Establishment and dominance of an introduced herbivore has limited impact on native host-parasitoid food webs

1. Shared enemies may link the dynamics of their prey. Recently, quantitative food webs have been used to infer that herbivorous insect species attacked by the same major parasitoid species will affect each other negatively through apparent competition. Nonetheless, theoretical work predicts several alternative outcomes, including positive effects. 2. In this paper, we use an experimental approach to link food web patterns to realized population dynamics. First, we construct a quantitative food web for three dominant leaf miner species on the oak Quercus robur. We then measure short- and long-term indirect effects by increasing leaf miner densities on individual trees. Finally, we test whether experimental results are consistent with natural leaf miner dynamics on unmanipulated trees. 3. The quantitative food web shows that all leaf miner species share a minimum of four parasitoid species. While only a small fraction of the parasitoid pool is shared among Tischeria ekebladella and each of two Phyllonorycter species, the parasitoid communities of the congeneric Phyllonorycter species overlap substantially. 4. Based on the structure of the food web, we predict strong short- and long-term indirect interactions between the Phyllonorycter species, and limited interactions between them and T. ekebladella. As T. ekebladella is the main source of its own parasitoids, we expect to find intraspecific density-dependent parasitism in this species. 5. Consistent with these predictions, parasitism in T. ekebladella was high on trees with high densities of conspecifics in the previous generation. Among leaf miner species sharing more parasitoids, we found positive rather than negative interactions among years. No short-term indirect interactions (i.e. indirect interactions within a single generation) were detected. 6. Overall, this study is the first to experimentally demonstrate that herbivores with overlapping parasitoid communities may exhibit independent population dynamics - or even apparent mutualism. Hence, it proves the potential for versatile indirect interactions in nature, and suggests that the link between patterns in food web structure and realized population dynamics should be verified by rigorous experiments.

Simple SummaryIn terrestrial natural ecosystems, more complex and diverse networks of plant–insect primary consumers and their predators are often more productive, stable, and resilient. Plant diversity often positively correlates to the diversity of phytophagous insects and their natural enemies generating multitrophic interactions with changing outcomes (bottom-up effects). The use of cover crops can promote natural enemy populations and their temporal synchronization with a target pest, resulting in greater pest control. Therefore, changes in the habitat conditions can alter food webs. In agroecosystems, characteristics of the food trophic webs, as connectance, measured as the proportion of realized links in the network, could be linked to the efficiency of pest control. In this study, we evaluated how the use of oat cover crops affects composition and structure in the aphid–parasitoid–hyperparasitoid food webs of plum orchards with different habitat management contexts: plums with inter-rows of oats as a cover crop (OCC) and plums with inter-rows with spontaneous vegetation (SV). Quantitative food web metrics differed significantly among treatments showing a higher generality, vulnerability, interaction evenness, and linkage density in SV, while OCC presented a higher degree of specialization.By increasing plant diversity in agroecosystems, it has been proposed that one can enhance and stabilize ecosystem functioning by increasing natural enemies’ diversity. Food web structure determines ecosystem functioning as species at different trophic levels are linked in interacting networks. We compared the food web structure and composition of the aphid– parasitoid and aphid-hyperparasitoid networks in two differentially managed plum orchards: plums with inter-rows of oats as a cover crop (OCC) and plums with inter-rows of spontaneous vegetation (SV). We hypothesized that food web composition and structure vary between OCC and SV, with network specialization being higher in OCC and a more complex food web composition in SV treatment. We found a more complex food web composition with a higher species richness in SV compared to OCC. Quantitative food web metrics differed significantly among treatments showing a higher generality, vulnerability, interaction evenness, and linkage density in SV, while OCC presented a higher degree of specialization. Our results suggest that plant diversification can greatly influence the food web structure and composition, with bottom-up effects induced by plant and aphid hosts that might benefit parasitoids and provide a better understanding of the activity, abundance, and interactions between aphids, parasitoids, and hyperparasitoids in plum orchards.

Seagrass beds are important habitats in coastal areas but increasingly decline in area and quality, thus conservation measures are urgently needed. Quantitative food webs, describing the biomass distribution and energy fluxes among trophic groups, reveal structural and functional aspects of ecosystems. Their knowledge can improve ecological conservation. For the recently discovered large warm-temperate seagrass (Zostera japonica) habitat in China’s Yellow River Delta wetland, we used δ13C and δ15N measurements and a Bayesian isotope mixing model to construct its food web diagram with quantitative estimations of consumer diet compositions, comprising detritus and 14 living trophic groups from primary producers to fish. We then estimated the quantitative food web fluxes based on biomass measurements and calculated corresponding ecosystem functions. Pelagic producers were significantly 13C-depleted compared to benthic sources. Consumers (except zooplankton) were increasingly 13C-depleted with increasing trophic positions even though the consumed benthic production surpassed the pelagic one. Bivalves dominated consumer biomasses and fluxes and were the first to connect the pelagic and benthic pathways, whereas zooplankton and gastropods were specialized on the two pathways, respectively. We found flat biomass and production pyramids indicating low trophic transfer efficiencies. Generally, the energetic structure of the quantitative food web was consistent with the stable isotope analysis, and the estimated net primary production and most estimated production to biomass ratios of the trophic groups fell within literature ranges. This study provides a systematical understanding of the quantitative trophic ecology of a seagrass bed and facilitates synergistic knowledge on management, conservation, and restoration.

Natural ecosystems comprise an innumerable amount of different organisms. These organisms are not separated, they interact and depend on each other. Today’s ecosystems are facing an enormous decline in biodiversity due to human impacts with thus far unknown consequences. One key objective of ecological research is to understand the mechanisms generating and maintaining this incredible amount of diversity. However, comprehensive analyses of natural ecosystems are impeded by their complexity and diversity. Food webs, therefore, provide an excellent tool to analyze the complexity of ecosystems. They depict the system‘s diversity and species interactions in a condensed form. Furthermore, food-web structure can help to predict the interaction strengths between species and the energy pathways through the system. In my thesis, I use food web structure to analyze structural properties which separate food webs from other network types and furthermore I investigate generalities and differences of food-web structure across different ecosystems. 
\nOne of the most important ecosystems is the soil ecosystem, as it provides the base for aboveground productivity. However, detailed soil food webs are scarce. In chapter 2, I assembled the complex food webs of 48 forest soil communities and analyzed if soil food webs differ in their topological parameters from those of other ecosystems. I found that soil food webs are characterized by a higher number of omnivorous and cannibalistic species. Moreover, they comprise more trophic chains and intraguild-predation motifs than food webs from other ecosystems. Finally, soil food webs showed high average and maximum trophic levels. These differences in network structure to other ecosystem types may be a result of ecosystem-specific constraints on hunting and feeding characteristics of the species that emerge as network parameters at the food-web level. Despite these differences, soil food webs showed the same scaling of their properties with connectance and size. In a second analysis of land-use effects, I found significant but only small differences of soil food web structure between different beech and coniferous forest types, which may be explained by generally strong selection effects of the soil that are independent of human land use. This study has unravelled systematic structures of soil food-webs, extending our mechanistic understanding how their environmental characteristics determine patterns at the community level. Additionally, I have shown that the general scaling laws also apply for soil food webs.
\nIn addition to purely topological properties, I analyzed another important aspect of food webs. The distributions of body masses and degrees across species are key determinants of food-web structure and dynamics. In chapter 3, I analyzed body masses of species and their systematic distributions across food-web structure. In particular, allometric degree distributions combine both aspects in the relationship between degrees and body masses. They are of critical importance for the stability of complex ecological networks. I used an entirely novel global body-mass database including food-web structures of four different ecosystem types to analyze body-mass distributions, cumulative degree distributions, and allometric degree distributions regarding differences among ecosystem types. My results demonstrate some general patterns across ecosystems: the body masses are either roughly log-normally (terrestrial and stream ecosystems) or multimodally (lake and marine ecosystems) distributed, and most networks exhibit exponential cumulative degree distributions except stream networks that most often possess uniform degree distributions. Additionally, with increasing species body masses we found significant decreases in vulnerability in 70% of the food webs and significant increases in generality in 80% of the food webs. Overall, these analyses document striking generalities in the body-mass and degree structure across ecosystem types as well as surprising exceptions (uniform degree distributions in stream ecosystems). This suggests general constraints of body masses on the link structure of natural food webs irrespective of ecosystem characteristics. 
\nWhile I revealed general patterns of food-web topology in chapter 2 and 3, I investigated the drivers of these general patterns in chapter 4. Therefore, I analyzed the influence of different external factors on community (beta diversity) and food-web structure. Two main theoretical bodies explain β-diversity, the niche theory and neutral theory. However, neutral theory predicts only distributions for trophically identical species, whereas influences of local niches or neutral effects on food-web structure as a crucial part of the multitrophic structure of ecosystems are not taken into account. In chapter 4, I therefore analyzed the effects of spatial distance and environmental dissimilarity on the species dissimilarity (beta diversity) and food web dissimilarity (structural dissimilarity) of multitrophic forest communities. I showed that the mechanisms proposed by neutral theory can adequately predict the beta diversity of multitrophic species communities. Furthermore, food-web structure was robust and affected neither by spatial distance (random dispersal, neutral theory) nor by environmental filtering (niche theory). I additionally analyzed model food webs (random and niche topology) and compared their dissimilarities to empirical food webs. The highest dissimilarity was reached by random food webs whereas niche model food webs were in between and the lowest distances were expressed by empirical food webs. Further, random food webs displayed the highest mean trophic level (115), while niche model food webs showed lower (5) and empirical food webs the lowest (4) mean trophic level values. Hence, food-web structure appears to be energetically optimized with local species adapted to energetic niches within the food web while species identity within these niches remains random. This suggests that different species could be adapted to the same energetic niches and, while following random drift, still assemble into similar food web structures.
\nAltogether, the results of this thesis demonstrate the practicality of food-web structure in unravelling generalities across different ecosystems. Furthermore, food-web structure explains species distributions across the environment and provides additional important information on the ecosystem. 
\nThe observed generalities indicate constraints on food-web structure. The allometric degree distributions demonsrate such constraints on food-web structure by distributing the links in dependence of the species body masses. Finally, my results from chapter 4 indicate that, additionally to global topological constraints, local communities have to meet certain energetic constraints to explain the similarity found across food webs.

Changes in species richness along elevational gradients are well documented. However, little is known about how trophic interactions between species and, in particular, the food webs that these interactions comprise, change with elevation. Here we present results for the first comparison of quantitative food webs in forest understorey and canopy along an elevational gradient. Replicate quantitative food webs were constructed for assemblages involving 23 species of cavity‐nesting Hymenoptera and 12 species of their parasitoids and kleptoparasites in subtropical rainforest in Australia. A total of 1589 insects were collected using trap nests across 20 plots distributed at sites ranging from 300 to 1100 m a.s.l. Insect abundance, insect diversity and parasitism rate generally decreased with increasing elevation. Food web structure significantly changed with elevation. In particular, weighted quantitative measures of linkage density, interaction evenness, nestedness (weighted NODF) and potential for enemy mediated interactions (PAC) decreased with increasing elevation, and network specialisation (H 2 ′) increased with increasing elevation, even after controlling for matrix size; but there was no change in weighted connectance. Changes in forest type and temperature along the elevational gradient are likely to be, at least partly, responsible for the patterns observed. We found no significant differences in insect abundance, insect diversity or parasitism rate between canopy and understorey. Furthermore, there were no differences in food web structure between strata. These results contribute further evidence to studies revealing changes in food web structure along natural environmental gradients and provide information that can potentially be used for predicting how communities may respond to climate change.

Nonpoint source pollution entering rivers will pollute water quality, degrading the health of aquatic ecosystems. However, owing to the lack of quantitative research on the effects of nonpoint source pollution on the structure of aquatic food webs, there is a lack of quantitative basis for river management. Nonpoint source pollution is not only difficult to control effectively, but also the success rate of water ecological restoration projects is low. With the increasing proportion of nonpoint source pollution in water environmental problems, it is urgent to quantitatively assess and predict the impact of nonpoint source pollution on the structure of food webs. Therefore, this thesis presents a method for quantitatively assessing and predicting the impact of nonpoint source pollution on the structure of food webs through using fuzzy clustering to screen the typical points of the impact of nonpoint source pollution, then using canonical correspondence analysis (CCA) and partial least squares regression analysis to comprehensively filtrate the driving factors affect food web that results in nonpoint source pollution, and then determining the impact of each driving factor on the structure of food webs. Finally, the change trend of food web structure is predicted. The results show that (1) the driving factors that the nonpoint source pollution that affects the food web structure is NH3‐N and chemical oxygen demand (COD). The increase in NH3‐N and COD promotes the growth of phytoplankton, causing the change of the primary productivity of the ecosystem, and ultimately changes the entire food web structure; (2) NH3‐N and COD affect the stability, maturity, connectivity and complexity of the aquatic food web structure. The increase of NH3‐N increases the connectivity and maturity of the food web structure but reduces complexity and stability; the increase of COD increases the connection of the food web structure, while reducing the other three indicators; (3) in some areas with good water quality, aquatic species diversity is high, the relationship of interspecies dietary is complex, food web structure level index is high and the structure of food web is stable. The food web structure in the rainy season will be better than that in the dry season. In some areas with severe water pollution and poor food web structure, the ability of the food web to resist external interference is weak. The food web structure in the rainy season will be worse than that in the dry season owing to rainfall into the river. The methods and conclusions in this treatise can provide a reliable and quantitative scientific basis for river ecosystem management and ecosystem restoration and can improve the success rate of ecological restoration projects.

Динамика популяции непарного шелкопряда в лесостепных дубравах Европейской России

Landscape management affects species interactions and can have notable effects on food web structure. Local parasitoid populations in greenhouses usually migrate from outside crops; biological control of greenhouse aphids may be thus highly dependent on the composition of surrounding landscape. However, it is less clear how surrounding landscape composition affects primary-hyperparasitoid food webs and pest control services in greenhouses. We investigated the food web of parasitoids on melon-cotton aphid (Aphis gossypii Glover) in watermelon greenhouses in two suburban Beijing counties over two years. We used the quantitative food web metrics (generality, vulnerability, link density, and interaction evenness) to assess the effects of landscape composition on primary-hyperparasitoid food web structure. We found that landscape with more cropland within 1–3 km tended to have more primary parasitoids per hyperparasitoid species (generality). Higher proportions of woodland at the 0.5 km scale were negatively correlated with the mean numbers of hyperparasitoid per primary parasitoid species (vulnerability), as well as with hyperparasitism rate and hyperparasitoid richness. Link density, interaction evenness and aphid mortality caused by parasitoids (parasitism rate) were not affected by landscape factors. However, active primary parasitism (biocontrol potential) increased with the proportion of woodland. This suggested that the bottom-up effect induced by primary parasitoids might benefit hyperparasitoids, thus exerting little influence of primary parasitoids on pest control. The top-down effect of hyperparasitoids may reduce with increasing woodland proportion. To enhance the effects of primary parasitoids, landscape management programs should also target, and thus limit the impact of hyperparasitoids.

Human impact on structure and functioning of ecosystems is rapidly increasing. Virtually all European forests are managed with major implications for diversity and structure of food webs. Centipedes (Chilopoda: Lithobiidae) are abundant arthropod predators in European temperate forest soils with a generalistic feeding behaviour. However, little is known on the variability in the prey spectrum of centipedes with land use and the responsible factors. Combining fatty acid (FA) analysis, which allows determination of the relative contribution of different prey to predator nutrition, and stable isotope analysis, providing insight into the trophic structure of decomposer food webs, we investigated variations in trophic niches of two dominant centipede species, Lithobius mutabilis and Lithobius crassipes , in differently aged beech and spruce forests. FA composition of the two centipede species differed significantly with bacterial marker FAs being more abundant in L. crassipes as compared to L. mutabilis . Differences were most pronounced in spruce as compared to beech forests. The results suggest that dense needle litter in coniferous forests may restrict prey availability to the larger L. mutabilis and confine foraging to the litter surface whereas the smaller L. crassipes is able to also exploit prey of deeper litter layers. Lithobius crassipes was significantly more enriched in 15 N and 13 C compared to L. mutabilis suggesting that, compared to L. mutabilis , the smaller L. crassipes occupies higher trophic levels and relies more on root derived carbon. The results indicate that trophic niches of centipedes vary in a species specific way between forest types with body size and habitat structure being major determinants of the variations in the prey spectrum. Combining techniques for delineating predator–prey interactions allowed insights into variations in trophic interrelationships and their driving forces in temperate forest soil food webs.

Food webs are usually regarded as snapshots of community feeding interactions. Here, we describe the yearly and cumulative structure of parasitoid-caterpillar food webs on soybean in central Argentina, analyzing parasitism rates and their variability in relation to parasitoid diversity and food web vulnerability in the system. Lepidoptera larvae were collected along four seasons from soybean crops and reared in laboratory to obtain and identify adults and parasitoids. Eleven species of defoliating Lepidoptera and ten parasitoid species were recorded. Food web statistics showed rather low annual variability, with most variation coefficients in the order of 0.20 and generality showing the most stable values. Parasitism showed the highest variability, which was independent of parasitoid diversity and food web vulnerability, although parasitism rates were negatively related to parasitoid richness. Our study highlights the need to consider food web structure and variability in order to understand the functioning of ecological communities in general and in extensive agricultural ecosystems in particular.

To test the effects of temperature on food web structure and productivity, Mary O'Connor (above, checking temperatures) and colleagues placed five microcosms of food webs (shielded from full sunlight and UV) in eight independent water tables, each filled with a temperature-conditioned water bath.

1. Consumer–resource species interactions form complex, dynamic networks, which may exhibit structural heterogeneity at various scales. This study set out to address whether host–parasitoid food web size and topology vary across forest canopy strata, and to what extent foliar resources and species abundances account for vertical patterns in network structure.2. The vertical stratification of leaf miner–parasitoid food webs was examined in two monotypic beech (Nothofagus pumilio) forests in northern Patagonia, Argentina. Quantitative food webs were constructed for separate canopy layers by sampling foliage from three tree‐height classes at 0.5–1, 2–3 and 5–6 m above ground.3. Leaf miner abundance per unit leaf mass and foliar damage (%) did not differ across strata, although foliage quality and quantity increased from the understorey to the upper canopy. Parasitism rates and food web complexity decreased with canopy height, as reflected by reduced linkage richness, linkage density, mean interaction strength, and host vulnerability.4. Null model analyses revealed that food web metrics, especially in the upper canopy, were often lower than expected when compared with randomly structured networks. Overall, these patterns held for two forests differing in vertical structure and in dominant miner morphotype and parasitoid species.5. These results suggest that vertical declines in network complexity may be driven by the parasitoids' limited functional response to host abundance and dispersal from pupation sites in the forest floor. A broader constraint on food web structure seemed to be imposed by host–parasitoid trait matching, a reflection of large‐scale assembly processes.

Food webs show the architecture of trophic relationships, revealing the biodiversity and species interactions in an ecosystem. Understanding which factors modulate the structure of food webs offers us the ability to predict how they will change when influential factors are altered. To date, most of the research about food webs has focused on species interactions whereas the influences of surrounding environments have been overlooked. Here, using network analysis, we identified how the structure of aquatic food webs varied across a range of geophysical conditions within a whole stream system. Within a headwater basin in the Cascade Mountain Range, Oregon, USA, macroinvertebrate and vertebrate composition was investigated at 18 sites. Predator–prey interactions were compiled based on existing literature and dietary analysis. Several structural network metrics were calculated for each food web. We show that the structure of food webs was predictable based on geophysical features at both local (i.e., slope) and broader (i.e., basin size) spatial extents. Increased omnivory, greater connectance, shorter path lengths, and ultimately greater complexity and resilience existed downstream compared to upstream in the stream network. Surprisingly, the variation in food web structure was not associated with geographic proximity. Structural metric values and abundance of omnivory suggest high levels of stability for these food webs. There is a predictable variation in the structure of food webs across the network that is influenced by both longitudinal position within streams and patchy discontinuities in habitat. Hence, findings illustrate that the slightly differing perspectives from the River Continuum Concept, Discontinuity Patch Dynamics, and Process Domains can be integrated and unified using food web networks. Our analyses extend ecologists’ understanding of the stability of food webs and are a vital step toward predicting how webs and communities may respond to both natural disturbances and current global environmental change.

Journal Article Regional Correlation of Gypsy Moth (Lepidoptera: Lymantriidae) Defoliation with Counts of Egg Masses, Pupae, and Male Moths Get access A. M. Liebhold, A. M. Liebhold Northeastern Forest Experiment Station, USDA Forest Service, 180 Canfield Street, Morgantown, WV 26505 Search for other works by this author on: Oxford Academic PubMed Google Scholar J. S. Elkinton, J. S. Elkinton 1 Northeastern Forest Experiment Station, USDA Forest Service, 180 Canfield Street, Morgantown, WV 26505 1Department of Entomology, University of Massachusetts, Amherst, MA 01003. Search for other works by this author on: Oxford Academic PubMed Google Scholar C. Zhou, C. Zhou 2 Northeastern Forest Experiment Station, USDA Forest Service, 180 Canfield Street, Morgantown, WV 26505 2Department of Mathematics, Branch Campus of Beijing University, Beijing 100083 China. Search for other works by this author on: Oxford Academic PubMed Google Scholar M. E. Hohn, M. E. Hohn 3 Northeastern Forest Experiment Station, USDA Forest Service, 180 Canfield Street, Morgantown, WV 26505 3West Virginia Geological and Economic Survey,P.O. Box 879, Morgantown, WV 26507-0879. Search for other works by this author on: Oxford Academic PubMed Google Scholar R. E. Rossi, R. E. Rossi 4 Northeastern Forest Experiment Station, USDA Forest Service, 180 Canfield Street, Morgantown, WV 26505 4Isaaks & Company, 654 Blair Island Road, Suite 205, Redwood City, CA 94063. Search for other works by this author on: Oxford Academic PubMed Google Scholar G. H. Boettner, G. H. Boettner 1 Northeastern Forest Experiment Station, USDA Forest Service, 180 Canfield Street, Morgantown, WV 26505 1Department of Entomology, University of Massachusetts, Amherst, MA 01003. Search for other works by this author on: Oxford Academic PubMed Google Scholar C. W. Boettner, C. W. Boettner 1 Northeastern Forest Experiment Station, USDA Forest Service, 180 Canfield Street, Morgantown, WV 26505 1Department of Entomology, University of Massachusetts, Amherst, MA 01003. Search for other works by this author on: Oxford Academic PubMed Google Scholar C. Burnham, C. Burnham 5 Northeastern Forest Experiment Station, USDA Forest Service, 180 Canfield Street, Morgantown, WV 26505 5Commonwealth of Massachusetts DEM, 100 Cambridge Street, Boston, MA 02202. Search for other works by this author on: Oxford Academic PubMed Google Scholar M. L. McManus M. L. McManus 6 Northeastern Forest Experiment Station, USDA Forest Service, 180 Canfield Street, Morgantown, WV 26505 6USDA Forest Service, Northeastern Forest Experiment Station, 51 Millpond Road, Hamden, CT 06514. Search for other works by this author on: Oxford Academic PubMed Google Scholar Environmental Entomology, Volume 24, Issue 2, 1 April 1995, Pages 193–203, https://doi.org/10.1093/ee/24.2.193 Published: 01 April 1995 Article history Received: 29 November 1993 Accepted: 20 July 1994 Published: 01 April 1995

This paper describes the effect of the colour of burlap bands (black versus naturally coloured or tan burlap) affixed to red oak, Quercus rubra (L.) (Fagaceae), and how it influences selection of larval resting site, pupation site, and egg-mass counts of gypsy moth, Lymantria dispar (L.). In field experiments with half black and half tan burlap bands, the mean number of larvae, pupae, and egg masses were significantly greater under the black section of burlap. Individual burlap bands composed of either black or tan burlap affixed to separate trees produced similar significant results for larvae and pupae. When two burlap bands composed of opposite colours (black versus tan) were affixed to the same tree, significantly more larvae were found under the upper band, regardless of colour. In contrast, pupa and egg-mass densities were significantly greater under black bands, regardless of band position.