersal ability can enable a species to colonize unsuitable sites through continuous immigration(Pulliam, 2000). Biotic interactions, including both competition and facilitation among plant species, as well as interactions with other trophic levels such as predation, herbivory, and pollination, can also influence species distribution and abundance (Meier et al., 2010).
Pollinators can mediate indirect biotic interactions by acting as vectors, even between non-neighboring individuals, due to their ability to move freely and cover long distances. While pollinators provide the essential function of pollination, they can also have negative effects. Recent studies have shown that among other things, pollinators can transfer viruses between different species (Fetters, 2023). More importantly, through the deposition of pollen mixes from different species on the stigma of a recipient flower, heterospecific pollen interference (HPI hereafter) can occur (T. L. Ashman & Arceo-Gómez, 2013). HPI refers to the reduction in reproductive output in the presence of heterospecific pollen (HP hereafter), despite the presence of conspecific pollen (CP hereafter) that could fertilize the ovules, potentially impacting the fitness of the recipient species (Morales & Traveset, 2008).
Previous studies have explored HPI between native and alien species (e.g. Suárez-Mariño, Arceo-Gómez, Sosenski, & Parra-Tabla, 2019, Malecore, Berthelot, Kleunen, & Razanajatovo, 2021). However, to the best of our knowledge, no study has specifically addressed the role of HPI between co-occurring rare and common species. Given that species distribution and abundance are crucial factors in determining a species’ endangerment status, understanding the mechanisms of heterospecific pollen interference for rare species could provide insights for both in-sit and ex-situ conservation strategies aimed at preserving plant communities.
To mitigate HPI, plant species can either avoid or reduce heterospecific pollen deposition or evolve tolerance to it (Arceo-Gómez, Raguso, & Geber, 2016; Streher, Bergamo, Ashman, Wolowski, & Sazima, 2020, Hao, Fang, & Huang, 2023). Avoidance or reduction mechanisms can occur at the pre-pollination stage through alterations in flower phenology, development of flower restrictiveness, reliance on specialized pollinators, or the use of different deposition sites on the pollinator’s body (Montgomery & Rathcke, 2012). Tolerance mechanisms occur at the post-pollination stage through pollen-stigma or pollen-pollen interactions. Tolerance is expected to evolve after exposure to heterospecific pollen. Therefore, in a plant community, if no avoidance or reduction mechanism prevents heterospecific pollen deposition, we can expect co-flowering species sharing common pollinators to evolve mechanisms to tolerate HPI.
In a co-flowering plant community, it is expected that common species receive more frequent visits from pollinators, while rare species receive fewer visits. Thus, according to the tolerance hypothesis (Hao et al., 2023), both rare and common species should be adapted to receive heterospecific pollen from other common species. On the other hand, both common and rare species should receive heterospecific pollen less frequently from other rare species. A reduced exposure means a lower need and chance to adapt to potential negative effects from heterospecific pollen. We predict that both common and rare species will experience HPI from rare donors but not from common donors.
The breeding system or self-compatibility of donor and recipient species could be another factor determining the strength of HPI for co-occurring species. Self-incompatible species present either mechanical or chemical mechanism to avoid self-pollination (Tom J. de Jong, Nickolas M. Waser, 1993) , and these mechanisms might similarly help in avoiding HPI. Thus, self-incompatible species could be better equipped against HPI. In a conservation context, self-compatibility could represent in some cases the only way for small populations to persist.
Another factor that has received attention in relation to HPI is the recipient-donor species relatedness. For example, due to similar recognition mechanism, it could be that only pollen from closely related species germinate on the stigma of the recipient species. Therefore, HPI might be reduced among distantly related species. While in a previous study we showed that the phylogenetic distance between recipient and donor species did not affect the overall strength of HPI (Malecore, Berthelot, Kleunen, & Razanajatovo, 2021), this pattern could change depending on the commonness or rarity of recipient and donor species.
In this study, we conducted hand-pollination experiments on a total of eight co-occurring and co-flowering species, collected from wild population. Five of these species are rare, and three are common in Switzerland. We will refer to species rarity or commonness with species status. We performed pairwise heterospecific pollen crosses as well as conspecific control treatments and measured seed set and seed number as our outcome variables. Seed set and seed number serve as proxies for reproductive success and thus of population viability. We asked following questions: 1) Does heterospecific pollen reduce seed set and seed number for common and rare recipient species and does this reduction depend on recipient and donor status? 2) Does heterospecific pollen reduce seed set for self-compatible and self-incompatible species and does this reduction depend on recipient and donor self-compatibility? 3) Does heterospecific pollen interference depend on recipient and donor relatedness? By addressing these questions, we aimed to shed new light on the complex interplay of factors that determine the distribution and abundance of plant species within co-flowering communities. Ultimately, gaining a deeper understanding of the mechanisms underlying heterospecific pollen interference could help inform conservation efforts aimed at preserving endangered species.