EEB Faculty Publications

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Above and below ground impacts of terrestrial mammals and birds in a tropical forest
(Blackwell Publishing, 2008) Dunham, Amy E.
Understanding the impact of losing trophic diversity has global significance for managing ecosystems as well as important theoretical implications for community and ecosystem ecology. In several tropical forest ecosystems, habitat fragmentation has resulted in declines and local extinctions of mammalian and avian terrestrial insectivores. To assess the ability of a tropical rainforest community in Ivory Coast to resist perturbation from such loss of trophic diversity, I traced feedbacks in above and below ground communities and measured changes in nutrient levels and herbivory rates in response to an experimental exclosure of avian and mammalian terrestrial insectivores. I present evidence that loss of this functional group may result in increased tree seedling herbivory and altered nutrient regimes through changes in the abundance and guild structure of invertebrates. Exclusion of top predators of the forest floor resulted in increased seedling herbivory rates and macro-invertebrate ( 5 mm) densities with strongest effects on herbivorous taxa, spiders and earthworms. Densities of microbivores including Collembola, Acarina and Sciaridae showed the opposite trend as did levels of inorganic phosphorus in the soil. Results were evaluated using path analysis which supported the presence of a top down trophic cascade in the detrital web which ultimately affected turnover of phosphorus, a limiting nutrient in tropical soils. Results illustrate the potential importance of vertebrate predators in both above and belowground food webs despite the biotic diversity and structural heterogeneity of the rainforest floor.
Adaptation of Enterococcus faecalis to Daptomycin Reveals an Ordered Progression to Resistance
(American Society for Microbiology, 2013) Miller, Corwin A.; Kong, Jiayi; Tran, Truc T.; Arias, Cesar A.; Saxer, Gerda; Shamoo, Yousif
With increasing numbers of hospital-acquired antibiotic resistant infections each year and staggering health care costs, there is a clear need for new antimicrobial agents, as well as novel strategies to extend their clinical efficacy. While genomic studies have provided a wealth of information about the alleles associated with adaptation to antibiotics, they do not provide essential information about the relative importance of genomic changes, their order of appearance, or potential epistatic relationships between adaptive changes. Here we used quantitative experimental evolution of a single polymorphic population in continuous culture with whole-genome sequencing and allelic frequency measurements to study daptomycin (DAP) resistance in the vancomycin-resistant clinical pathogen Enterococcus faecalis S613. Importantly, we sustained both planktonic and nonplanktonic (i.e., biofilm) populations in coculture as the concentration of antibiotic was raised, facilitating the development of more ecological complexity than is typically observed in laboratory evolution. Quantitative experimental evolution revealed a clear order and hierarchy of genetic changes leading to resistance, the signaling and metabolic pathways responsible, and the relative importance of these mutations to the evolution of DAP resistance. Despite the relative simplicity of this ex vivo approach compared to the ecological complexity of the human body, we showed that experimental evolution allows for rapid identification of clinically relevant adaptive molecular pathways and new targets for drug design in pathogens.
Amino Acid Repeats Cause Extraordinary Coding Sequence Variation in the Social Amoeba Dictyostelium discoideum
(Public Library of Science, 2012-09) Scala, Clea; Tian, Xiangjun; Mehdiabadi, Natasha J.; Smith, Margaret H.; Saxer, Gerda; Stephens, Katie; Buzombo, Prince; Strassmann, Joan E.; Queller, David C.
Protein sequences are normally the most conserved elements of genomes owing to purifying selection to maintain their functions. We document an extraordinary amount of within-species protein sequence variation in the model eukaryote Dictyostelium discoideum stemming from triplet DNA repeats coding for long strings of single amino acids. D. discoideum has a very large number of such strings, many of which are polyglutamine repeats, the same sequence that causes various human neurological disorders in humans, like Huntington's disease. We show here that D. discoideum coding repeat loci are highly variable among individuals, making D. discoideum a candidate for the most variable proteome. The coding repeat loci are not significantly less variable than similar non-coding triplet repeats. This pattern is consistent with these amino-acid repeats being largely non-functional sequences evolving primarily by mutation and drift.
Balancing anti-herbivore benefits and anti-pollinator costs of defensive mutualists
(Ecological Society of America, 2014) Ohm, Johanna R.; Miller, Tom E.X.
Quantifying costs and benefits of ostensibly mutualistic interactions is an important step toward understanding their evolutionary trajectories. In food-for-protection interactions between ants and extrafloral nectar (EFN)-bearing plants, tending by aggressive ants may deter herbivores, but it may also deter pollinators. The fitness costs of pollinator deterrence are not straightforward for long-lived iteroparous plants, because reproductive vital rates often contribute weakly to fitness relative to growth and survival (vital rates that may be enhanced by ant defense). We used field manipulations of ant and pollinator activity and demographic modeling to examine how pollination costs of ant defense translate to plant fitness, given the benefits of ant defense elsewhere in the plant life cycle. We contrasted the net fitness effects of alternative ant partner species. Our work focused on the tree cholla cactus, Opuntia imbricata, an EFN-bearing plant associated with two ant species (Crematogaster opuntiae and Liometopum apiculatum) that differ in quality of defense against insect herbivores. We found that ant defense imposed pollination costs, despite evidence for ant-repellent floral volatiles and temporal partitioning of ant and pollinator activity. The two partner species similarly reduced pollinator visitation and seed mass, and one (C. opuntiae) additionally reduced seed number. We used the experimental data and other long-term demographic data to parameterize an integral projection model that integrated costs and benefits of ant defense over the complete plant life cycle. Model results indicated that the pollination costs of L. apiculatum were balanced by beneficial effects on growth, leading to a net fitness effect that was neutral to positive. By contrast, pollination costs outweighed benefits for C. opuntiae, the weaker defender, rendering this species a reproductive parasite. Thus, we infer that pollination costs destabilize mutualism with one partner species, but are offset by strong defensive benefits provided by the other, leading to contrasting selective pressures imposed by alternative associations. Accounting for ontogenetic turnover in ant partner identity indicated that most plants avoid the parasitic effects of C. opuntiae by associating nonrandomly with L. apiculatum at reproductive life stages. Our results highlight the value of a demographic approach to quantifying the costs and benefits of mutualism.
Battle of the sexes: Cost asymmetry explains female dominance in lemurs
(Elsevier, 2008) Dunham, Amy E.
Biodiversity maintenance in food webs with regulatory environmental feedbacks
(Elsevier, 2007) Rauscher, Daniel; Brown, Christopher G.; Dunham, Amy E.; Bagdassarian, Carey K.
Although the food web is one of the most fundamental and oldest concepts in ecology, elucidating the strategies and structures by which natural communities of species persist remains a challenge to empirical and theoretical ecologists. We show that simple regulatory feedbacks between autotrophs and their environment when embedded within complex and realistic food-web models enhance biodiversity. The food webs are generated through the niche-model algorithm and coupled with predator–prey dynamics, with and without environmental feedbacks at the autotroph level. With high probability and especially at lower, more realistic connectance levels, regulatory environmental feedbacks result in fewer species extinctions, that is, in increased species persistence. These same feedback couplings, however, also sensitize food webs to environmental stresses leading to abrupt collapses in biodiversity with increased forcing. Feedback interactions between species and their material environments anchor food-web persistence, adding another dimension to biodiversity conservation. We suggest that the regulatory features of two natural systems, deep-sea tubeworms with their microbial consortia and a soil ecosystem manifesting adaptive homeostatic changes, can be embedded within niche-model food-web dynamics. r 2007 Elsevier Ltd. All rights reserved.
Chinese Tallow Trees (Triadica sebifera) from the Invasive Range Outperform Those from the Native Range with an Active Soil Community or Phosphorus Fertilization
(Public Library of Science, 2013) Zhang, Ling; Zhang, Yaojun; Wang, Hong; Zou, Jianwen; Siemann, Evan
Two mechanisms that have been proposed to explain success of invasive plants are unusual biotic interactions, such as enemy release or enhanced mutualisms, and increased resource availability. However, while these mechanisms are usually considered separately, both may be involved in successful invasions. Biotic interactions may be positive or negative and may interact with nutritional resources in determining invasion success. In addition, the effects of different nutrients on invasions may vary. Finally, genetic variation in traits between populations located in introduced versus native ranges may be important for biotic interactions and/or resource use. Here, we investigated the roles of soil biota, resource availability, and plant genetic variation using seedlings of Triadica sebifera in an experiment in the native range (China). We manipulated nitrogen (control or 4 g/m2), phosphorus (control or 0.5 g/m2), soil biota (untreated or sterilized field soil), and plant origin (4 populations from the invasive range, 4 populations from the native range) in a full factorial experiment. Phosphorus addition increased root, stem, and leaf masses. Leaf mass and height growth depended on population origin and soil sterilization. Invasive populations had higher leaf mass and growth rates than native populations did in fresh soil but they had lower, comparable leaf mass and growth rates in sterilized soil. Invasive populations had higher growth rates with phosphorus addition but native ones did not. Soil sterilization decreased specific leaf area in both native and exotic populations. Negative effects of soil sterilization suggest that soil pathogens may not be as important as soil mutualists for T. sebifera performance. Moreover, interactive effects of sterilization and origin suggest that invasive T. sebifera may have evolved more beneficial relationships with the soil biota. Overall, seedlings from the invasive range outperformed those from the native range, however, an absence of soil biota or low phosphorus removed this advantage.
Climate warming increases biological control agent impact on a non-target species
(Wiley, 2015) Lu, Xinmin; Siemann, Evan; He, Minyan; Wei, Hui; Shao, Xu; Ding, Jianqing
Climate change may shift interactions of invasive plants, herbivorous insects and native plants, potentially affecting biological control efficacy and non-target effects on native species. Here, we show how climate warming affects impacts of a multivoltine introduced biocontrol beetle on the non-target native plant Alternanthera sessilis in China. In field surveys across a latitudinal gradient covering their full distributions, we found beetle damage on A. sessilis increased with rising temperature and plant life history changed from perennial to annual. Experiments showed that elevated temperature changed plant life history and increased insect overwintering, damage and impacts on seedling recruitment. These results suggest that warming can shift phenologies, increase non-target effect magnitude and increase non-target effect occurrence by beetle range expansion to additional areas where A. sessilis occurs. This study highlights the importance of understanding how climate change affects species interactions for future biological control of invasive species and conservation of native species.
Co-pollinators and specialization in the pollinating seed-consumer mutualism between senita cacti and senita moths
(Springer-Verlag, 2002) Fleming, Theodore H.; Holland, J. Nathaniel
Specialization of a plant on a particular pollinator may not evolve if co-pollinators are effective and abundant. This is particularly evident if fruit set is resource limited and cannot be increased above the levels produced by the actions of co-pollinators. The pollinating seed-consuming interaction between senita cacti and senita moths in the Sonoran Desert presents a paradox because it exhibits many traits resembling those of the highly specialized yucca/yucca moth system, but also involves co-pollinators. For 6 years, we studied how contributions of nocturnal senita moths and diurnal co-pollinating bees to fruit set depended on resource and pollen limitation, time of flower closing, and the onset and phenology of flowering. Fruit set was typically resource limited. Fruit set of flowers exposed only to senita moths was not different from resource-limited fruit set of control flowers. When only co-pollinating bees were allowed to visit flowers, however, fruit set became pollen limited. Only in one year when fruit set was pollen limited were bees able to increase fruit set beyond the level resulting from senita moth pollination. High temperatures commonly induced flowers to close before sunrise so that diurnal bees were unable to visit flowers. This was particularly important from 1998 to 2000, when flowering did not begin until late in spring when temperatures were already high enough to induce flowers to close before sunrise. Bees were typically functionally redundant with senita moths; excluding bees from visiting flowers did not alter fruit set. Nevertheless, extreme specialization of floral traits to exclude co-pollinators has not evolved in senita, possibly because there are times when bees do increase fruit set. This can occur when senita moths are rare, fruit set is pollen limited, cool temperatures prevent flowers from closing before sunrise, and flowering begins early in spring.
Computational approaches to species phylogeny inference and gene tree reconciliation
(Elsevier, 2013) Nakhleh, Luay
An intricate relationship exists between gene trees and species phylogenies, due to evolutionary processes that act on the genes within and across the branches of the species phylogeny. From an analytical perspective, gene trees serve as character states for inferring accurate species phylogenies, and species phylogenies serve as a backdrop against which gene trees are contrasted for elucidating evolutionary processes and parameters. In a 1997 paper, Maddison discussed this relationship, reviewed the signatures left by three major evolutionary processes on the gene trees, and surveyed parsimony and likelihood criteria for utilizing these signatures to computationally elucidate this relationship. Here, we review progress that has been made on developing computational methods for analyses under these two criteria, and survey remaining challenges.
Confronting climate change in the Gulf region: prospects for sustaining our ecological heritage
(Union of Concerned Scientists, 2001-10) Zimmerman, R.J.; Wetzel, R.G.; Siemann, E.H.; Reed, J.B.; Miller, R.L.; Harwell, M.A.; Gholz, M.A.; Barron, E.J.; Twilley, R.B.
From Texas to Florida, the Gulf coast region is rich with ecological resources that support the region’s economic wealth. Over time, human activities from dam construction to shoreline development have dramatically altered natural landscapes, waterways, and ecological processes. Pressures from human activities remain the most important agents of ecological change in the region today. Over the century ahead, land-use changes are likely to increase as rapid population growth continues. Global climate change, driven by rising levels of carbon dioxide and other heat-trapping greenhouse gases in the atmosphere, will interact with, and magnify, other human stresses on Gulf Coast ecosystems and the goods and services they provide. Confronting Climate Change in the Gulf Coast Region explores the potential risks of climate change to Gulf Coast ecosystems in the context of pressures from land use. Its purpose is to help the public and policymakers understand the most likely ecological consequences of climate change in the region over the next 50 to 100 years and prepare to safeguard the economy, culture, and natural heritage of the Gulf Coast. This summary highlights key findings.
Consequences of sex-selective harvesting and harvest refuges in experimental meta-populations
(Nordic Society Oikos, 2013) Snyder, Kate T.; Freidenfelds, Nicole A.; Miller, Tom E.X.
Harvesting for food or sport is often non-random with respect to demographic state, such as size or life stage. The population- level consequences of such selective harvesting depend upon which states are harvested and how those states contribute to population dynamics. We focused on a form of selective harvesting that has not previously been investigated in an experimental context: sex-selective harvesting, a common feature of exploited, dioecious populations. Using simple metapopulations (two patches connect by dispersal) of sexually dimorphic Bruchid beetles in the laboratory, we contrasted the effects of female-selective, male-selective, and non-selective harvesting over six generation of population dynamics. We also tested the ability of a harvest refuge (one patch of the metapopulation free from harvesting) to mitigate the effects of harvesting, and whether refuge effects interacted with sex selectivity. Sex-selective harvesting significantly perturbed operational sex ratios and harvest refuges dampened these perturbations. Metapopulations assigned to male-selective and non-selective treatments were able to fully compensate for harvesting, such that their dynamics did not differ from non-harvested controls. Only female-selective harvesting led to significant reductions in population size and this effect was completely offset by dispersal from a harvest refuge. A two-sex model confirmed that population dynamics are more sensitive to female vs. male harvesting, but suggested that higher levels of male harvest than included in our experiment would cause population decline. We discuss the roles of density-dependent competition and frequency-dependent sexual processes in the population response to sex-selective harvesting.
Conspecific Plasticity and Invasion: Invasive Populations of Chinese Tallow (Triadica sebifera)ﾠHave Performance Advantage over Native Populations Only in Low Soil Salinity
(Public Library of Science, 2013) Chen, Leiyi; Tiu, Candice J.; Peng, Shaolin; Siemann, Evan
Global climate change may increase biological invasions in part because invasive species may have greater phenotypic plasticity than native species. This may be especially important for abiotic stresses such as salt inundation related to increased hurricane activity or sea level rise. If invasive species indeed have greater plasticity, this may reflect genetic differences between populations in the native and introduced ranges. Here, we examined plasticity of functional and fitness-related traits of Chinese tallow (Triadica sebifera) populations from the introduced and native ranges that were grown along a gradient of soil salinity (control: 0 ppt; Low: 5 ppt; Medium: 10 ppt; High: 15 ppt) in a greenhouse. We used both norm reaction and plasticity index (PIv) to estimate the conspecific phenotypic plasticity variation between invasive and native populations. Overall, invasive populations had higher phenotypic plasticity of height growth rate (HGR), aboveground biomass, stem biomass and specific leaf area (SLA). The plasticity Index (PIv) of height growth rate (HGR) and SLA each were higher for plants from invasive populations. Absolute performance was always comparable or greater for plants from invasive populations versus native populations with the greatest differences at low stress levels. Our results were consistent with the ?Master-of-some? pattern for invasive plants in which the fitness of introduced populations was greater in more benign conditions. This suggests that the greater conspecific phenotypic plasticity of invasive populations compared to native populations may increase invasion success in benign conditions but would not provide a potential interspecific competitive advantage in higher salinity soils that may occur with global climate change in coastal areas.
Daptomycin Resistance in Enterococci Is Associated with Distinct Alterations of Cell Membrane Phospholipid Content
(Public Library of Science, 2012) Mishra, Nagendra N.; Bayer, Arnold S.; Tran, Truc T.; Shamoo, Yousif; Mileykovskaya, Eugenia; Dowhan, William; Guan, Ziqiang; Arias, Cesar A.
Background: The lipopeptide antibiotic, daptomycin (DAP) interacts with the bacterial cell membrane (CM). Development of DAP resistance during therapy in a clinical strain of Enterococcus faecalis was associated with mutations in genes encoding enzymes involved in cell envelope homeostasis and phospholipid metabolism. Here we characterized changes in CM phospholipid profiles associated with development of DAP resistance in clinical enterococcal strains. Methodology: Using two clinical strain-pairs of DAP-susceptible and DAP-resistant E. faecalis (S613 vs. R712) and E. faecium (S447 vs. R446) recovered before and after DAP therapy, we compared four distinct CM profiles: phospholipid content, fatty acid composition, membrane fluidity and capacity to be permeabilized and/or depolarized by DAP. Additionally, we characterized the cell envelope of the E. faecium strain-pair by transmission electron microscopy and determined the relative cell surface charge of both strain-pairs. Principal Findings: Both E. faecalis and E. faecium mainly contained four major CM PLs: phosphatidylglycerol (PG), cardiolipin, lysyl-phosphatidylglycerol (L-PG) and glycerolphospho-diglycodiacylglycerol (GP-DGDAG). In addition, E. faecalis CMs (but not E. faecium) also contained: i) phosphatidic acid; and ii) two other unknown species of amino-containing PLs. Development of DAP resistance in both enterococcal species was associated with a significant decrease in CM fluidity and PG content, with a concomitant increase in GP-DGDAG. The strain-pairs did not differ in their outer CM translocation (flipping) of amino-containing PLs. Fatty acid content did not change in the E. faecalis strain-pair, whereas a significant decrease in unsaturated fatty acids was observed in the DAP-resistant E. faecium isolate R446 (vs S447). Resistance to DAP in E. faecium was associated with distinct structural alterations of the cell envelope and cell wall thickening, as well as a decreased ability of DAP to depolarize and permeabilize the CM. Conclusion: Distinct alterations in PL content and fatty acid composition are associated with development of enterococcal DAP resistance.
Daptomycin-Resistant Enterococcus faecalis Diverts the Antibiotic Molecule from the Division Septum and Remodels Cell Membrane Phospholipids
(American Society for Microbiology, 2013) Tran, Truc T.; Panesso, Diana; Mishra, Nagendra N.; Mileykovskaya, Eugenia; Guan, Ziqiang; Munita, Jose M.; Reyes, Jinnethe; Diaz, Lorena; Weinstock, George M.; Murray, Barbara E.; Shamoo, Yousif; Dowhan, William; Bayer, Arnold S.; Arias, Cesar A.
Treatment of multidrug-resistant enterococci has become a challenging clinical problem in hospitals around the world due to the lack of reliable therapeutic options. Daptomycin (DAP), a cell membrane-targeting cationic antimicrobial lipopeptide, is the only antibiotic with in vitro bactericidal activity against vancomycin-resistant enterococci (VRE). However, the clinical use of DAP against VRE is threatened by emergence of resistance during therapy, but the mechanisms leading to DAP resistance are not fully understood. The mechanism of action of DAP involves interactions with the cell membrane in a calciumdependent manner, mainly at the level of the bacterial septum. Previously, we demonstrated that development of DAP resistance in vancomycin-resistant Enterococcus faecalis is associated with mutations in genes encoding proteins with two main functions, (i) control of the cell envelope stress response to antibiotics and antimicrobial peptides (LiaFSR system) and (ii) cell membrane phospholipid metabolism (glycerophosphoryl diester phosphodiesterase and cardiolipin synthase). In this work, we show that these VRE can resist DAP-elicited cell membrane damage by diverting the antibiotic away from its principal target (division septum) to other distinct cell membrane regions. DAP septal diversion by DAP-resistant E. faecalis is mediated by initial redistribution of cell membrane cardiolipin-rich microdomains associated with a single amino acid deletion within the transmembrane protein LiaF (a member of a three-component regulatory system [LiaFSR] involved in cell envelope homeostasis). Full expression of DAP resistance requires additional mutations in enzymes (glycerophosphoryl diester phosphodiesterase and cardiolipin synthase) that alter cell membrane phospholipid content. Our findings describe a novel mechanism of bacterial resistance to cationic antimicrobial peptides.
Decomposition of Phragmites australis litter retarded by invasive Solidago canadensis in mixtures: an antagonistic non-additive effect
(Macmillan Publishers Limited, 2014) Zhang, Ling; Zhang, Yaojun; Zou, Jianwen; Siemann, Evan
Solidago canadensis is an aggressive invader in China. Solidago invasion success is partially attributed to allelopathic compounds release and more benefits fromAMfungi, which potentially makes the properties of Solidago litter different from co-occurring natives. These properties may comprehensively affect litter decomposition of co-occurring natives. We conducted a field experiment to examine litter mixing effects in a Phragmites australis dominated community invaded by Solidago in southeast China. Solidago had more rapid mass and N loss rate than Phragmites when they decomposed separately. Litter mixing decreased N loss rate in Phragmites litter and increased that of Solidago. Large decreases in Phragmites mass loss and smaller increases in Solidago mass loss caused negative non-additive effect. Solidago litter extracts reduced soil C decomposition and N processes, suggested an inhibitory effect of Solidago secondary compounds. These results are consistent with the idea that nutrient transfer and secondary compounds both affected litter mixtures decomposition.
Disturbance Driven Colony Fragmentation as a Driver of a Coral Disease Outbreak
(2013-02-20) Brandt, Marilyn E.; Smith, Tyler B.; Correa, Adrienne M.S.; Vega-Thurber, Rebecca
In September of 2010, Brewer’s Bay reef, located in St. Thomas (U.S. Virgin Islands), was simultaneously affected by abnormally high temperatures and the passage of a hurricane that resulted in the mass bleaching and fragmentation of its coral community. An outbreak of a rapid tissue loss disease among coral colonies was associated with these two disturbances. Gross lesion signs and lesion progression rates indicated that the disease was most similar to the Caribbean coral disease white plague type 1. Experiments indicated that the disease was transmissible through direct contact between colonies, and five-meter radial transects showed a clustered spatial distribution of disease, with diseased colonies being concentrated within the first meter of other diseased colonies. Disease prevalence and the extent to which colonies were bleached were both significantly higher on unattached colony fragments than on attached colonies, and disease occurred primarily on fragments found in direct contact with sediment. In contrast to other recent studies, disease presence was not related to the extent of bleaching on colonies. The results of this study suggest that colony fragmentation and contact with sediment played primary roles in the initial appearance of disease, but that the disease was capable of spreading among colonies, which suggests secondary transmission is possible through some other, unidentified mechanism.
Does phylogeny matter? Assessing the impact of phylogenetic information in ecological meta-analysis
(Blackwell Publishing Ltd/CNRS, 2012) Chamberlain, Scott A.; Hovick, Stephen M.; Dibble, Christopher J.; Rasmussen, Nick L.; Van Allen, Benjamin G.; Maitner, Brian S.; Ahern, Jeffrey R.; Bell-Dereske, Lukas P.; Roy, Christopher L.; Meza-Lopez, Maria; Carrillo, Juli; Siemann, Evan; Lajeunesse, Marc J.; Whitney, Kenneth D.
The Effect of Diet and Opponent Size on Aggressive Interactions Involving Caribbean Crazy Ants (Nylanderia fulva)
(Public Library of Science, 2013) Horn, Katherine C.; Eubanks, Micky D.; Siemann, Evan
Biotic interactions are often important in the establishment and spread of invasive species. In particular, competition between introduced and native species can strongly influence the distribution and spread of exotic species and in some cases competition among introduced species can be important. The Caribbean crazy ant, Nylanderia fulva, was recently introduced to the Gulf Coast of Texas, and appears to be spreading inland. It has been hypothesized that competition with the red imported fire ant, Solenopsis invicta, may be an important factor in the spread of crazy ants. We investigated the potential of interspecific competition among these two introduced ants by measuring interspecific aggression between Caribbean crazy ant workers and workers of Solenopsis invicta. Specifically, we examined the effect of body size and diet on individual-level aggressive interactions among crazy ant workers and fire ants. We found that differences in diet did not alter interactions between crazy ant workers from different nests, but carbohydrate level did play an important role in antagonistic interactions with fire ants: crazy ants on low sugar diets were more aggressive and less likely to be killed in aggressive encounters with fire ants. We found that large fire ants engaged in fewer fights with crazy ants than small fire ants, but fire ant size affected neither fire ant nor crazy ant mortality. Overall, crazy ants experienced higher mortality than fire ants after aggressive encounters. Our findings suggest that fire ant workers might outcompete crazy ant workers on an individual level, providing some biotic resistance to crazy ant range expansion. However, this resistance may be overcome by crazy ants that have a restricted sugar intake, which may occur when crazy ants are excluded from resources by fire ants.
Effects of above-ground herbivory on soil microbial biomass in conventional and no-tillage agroecosystems
(Elsevier, 1995) Holland, J. Nat
The effects of above-ground herbivory on soil microbial biomass associated with maize (Zea muys) roots were measured in field experiments. Different grazing intensities were established by placing grasshoppers (0, 5, 10 and 20) in cages around individual maize plants, and allowing them to graze for 5 days. Soil samples were taken 12 days later at a depth of between 8 and 12 cm. Soil microbial biomass was measured using the chloroform fumigation direct extraction method. Results indicated that intermediate levels of herbivory increased soil microbial biomass in the no-tillage system. However, in a conventional tillage system no significant differences were found among the grazing treatments. It is hypothesized that increases in soil microbial biomass in the no-tillage system were attributable to increases in root exudates resulting from above-ground herbivory.

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