David A. Holway’s research while affiliated with University of California System and other places

Intertidal environments receive energy from marine ecosystems in the form of marine wrack, which makes up the base of a food web that includes both intertidal and terrestrial consumers. Consumption of wrack by terrestrial consumers can elevate their abundance and alter how they interact with organisms in adjacent terrestrial environments. Although rarely documented, terrestrial invaders may exploit marine wrack subsides and potentially disrupt intertidal and terrestrial food webs. Here, we examine consumption of marine wrack resources by the introduced Argentine ant (Linepithema humile), which occurs commonly on beaches in southern California. In controlled trials the Argentine ant readily scavenged arthropod detritivores (amphipods and flies) abundant in wrack. In spite of obvious risks (e.g., exposure to tides, desiccation, thermal stress) associated with intertidal foraging, Argentine ant activity on beaches was comparable to that in spatially-paired, scrub environments. Foraging on beaches allowed ants to access higher densities of arthropod prey and carrion compared to those found in scrub environments. Stable isotope analyses provide evidence for extensive assimilation of marine-derived resources. Values of δ¹⁵N and δ¹³C for the Argentine ant were higher at beach sites than at scrub sites, and Argentine ant δ¹⁵N values broadly overlapped those of intertidal consumers at beach sites. Although ants are known to forage in intertidal environments, this study provides a novel example of an introduced ant species exploiting a cross-boundary subsidy.

Island biotas provide unparalleled opportunities to examine evolutionary processes. Founder effects and bottlenecks, for example, typically decrease genetic diversity in island populations, while selection for reduced dispersal can increase population structure. Given that support for these generalities mostly comes from single-species analyses, assemblage-level comparisons are needed to clarify how (i) colonization affects the gene pools of interacting insular organisms, and (ii) patterns of genetic differentiation vary within assemblages of organisms. Here, we use genome-wide sequence data from ultraconserved elements (UCEs) to compare genetic diversity and population structure of mainland and island populations of nine ant species in coastal southern California. As expected, island populations (from Santa Cruz Island) had lower expected heterozygosity and Watterson’s theta compared to mainland populations (from the Lompoc Valley). Island populations, however, exhibited smaller genetic distances among samples, indicating less population subdivision. Within the focal assemblage, pairwise Fst values revealed pronounced interspecific variation in mainland-island differentiation, which increases with gyne body size. Our results reveal population differences across an assemblage of interacting species, and illuminate general patterns of insularization in ants. Compared to single-species studies, our analysis of nine conspecific population pairs from the same island-mainland system offers a powerful approach to studying fundamental evolutionary processes.

Climate change alters environmental conditions in ways that directly and indirectly affect plants. Flowering plants, for example, modify reproductive allocation in response to heat and drought stress, and such changes can in turn affect pollinator visitation and, ultimately, plant reproduction. Although the individual effects of warming and drought on plant reproductive allocation are well known, these factors may interact to influence reproduction. Here, we conducted a fully crossed temperature by irrigation manipulation in squash (Cucurbita pepo) to test how temperature and soil moisture variation affect pollinator‐mediated reproduction. To tease apart the direct and indirect effects of temperature and soil moisture, we compared hand‐pollinated plants to bee‐pollinated plants and restricted bee foraging (i.e., pollen transfer) to one experimental group per day. Temperature and soil‐moisture limitation acted independently of one another: warming decreased flower size and increased pollen production, whereas the effects of soil‐moisture limitation were uniformly inhibitory. While treatments did not change squash bee (Xenoglossa spp.) behavior, floral visitation by the honey bee (Apis mellifera) increased with temperature in male flowers and decreased with soil moisture in female flowers. Pollen deposition by bees was independent of plant soil moisture, yet reducing soil moisture increased pollen limitation. This result stemmed at least in part from the effects of soil‐moisture limitation on pollen viability; seed set declined with increasing deposition of fluorescent pigment (a proxy for pollen) from plants experiencing decreased soil moisture. These findings suggest that the transfer of lower‐quality pollen from plants experiencing soil‐moisture limitation led to drought‐induced pollen limitation. Similar effects may occur in a wide variety of flowering plant species as climate warming and drought increasingly impact animal‐pollinated systems.

The San Bernardino flying squirrel (SBFS) is an isolated subspecies of Humboldt's flying squirrel, occurring in montane sky islands in the San Bernardino and San Jacinto Mountains in Southern California, USA. Recent small mammal surveys in the San Jacinto Mountains suggest the squirrel is extirpated. Our objectives were (1) determine habitat features, including forest metrics and topographical factors, that influence SBFS presence, in the San Bernardino Mountains; (2) use information collected in the San Bernardino Mountains to confirm squirrel occurrence and habitat preference in the San Jacinto Mountains; and (3) assess habitat and climatic differences between the two mountain ranges that could explain species persistence in one mountain range but not the other. We surveyed for SBFS using camera traps at 54 sites in the San Bernardino Mountains and 34 sites in the San Jacinto Mountains using both camera traps and acoustics. In the San Bernardino Mountains, we detected squirrels in sites that were more mesic, had higher structural heterogeneity, and had greater amounts of downed woody material compared to non‐detection sites. Habitat parameters were similar between the two ranges; however, squirrels were not detected in the San Jacinto Mountains. Conditions in the San Jacinto Mountains were hotter and drier. Increased temperatures due to climate change could potentially explain the absence of flying squirrels in the San Jacinto Mountains.

Drought is an increasingly important consequence of climate change. Drought often causes plants to alter patterns of resource allocation, which in turn can affect how plants interact with other species. How these altered interactions subsequently influence plant reproductive success remains incompletely understood and may depend on the degree of specialization exhibited by antagonists and mutualists. Specialist pollinators, for example, are dependent on floral resources from their obligate hosts and under drought conditions may thus indiscriminately visit these hosts (at least in certain circumstances). Generalist pollinators, in contrast, may only forage on host plants in good condition, given that they can forage on other plant species. We tested this hypothesis and its consequences for plant reproduction in squash (Cucurbita pepo) grown along an experimental moisture gradient ranging from dry (growth and flowering compromised) to wet conditions. Floral visitation increased with plant soil moisture for generalist honey bees but was independent of plant soil moisture for specialist squash bees. Pollen production increased with plant soil moisture, and fluorescent pigments placed on flowers revealed that pollinators primarily moved pollen from male flowers on well‐watered plants to the stigmas of female flowers on well‐watered plants. Seed set increased with increasing plant soil moisture but, notably, was higher in bee‐pollinated plants compared to plants pollinated by hand with an even mix of pollen from plants grown at either end of the experimental moisture gradient. These results suggest that superior pollen rewards, perhaps combined with selective foraging by generalists, enhanced reproductive success in C. pepo when plant soil moisture was high and more generally illustrate that pollinator behavior may contribute to how drought conditions affect plant reproduction.

The ecological effects of species introductions can change over time, but an understanding of how and why they do remains hindered by the lack of long-term data sets that permit investigation into underlying causes. We employed stable isotope analysis to estimate how trophic position changes as a function of time for the Argentine ant, a widespread, abundant, and ecologically disruptive introduced species. Previous research at a site in southern California (Rice Canyon, San Diego Co.) found that Argentine ant δ15N values were higher at the leading edge of invasion than at those same sites in years subsequent to invasion (i.e., after the invasion front had advanced). To assess if a reduction in relative trophic position over time is a typical feature of Argentine ant invasions, we expanded the temporal and spatial scale of sampling and measured δ15N values of the Argentine ant at three locations with a known or inferred history of invasion: Rice Canyon (the location of the original study), the Sacramento River Valley (Yolo and Solano Cos., CA), and San Nicolas Island (Ventura Co., CA). Resampling Rice Canyon in 2019, 16 years after the original survey, revealed a significant increase in Argentine ant δ15N values. At the two other locations, Argentine ant δ15N values were independent of time since invasion (Sacramento River Valley) or position relative to the invasion front (San Nicolas Island). These findings suggest that post-invasion reductions in trophic position may not be a general phenomenon or could reflect transitory ecological processes that require finer-scale temporal sampling than was possible to achieve in the present study. Our findings are nonetheless consistent with the results of recent studies, which found that the effects of Argentine ant invasions persist over decadal time scales.

Non-native plant species can disrupt plant–pollinator interactions by altering pollinator foraging behavior, which can in turn affect levels of interspecific pollen transfer between native and non-native plant species. These processes may be amplified in cases where introduced plant species act as magnet taxa that enhance pollinator visitation to other plant species. We investigated these interactions on Santa Cruz Island (Santa Barbara Co., California) between non-native fennel (Foeniculum vulgare), a widespread and abundant invader, and the endemic Santa Cruz Island buckwheat (Eriogonum arborescens), which broadly overlaps fennel in its local distribution and blooming phenology. A fennel flower removal experiment revealed that this invader acts as a magnet species by increasing insect visitation to adjacent buckwheat flowers. Analysis of the amount of pollen carried on the bodies of insect pollinators (i.e., pollen transport) revealed that 96% of visitors to buckwheat flowers carried fennel pollen and 72% of visitors to fennel flowers carried buckwheat pollen. Pollen transport analyses and visitation rate data further suggest that members of three bee genera (primarily Augochlorella) may be responsible for the majority of fennel pollen deposited on the stigmas of buckwheat flowers (i.e., pollen transfer) and vice versa. Lastly, fennel pollen transport appeared to occur at a larger spatial scale than the magnet effect that fennel plants exert on floral visitors to neighboring buckwheat plants. The ability of fennel to act as a magnet species, coupled with the fact that it is widespread invader with known allelopathic capacities, suggests that future studies could evaluate if the transfer of fennel pollen adversely affects native plant reproduction in areas where fennel is introduced.

The ecological effects of species introductions can change over time, but an understanding of how and why they do remains hindered by the lack of long-term data sets that permit investigation into underlying causes. We employed stable isotope analysis to estimate how trophic position changes as a function of time for the Argentine Ant, a widespread, abundant, and ecologically disruptive introduced species. Previous research at a site in southern California (Rice Canyon, San Diego Co.) found that Argentine Ant δ15N values were higher at the leading edge of invasion than at those same sites in years subsequent to invasion. To assess if a reduction in relative trophic position over time is a typical feature of ant invasions, we expanded the temporal and spatial scale of sampling and measured δ15N values of the Argentine ant at three locations with a known or inferred history of invasion: Rice Canyon (the site of the original study), the Sacramento River Valley (Yolo and Solano Cos., CA), and San Nicolas Island (Ventura Co., CA). Resampling Rice Canyon in 2019, 16 years after the original survey, revealed a significant increase in Argentine ant δ15N values. At the two other locations, Argentine ant δ15N values were independent of time since invasion (Sacramento River Valley) or position relative to the invasion front (San Nicolas Island). These findings suggest that post-invasion reductions in trophic position may not be a general phenomenon or could reflect transitory ecological processes that require finer-scale temporal sampling than was possible to achieve in the present study. Our findings are nonetheless consistent with the results of recent studies, which found that the effects of Argentine ant invasions persist over decadal time scales.

Global climate change is causing more frequent and severe droughts, which could have serious repercussions for the maintenance of biodiversity. Here, we compare native bee assemblages collected via bowl traps before and after a severe drought event in 2014 in San Diego, California, and examine the relative magnitude of impacts from drought in fragmented habitat patches versus unfragmented natural reserves. Bee richness and diversity were higher in assemblages surveyed before the drought compared to those surveyed after the drought. However, bees belonging to the Lasioglossum subgenus Dialictus increased in abundance after the drought, driving increased representation by small-bodied, primitively eusocial, and generalist bees in post-drought assemblages. Conversely, among non-Dialictus bees, post-drought years were associated with decreased abundance and reduced representation by eusocial species. Drought effects were consistently greater in reserves, which supported more bee species, than in fragments, suggesting that fragmentation either had redundant impacts with drought, or ameliorated effects of drought by enhancing bees’ access to floral resources in irrigated urban environments. Shifts in assemblage composition associated with drought were three times greater compared to those associated with habitat fragmentation, highlighting the importance of understanding the impacts of large-scale climatic events relative to those associated with land use change.

Recent work underscores that ants are highly proficient and ubiquitous scavengers. These tendencies extend to numerically and behaviorally dominant introduced ants, which exhibit a suite of traits that allow them to exploit and monopolize carrion to a greater extent than is widely appreciated. We thus contend that an understanding of how introduced ants fit into food webs remains incomplete. Monopolization of carrion resources by introduced ants could increase worker production, enhance the ability of these species to compete with and prey upon other organisms, and alter the strength of direct and indirect interactions within food webs. Future work should consider how ant invasions influence energy transfer within and between green and brown food webs.