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.