Entomology

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    Plant-insect interactions in a shifting coastal ecosystem: Avicennia germinans and its associated arthropods
    (2020) Nathan, Mayda; Gruner, Daniel S; Entomology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The climate’s role in determining where species occur is increasingly well understood, but our ability to predict how biotic interactions both influence and respond to species’ range shifts remains poor. This is particularly important when considering climate-change-driven range shifts in habitat-forming species like mangroves, given their impact on ecosystem structure and function. In this dissertation, I consider the arthropods associated with the black mangrove, Avicennia germinans, to explore whether patterns of arthropod diversity affect the rate of a plant’s range expansion, and, in turn, how a range-expanding plant alters arthropod communities in habitats where it is invading. Among arthropods with the potential to influence plants’ range dynamics, pollinators can directly affect plant reproduction and ability to spread into new territory. Breeding system experiments reveal that A. germinans relies on pollinators for full fruit set, and surveys along the Florida coast show a substantial northward decline in the overall frequency of pollinator visits to A. germinans flowers. However, the decline in abundance of some common pollinator taxa is partly offset by an increase in the frequency of other highly effective taxa. Furthermore, range-edge A. germinans produce more flowers than southern individuals, contributing to high range-edge fecundity and enabling range expansion. As a woody plant with nectar-producing flowers, A. germinans is a novel resource for arthropods in the salt marshes where it is encroaching. To understand arthropod community assembly on these frontier mangroves, and how mangrove presence affects marsh arthropod community composition, I compare arthropod communities in these adjacent vegetation types. Arthropods form distinct communities on mangroves and marsh vegetation, with at least one A. germinans specialist already present in this range-edge population. However, neither mangrove proximity nor the abundance of mangrove flowers appears to influence salt marsh arthropod community structure, indicating that mangrove encroachment may lead to a net increase in arthropod diversity in coastal regions by increasing habitat heterogeneity. In sum, plants that rely on pollinators can avoid range-edge reproductive failure by attracting a diverse group of pollinating taxa, and range-expanding plants can rapidly alter invaded communities by shaping diversity at very local scales.
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    Linking detritus and primary producer based communities
    (2008-03-25) Hines, Jessica; Denno, Robert F; Entomology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Terrestrial food-web theory has been developed largely by examining species interactions in primary producer food webs. However, the decomposer subsystem can have strong influences on aboveground communities and ecosystem functioning. Here I examine, at several spatial scales, the complexity of terrestrial food-web interactions by considering interactions between species in detritivore and primary-producer food webs. I focused on Spartina alterniflora marshes and interactions among the numerically dominant herbivore Prokelisia dolus, its major spider predator Pardosa littoralis, and several detritivores (Littorophiloscia vittata, Orchestia grillus, Melampus bidentatus and Littoraria irrorata). I found that predator-detritivore interactions have weak indirect effects on plant growth and decomposition (Chapter 1). Furthermore, by serving as alternative prey, detritivores can influence the strength of predator-herbivore interactions. However, the strength of predator-herbivore-detritivore interactions was species-specific and depended on habitat structure (leaf litter - Chapter 1) and detritivore identity (Chapter 2). Although detritivore species are often functionally redundant in soil communities, changes in detritivore species composition can have divergent influences on aboveground predator-herbivore interactions (Chapter 2). Whereas some detritivores (Littorophiloscia vittatta) promote herbivore and predator survival, other detritivores (Littoraria irroratta) reduce predator and herbivore densities. Moreover, the geographic distribution of detritivores influences the relative strength of predator-herbivore interactions across broader spatial scales (Chapter 3). I found a shift in the relative abundance of dominant detritivore, herbivore, and predator species across a 1660 km latitudinal gradient. Detritivorous Littoraria snails that abound on low-latitude marshes modify Spartina vegetation structure and create an unfavorable habitat for Pardosa spiders. Pardosa exert stronger predation pressure on Prokelisia planthoppers on high-latitude marshes where spiders are abundant. Changes in global carbon cycles may influence the strength of linkages between primary production and decomposition food webs. I examined how changes in the detritivore food chain influenced the growth of two plant species (Scirpus olneyi and Spartina patens) under elevated and ambient CO2 conditions. I found limited and species-specific support for the increased importance of the decomposition pathway under elevated CO2 conditions. Overall, detritivores modified predator-herbivore interactions, live plant growth, and decomposition. The strength of these interactions changed with the composition of the detritivore community, latitude, and atmospheric CO2 conditions.
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    Patterns of Diversification in Phytophagous Insects: Phylogeny and Evolution of Phytomyza Leaf-mining Flies (Diptera: Agromyzidae)
    (2008-01-22) Winkler, Isaac Scott; Mitter, Charles; Scheffer, Sonja J.; Entomology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Plant-feeding insects account for about one fourth of macroscopic biodiversity. This study aims to document factors contributing to this diversity by investigating phylogenetic relationships within a large radiation of herbivorous insects, Phytomyza leaf-mining flies (Diptera: Agromyzidae). After a brief introduction (Chapter 1), a general overview of phylogenetic patterns in phytophagous insects is presented, based on over 200 phytophagous insect phylogenies from the recent literature (Chapter 2). A few salient results include 1) host use conservatism at the family level predominates, with shifts occurring at about 5% of speciation events; 2) host shifts are a major contributor to speciation, occuring in about half of 145 speciation events tabulated; 3) insect-host associations mostly reflect colonization of already diversified host plant clades; and 4) variation in diversification rates is not yet well-documented for phytophagous insects, except at the broadest scale. Chapter 3 is a phylogenetic study of the genus Phytomyza sensu lato, using over 3,000 nucleotides of DNA sequence data from three genes. Results indicate that the genus Chromatomyia, considered by some as synonymous with Phytomyza, is in fact polyphyletic and nested within Phytomyza. Possible parallelism in a biological trait (internal pupation in leaf tissue) which is one of the defining traits of species in the former Chromatomyia is discussed. In addition, the internal classification of Phytomyza is assessed and revised insofar as the data permit. Divergence times for the Agromyzidae, and also for Phytomyza and related genera, were estimated using a molecular phylogeny calibrated by three agromyzid fossils (Chapter 4). Results suggest that the temperate Phytomyza group of genera originated in the relatively warm Eocene epoch. Ranunculaceae, a primitive plant family, is inferrred as the ancestral host for a clade including most Phytomyza species, but is probably secondary to feeding on more derived plant families ("asterid clade"). Ten clades were identified for comparison of diversification rates between Ranunculaceae- and asterid-feeding lineages, which showed that asterid-feeding clades exhibit higher rates of diversification. Phytomyza originated approximately at the early Oligocene global cooling event, but contrary to expectations, diversification significantly slowed during the Oligocene cool period, when suitable habitats for Phytomyza were presumably widespread.