About

David Holway is a researcher whose work focuses on the ecological and evolutionary impacts of invasive species, habitat fragmentation, and plant-pollinator interactions. His research includes studying the effects of non-native ants on native ecosystems, particularly their role in disrupting pollination mutualisms and their influence on plant reproductive success. Holway has contributed to understanding how habitat fragmentation affects native bee diversity and plant-pollinator network structure, emphasizing the importance of large, intact reserves for conserving regional bee faunas. His work also explores the behavioral and ecological dynamics of invasive predators such as yellowjackets in Hawaiian ecosystems, and the effects of environmental stressors like drought on pollination systems. Holway's research combines field experiments, genetic analyses, and ecological modeling to address questions related to biodiversity conservation, invasive species management, and ecological resilience.

Research topics

• Computer Science
• Geography
• Ecology
• Biology
• Demography
• Medicine
• Geometry
• Mathematics
• Simulation
• Cartography
• Physics
• Physical medicine and rehabilitation

Selected publications

• Pollen exploitation by non‐native, feral honey bees: Potential consequences for interspecific competition

  Insect Conservation and Diversity · 2025-07-07 · 5 citations

  articleOpen access

  Abstract The globally distributed western honey bee ( Apis mellifera ) can reach high local abundances in both its native and introduced ranges. For instance, previous studies documented that honey bees outnumber the sum of all other floral visitors to native vegetation in Southern California. To quantify potential resource competition exerted by honey bees, we used plant‐pollinator network data to estimate the abundance and biomass of honey bees and native bees, and pollen removal experiments to estimate the amount of pollen removed by honey bees. Honey bee biomass constituted 98% of total bee biomass across our study sites. Just two visits by honey bees resulted in the removal of more than 60% of available pollen produced by three common native plants ( Salvia mellifera, S. apiana and Phacelia distans ), and 80% of pollen was removed by the end of one day for each of the three species. Based on documented relationships between pollen provision mass and bee body mass, we estimated that pollen removed by honey bees from these three plant species could produce many dozens to thousands of native bees of average size per hectare per day. Our findings illustrate the potential for strong exploitative competition between honey bees and native bees, and underscore that resource consumption by honey bees should receive greater attention as a potential factor contributing to pollinator declines.

  PublisherOA PDFDOI

• Historical resurveys provide support for abiotic limits to Argentine ant invasion in southern California

  Biological Invasions · 2025-04-19

  articleOpen accessSenior author

  Abstract Predicting where introduced species will establish and spread remains a central goal of invasion biology. While climate data are often used to forecast potential occurrence at regional scales, environmental filtering can limit susceptibility to invasion at finer scales. Factors underlying such filtering are important to identify, especially when they influence spread into protected areas set aside for conservation. Useful information about factors limiting invasion can be obtained from historical resurveys. Between 2022 and 2024, we used pitfall traps and visual surveys to resurvey 111 sampling points in two protected areas in coastal San Diego County, California (Torrey Pines State Reserve and the Point Loma Ecological Conservation Area) that were originally surveyed for the non-native Argentine ant between 1995 and 1997. The multi-decade time span between the surveys coupled with the observed distributional limits, which have either contracted (Torrey Pines) or appear static (Point Loma), indicate that the Argentine ant has reached the limits of its ability to invade these sites. At Torrey Pines the soil types with the lowest water retention values were the least invaded in the original survey and were overrepresented among retractions observed in the resurvey. These findings are consistent with experimental work demonstrating the central role of soil moisture in limiting Argentine ant spread in seasonally dry areas. Variation in precipitation combined with changes in human water use will likely continue to influence the distribution of the Argentine ant in semiarid regions. These results generally illustrate the value of historical resurveys in clarifying limits to invasion.

  PublisherOA PDFDOI

• Invasion of the big-headed ant (Pheidole megacephala) in southern California: implications of future expansion

  Biological Invasions · 2025-05-24 · 1 citations

  articleOpen accessSenior author

  Abstract The big-headed ant, Pheidole megacephala , is an ecologically disruptive invader of tropical and subtropical environments worldwide. In April 2014 an established infestation of P. megacephala was discovered in a residential neighborhood in Costa Mesa, Orange County, California, and in 2019 a second infestation was found in a residential neighborhood (Talmadge / City Heights) in San Diego, San Diego County, California. Although big-headed ants are regularly detected in commerce in California, the records from Costa Mesa and Talmadge / City Heights represent the first established infestations documented from the state. In 2024 and 2025, four additional infestations were discovered or confirmed in other residential neighborhoods in San Diego. To assess whether or not P. megacephala will expand its range in this region, we delineated infestations in Costa Mesa and Talmadge / City Heights in 2023 and 2024 and compared this species to another widespread invader, the Argentine ant ( Linepithema humile ), with respect to desiccation tolerance and δ15N. The delineated P. megacephala infestations extend over multiple hectares of suburban and urban development, with the Talmadge / City Heights infestation exceeding 100 ha and the Costa Mesa infestation exceeding 10 ha. Between 2023 and 2024 the size of the Talmadge / City Heights infestation increased by 12 ha. Comparisons of the two focal species revealed overlapping δ15N values and estimates of desiccation tolerance. Our findings indicate that established populations of P. megacephala will continue to spread in urban environments in coastal southern California and potentially cause impacts comparable to those resulting from invasion by the Argentine ant.

  PublisherOA PDFDOI

• An assemblage-level comparison of genetic diversity and population genetic structure between island and mainland ant populations

  Evolution · 2024-07-09 · 1 citations

  articleSenior author

  Island biotas provide unparalleled opportunities to examine evolutionary processes. Founder effects and bottlenecks, e.g., 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 the 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.

  PublisherDOI

• Temperature and soil moisture manipulation yields evidence of drought‐induced pollen limitation in bee‐pollinated squash

  Ecology and Evolution · 2024-06-01 · 7 citations

  articleOpen accessSenior author

  Abstract 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.

  PublisherOA PDFDOI

• Exploitation of a marine subsidy by a terrestrial invader

  Biological Invasions · 2024-12-11 · 1 citations

  articleOpen accessSenior author

  Abstract 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 δ 15 N and δ 13 C for the Argentine ant were higher at beach sites than at scrub sites, and Argentine ant δ 15 N 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.

  PublisherOA PDFDOI

• Habitat parameters influencing the distribution of a geographically isolated flying squirrel

  Conservation Science and Practice · 2024-05-30 · 1 citations

  articleOpen access

  Abstract 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.

  PublisherOA PDFDOI

• Divergent responses of generalist and specialist pollinators to experimental drought: Outcomes for plant reproduction

  Ecology · 2023-05-27 · 14 citations

  articleOpen accessSenior author

  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.

  PublisherOA PDFDOI

• Variation in Argentine ant (Linepithema humile) trophic position as a function of time

  Biological Invasions · 2022-09-03 · 3 citations

  articleSenior author
  PublisherDOI

• Interspecific pollen transport between non-native fennel and an island endemic buckwheat: assessment of the magnet effect

  Biological Invasions · 2021-09-23 · 14 citations

  articleSenior author
  PublisherDOI

Recent grants

• LTREB Collaborative Proposal: Large-scale removal of introduced ants as a test of community reassembly

  NSF · $214k · 2017–2022

• DISSERTATION RESEARCH: The effects of pollinator diversity loss on plant-pollinator interaction networks and native plant reproduction in scrub ecosystems of Southern California

  NSF · $18k · 2015–2017

• Collaborative Research: Do Positive Species Interactions Promote Invasions? Effects of Ant-hemipteran Mutualisms on the Success and Consequences of Ant Invasions

  NSF · $170k · 2007–2010

Frequent coauthors

• Andrew V. Suarez

  University of Illinois Urbana-Champaign

  48 shared

• Ted J. Case

  University of California, San Diego

  27 shared

• Neil D. Tsutsui

  University of California, Berkeley

  26 shared

• Keng‐Lou James Hung

  University of Oklahoma

  21 shared

• Ida Naughton

  University of California, Berkeley

  19 shared

• Lori Lach

  James Cook University

  16 shared

• Christina L. Boser

  The Nature Conservancy

  14 shared

• Cause Hanna

  California State University, Channel Islands

  13 shared

Labs

• Holway LabPI