Past Members


Ida Naughton (PhD 2021)

Ida employed phylogenomics (based on ultra-conserved elements) to test several hypotheses regarding the evolutionary history of ants on the California Channel Islands. Ida’s research revealed that endemic ants on the Channel Islands are the products of both allochthonous and autochthonous endemism. Moreover, in an assemblage-level comparison of native ant species on Santa Cruz Island and a mainland area of comparable size, Ida found that island populations did not significantly differ from mainland populations in terms of genetic diversity but did exhibit decreased population genetic structure. These findings run counter to the more commonly observed patterns of island populations supporting less genetic diversity and showing more structure compared to mainland populations.

Ida contributed to the Santa Cruz Island Argentine ant eradication project as a researcher and field crew leader throughout her doctoral studies. She continues to conduct long-term research on the capacity of native ant assemblages to recover structure and function following landscape-scale removal of the Argentine ant from Santa Cruz Island, California. Ida’s research was supported by The Nature Conservancy, The National Institute of Health CMG Training Grant, and an LTREB award from the National Science Foundation.

Naughton, I., C. Boser, N.D. Tsutsui & D.A. Holway. 2020. Direct evidence of native ant displacement by the Argentine ant in island ecosystems. Biological Invasions 22:681-691.

Naughton, I., B-H Chiang, N.D. Tsutsui, P.S. Ward & D.A. Holway. Endemic ants from the California Channel Islands result from divergent evolutionary processes. In preparation.

Naughton, I., N.D. Tsutsui, P.S. Ward & D.A. Holway. A multi-species comparison of genetic diversity between island and mainland ant populations. In preparation.

Jess Gambel (PhD 2020)

Abiotic stress directly influences plant performance and survival, but plants may also be affected by how pollinators respond to floral traits compromised by changing environmental conditions. Using field manipulations of cultivated squash (Cucurbita pepo), Jess investigated how drought and warming affect the mutualistic relationships between plants and their pollinators. These experiments revealed that low soil moisture levels inhibited different aspects of reproductive allocation. Comparisons of squash pollinated by hand versus by bees showed that bee-pollinated plants experienced increasing levels of pollen limitation (estimated based on seed set) in response to decreasing soil moisture. Moreover, additional experiments revealed how differences in visitation by specialist (squash bees) and generalist (honey bees) pollinators in response to squash grown under experimental drought impact plant reproductive success. Jess also compared the performance of squash bees and honey bees in terms of their effectiveness as pollinators of cultivated squash and free-living buffalo gourd.

Jess’s dissertation was supported by the National Institute of Health CMG Training Grant and the University of California Climate Initiative and Global Food Initiative Fellowships. Following graduate school, she lectured in the Division of Biological Sciences at UC San Diego and is currently conducting postdoctoral research in pollination systems with the US Department of Agriculture (Madison, WI).

Gambel, J. & D.A. Holway. 2023. Divergent responses of generalist and specialist pollinators to experimental drought: outcomes for plant reproduction. Ecology 104:e4111.

Gambel, J., M. Martinez, J. Davids, K. Eastwood, J. Warner, J. Kohn & D.A. Holway. The performance of specialist and generalist pollinators on cultivated squash and wild gourds. Manuscript in preparation.

Keng-Lou James Hung (PhD 2017)

Keng-Lou James Hung (PhD 2017) studied the effects of habitat fragmentation and introduced species on the structure and function of plant-pollinator interactions. Few studies have examined how fragmentation impacts pollinator temporal and functional diversity, or how fragmentation-induced changes to pollinator diversity influence the structure and function of plant-pollinator networks. Working in the native bee-rich scrub ecosystems of southern California, James compared fragments and reserves (un-fragmented habitat) with respect to different components of native bee diversity and to the structure of floral visitation networks. Fragments and reserves harbored similar abundances of bees, but fragments supported bee assemblages that were taxonomically and functionally distinct from those in reserves, with consistently lower taxonomic diversity throughout the study season, less species-level turnover as the season progressed, and lower functional diversity. Plant-pollinator networks in fragments exhibited lower interaction selectivity and higher nestedness compared to those in reserves, but networks in fragments and reserves were otherwise structurally similar.

The patterns of pollinator diversity loss that James documented suggest that large, intact natural reserves are essential for conserving the regional distinctiveness of bee faunas of southern California. On the other hand, conserving plant-pollinator interactions in habitat fragments may sometimes be possible given that these areas can support structurally robust plant-pollinator interaction networks as well as native bee assemblages with relatively high functional and taxonomic diversity.

James’s research was supported by a Doctoral Dissertation Improvement Grant from the National Science Foundation. While in graduate school James compiled a checklist of the bees of San Diego County ( and discovered several undescribed species. James conducted postdoctoral research in the Department of Evolution, Ecology, and Organismal Biology at Ohio State University and in the Department of Ecology and Evolutionary Biology at the University of Toronto (supported by an NSERC postdoctoral fellowship award). He is currently an assistant professor in the Oklahoma Biological Survey at the University of Oklahoma. For more information about James’s research, see the following:

Hung, K.-L.J., S.S. Sandoval, J.S. Ascher & D.A. Holway. 2021. Joint impacts of drought and habitat fragmentation on native bee assemblages in a California biodiversity hotspot. Insects 12:135.

Hung, K.-L. J., J.S. Ascher, J.A. Davids & D.A. Holway. 2019. Ecological filtering in scrub fragments shapes the taxonomic and functional structure of native bee assemblages. Ecology 100:e02654. (Cover article.)

Hung, K.-L. J., J.M. Kingston, A. Lee, D.A. Holway & J.R. Kohn. 2019. Non-native honey bees disproportionately dominate abundant floral resources in a hotspot of pollinator diversity. Proceedings of the Royal Society B 286:20182901.

Hung, K.-L. J., J.M. Kingston, M. Albrecht, D.A. Holway & J.R. Kohn. 2018. The worldwide importance of honey bees as pollinators in natural habitats. Proceedings of the Royal Society B 285: 20172140.

Hung, K.-L. J., J.S. Ascher & D.A. Holway. 2017. Urbanization-induced habitat fragmentation erodes multiple components of temporal diversity in a Southern California native bee assemblage. PLoS One 12:e0184136.

Rightmyer, M.G., Y. Kono, J.R. Kohn & K-L.J. Hung. 2014. A new species of Triepeolus (Hymenoptera: Apidae), with comments on T. utahensis (Cockerell) and T. melanarius Rightmyer. Zootaxa 3872:48-56.

Katherine LeVan (PhD 2013)

Katherine LeVan (PhD 2013) investigated the ecological effects of floral visitation by non-native ants. Ants commonly visit flowers but have only seldom been documented to provide effective pollination services. Floral visitation by introduced ants may be especially likely to disrupt pollination mutualisms. A lack of evolutionary history between ants and plants, for example, can increase the likelihood of floral visitation by ants, and some introduced ant species attain high levels of abundance, exhibit strong preferences for carbohydrate-rich nectar, and interfere with other floral visitors. Katie’s research demonstrated that floral visitation by the introduced Argentine ant negatively affected plant reproductive performance (seed set) and suggests that negative effects of floral visitation by introduced results are likely more widespread than previously thought.

Katie’s dissertation was supported by a Graduate Research Fellowship from the National Science Foundation and an ARCS fellowship. Katie works for the National Science Foundation.

LeVan, K.E., S.K. Barney & E.E. Wilson Rankin. 2018. Introduced ants reduce interaction diversity in a multi-species, ant-aphid mutualism. Oikos 127:1132-1141.

LeVan, K.E. & D.A. Holway. 2015. Ant-aphid interactions increase ant floral visitation and reduce plant reproduction via decreased pollinator visitation. Ecology 96:1620-1630.

LeVan, K.E., K-L.J. Hung, K.R. McCann, J. Ludka & D.A. Holway. 2014. Floral visitation by the Argentine ant reduces pollinator visitation and seed set in the coast barrel cactus, Ferocactus viridescens. Oecologia 174:163-171.

Erin Wilson-Rankin (PhD 2009)

Erin Wilson (PhD 2009) combined behavioral, ecological, physiological and genetic approaches to investigate the effects of the western yellowjacket (Vespula pensylvanica) invasions in Hawaiian ecosystems (Haleakala and Hawaii Volcanoes National Parks). These aggressive scavenging predators have become abundant in Hawaii (a region without any native eusocial insects) and pose a serious threat to endemic arthropods. Erin used DNA sequence data from diet items retrieved from incoming yellowjacket foragers to document that Vespula consumes a wide variety of endemic arthropods. Although yellowjackets scavenge some of what they eat, results of colony-level removal experiments demonstrate that yellowjackets are important predators of numerous arthropod groups. The effects of this invasion are compounded by the tendency of introduced yellowjacket populations to form large, perennial colonies; native populations typically form small, annual colonies.

Erin’s dissertation was supported by a STAR fellowship from the Environmental Protection Agency, a Doctoral Dissertation Improvement Grant from the National Science Foundation, and an ARCS fellowship. After doing a postdoc at the University of Maryland, Erin joined the Department of Entomology at University of California at Riverside, where she is an Associate Professor (

Wilson, E.E, C. Sidhu, K.E. LeVan & D.A. Holway. 2010. Pollen foraging behavior of solitary Hawaiian bees revealed through DNA barcoding. Molecular Ecology 19:4823-4829.

Wilson, E.E. & D.A. Holway. 2010. Multiple mechanisms underlie displacement of solitary Hawaiian Hymenoptera by an invasive social wasp. Ecology 91:3294-3302.

Wilson, E.E., C.V. Young & D.A. Holway. 2010. Predation or scavenging? Thoracic muscle pH and rates of water loss reveal cause of death in arthropods. Journal of Experimental Biology 213:2640-2646.

Wilson, E.E., L.M. Mullen & D.A. Holway. 2009. Life history plasticity magnifies the ecological effects of a social wasp invasion. Proceedings of the National Academy of Sciences 106:12809-12813.

Sean Menke (PhD 2007)
Sean Menke (PhD 2007) studied the factors controlling the spread of the non-native Argentine ant. To achieve this goal, Sean employed a combination of manipulative field experiments and geographic information systems (GIS) analyses. The experimental work clearly illustrated how vulnerability to invasion hinges on fine-scale changes in environmental conditions, such that soil moisture largely controls whether or not the Argentine ant penetrates native ant assemblages in southern California. Sean also tested the common assumption that the abundance of introduced species and the diversity of native species respond in parallel to environmental variation. Field manipulations and GIS-based models demonstrate how this assumption is invalidated in this system and how the factors influencing invader occurrence change with spatial scale. Sean’s GIS-based research further demonstrated that environmental niche models, which are commonly used to predict the spread of non-native species, have potentially general limitations, especially when they are used to predict species occurrence in novel environments.

Sean’s dissertation was supported by a research fellowship from the National Oceanic and Atmospheric Administration (Estuarine Reserves Division), and he was awarded the Murray F. Buell Award from the Ecological Society of America for the best student presentation at the 2005 ESA meeting in Montreal. After a postdoctoral stint at North Carolina State University, Sean joined the faculty of Lake Forest College where he is currently an associate professor (

Menke, S.B., A.V. Suarez, C.V. Tillberg, C.T. Chou & D.A. Holway. 2010. Trophic ecology of the invasive Argentine ant: spatio-temporal variation in resource assimilation. Oecologia 164:763-773.

Menke, S.B., D.A. Holway, R.N. Fisher & W. Jetz. 2009. Characterizing and predicting species distributions across environments and scales: Argentine ant occurrences in the eye of the beholder. Global Ecology and Biogeography 18:50-63.

Menke, S.B., R.N. Fisher, W. Jetz & D.A. Holway. 2007. Biotic and abiotic controls of Argentine ant invasion success at local and landscape scales. Ecology 88:3164-3175.

Menke, S.B. & D.A. Holway. 2006. Abiotic factors control invasion by ants at the community scale. Journal of Animal Ecology 75:368-376.


Ho Jung Yoo (2007-2010)
Yoo, H.J., M.C. Kizner & D.A. Holway. 2013. Ecological effects of multi-species, ant-hemipteran mutualisms in citrus. Ecological Entomology 38:505-514.

Yoo, H.J. & D.A. Holway. 2011. Context-dependence in an ant-aphid mutualism: direct effects of tending intensity on aphid performance. Ecological Entomology 36:450-458.

Ed LeBrun (2003-2005)
LeBrun, E.G., C.V. Tillberg, A.V. Suarez, P. Folgarait, C.R. Smith & D.A. Holway. 2007. An experimental study of competition between fire ants and Argentine ants in their native range. Ecology 88:63-75.

Tillberg, C.V., D.A. Holway, E.G. LeBrun & A.V. Suarez. 2007. Trophic ecology of Argentine ants in their native and introduced ranges. Proceedings of the National Academy of Sciences 104:20856-20861.

Melissa Thomas (2002-2004)
Thomas, M.L., C.M. Payne, A.V. Suarez, N.D. Tsutsui & D.A. Holway. 2007. Contact between supercolonies elevates aggression in Argentine ants. Insectes Sociaux 54:225-233.

Thomas, M.L., C.M. Payne, A.V. Suarez, N.D. Tsutsui & D.A. Holway. 2006. When supercolonies collide: territorial aggression in an invasive and unicolonial social insect. Molecular Ecology 15:4303-4315.

Thomas, M.L., A. Dixson, V. Coggins & D.A. Holway. 2005. Food availability and brood number do not influence intraspecific aggression in Argentine ants. Insectes Sociaux 52:108-113.

Thomas, M.L, N.D. Tsutsui & D.A. Holway. 2005. Intraspecific competition influences the symmetry and intensity of aggression in the invasive Argentine ant. Behavioral Ecology 16:472-481.

Thomas, M.L. & D.A. Holway. 2005. Condition-dependent competition between invasive Argentine ants and Australian Iridomyrmex. Journal of Animal Ecology 74:532-542.

Pablo Schilman (2002-2004)
Schilman, P.E., J.R.B. Lighton & D.A. Holway. 2007. Water balance in the Argentine ant (Linepithema humile) compared to five common native ant species from southern California. Physiological Entomology 32:1-7.

Schilman, P.E., J.R.B. Lighton & D.A. Holway. 2005. Respiratory and cuticular water loss in insects with continuous gas exchange: comparison across five ant species. Journal of Insect Physiology 51:1295-1305.

Lighton, J.R.B., P.E. Schilman & D.A. Holway. 2004. The hyperoxic switch: assessing respiratory water loss rates in tracheate arthropods with continuous gas exchange. Journal of Experimental Biology 207:4463-4471.


Our lab supports students participating in the contiguous BS/MS program sponsored by the Division of Biological Sciences. BS/MS students complete at least one quarter of BISP199 prior to applying to the program and typically work closely with other graduate students or postdoctoral researchers. Master’s projects range from highly controlled, lab-based studies to work that is primarily field oriented. Research highlights and publications of former students are provided below.

Rebecca Stiling (MS 2023)

Although ants are known to forage in intertidal environments, impacts caused by introduced ants in these habitats are largely unstudied. To learn more about the ecological effects of Argentine ant foraging in salt marshes of San Diego County, we combined field surveys, a lab experiment, and stable isotope analyses. An activity survey using baits to measure recruitment in marsh and adjacent upland habitat revealed that Argentine ant occurs commonly intertidal marsh habitat with activity levels not different from those observed in adjacent upland habitat. A lab experiment showed that colony growth is unaffected by a realistic range of soil salinity levels. Lastly, we used stable isotope analysis to determine the extent to which the Argentine ant assimilates resources from intertidal environments. These analyses demonstrated that the Argentine ant had a significantly higher δ15N value and a significantly lower δ13C compared to a native ant (Dorymyrmex insanus) that also nests near salt marshes.

Chris Winters (MS 2023)

Although rarely documented, terrestrial invaders may exploit marine subsides and in doing so alter intertidal food webs with potentially disruptive effects in both intertidal and terrestrial environments. We employed field sampling and stable isotope analysis to examine exploitation of marine wrack by the introduced Argentine ant (Linepithema humile), which occurs commonly in intertidal beach environments in southern California. The Argentine ant readily scavenges arthropod detritivores abundant in wrack and is most active on beaches in summer when arthropod biomass (potential prey and carrion) is also high. These patterns contrast with paired terrestrial environments (coastal sage scrub) where the Argentine ant is more active in fall than in summer, when arthropod biomass (potential prey and carrion) is low. Stable isotope comparisons of organisms at paired wrack and scrub sites reveal that the diet of the Argentine ant on beaches primarily consists of marine resources. Values of δ15N and δ15C for the Argentine ant are higher in beach environments than in scrub environments and are seasonally invariant. This study provides a novel example of an introduced species exploiting a cross-boundary subsidy. Such phenomena may be underappreciated and warrant further study.

Edward Chen (MS 2021)

Edward used stable isotope analysis (variation in δ15N) to estimate temporal variation in relative trophic positions for an assemblage of native ant species from Santa Cruz Island, California. Soil δ15N and plant δ15N values were positively related to one another, and plant δ15N values were thus used as a baseline in interspecific comparisons of ant δ15N values. Overall, there was significant variation in trophic positions among native ant species but temporal constancy (2017 vs. 2019) within species. Functional traits likely underlie interspecific differences in δ15N values, and interspecific competition may reduced as a result of these differences.

Evelyne Baratelli (MS 2021)

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. Evelyne employed stable isotope analysis to estimate how trophic position changes as a function of time since invasion for the Argentine Ant, a widespread, abundant, and ecologically disruptive invader. 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 the years subsequent to invasion. To assess if a reduction in relative trophic position over time is a typical feature of ant invasions, Evelyne 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. These findings suggest that post-invasion reductions in trophic position may not be a general phenomenon and could reflect transitory ecological processes that require finer-scale temporal sampling than was possible to achieve in the present study. These findings are nonetheless consistent with the results of previous studies, which found that the effects of Argentine ant invasions persist over decadal time scales.

Baratelli, E., I. Naughton, A.V. Suarez, C.V. Tillberg, S.B. Menke & D.A. Holway. 2023. Variation in Argentine Ant trophic position in relation to time since invasion. Biological Invasions 25:133-140.

Bryan Huang (MS 2020)

Bryan examined mitochondrial DNA variation exhibited by Aphaenogaster patruelis, an ant species that is confined to five oceanic islands off the coast of southern California and Baja California: San Clemente Island, Santa Catalina Island, San Nicolas Island, Santa Barbara Island, and Isla Guadalupe. The aims of this study were to assess inter-island evolutionary relationships and to compare these findings with previous analyses based on nuclear DNA variation. Samples were collected from each of the islands, and DNA extraction was performed to sequence the 12S and CO1 mitochondrial genes. Based on these sequence data, Bryan constructed median-joining haplotype networks and a CO1 phylogenetic tree. Samples on three islands were represented by a single haplotype, but samples on two islands were represented by two, distinct haplotypes. The presence of two, distinct haplotypes suggests multiple colonization events. Given the lack of an extant mainland population of Aphaenogaster patruelis, reconstructing evolutionary relationships among island populations is not possible, but our results suggest repeated colonization of individual islands, either through movements between islands or between islands and a now extinct mainland population.

Maria Martinez (MS 2020)

Maria compared the effectiveness of specialist (squash bees) and generalist (honey bees) pollinators of cultivated squash (Cucurbita pepo). Single and multiple visit trials by honey bees and squash bees revealed that visitation by female squash bees had a greater positive effect on pollen removal, pollen deposition, fruit set, and seed set (i.e. seed number) compared to honey bee visitation. These findings were consistent with the behavior of the pollinator species. Cumulative duration of stigmatic and anther contact by female squash bees had a greater effect on pollen deposition and pollen removal compared to the same behaviors in honey bees. Previous studies on cultivated squash conducted in agricultural settings have found that the effectiveness of honey bees and squash bees appears more similar than what we report here. These contrasting results could be explained by differences in resource availability in large-scale agricultural settings versus small-scale plots (used here) and also by including male squash bees in estimates of pollinator importance.


Gambel, J., M. Martinez, J. Davids, K. Eastwood, J. Warner, J. Kohn & D.A. Holway. The performance of specialist and generalist pollinators on cultivated squash and wild gourds. Manuscript in preparation.

Kylie Etter (MS 2019)

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 species that enhance pollinator visitation to other plant species. Kylie investigated these interactions on Santa Cruz Island (Santa Barbara County, CA) 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 terms of its local distribution and blooming phenology. Fennel flower removal experiments revealed that fennel acts as a magnet species that enhances insect visitation to adjacent buckwheat flowers. Pollen transport analysis further revealed that 87% of insects collected on the two plant species carried pollen from the opposing plant species. Moreover, a higher proportion of insects visiting buckwheat carried fennel pollen than the converse, suggesting more frequent transition of pollinators from fennel to buckwheat than vice versa. Pollen transport analyses and visitation rate data suggest that members of three bee genera (primarily Augochlorella) may be responsible for the majority of fennel pollen transferred to buckwheat flowers and vice versa. Our findings demonstrate how a non-native plant species that enhances visitation to neighboring native species could negatively impact native plant species.

Etter, K.J, G. Junquera, J. Horvet, R. Alarcon, K.-L.J. Hung & D.A. Holway. 2022. Interspecific pollen transport between non-native fennel and an island endemic buckwheat: assessment of the magnet effect. Biological Invasions 24:139-155.

Mohamed Musse (MS 2019)

Volatile organic compounds (VOCs) emitted by flowers contribute to pollinator attraction. If VOC production changes as a function of environmental variation (e.g., drought, high temperatures), then floral attractiveness may be affected. Mohamed experimentally tested how drought stress affected floral VOC emission in squash (Cucurbita pepo) by growing squash plants under an experimental moisture gradient and then measuring VOC emissions from male and female flowers on these plants. Mohamed focused on four VOCs that previous studies have shown to be important in attracting floral visitors to squash flowers: 1,4-dimethoxybenzene, 1,2,4-trimethoxybenzene, alpha-gurjunene, and cinnamaldehyde. Using gas chromatography-mass spectrometry to quantify floral VOC emission in flowers, Mohamed found that alpha-gurjunene emissions from female squash flowers decreased with decreasing soil moisture levels experienced by squash plants. These findings illustrate a potential underlying mechanism for altered pollinator visitation to flowers on plants experiencing drought stress.

Bo-Huey Chiang (MS 2018)

Bo-huey used high-throughput DNA sequences of ultra-conserved elements (UCEs) to conduct a phylogeographic study of Aphaenogaster patruelis, an ant species endemic to the southern California Channel Islands and Isla Guadalupe (but absent from the continental mainland). Using samples obtained from each island, Bo-huey extracted total genomic DNA from each sample, constructed dual indexed DNA libraries, and then enriched pooled DNA libraries for UCE’s. After quality control and assembly, 847 UCE loci were recovered. Bo-huey analyzed aligned sequence data using three methods: PAUP, RAxML, and MrBayes. The resulting trees from all analyses were concordant in major splits among island populations. These analyses suggest that the colonization of the southern California Channel Islands and Isla Guadalupe may have resulted from island hopping from Santa Catalina Island to the other islands where this species occurs.

Jessica Davids (MS 2018)

Jess compared the performance of non-native western honey bees (a generalist pollinator) and native squash bees (a specialist pollinator) as pollinators of squash. Previous studies have estimated pollinator effectiveness of honey bees and squash bees in agricultural Cucurbita species, but behavioral differences that underlie variation in effectiveness have not been examined in detail. Female squash bees spent more than seven times longer per single visit in contact with squash stigmas and in turn deposited more than ten times more pollen compared to generalist honey bees. For all trials combined, pollen deposition and fruit set increased with time bees spent on receptive stigmatic surfaces. Single visits by squash bees were more likely to result in fruit set compared to honey bees (85% vs. 12%). In terms of single visits, these results indicate that honey bees are less effective at pollinating acorn squash compared to female squash bees.


Gambel, J., M. Martinez, J. Davids, K. Eastwood, J. Warner, J. Kohn & D.A. Holway. The performance of specialist and generalist pollinators on cultivated squash and wild gourds. Manuscript in preparation.

Sara Sandoval (MS 2018)

Sara investigated interactions between the non-native western honey bee, which is a super generalist pollinator, and native squash bees, which are specialists that solely depend on squash pollen to reproduce. Using videos to record pollinator behavior in the flowers of acorn squash (Cucurbita pepo), Sara found that honey bees, compared to squash bees, had higher visitation rates and more frequently occupied flowers with multiple individuals. Accordingly, intra-floral interactions involving multiple honey bee individuals (both aggressive and non-aggressive) were more common than were interactions between squash bees and honey bees. Honey bees increased their visitation rates in response to increasing nectar volume unlike squash bees, which exhibited an independent relationship between visitation rate and foraging. Interestingly, honey bee foraging appeared to have both positive and negative effects on plant reproductive success. Seed number was negatively related to the cumulative time that honey bees spent on stigmas, whereas mean seed weight and fruit volume both increased with the frequency of aggressive interactions between honey bee individuals.

Jillian Schat (MS 2018)

Functional traits can be used to infer how species obtain their food and interact with competitors. Jillian investigated relationships between morphology and trophic ecology (based on stable isotope ratios) in a native ant assemblage from Santa Cruz Island. Ant species mostly overlapped in terms of their stable isotope values, but significant relationships existed between morphological traits and stable isotope values. Jillian did not find relationships between morphological traits and interspecific measures of predation / scavenging ability.

Jeremy Warner (MS 2017)

Plants in the genus Cucurbita (Cucurbitaceae) are visited by generalist pollinators, such as honey bees, and by specialist pollinators, such as squash bees. Previous studies have examined pollinator effectiveness between honey bees and squash bees on agricultural Cucurbita species, but few have investigated the effectiveness of these pollinators in a non-agricultural context. Jeremy investigated the effectiveness of honey bees and squash bees as pollinators of free-living buffalo gourd (Cucurbita foetidissima). The percentage of fruit set from single visits by female squash bees (57.9%) was higher than that from single visits by male squash bees (23.5%). Single visits by honey bees never resulted in successful fruit set. Control fruit, by comparison, set at a percentage of 85.7%. The average weight of seeds from fruit resulting from single visits by female squash bees was significantly higher (by 28.6%) than for single visits by males. These results indicate that honey bees and male squash bees are less effective at pollinating buffalo gourd compared to female squash bees. Differences in how these pollinators gather nectar from Cucurbita may be one explanation for differences in fruit set.


Gambel, J., M. Martinez, J. Davids, K. Eastwood, J. Warner, J. Kohn & D.A. Holway. The performance of specialist and generalist pollinators on cultivated squash and wild gourds. Manuscript in preparation.

Adrienne Lee (MS 2016)

Adrienne Lee (MS 2016) investigated mechanistic links between habitat fragmentation, a proxy for pollinator diversity loss, and the quantity (conspecific pollen deposition) and quality (heterospecific pollen proportion) of pollination services. Working in coastal sage scrub ecosystems in San Diego, Adrienne compared pollinator visitation and pollen deposition across ten focal plant species in scrub fragments and reserves (expanses of un-fragmented scrub). At the level of the community, habitat fragmentation per se was not a significant driver of conspecific pollen deposition or heterospecific pollen proportion. However, habitat type (reserves versus fragments) formed statistically significant interactions with other variables, suggesting that fragmentation can indirectly affect pollination services.

Hung, K.-L. J., J.M. Kingston, A. Lee, D.A. Holway & J.R. Kohn. 2019. Non-native honey bees disproportionately dominate abundant floral resources in a hotspot of pollinator diversity. Proceedings of the Royal Society B 286:20182901.


Henry Cen (MS 2015)

Henry Cen (MS 2015) examined how habitat fragmentation affects the reproductive performance of two native plant species: California poppy and clustered tarweed. Working in coastal sage scrub ecosystems in San Diego, Henry compared plant reproductive performance (seed set) between plants in scrub fragments and plants in reserves (expanses of un-fragmented scrub). Plants in the two types of sites did not differ with respect to seed set, and fragments and reserves also did not differ with respect to the abundance or richness of floral visitors. More species of native bee, however, were found visiting tarweed flowers in reserves than in fragments. These results suggest that floral visitation at our fragment sites appeared sufficient for certain native plant species to reproduce successfully.

Nabors, A.J., H.J. Cen, K-L.J. Hung, J.R. Kohn & D.A. Holway. 2018. The effect of removing numerically dominant, non-native honey bees on seed set of a native plant. Oecologia 186:281-289.

Annika Nabors (MS 2015)

Annika Nabors (MS 2015) investigated how pollination services provided to a native plant change upon removal of the non-native, western honey bee – a numerically dominant floral visitor in the native bee-rich ecosystems of southern California. Annika focused on services provided to clustered tarweed, a native, annual forb that benefits from outcross pollination. To test the ability of native insects to provide pollination services, we performed honey bee removals within small experimental plots that contained approximately 20 tarweed plants and compared visitation and seed set between plants in removal and paired control plots. Despite that 92% of observed floral visits to control plots were from honey bees, their removal only reduced seed production by 14% relative to plants in control plots. These results indicate that native insects contribute important pollination services even in ecosystems numerically dominated by introduced pollinators.

Nabors, A.J., H.J. Cen, K-L.J. Hung, J.R. Kohn & D.A. Holway. 2018. The effect of removing numerically dominant, non-native honey bees on seed set of a native plant. Oecologia 186:281-289.


Amanda Schochet (MS 2014)
Amanda Schochet (MS 2014) examined the responses of bumble bees to urbanization by combining field surveys with the quantification of local- and landscape-level variables using Geographic Information Systems (GIS). Amanda sampled 57, one-hectare plots in San Diego that spanned a gradient of urbanization and then used species-level, presence / absence data and logistic regressions to identify correlates of bumble bee occurrence. All bumble bee species were negatively affected by impermeable surface cover within study plots, but species responded uniquely to other plot-scale variables and exhibited distinctive patterns of scale-dependency with respect to impermeable surface cover surrounding plots. Species-specific responses presumably reflect interspecific trait differences (e.g., body size, tongue length, foraging behavior). Unique responses to urbanization caution against pooling species into functional groups based merely on taxonomic relationships or perceived ecological similarities.

Schochet, A.B., K-L J. Hung, & D.A. Holway. 2016. Bumble bee species exhibit divergent responses to urbanization in a Southern California landscape. Ecological Entomology 41:685-692.

Kyle McCann (MS 2011)
Kyle McCann (MS 2011) examined how floral visitation by ants affects pollination services. Kyle focused on what happens the non-native Argentine ant replaces native ant species in a food-for-protection mutualism with the coast barrel cactus, which, like certain other barrel cacti, produces extrafloral nectar attractive to ants. Compared to the most prevalent native ant species in this system, the Argentine ant visited flowers more often and was present in higher numbers in flowers. Cactus bees, the key pollinators in this system, spent less time in flowers when cacti were occupied by the Argentine ant compared to when cacti were occupied by native ants. Presumably as a consequence of decreased duration of floral visits by cactus bees, cacti occupied by the Argentine ant set fewer seeds per fruit and produced fewer seeds overall compared to cacti occupied by native ants.

LeVan, K.E., K-L.J. Hung, K.R. McCann, J. Ludka & D.A. Holway. 2014. Floral visitation by the Argentine ant reduces pollinator visitation and seed set in the coast barrel cactus, Ferocactus viridescens. Oecologia 174:163-171.

Justin Scioli (MS 2013)
Justin Scioli (MS 2014) investigated differences in the diversity and composition of native bee assemblages between vernal pool habitat and adjacent patches of scrub habitat. At the community level, these two habitats did not differ in terms of bee richness or species composition but several native bee species appeared more common in the vicinity of vernal pools. These same bee species were observed to visit plant species, such as San Diego mesa mint, that are restricted in their distribution to vernal pool locations.
Michelle Kizner (MS 2010)
Michelle Kizner (MS 2010) investigated how multi-species mutualisms between ants and honeydew-producing insects structure arthropod food webs. Working in an organic lemon orchard, we experimentally removed ants or honeydew-producing insects from individual lemon trees and then over a two-year period estimated the abundances of the following: the Argentine ant, five species of honeydew-producing hemipterans, and California red scale. Red scale produces no honeydew but indirectly benefits from the presence of ants, which disrupt parasitism by Aphytis wasps. Removals revealed that mutualism between ants and honeydew-producing hemipterans indirectly and positively affected red scale. These results argue for an appreciation of how mutualist diversity affects the ecological consequences of mutualisms.

Yoo, H.J., M.C. Kizner & D.A. Holway. 2013. Ecological effects of multi-species, ant-hemipteran mutualisms in citrus. Ecological Entomology 38:505-514.

John Ludka (MS 2008)
John Ludka (MS 2008) studied interactions between ants and the coast barrel cactus, which produces extra-floral nectar attractive to ants. The ants in turn protect cacti from insect herbivores. More than twenty native ant species protect cacti but few species co-occur with the non-native Argentine Ant. Over the spring and summer, cacti vary in the diversity of ant bodyguards present, with cacti in invaded areas supporting the fewest ant species. For individual cacti, seed size increases with the number of ant species present possibly because cacti visited by multiple ant species receive more consistent protection over time.

Ludka, J., K.E. LeVan & D.A. Holway. 2015. Infiltration of a facultative ant-plant mutualism by the introduced Argentine ant: effects on mutualist diversity and mutualism benefits. Ecological Entomology 40:437-443.

Gene Chou (MS 2009)
Gene Chou (MS 2009) investigated how nitrogen isotopic data can be used to study assimilation of plant-based resources by ant colonies. In a greenhouse-based, diet-manipulation experiment, we reared Argentine ant colonies with and without honeydew-producing aphids but fed all colonies insects. Workers from colonies without aphids had δ15N values that were, on average, 6.3% higher compared to those from colonies with aphids. These data thus illustrate how isotopic data can clarify whether or not ants assimilate plant-based, carbohydrate resources. The monopolization of such resources (especially honeydew produced by Hemiptera) is linked to invasion success.

Menke, S.B., A.V. Suarez, C.V. Tillberg, C.T. Chou & D.A. Holway. 2010. Trophic ecology of the invasive Argentine ant: spatio-temporal variation in resource assimilation. Oecologia 164:763-771.

Crystal Grover (MS 2007)
Crystal Grover (MS 2007) investigated how the availability of carbohydrate resources affects Argentine ant behavior. In a lab-based, diet-manipulation experiment, we discovered that levels of insect prey and sucrose both influenced brood production and worker survival, but that colonies became less aggressive and less active only when deprived of sucrose. These findings illustrate in principle how access to carbohydrates may alter behavioral investments that contribute to competitive performance. A related study investigated how honeydew-producing aphids affect ant foraging behavior. Field and greenhouse experiments demonstrated that levels of foliar foraging by ants increases with aphid abundance.

Grover, C.D., K.C. Dayton, S.B. Menke & D.A. Holway. 2008. Effects of aphids on foliar foraging by Argentine ants and the resulting effects on other arthropods. Ecological Entomology 33:101-106.

Grover, C.D., A.D. Kay, J.A. Monson, T.C. Marsh & D.A. Holway. 2007. Linking nutrition and behavioral dominance: carbohydrate scarcity limits aggression and activity in Argentine ants. Proceedings of the Royal Society Series B 274:2951-2957.

Stephanie Glenn (MS 2005)
Stephanie Glenn (MS 2005) studied how pit-building ant lions respond to the loss of native ants following invasion by the Argentine ant. With respect to a range of growth parameters, larval ant lions in invaded areas performed at least as well, if not better, than they did in areas with native ants. This performance disparity may result from the Argentine ant’s high susceptibility to capture by ant lions. If invaded areas are indeed more favorable environments for larval ant lions, then the direct effects of this invasion might also include indirect effects that result from the Argentine ant’s positive influence on a predator it shares with native ants.

Glenn, S. & D.A. Holway. 2008. Consumption of introduced prey by native predators: Argentine ants and pit-building ant lions. Biological Invasions 10:273-280.

Jennifer Zee (MS 2003)
Jennifer Zee (MS 2003) investigated nest-raiding behavior of the Argentine ant. Raids blur the distinction between predation and competition, and contribute to the Argentine ant’s ability to displace native ants. The disappearance of seed-eating harvester ants from areas with high Argentine ant densities seems especially likely to be the result of nest raiding because the Argentine ant and most harvester ant species largely consume different types of food. Incipient harvester ant colonies appear most vulnerable to marauding raids.

Zee J. & D.A. Holway. 2006. Nest raiding by the invasive Argentine ant on colonies of the harvester ant, Pogonomymex subnitidus. Insectes Sociaux 53:161-167.