Entomology Research Works

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Two ant species (Hymenoptera: Formicidae) new to the Hawaiian Islands
(Bishop Museum Occasional Papers, 2003) Gruner, Daniel; Heu, Ron; Chun, Marianne
Two 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.
Regressions of length and width to predict arthropod biomass in the Hawaiian Islands
(Pacific Science, 2003) Gruner, Daniel
Biologists in many fields use published regression equations to predict biomass from simple linear body measurements. Power functions are used with arthropods, facilitating biomass estimation of a sample when destructive techniques are not feasible. Resulting predictive coefficients vary widely depending on region and taxa. There are no published biomass regressions for oceanic island fauna, despite the widely accepted conclusion that their arthropod assemblages are unusual in composition. I present a suite of general and taxonomically and morphologically restricted regression equations developed for arthropods in the Hawaiian Islands. General regression equations were highly significant when only length was used to predict biomass, but fits were usually improved by including body width. In regressing restricted sets of taxa, the addition of width did little to improve the fit of the functions. Thus, the choice of regression equations involves a trade-off in taxonomic resolution: precise biomass estimates will come either from (1) low taxonomic resolution measured for both length and width, or (2) high taxonomic resolution measured only for body length. These equations have a high predictive capacity for a broad range of arthropod taxa common in the Hawaiian Islands and, in the absence of locally developed equations, the arthropods of other oceanic islands.
Arthropods from ‘ōhi‘a lehua (Myrtaceae: Metrosideros polymorpha), with new records for the Hawaiian Islands
(Bishop Museum Occasional Papers, 2004) Gruner, Daniel
This 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.
Attenuation of top-down and bottom-up forces in a complex terrestrial community
(Ecology, 2004) Gruner, Daniel
Carnivore (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.
The effects of foliar pubescence and nutrient enrichment on arthropod communities of Metrosideros polymorpha (Myrtaceae)
(Ecological Entomology, 2005) Gruner, Daniel; Taylor, Andrew; Forkner, Rebecca
1. 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.
Biotic resistance to an invasive spider conferred by generalist insectivorous birds on the island of Hawai‘i
(Biological Invasions, 2005) Gruner, Daniel
A 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.
Richness and species composition of arboreal arthropods affected by nutrients and predators: a press experiment
(Oecologia, 2006) Gruner, Daniel; Taylor, Andrew
A 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.
All wet or dried up? Real differences between aquatic and terrestrial food webs
(Proceedings of the Royal Society B: Biological Sciences, 2006) Shurin, Jonathan; Gruner, Daniel; Hillebrand, Helmut
Ecologists have greatly advanced our understanding of the processes that regulate trophic structure and dynamics in ecosystems. However, the causes of systematic variation among ecosystems remain controversial and poorly elucidated. Contrasts between aquatic and terrestrial ecosystems in particular have inspired much speculation, but only recent empirical quantification. Here, we review evidence for systematic differences in energy flow and biomass partitioning between producers and herbivores, detritus and decomposers, and higher trophic levels. The magnitudes of different trophic pathways vary considerably, with less herbivory, more decomposers and more detrital accumulation on land. Aquatic– terrestrial differences are consistent across the global range of primary productivity, indicating that structural contrasts between the two systems are preserved despite large variation in energy input. We argue that variable selective forces drive differences in plant allocation patterns in aquatic and terrestrial environments that propagate upward to shape food webs. The small size and lack of structural tissues in phytoplankton mean that aquatic primary producers achieve faster growth rates and are more nutritious to heterotrophs than their terrestrial counterparts. Plankton food webs are also strongly size-structured, while size and trophic position are less strongly correlated in most terrestrial (and many benthic) habitats. The available data indicate that contrasts between aquatic and terrestrial food webs are driven primarily by the growth rate, size and nutritional quality of autotrophs. Differences in food web architecture (food chain length, the prevalence of omnivory, specialization or anti-predator defences) may arise as a consequence of systematic variation in the character of the producer community.
Arthropod surveys on Palmyra Atoll, Line Islands, and insights into the decline of the native tree Pisonia grandis (Nyctaginaceae)
(Pacific Science, 2007) Handler, Alex; Gruner, Daniel; Haines, William; Lange, Matthew; Kaneshiro, Ken
Palmyra Atoll, in the Line Islands of the equatorial Pacific, supports one of the largest remaining native stands of Pisonia grandis forest in the tropical Pacific Ocean. In 2003, we surveyed terrestrial arthropods in order to document extant native and introduced species richness, compare these lists to historical records, and assess potential threats to native species and ecosystem integrity. In total, 115 arthropod taxa were collected, bringing the total number of taxa recorded since 1913 to 162. Few native species were collected; most taxa were accidental introductions also recorded from the Hawaiian Islands, the presumed main source of introductions to Palmyra. The overlap with previous historical surveys in 1913 and 1948 was low (<40%) and new species continue to establish, with one species of whitefly reaching pest status between 2003 and 2005. We observed numerous dead or dying large Pisonia grandis, and the green scale Pulvinaria urbicola (Coccidae) was particularly abundant on trees of poor health. Abundant introduced ants, particularly Pheidole megacephala, tended this and other hemipterans feeding on both native and introduced plants. We hypothesize that the Pheidole--Pulvinaria facultative mutualism is causing the decline of Pisonia grandis. Because of the unique properties of Pisonia grandis forest on oceanic atolls, its importance for nesting seabirds, and its alarming global decline, immediate conservation efforts should be directed at controlling introduced Hemiptera and disrupting their mutualisms with non-native ants on Palmyra Atoll.
Consumer versus resource control of producer diversity depends on ecosystem type and producer community structure
(Proceedings of the National Academy of Sciences of the USA, 2007) Hillebrand, Helmut; Gruner, Daniel; Borer, Elizabeth; Bracken, Matthew; Cleland, Elsa; Harpole, Stanley; Ngai, Jacqueline; Seabloom, Eric; Shurin, Jonathan; Smith, Jennifer
Consumer and resource control of diversity in plant communities have long been treated as alternative hypotheses. However, experimental and theoretical evidence suggests that herbivores and nutrient resources interactively regulate the number and relative abundance of coexisting plant species. Experiments have yielded divergent and often contradictory responses within and among ecosystems, and no effort has to date reconciled this empirical variation within a general framework. Using data from 274 experiments from marine, freshwater, and terrestrial ecosystems, we present a cross-system analysis of producer diversity responses to local manipulations of resource supply and/or herbivory. Effects of herbivory and fertilization on producer richness differed substantially between systems: (i) herbivores reduced species richness in freshwater but tended to increase richness in terrestrial systems; (ii) fertilization increased richness in freshwater systems but reduced richness on land. Fertilization consistently reduced evenness, whereas herbivores increased evenness only in marine and terrestrial ecosystems. Producer community evenness and ecosystem productivity mediated fertilization and herbivore effects on diversity across ecosystems. Herbivores increased producer richness in more productive habitats and in producer assemblages with low evenness. These same assemblages also showed the strongest reduction in richness with fertilization, whereas fertilization increased (and herbivory decreased) richness in producer assemblages with high evenness. Our study indicates that system productivity and producer evenness determine the direction and magnitude of top-down and bottom-up control of diversity and may reconcile divergent empirical results within and among ecosystems.
Geological age, ecosystem development, and resource constraints on arthropod community structure in the Hawaiian Islands
(Biological Journal of the Linnean Society, 2007) Gruner, Daniel
An 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.
Soil mediates the interaction of coexisting entomopathogenic nematodes with an insect host
(Journal of Invertebrate Pathology, 2007) Gruner, Daniel; Ram, Karthik; Strong, Donald
We tested for soil substrate effects on the movement and infectivity of naturally co-occurring entomopathogenic nematodes Steinernema feltiae and Heterorhabditis marelatus, alone and in combination. We manipulated the presence and bulk density of soil and added Galleria mellonella baits within capped and perforated 15 mL centrifuge tubes. Sampling tubes were then deployed in situ into field and laboratory settings as experimental traps for infective juveniles. In comparisons with standard soil collections from Lupinus arboreus rhizospheres, sampling tubes were equally sensitive to the presence of H. marelatus and more sensitive to S. feltiae. In laboratory microcosms, both EPN species infected Galleria at high frequencies in tubes lacking soil and in the absence of heterospecifics. Infection frequency of S. feltiae was unaffected by the presence of H. marelatus, but it declined with higher soil bulk density inside tubes. In contrast, detectable infection frequency by H. marelatus was reduced only marginally by the presence of soil but severely by the presence of S. feltiae. Thus, the presence of soil in tubes reversed the identity of dominant species infecting Galleria in tubes, an effect magnified when soils were compacted. Moreover, S. feltiae rarely moved into tubes lacking Galleria baits, whereas H. marelatus colonized unbaited tubes 4- to 5-fold more frequently than S. feltiae. In situ, sampling tubes acted as filters to reduce interference and contamination by fungal pathogens common in field soils. The method allows precision sampling with minimal soil disturbance while protecting bait insects from scavengers. Manipulation of tube design may allow selective sampling of EPN species, depending on the abiotic characteristics of soils, and the biology, behavior, and interspecific interactions of coexisting species.
Global analysis of nitrogen and phosphorus limitation of primary producers in freshwater, marine and terrestrial ecosystems
(Ecology Letters, 2007-12) Elser, James; Bracken, Matthew; Cleland, Elsa; Gruner, Daniel; Harpole, Stanley; Hillebrand, Helmut; Ngai, Jacqueline; Seabloom, Eric; Shurin, Jonathan; Smith, Jennifer
The cycles of the key nutrient elements nitrogen (N) and phosphorus (P) have been massively altered by anthropogenic activities. Thus, it is essential to understand how photosynthetic production across diverse ecosystems is, or is not, limited by N and P. Via a large-scale meta-analysis of experimental enrichments, we show that P limitation is equally strong across these major habitats and that N and P limitation are equivalent within both terrestrial and freshwater systems. Furthermore, simultaneous N and P enrichment produces strongly positive synergistic responses in all three environments. Thus, contrary to some prevailing paradigms, freshwater, marine and terrestrial ecosystems are surprisingly similar in terms of N and P limitation.
Does species richness drive speciation? A reassessment with the Hawaiian biota
(Ecography, 2008) Gruner, Daniel; Gotelli, Nicholas; Price, Jonathan; Cowie, Robert
Potential for entomopathogenic nematodes in biological control: a meta-analytical synthesis and insights from trophic cascade theory
(2008) Denno, Robert; Gruner, Daniel; Kaplan, Ian
Entomopathogenic nematodes (EPN) are ubiquitous and generalized consumers of insects in soil food webs, occurring widely in and agricultural ecosystems on all continents. Augmentative releases of EPN have been used to enhance biological control of pests in agroecosystems. Pest managers strive to achieve a trophic cascade whereby natural-enemy effects permeate down through the food web to suppress host herbivores and increase crop production. Although trophic cascades have been studied in diverse aboveground arthropod-based systems, they are infrequently investigated in soil systems. Moreover, no overall quantitative assessment of the effectiveness of EPN in suppressing hosts with cascading benefits to plants has been made. Toward synthesizing the available but limited information on EPN and their ability to suppress prey and affect plant yield, we surveyed the literature and performed a meta-analysis of 35 published studies. Our analysis found that effect sizes for arthropod hosts as a result of EPN addition were consistently negative and indirect effects on plants were consistently positive. Results held across several different host metrics (abundance, fecundity and survival) and across several measures of plant performance (biomass, growth, yield and survival). Moreover, the relationship between plant and host effect size was strikingly and significantly negative. That is, the positive impact on plant responses generally increased as the negative effect of EPN on hosts intensified, providing strong support for the mechanism of trophic cascades. We also review the ways in which EPN might interact antagonistically with each other and other predators and pathogens to adversely affect host suppression and dampen trophic cascades. We conclude that the food web implications of multiple-enemy interactions involving EPN are poorly studied, but, as management techniques that promote the long-term persistence of EPN are improved, antagonistic interactions are more likely to arise. We hope that the likely occurrence of antagonistic interactions in soil food webs should stimulate researchers to conduct field experiments explicitly designed to examine multiple-enemy interactions involving EPN and their cascading effects to hosts and plants.
Dynamics of a subterranean trophic cascade in space and time
(2008) Ram, Karthik; Gruner, Daniel; McLaughlin, John; Preisser, Evan; Strong, Donald
Trophic cascades, whereby predators indirectly benefit plant biomass by reducing herbivore pressure, form the mechanistic basis for classical biological control of pest insects. Entomopathogenic nematodes (EPN) are lethal to a variety of insect hosts with soil-dwelling stages, making them promising biocontrol agents. EPN biological control programs, however, typically fail because nematodes do not establish, persist and/or recycle over multiple host generations in the field. A variety of factors such as local abiotic conditions, host quantity and quality, and rates of movement affect the probability of persistence. Here, we review results from 13 years of study on the biology and ecology of an endemic population of Heterorhabditis marelatus (Rhabditida: Heterorhabditidae) in a California coastal prairie. In a highly seasonal abiotic environment with intrinsic variation in soils, vegetation structure, and host availability, natural populations of H. marelatus persisted at high incidence at some but not all sites within our study area. Through a set of field and lab experiments, we describe mechanisms and hypotheses to understand the persistence of H. marelatus. We suggest that further ecological study of naturally occurring EPN populations can yield significant insight to improve the practice and management of biological control of soil-dwelling insect pests.
Evidence for divergent selection between the molecular forms of Anopheles gambiae: role of predation
(Springer Nature, 2008-01-11) Diabaté, Abdoulaye; Dabiré, Roch K; Heidenberger, Kyle; Crawford, Jacob; Lamp, William O; Culler, Lauren E; Lehmann, Tovi
The molecular forms of Anopheles gambiae are undergoing speciation. They are characterized by a strong assortative mating and they display partial habitat segregation. The M form is mostly found in flooded/irrigated areas whereas the S form dominates in the surrounding areas, but the ecological factors that shape this habitat segregation are not known. Resource competition has been demonstrated between species undergoing divergent selection, but resource competition is not the only factor that can lead to divergence. In a field experiment using transplantation of first instar larvae, we evaluated the role of larval predators in mediating habitat segregation between the forms. We found a significant difference in the ability of the molecular forms to exploit the different larval sites conditioned on the presence of predators. In absence of predation, the molecular forms outcompeted each other in their respective natural habitats however, the developmental success of the M form was significantly higher than that of the S form in both habitats under predator pressure. Our results provide the first empirical evidence for specific adaptive differences between the molecular forms and stress the role of larval predation as one of the mechanisms contributing to their divergence.
Birds as predators in tropical agroforestry systems
(2008-04) Van Bael, Sunshine; Philpott, Stacy; Greenberg, Russell; Bichier, Peter; Barber, Nicholas; Mooney, Kailen; Gruner, Daniel
Insectivorous birds reduce arthropod abundances and their damage to plants in some, but not all, studies where predation by birds has been assessed. The variation in bird effects may be due to characteristics such as plant productivity or quality, habitat complexity, and/or species diversity of predator and prey assemblages. Since agroforestry systems vary in such characteristics, these systems provide a good starting point for understanding when and where we can expect predation by birds to be important. We analyze data from bird exclosure studies in forests and agroforestry systems to ask whether birds consistently reduce their arthropod prey base and whether bird predation differs between forests and agroforestry systems. Further, we focus on agroforestry systems to ask whether the magnitude of bird predation (1) differs between canopy trees and understory plants, (2) differs when migratory birds are present or absent, and (3) correlates with bird abundance and diversity. We found that, across all studies, birds reduce all arthropods, herbivores, carnivores, and plant damage. We observed no difference in the magnitude of bird effects between agroforestry systems and forests despite simplified habitat structure and plant diversity in agroforests. Within agroforestry systems, bird reduction of arthropods was greater in the canopy than the crop layer. Top-down effects of bird predation were especially strong during censuses when migratory birds were present in agroforestry systems. Importantly, the diversity of the predator assemblage correlated with the magnitude of predator effects; where the diversity of birds, especially migratory birds, was greater, birds reduced arthropod densities to a greater extent. We outline potential mechanisms for relationships between bird predator, insect prey, and habitat characteristics, and we suggest future studies using tropical agroforests as a model system to further test these areas of ecological theory.
A cross-system synthesis of herbivore and nutrient resource control on producer biomass
(2008-07) Gruner, Daniel; Smith, Jennifer; Seabloom, Eric; Sandin, Stuart; Ngai, Jacqueline; Hillebrand, Helmut; Harpole, Stanley; Elser, James; Cleland, Elsa; Bracken, Matthew; Borer, Elizabeth; Bolker, Benjamin
Nutrient availability and consumption by herbivores control the biomass of primary producer communities to varying degrees across ecosystems. Ecological theory, individual experiments in many different systems, and system-specific quantitative reviews have suggested that 1) bottom-up control is pervasive but top-down control is more influential in aquatic habitats relative to terrestrial systems, and 2) bottom-up and top-down forces often interact to synergize or dampen relative influences on producer biomass. We use a simple set of dynamic models to review mechanistic hypotheses for these questions, and compare model predictions to empirical data from a comprehensive meta-analysis of 191 factorial manipulations of herbivores and nutrients from freshwater, marine and terrestrial ecosystems. Parameterized model equilibria suggest that interactive outcomes should be weak and less common than strict additivity. Producer community biomass responded positively to fertilization across all systems, although effects were most pronounced in freshwater. Herbivores suppressed producer biomass in both freshwater and marine systems, but effects were inconsistent on land. Importantly, we observed a striking absence of either synergistic or dampening interactive effects of nutrients and herbivores across ecosystem types and within most habitats. Marine temperate rocky reef systems, which showed superadditive synergism of nutrient and herbivore controls, represented an exception to this pattern. Experimental studies showed limited support for emergent interactive effects on producer community-level biomass. We suggest that compensation by multiple herbivore guilds, top-down control of herbivores, spatial and temporal heterogeneity, and herbivore-mediated nutrient recycling tend to reduce the expectation for consistent interactive effects on producer biomass. Continuing studies should expand the temporal and spatial scales of experiments, particularly in understudied terrestrial systems; broaden factorial designs to manipulate independently both multiple producer resources (e.g. nitrogen, phosphorus, light) and multiple herbivore taxa or guilds (e.g. vertebrates and invertebrates); and simultaneously assess the effects on not only producer biomass but also species diversity, community composition and structure, and nutrient status.
Metapopulation dynamics override local limits on long-term parasite persistence
(2008-12) Ram, Karthik; Preisser, Evan; Gruner, Daniel; Strong, Donald
A simple null model, particularly germane to small and vulnerable organisms such as parasites, is that local conditions set a stage upon which larger-scale dynamics play out. Soil moisture strongly influences survival of entomopathogenic nematodes (EPN), which in turn drive trophic cascades by protecting vegetation from root-feeding herbivores. In this study, we examine the mechanisms responsible for patchy occurrence of an entomopathogenic nematode, Heterorhabditis marelatus, in a California coastal prairie. One hypothesis proposes that biotic factors such as competition and natural enemies could regulate occurrence of EPN populations. We found that fungi and other enemies of EPN, although locally potent, did not explain the patterns of incidence across sites. Abiotic factors also have strong effects on EPN persistence, especially for vulnerable free-living stages. Thus, we tested the hypothesis that patchy occurrence of EPN on a large landscape was driven by differences in soil moisture. Our research uses long-term data on nematode incidence in combination with a landscapelevel experiment to demonstrate the lack of a correlation between soil moisture and long-term persistence. A year-long experiment showed EPN mortality was weakly correlated with soil moisture among our study sites. Thirteen years of data, however, showed that colonization rates were highly correlated with long-term persistence. Sites with highest long-term persistence experienced the highest rates of rhizosphere colonization, extinction, and turnover. As a result, we concluded that metapopulation dynamics override limitations set by local and short-term abiotic conditions to determine long-term persistence in this parasite-driven trophic cascade.