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.