Introduction

Different aspects of the drivers of plant species distribution and abundance have been studied. 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 and 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 at other trophic levels such as predation, herbivory, and pollination, can also influence species distribution and abundance (Meier et al. 2010). Pollinators can mediate indirect plant-plant 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, pollen mixes from different species can move between different donor and recipient species, a mechanism known as interspecific pollen transfer. Interspecific pollen transfer has two components: conspecific pollen loss and heterospecific pollen interference (HPI hereafter) (Morales and Traveset 2008; Ashman and Arceo-Gómez 2013). Conspecific pollen loss refers to the reduction of pollen transferred between conspecific flowers due to loss to a heterospecific recipient. 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. HPI thus can potentially impact the female fitness of the recipient species through reduced seed set (Morales and Traveset 2008), while conspecific pollen loss can impact the male fitness of the donor species (Waser 1978) through reduced pollen transfer. 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 interspecific pollen transfer and in particular of HPI between co-occurring rare and common native 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 populations. 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 and 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 overall common and abundant 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 (de Jong & 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, thus a higher susceptibility to HPI for rare self-compatible species could further endanger them. 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 populations in Switzerland. Five of these species are rare, and three are common in Switzerland. We will refer to species rarity or commonness as 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. 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 recipient 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.