Introduction
Plant-arthropod interactions, through processes such as pollination, herbivory, and frugivory, play an important role in the reproductive success of plant species (Strong et al. 1995, Nemec and Bragg, 2008; Schweizer et al. 2013). Low pollinator visitation diversity or abundance can explain reduced distribution, low reproductive output, or failure to establish novel plant populations in habitats that are otherwise abiotically suitable (Karron, 1987; Kearns, Inouye and Waser, 1998). Herbivorous arthropods meanwhile have been shown to impact establishment and seed production of plants (Bevill, Louda and Stanforth, 1999; Münzbergová and Herben, 2005). Both the mutualistic and antagonistic interactions between plants and arthropods are important drivers in determining the survival of a plant (Stahl, Hilfiker and Reymond, 2018).
Despite the importance of arthropod interactions to plant species, current conservation efforts to restore endangered plants often prioritize the presence of appropriate abiotic conditions to select reintroduction sites (Falk, Millar and Olwell, 1996; Guerrant and Kaye, 2007; Godefroid et al. , 2011). Knowledge of pollination and herbivory has been shown to be important in maximizing reproduction and increasing establishment success of rare plant species (Archer and Pyke, 1991; Kay, 2008; Ancheta and Heard, 2011). Pollination is especially important to endangered species with small populations, not only to maintain the population size, but also to increase outcrossing of individuals (Steffan-Dewenter, Mïnzenberg and Tscharntke, 2001; Reiteret al. , 2017; Horth, 2019). Maintaining genetic diversity and reducing the likelihood of inbreeding depression is essential to the conservation of rare plant species (Falk, 1990; Lee et al. , 2018).
Lupinus nipomensis Eastw. (Fabaceae, Nipomo Mesa lupine) is a state and federally endangered annual forb endemic to Nipomo, California (USFWS 2009; Fig. 1). Historically, L. nipomensis has occurred at low densities in back dunes and inter-dune habitat. The loss of coastal back dune habitat due to land conversion, fragmentation, and competition with the invasive perennial veldt grass (Ehrharta calycina Sm., Poaceae) limit the range and potential for natural regeneration of theL. nipomensis populations (Skinner and Pavlik, 1994). The entire extant population is geographically isolated within a 5 km2 area along the central California coast in the Guadalupe-Nipomo Dune Complex and is comprised of seven dispersed colonies. Total population size is dependent on winter and spring climatic conditions and ranges between 139 and 771 individuals per year (USFWS, 2019).
The reintroduction of Nipomo Mesa lupine, and other rare plants with a limited initial population size, calls for abiotic requirements to maximize reproductive output from reintroduction efforts. Luong, Nolan, and Stratton (2019) provides an overview of abiotic microhabitat characteristics (i.e., landscape slope and aspect) and seed treatment relevant to L. nipomensis fecundity. A foundational study ofL. nipomensis by Walters and Walters (1989) primarily focused on abiotic drivers of reproduction, specifically changes observed in flowering and fruit set as a factor of rainfall received; this publication also includes a record of herbivorous arthropod interactions, but did not specifically include pollinator observations. Although other annual lupines have had interactions with known pollinators, the absence of pollinator observations has led researchers to hypothesize that L. nipomensis is capable of both self-pollinating as well as outcrossing (USFWS 2019).
In this study, we sought to establish a baseline of plant-arthropod interactions of in situ L. nipomensis to inform future restoration efforts and especially targeted pollinator observations, which have not previously been studied. We surveyed arthropod use ofL. nipomensis and classified plant visitors as potential pollinators when observed on or in the flowers. The primary goal of this study was to create an inventory of observed arthropod interactions withL. nipomensis to inform future research and conservation efforts.