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.