Entomology
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Item Does species richness drive speciation? A reassessment with the Hawaiian biota(Ecography, 2008) Gruner, Daniel; Gotelli, Nicholas; Price, Jonathan; Cowie, RobertItem Arthropods from ‘ōhi‘a lehua (Myrtaceae: Metrosideros polymorpha), with new records for the Hawaiian Islands(Bishop Museum Occasional Papers, 2004) Gruner, DanielThis paper presents new records, range extensions, and a checklist of arthropod species found associated with the most common and widespread native tree in the Hawaiian Islands, ‘öhi‘a lehua (Myrtaceae: Metrosideros polymorpha Gaudichaud-Beaupré). Metrosideros polymorpha is found on all the main islands, naturally occurs from sea level to tree line (>2000 m), in dry, mesic, and wet forests, and is the canopy dominant in old growth and the first woody colonist on recent basaltic lava flows (Dawson & Stemmerman, 1990). Numerous insect species use ‘öhi‘a lehua as a resource for either food or habitat space, and it may have the largest fauna of any native plant (Southwood, 1960; Stein, 1983). Metrosideros is an important, year-round nectar resource for native bees, moths, thrips and other insects, and for native nectarivorous birds, such as the ‘apapane (Himatione sanguinea), ‘i‘iwi (Vestiara coccinea), and ‘akohekohe (Palmeria dolei). Metrosideros also provides important habitat for birds that forage for arthropod prey in the foliage (e.g., ‘akepa [Loxops coccineus]) and bark (e.g., Hawai‘i creeper [Oreomystis mana]). It can be argued that M. polymorpha is the backbone of Hawaiian forests and one of the most important resources for the long-term stability of ecosystems and watersheds in the islands.Item Richness and species composition of arboreal arthropods affected by nutrients and predators: a press experiment(Oecologia, 2006) Gruner, Daniel; Taylor, AndrewA longstanding goal for ecologists is to understand the processes that maintain biological diversity in communities, yet few studies have investigated the combined effects of predators and resources on biodiversity in natural ecosystems. We fertilized nutrient limited plots and excluded insectivorous birds in a randomized block design, and examined the impacts on arthropods associated with the dominant tree in the Hawaiian Islands, Metrosideros polymorpha (Myrtaceae). After 33 mo, the species load (per foliage mass) of herbivores and carnivores increased with fertilization, but rarified richness (standardized to abundance) did not change. Fertilization depressed species richness of arboreal detritivores, and carnivore richness dropped in caged, unfertilized plots, both because of the increased dominance of common, introduced species with treatments. Herbivore species abundance distributions were more equitable than other trophic levels following treatments, and fertilization added specialized native species without changing relativized species richness. Overall, bird removal and nutrient addition treatments on arthropod richness acted largely independently, but with countervailing influences that obscured distinct top-down and bottom-up effects on different trophic levels. This study demonstrates that species composition, biological invasions, and the individuality of species traits may complicate efforts to predict the interactive effects of resources and predation on species diversity in food webs.Item The effects of foliar pubescence and nutrient enrichment on arthropod communities of Metrosideros polymorpha (Myrtaceae)(Ecological Entomology, 2005) Gruner, Daniel; Taylor, Andrew; Forkner, Rebecca1. Nutrient resource availability and host-plant foliar pubescence both influence arthropod food webs, but multifactor studies are needed to understand their interdependence and relative importance. Arthropods were sampled by clipping foliage from Metrosideros polymorpha (Myrtaceae) trees of pubescent, glabrous, and intermediate leaf forms on fertilised and unfertilised plots. 2. Fertilisation decreased leaf mass per area (LMA) but did not change the relative mass of pubescence within leaf morphological classes. 3. Fertilisation increased densities of individuals in four taxonomic orders, densities of individuals and species of all trophic levels, and the biomass of Collembola and Homoptera. Herbivore relative diversity (Shannon H0) also increased with fertilisation, but detritivore diversity declined due to increasing dominance of Salina celebensis (Schaeffer) (Collembola). 4. Detritivore density, driven again by S. celebensis, increased with decreasing leaf pubescence, but Heteroptera and Acari were most abundant on the intermediate pubescence class, and Psocoptera density and biomass increased with increasing pubescence. Trophic-level species density did not change with leaf morphological class, but relative diversity of all arthropods and of detritivores increased with increasing pubescence. 5. Both resource availability and leaf pubescence affected Metrosideros arthropod communities. However, the pervasive positive influence of fertilisation did not translate to compositional shifts, and there were no interactions with leaf morphological class. In contrast, the effects of leaf pubescence on arthropod density, biomass, and diversity were more restricted taxonomically, and nonparametric MANOVA and redundancy analyses demonstrated significant differentiation in community composition on the pubescent morphology.Item Geological age, ecosystem development, and resource constraints on arthropod community structure in the Hawaiian Islands(Biological Journal of the Linnean Society, 2007) Gruner, DanielAn ongoing debate in evolutionary ecology concerns the relative role of contemporary versus historical processes in determining local species richness and community structure. At sites along a 4-Myr geological chronosequence on Hawai‘i, Moloka‘i and Kaua‘i, numerous extrinsic factors can be held constant, but ecosystem fertility and nutrient availability are low at both very young and very old sites, peaking at intermediate geological age across islands. Thus, contemporary resource traits are shared among sites with different biogeographic legacies, and these opposing gradients allowed a test of their relative importance for arboreal arthropod community structure. Pyrethrum knockdown was used to sample arboreal arthropods from Metrosideros polymorpha (Myrtaceae), the dominant tree throughout the Hawaiian Islands. Arthropod abundances and sample-based species richness peaked at more productive, intermediate-aged sites, but did not correlate with geological age. The proportions of individuals and biomass in trophic groups and in different taxonomic orders differed widely across sites, but proportions of species in trophic groups were more regular than chance expectation. Species richness in local communities did not accumulate or pack more tightly with increasing geological age to the oldest island. Intermediate-aged islands may be contemporary peaks of richness, mediated by ecosystem development and senescence. Although historical and evolutionary processes generate diversity at broad scales, local communities converged in trophic structure and composition, and ecosystem resource availability constrained arthropod numbers and richness at local scales.Item Biotic resistance to an invasive spider conferred by generalist insectivorous birds on the island of Hawai‘i(Biological Invasions, 2005) Gruner, DanielA central problem for ecology is to understand why some biological invasions succeed while others fail. Species interactions frequently are cited anecdotally for establishment failure, but biotic resistance is not well supported by quantitative experimental studies in animal communities. In a 33-month experiment on Hawaii Island, exclusion of native and alien forest birds resulted in a 25- to 80-fold increase in the density of a single non-indigenous spider species (Theridiidae: Achaearanea cf. riparia). Caged plots held large aggregations of juveniles and more large-bodied individuals, suggesting potential reproductive individuals are more susceptible to bird predation. Most examples of biotic resistance involve competition for limiting resources among sessile marine animals or terrestrial plants. The present results show that generalist predators can limit the success of introductions, even on oceanic islands, generally assumed less resistant to invasion.Item Two ant species (Hymenoptera: Formicidae) new to the Hawaiian Islands(Bishop Museum Occasional Papers, 2003) Gruner, Daniel; Heu, Ron; Chun, MarianneTwo new ant species records are reported for the Hawaiian Islands. Specimens for both species were first collected in the spring of 2000 by K-12 students and classes as part of an ongoing survey for the little fire ant, Wasmannia auropunctata (Roger), on Hawai‘i Island. Discovery of the little fire ant on Hawai‘i and Kaua‘i (Conant & Hirayama, 2000) elicited survey and control activities by the Hawaii Department of Agriculture (HDOA) and stimulated the creation of a traveling educational curriculum through the University of Hawai‘i. Intermediate and high school students collected ants from their backyard environment with the goal of finding additional infestations of W. auropunctata. One of us (DSG) analyzed and identified all ants in these samples, which contained the two species new to Hawai‘i, and mapped their distributions. Details of this program will be reported elsewhere (D. Gruner, unpubl.). Concurrently, HDOA (RAH. & MEC) discovered one of these ant species during surveys on the island of O‘ahu.Item Attenuation of top-down and bottom-up forces in a complex terrestrial community(Ecology, 2004) Gruner, DanielCarnivore (top-down) and resource (bottom-up) influences in food webs are strong and pervasive, but few studies have investigated their interactive effects in species-rich terrestrial ecosystems. This study focused on arthropods associated with the dominant tree species, Metrosideros polymorpha (Myrtaceae), in Hawaiian forests. Severe soil nutrient limitation on a 120-yr-old lava flow was removed by fertilization and combined with bird predator exclusion cages in a randomized block design. Arthropod densities were measured from clipped foliage at the outset and conclusion of a 33-mo experiment, and their biomass was estimated using regression equations. Metrosideros foliar nitrogen, tree growth, and biomass increased directly in response to fertilization but did not change with bird exclusion. Fertilization increased detritivore densities but not biomass, and both density and biomass of herbivores, while bird exclusion increased both density and biomass of carnivores. Fertilization also increased spider density and biomass, but bird exclusion increased spider numbers (15 species) only in high resource plots. Overall, trophic level biomass responses were less pronounced than density because smaller bodied individuals responded more to enrichment. Bottom-up factors controlled basal trophic levels, and detritivores comprised the largest fraction of arthropod density and biomass. Conversely, top-down impacts were apparent but variable, limited to higher order consumers, and did not cascade to the level of primary producers. These experimental results were consistent with the view that complex forest ecosystems are structured on a bottom-up template.