Discussion
Plant arthropod interaction studies associated with restoration efforts
are becoming more common due to greater appreciation of the role
arthropods play in the survival and reproductive output of plants
through positive (pollination) and antagonistic (herbivory) interactions
(Bucharova et al. , 2021; Cariveau, Bruninga-socolar and Pardee,
2021; Sabatino, Rovere and Meli, 2021). While our study did not detect
obvious pollinators, we recorded a high number of arthropod individuals
that could be incidental pollinators and inadvertently pollinateL. nipomensis while feeding on pollen or other plant resources
(Gill, 1991). We observed over 200 Thysanoptera individuals present inL. nipomensis flowers (Table 1). Thysanoptera have been known to
pollinate members of several angiosperm families, including fabaceous
plants (Velayudhan and Annadurai, 1986; Varatharajan et al. ,
2016). However, Thysanoptera are well known flower pests that consume
pollen, potentially causing withering of flowers and lowering plant
reproductivity (Reitz, 2009). We also observed arthropods known to be
important pollinators (i.e., Apis mellifera Linnaeus (Apidae))
visiting neighboring plants, including another species of lupine, but
never interacting with L. nipomensis (Aslan, Galindo and Service,
2016; Hung et al. , 2018). The record of these arthropods visiting
other, nearby plants indicates that while pollinators such as Apis
mellifera were present, these arthropods were not visiting L.
nipomensis . If cross-pollination is occurring between L.
nipomensis individuals, this service is being performed by incidental
pollinators, such as Thysanoptera, and not being performed by arthropods
such as Apidae, known to pollinate other lupine species (Williams,
1987).
Arthropod genera were observed that could affect the reproductive
success of L. nipomensis due to herbivory. The Dipteran found,Delia lupini (Anthomyiidae), was collected from a gall present on
a L. nipomensis individual and has been observationally
implicated to reduce fecundity in a previous study of L.
nipomensis (Walters and Walters, 1988). While some galls aid in
pollination and nutritional services, most are detrimental to plant
health and in some cases, have been shown to threaten endangered plant
species (Kolesik et al. , 2019; Harris and Pitzschke, 2020). The
impact of D. lupini gall presence on L. nipomensisreproductivity was not quantified in this study; however, it is possible
there were additional individuals we did not observe that are affecting
the fecundity of this lupine. Formicidae species we observed includedLinepithena humile , the invasive Argentine ant (Holway, 1999).
Argentine ants have been shown to impact floral visitation patterns and
nesting success of other arthropods with the potential to create
cascading, negative effects and reduce pollinator visitation (Underwood
and Fisher, 2006; Sahli et al. , 2016; Plentovich et al. ,
2021). The presence of a gall-inducing and invasive arthropods may
further inhibit recruitment within this singular extant population ofL. nipomensis .
Plants face both biotic and abiotic barriers to reproductive success,
and for rare, endangered species, these barriers can ultimately result
in extirpation or extinction (Rejmánek, 2018). Small populations, like
that of L. nipomensis , can face pollination limitation, as small
plant populations often do not attract pollinators due to low pollen
rewards (Shi, Michaels and Mitchell, 2005). A reduction in pollination
leads to reduced outcrossing, ultimately resulting in a lower genetic
diversity that further threatens already small, rare plant populations
(Gray, 2019). Simultaneously, herbivorous arthropods can impact seed
production and recruitment of plant species (Lucas-Barbosa, 2016).
Active intervention may be necessary to promote outcrossing via hand
pollination, as well as protect L. nipomensis from herbivorous
arthropods to ensure the genetic diversity and successful establishment
of new individuals (Walsh et al. , 2019; Serrano et al. ,
2021).
Our results suggest that if cross-pollination is occurring at all amongL. nipomensis individuals, it is only being performed by
incidental pollinators, and that herbivorous arthropods are present that
may threaten fecundity of this lupine. Additional work is necessary
determine the frequency of cross-pollination and whether the potential
threats this lupine faces from herbivorous arthropods will affect the
establishment of novel populations during restoration efforts.