Effects of Tree Genotypic Diversity and Species Diversity on the Arthropod Community Associated with Big‐leaf Mahogany

Despite potential interactive effects of plant species and genotypic diversity (SD and GD, respectively) on consumers, studies have usually examined these effects separately. We evaluated the individual and combined effects of tree SD and mahogany (Swietenia macrophylla) GD on the arthropod community associated with mahogany. We conducted this study within the context of a tree diversity experiment consisting of 74 plots with 64 saplings/plot. We sampled 24 of these plots, classified as monocultures of mahogany or polycultures of four species (including mahogany). Within each plot type, mahogany was represented by either one or four maternal families. We surveyed arthropods on mahogany and estimated total arthropod abundance and species richness, as well as abundance and richness separately for herbivorous and predatory arthropods. Overall tree SD and mahogany GD had positive effects on total arthropod species richness and abundance on mahogany, and also exerted interactive effects on total species richness (but not abundance). Analyses conducted by trophic level group showed contrasting patterns; SD positively influenced herbivore species richness but not abundance, and did not affect either predator richness or abundance. GD influenced predator species richness but not abundance, and did not influence herbivore abundance or richness. There were interactive effects of GD and SD only for predator species richness. These results provide evidence that intra‐ and inter‐specific plant diversity exert interactive controls on associated consumer communities, and that the relative importance of SD and GD may vary among higher trophic levels, presumably due to differences in the underlying mechanisms or consumer traits.     

Effects of tree species diversity on a community of weaver spiders in a tropical forest plantation

The effects of producer diversity on predators have received little attention in arboreal plant communities, particularly in the tropics. This is particularly true in the case of tree diversity effects on web‐building spiders, one of the most important groups of invertebrate predators in terrestrial plant communities. We evaluated the effects of tree species diversity on the community of weaver spiders associated with big‐leaf mahogany (Swietenia macrophylla) in 19, 21 × 21‐m plots (64 plants/plot) of a tropical forest plantation which were either mahogany monocultures (12 plots) or polycultures (seven plots) that included mahogany and three other tree species. We conducted two surveys of weaver spiders on mahogany trees to evaluate the effects of tree diversity on spider abundance, species richness, diversity, and species composition associated with mahogany. Our results indicated that tree species mixtures exhibited significantly greater spider abundance, species richness, and diversity, as well as differences in spider species composition relative to monocultures. These results could be due to species polycultures providing a broader range of microhabitat conditions favoring spider species with different habitat requirements, a greater availability of web‐building sites, or due to increased diversity or abundance of prey. Accordingly, these results emphasize the importance of mixed forest plantations for boosting predator abundance and diversity and potentially enhancing herbivore pest suppression. Future work is necessary to determine the specific mechanisms underlying these patterns as well as the top‐down effects of increased spider abundance and species richness on herbivore abundance and damage.     

Predatory arthropod community composition in apple orchards: Orchard management,landscape structure and sampling method

Studies on predatory arthropods in agricultural areas seldom include Diptera other than hoverflies, partly because common sampling methods are less effective for capturing species that easily fly off when disturbed. To study the effect from this bias when describing the predator community, we compared traditional beat sampling of branches and suction sampling for describing the community of predatory arthropods in Swedish apple orchards, both organic orchards and orchards using integrated pest management (IPM). Our results indicate that the proportion of both predatory dipterans and parasitic hymenopterans increase dramatically when using suction sampling (Diptera: 32% vs. 20%, Hymenoptera: 25% vs. 7%). In fact, predatory dipterans were the most abundant predatory group when using suction sampling, in contrast to beat sampling where spiders were the most abundant group. One group of predatory flies that was particularly rich in both species and individuals in the surveyed apple orchards was dance flies in the family Hybotidae. Even though the bias of sampling method was evident, it was encouraging that the method choice did not affect the conclusions concerning management on predatory arthropod communities. With both methods, dipteran and coleopteran predators were more abundant in organic apple orchards whereas opilionids were more abundant in orchards managed according to IPM. The inclusion of landscape variables further indicated effects of landscape diversity and of deciduous forest cover, but the response varied in sign between predatory groups. Whereas both Coleoptera and Heteroptera were more abundant in orchards surrounded by more complex landscapes (high landscape diversity and/or high deciduous forest cover), spiders, opilionids and dipterans were rather less abundant in these orchards. To conclude, our study points to the potential importance of predatory dipterans in apple orchards, and we highly recommend future studies of arthropod predators in apple and other crops to actively include predatory Diptera.     

Predator diversity and abundance provide little support for the enemies hypothesis in forests of high tree diversity

Predatory arthropods can exert strong top-down control on ecosystem functions. However, despite extensive theory and experimental manipulations of predator diversity, our knowledge about relationships between plant and predator diversity--and thus information on the relevance of experimental findings--for species-rich, natural ecosystems is limited. We studied activity abundance and species richness of epigeic spiders in a highly diverse forest ecosystem in subtropical China across 27 forest stands which formed a gradient in tree diversity of 25-69 species per plot. The enemies hypothesis predicts higher predator abundance and diversity, and concomitantly more effective top-down control of food webs, with increasing plant diversity. However, in our study, activity abundance and observed species richness of spiders decreased with increasing tree species richness. There was only a weak, non-significant relationship with tree richness when spider richness was rarefied, i.e. corrected for different total abundances of spiders. Only foraging guild richness (i.e. the diversity of hunting modes) of spiders was positively related to tree species richness. Plant species richness in the herb layer had no significant effects on spiders. Our results thus provide little support for the enemies hypothesis--derived from studies in less diverse ecosystems--of a positive relationship between predator and plant diversity. Our findings for an important group of generalist predators question whether stronger top-down control of food webs can be expected in the more plant diverse stands of our forest ecosystem. Biotic interactions could play important roles in mediating the observed relationships between spider and plant diversity, but further testing is required for a more detailed mechanistic understanding. Our findings have implications for evaluating the way in which theoretical predictions and experimental findings of functional predator effects apply to species-rich forest ecosystems, in which trophic interactions are often considered to be of crucial importance for the maintenance of high plant diversity.     

Tree species diversity influences herbivore abundance and damage: meta-analysis of long-term forest experiments

Plant monocultures are commonly believed to be more susceptible to herbivore attacks than stands composed of several plant species. However, few studies have experimentally tested the effects of tree species diversity on herbivory. In this paper, we present a meta-analysis of uniformly collected data on insect herbivore abundance and damage on three tree species (silver birch, black alder and sessile oak) from seven long-term forest diversity experiments in boreal and temperate forest zones. Our aim was to compare the effects of forest diversity on herbivores belonging to different feeding guilds and inhabiting different tree species. At the same time we also examined the variation in herbivore responses due to tree age and sampling period within the season, the effects of experimental design (plot size and planting density) and the stability of herbivore responses over time. Herbivore responses varied significantly both among insect feeding guilds and among host tree species. Among insect feeding guilds, only leaf miner densities were consistently lower and less variable in mixed stands as compared to tree monocultures regardless of the host tree species. The responses of other herbivores to forest diversity depended largely on host tree species. Insect herbivory on birch was significantly lower in mixtures than in birch monocultures, whereas insect herbivory on oak and alder was higher in mixtures than in oak and alder monocultures. The effects of tree species diversity were also more pronounced in older trees, in the earlier part of the season, at larger plots and at lower planting density. Overall our results demonstrate that forest diversity does not generally and uniformly reduce insect herbivory and suggest instead that insect herbivore responses to forest diversity are highly variable and strongly dependent on the host tree species and other stand characteristics as well as on the type of the herbivore.     

Herbivore and predator diversity interactively affect ecosystem properties in an experimental marine community

Interacting changes in predator and prey diversity likely influence ecosystem properties but have rarely been experimentally tested. We manipulated the species richness of herbivores and predators in an experimental benthic marine community and measured their effects on predator, herbivore and primary producer performance. Predator composition and richness strongly affected several community and population responses, mostly via sampling effects. However, some predators survived better in polycultures than in monocultures, suggesting complementarity due to stronger intra- than interspecific interactions. Predator effects also differed between additive and substitutive designs, emphasizing that the relationship between diversity and abundance in an assemblage can strongly influence whether and how diversity effects are realized. Changing herbivore richness and predator richness interacted to influence both total herbivore abundance and predatory crab growth, but these interactive diversity effects were weak. Overall, the presence and richness of predators dominated biotic effects on community and ecosystem properties.     

Tree diversity drives diversity of arthropod herbivores,but successional stage mediates detritivores

The high tree diversity of subtropical forests is linked to the biodiversity of other trophic levels. Disentangling the effects of tree species richness and composition, forest age, and stand structure on higher trophic levels in a forest landscape is important for understanding the factors that promote biodiversity and ecosystem functioning. Using a plot network spanning gradients of tree diversity and secondary succession in subtropical forest, we tested the effects of tree community characteristics (species richness and composition) and forest succession (stand age) on arthropod community characteristics (morphotype diversity, abundance and composition) of four arthropod functional groups. We posit that these gradients differentially affect the arthropod functional groups, which mediates the diversity, composition, and abundance of arthropods in subtropical forests. We found that herbivore richness was positively related to tree species richness. Furthermore, the composition of herbivore communities was associated with tree species composition. In contrast, detritivore richness and composition was associated with stand age instead of tree diversity. Predator and pollinator richness and abundance were not strongly related to either gradient, although positive trends with tree species richness were found for predators. The weaker effect of tree diversity on predators suggests a cascading diversity effect from trees to herbivores to predators. Our results suggest that arthropod diversity in a subtropical forest reflects the net outcome of complex interactions among variables associated with tree diversity and stand age. Despite this complexity, there are clear linkages between the overall richness and composition of tree and arthropod communities, in particular herbivores, demonstrating that these trophic levels directly impact each other.     

Increasing vole numbers cause more lethal damage to saplings in tree monocultures than in mixed stands

Overall, mammalian herbivores are more harmful in mixed plantations than in monocultures, but the effect of herbivore abundance has not been experimentally tested in this context. It has been proposed that there is a critical threshold density where herbivore pressure spreads from preferred plants to everything edible, leading to non-linear density effects on low-quality plants. We experimentally investigated whether survival of an unpalatable plant is similarly related to herbivore density in both monocultures and mixed stands. This we did by establishing monocultures of unpalatable black alder (Alnus glutinosa) and mixed stands of black alder and five more palatable tree species in enclosures, where Microtus voles were introduced and their abundances monitored.The effect of stand diversity tended to depend on vole abundance. Vole damage of tree saplings did not differ between monocultures and mixed stands, but at higher vole abundances attacks had a stronger effect on sapling survival in the monocultures. Sapling survival showed a significant drop in the monocultures at peak abundance of approximately 300 voles ha^?1. In monocultures herbivores do not have alternatives and therefore are forced to become deadlier consumers.     

Positive effects of wheat variety mixtures on aboveground arthropods are weak and variable

Although modern agriculture generally relies on homogeneous varieties that are usually grown in pure stands, crop variety mixtures have been used for a long time, notably to improve resistance to fungal diseases. A growing number of studies suggest that intraspecific plant diversity may also enhance the abundance and diversity of wild species and thereby some ecosystem services such as biological control by natural predators. However, positive effects of the genetic diversity of plant species on the diversity of associated communities have mostly been documented in natural systems, with only a handful of studies targeting crop species in agroecosystems. Here, we investigated the ecological effects of the number of winter wheat varieties (Triticum aestivum) on aboveground arthropods and particularly predatory species. We manipulated the number of wheat varieties (1, 2, 4 or 8) in 120 plots (80 m² each) to examine how wheat diversity and stand characteristics impact communities of three dominant aboveground arthropod groups that include many predatory species: ground beetles, rove beetles and spiders. The number of wheat varieties had a weak, but positive effect on predator abundance, notably spider abundance. In contrast, wheat functional diversity, as assessed by the number of wheat functional groups, was only negatively related to the diversity of spiders. Among wheat stand characteristics, the variance in plant height, wheat biomass and the Green Area Index were weakly correlated with ground beetle, rove beetle and predatory diversity, respectively. The Green Area Index was also weakly correlated with ground beetle abundance. Our study suggests that wheat variety mixtures have variable and limited effects on aboveground arthropods and probably low effectiveness to enhance biological control, but these results should be further tested under low-input agriculture in real fields.     

Woody plant phylogenetic diversity mediates bottom–up control of arthropod biomass in species-rich forests

Global change is predicted to cause non-random species loss in plant communities, with consequences for ecosystem functioning. However, beyond the simple effects of plant species richness, little is known about how plant diversity and its loss influence higher trophic levels, which are crucial to the functioning of many species-rich ecosystems. We analyzed to what extent woody plant phylogenetic diversity and species richness contribute to explaining the biomass and abundance of herbivorous and predatory arthropods in a species-rich forest in subtropical China. The biomass and abundance of leaf-chewing herbivores, and the biomass dispersion of herbivores within plots, increased with woody plant phylogenetic diversity. Woody plant species richness had much weaker effects on arthropods, but interacted with plant phylogenetic diversity to negatively affect the ratio of predator to herbivore biomass. Overall, our results point to a strong bottom–up control of functionally important herbivores mediated particularly by plant phylogenetic diversity, but do not support the general expectation that top–down predator effects increase with plant diversity. The observed effects appear to be driven primarily by increasing resource diversity rather than diversity-dependent primary productivity, as the latter did not affect arthropods. The strong effects of plant phylogenetic diversity and the overall weaker effects of plant species richness show that the diversity-dependence of ecosystem processes and interactions across trophic levels can depend fundamentally on non-random species associations. This has important implications for the regulation of ecosystem functions via trophic interaction pathways and for the way species loss may impact these pathways in species-rich forests.     

Organic versus conventional management in an apple orchard: effects of fertilization and tree-row management on ground-dwelling predaceous arthropods

1 Organic and conventional management of apple orchards may have a different effect on arthropod communities.
2 We conducted a 3-year study to assess the effect of two strategies of fertilizer treatment (organic versus chemical) and three tree-row management systems (straw mulching, tillage and herbicide) on activity-density and biodiversity of epigeic predators. Ground beetles (Carabidae), rove beetles (Staphylinidae), ants (Formicidae) and spiders (Araneae) were sampled monthly with pitfall traps in the same apple orchard during 2003, 2004 and 2005.
3 A total of 4978 individuals were collected. Carabids (56.8% of the total catches) were the most abundant taxonomic group, followed by spiders (20.7%), ants (14.8%) and rove beetles (7.7%). Tree-row management had a greater influence on predator catches than fertilizer treatment. Total predator catches were lower under the mulch. Mulching also reduced carabid abundance, but increased staphylinid catches.
4 Tree row management also had a significant effect on biodiversity parameters. Species richness did not significantly differ among treatments for ants, spiders or the total catches, but was higher on herbicide-treated plots for carabids and on mulched plots for staphylinids. Shannon–Wiener's diversity index was significantly greater in the mulched and herbicide treated plots for total predators and carabids. For staphylinids, this index was significantly greater on the mulched plots. Fertilizer application strategy only influenced the species richness of rove beetles, which was greater in the chemically-treated plots.
5 The results showed that a change from conventional to organic fertilizer treatment of apple trees may be performed without differential effects on predator activity-density or biodiversity. However, a change from herbicide treatment to mulching or mechanical weed control may be significant, depending on the taxonomic group.     

Factors shaping latitudinal patterns in communities of arboreal spiders in northern Europe

Predator effects on herbivores are often referred to as examples of biotic interactions that weaken with latitude, but more studies are needed to test for the generality of this pattern. To further the understanding of large‐scale geographical patterns in abundance and diversity of predatory arthropods, from 2008–2011 we explored spider communities in the canopies of primary forest trees of the boreal zone (Pinus sylvestris, Picea abies, Betula pubescens and B. pendula) along five latitudinal gradients in northern Europe, from 59 to 70°N and from 10 to 60°E. The abundance of arboreal spiders in Norway and Finland was about a half of that in Russia, presumably due to more intensive forest management in Scandinavia. The abundance, taxonomic and functional diversity of arboreal spiders generally decreased with latitude; however, actual weather conditions during the study years had little effect on this pattern. Coniferous species supported higher abundance and diversity of arboreal spiders than birches, but the poleward decrease in either abundance or taxonomic diversity of arboreal spiders did not differ between coniferous and deciduous tree species. In contrast, functional diversity on birches decreased with latitude greater than on coniferous trees. Euryphagous spiders showed stronger decrease with latitude in terms of both abundance and taxonomic diversity than more specialized (steno‐ and oligophagous) spiders. We attribute the general decrease in density and diversity of spiders with latitude to an increased pressure from apex predators (birds) rather than to direct effects of climate or changes in prey abundance. The abundance of spiders declined with the latitude to the greater extent than the densities of their potential prey did, suggesting a decrease in the strength of predator–prey interactions towards the north.     

Going undercover: increasing canopy cover around a host tree drives associational resistance to an insect pest

Neighbouring heterospecific plants are often observed to reduce the probability of herbivore attack on a given focal plant. While this pattern of associational resistance is frequently reported, experimental evidence for underlying mechanisms is rare particularly for potential plant species diversity effects on focal host plants and their physical environment. Here, we used an established forest diversity experiment to determine whether tree diversity effects on an important insect pest are driven by concomitant changes in host tree growth or the light environment. We examined the effects of tree species richness, canopy cover and tree growth on the probability of occurrence, the abundance, and volume of galls caused by the pineapple gall adelgid Adelges abietis on Norway spruce. Although tree diversity had no effect on gall abundance, we observed that both the probability of gall presence and gall volume (an indicator of maternal fecundity) decreased with tree species richness and canopy cover around host spruce trees. Structural equation models revealed that effects of tree species richness on gall presence and volume were mediated by concurrent increases in canopy cover rather than changes in tree growth or host tree density. As canopy cover did not influence tree or shoot growth, patterns of associational resistance appear to be driven by improved host tree quality or more favourable microclimatic conditions in monocultures compared to mixed‐stands. Our study therefore demonstrates that changes in forest structure may be critical to understanding the responses of herbivores to plant diversity and may underpin associational effects in forest ecosystems.     

Meta-analysis of tree diversity effects on the abundance,diversity and activity of herbivores' enemies

The natural enemies hypothesis predicts that the abundance and diversity of antagonists such as predators and parasitoids of herbivores increases with the diversity of plants, which can lead to more effective top-down control of insect herbivores. However, although the hypothesis has received large support in agricultural systems, fewer studies have been conducted in forest ecosystems and a comprehensive synthesis of previous research is still lacking.We conducted a meta-analysis of 65 publications comparing the diversity, abundance or activity of various groups of natural enemies (including birds, bats, spiders and insect parasitoids) in pure vs. mixed forest stands. We tested the effects of forest biome, natural enemy taxon and type of study (managed vs experimental forest).We found a significant positive effect of forest tree diversity on natural enemy abundance and diversity but not on their activity. The effect of tree diversity on natural enemies was stronger towards lower latitudes but was not contingent on the natural enemy taxon level.Overall, our study contributes toward a better understanding of the “natural enemies hypothesis” in forest systems and provides new insights about the mechanisms involved. Furthermore, we outline potential avenues for strengthening forest resistance to the growing threat of herbivorous insects.     

The impact of high-altitude ski pistes on ground-dwelling arthropods in the Alps

The development of winter sport resorts above the timberline may affect every ecosystem component. We analyzed the effect of ski-pistes on the abundance and species richness of arthropods (namely carabids, spiders, opilionids, and grasshoppers) trapped in grasslands adjacent to the ski-run, on ski-pistes and at the edge between these two habitat types. Our results showed that diversity of brachypterous carabids, spiders, and grasshoppers decreased significantly from natural grasslands to ski-pistes. This was not true for the macropterous carabid guild, which included species with contrasting ecological requirements. The analysis of indicator species (IndVal) showed that most of the species (some of them precinctive to restricted areas in the north-western Alps) had clear preferences for natural grassland and few taxa were limited to ski-pistes. Generalized linear models suggested that the local extent of grass and rock cover can significantly affect assemblages: the low grass cover of ski-pistes, in particular, was a serious hindrance to colonization by spider, grasshopper, brachypterous, and some macropterous carabid species. The results obtained, support concerns over the possible disruption of local ecosystem functionality and over the conservation of arthropod species which are endemic to restricted alpine areas. In order to retain arthropod ground-dwelling fauna we suggest that: (i) new, environmentally friendly ways of constructing pistes should be developed to preserve as much soil and grass cover as possible; (ii) existing ski-pistes should be restored through management to promote the recovery of local vegetation.     

How tree species identity and diversity affect light transmittance to the understory in mature temperate forests

Light is a key resource for plant growth and is of particular importance in forest ecosystems, because of the strong vertical structure leading to successive light interception from canopy to forest floor. Tree species differ in the quantity and heterogeneity of light they transmit. We expect decreases in both the quantity and spatial heterogeneity of light transmittance in mixed stands relative to monocultures, due to complementarity effects and niche filling. We tested the degree to which tree species identity and diversity affected, via differences in tree and shrub cover, the spatiotemporal variation in light availability before, during, and after leaf expansion. Plots with different combinations of three tree species with contrasting light transmittance were selected to obtain a diversity gradient from monocultures to three species mixtures. Light transmittance to the forest floor was measured with hemispherical photography. Increased tree diversity led to increased canopy packing and decreased spatial light heterogeneity at the forest floor in all of the time periods. During leaf expansion, light transmittance did differ between the different tree species and timing of leaf expansion might thus be an important source of variation in light regimes for understory plant species. Although light transmittance at the canopy level after leaf expansion was not measured directly, it most likely differed between tree species and decreased in mixtures due to canopy packing. A complementary shrub layer led, however, to similar light levels at the forest floor in all species combinations in our plots. Synthesis. We find that a complementary shrub layer exploits the higher light availability in particular tree species combinations. Resources at the forest floor are thus ultimately determined by the combined effect of the tree and shrub layer. Mixing species led to less heterogeneity in the amount of light, reducing abiotic niche variability.     

Invasive plant removal increases insect herbivory pressure on a native tree due to an increase in resource quality

Insect predator densities and plant nutritional status can affect insect herbivore densities, but these effects have not yet been assessed in the context of an invasive species management. This study investigated the influence of (i) altered predatory arthropod abundance and community composition (top-down-effects); and (ii) altered leaf nitrogen and phosphorus levels (bottom-up-effects) as possible causes of fluctuations in herbivore abundance and herbivory on the native tree Brabejum stellatifolium in riparian sites that differ in invasion status (near pristine, heavily invaded by Acacia mearnsii, and restored). Species richness, abundance and composition of herbivorous and predatory arthropods were compared between categories. Herbivore and predator abundance, species richness and assemblage composition were significantly influenced by invasion status. As expected, herbivore abundance was significantly and positively correlated to levels of leaf damage. Leaf nitrogen levels were the highest at restored sites where herbivorous arthropods were also most abundant and the abundance of predators was intermediate. These results suggest that altered leaf nitrogen content is likely a key influence for the increased herbivore pressure on B. stellatifolium at sites under pressure from invasive A. mearnsii and its status. Bottom-up influences therefore may be more important than top-down effects on controlling herbivore abundances in these altered riparian environments.

     

Plant species loss decreases arthropod diversity and shifts trophic structure

Plant diversity is predicted to be positively linked to the diversity of herbivores and predators in a foodweb. Yet, the relationship between plant and animal diversity is explained by a variety of competing hypotheses, with mixed empirical results for each hypothesis. We sampled arthropods for over a decade in an experiment that manipulated the number of grassland plant species. We found that herbivore and predator species richness were strongly, positively related to plant species richness, and that these relationships were caused by different mechanisms at herbivore and predator trophic levels. Even more dramatic was the threefold increase, from low- to high-plant species richness, in abundances of predatory and parasitoid arthropods relative to their herbivorous prey. Our results demonstrate that, over the long term, the loss of plant species propagates through food webs, greatly decreasing arthropod species richness, shifting a predator-dominated trophic structure to being herbivore dominated, and likely impacting ecosystem functioning and services.     

Comparisons of arthropod assemblages on an invasive and native trees: abundance,diversity and damage

The success of exotic plants may be due to lower herbivore loads than those on native plants (Enemies Release Hypothesis). Predictions of this hypothesis include lower herbivore abundances, diversity, and damage on introduced plant species compared to native ones. Greater density or diversity of predators and parasitoids on exotic versus native plants may also reduce regulation of exotic plants by herbivores. To test these predictions, we measured arthropod abundance, arthropod diversity, and foliar damage on invasive Chinese tallow tree (Triadica sebifera) and three native tree species: silver maple (Acer saccharinum), sycamore (Platanus occidentalis), and sweetgum (Liquidambar styraciflua). Arthropod samples were collected with canopy sweep nets from six 20 year old monoculture plots of each species at a southeast Texas site. A total of 2,700 individuals and 285 species of arthropods were caught. Overall, the species richness and abundance of arthropods on tallow tree were similar to the natives. But, ordination (NMS) showed community composition differed on tallow tree compared to all three native trees. It supported an arthropod community that had relatively lower herbivore abundance but relatively more predator species compared to the native species examined. Leaves were collected to determine damage. Tallow tree experienced less mining damage than native trees. The results of this study supported the Enemies Release Hypothesis predictions that tallow tree would have low herbivore loads which may contribute to its invasive success. Moreover, a shift in the arthropod community to fewer herbivores without a reduction in predators may further limit regulation of this exotic species by herbivores in its introduced range.     

Plant diversity and identity effects on predatory nematodes and their prey

There is considerable evidence that both plant diversity and plant identity can influence the level of predation and predator abundance aboveground. However, how the level of predation in the soil and the abundance of predatory soil fauna are related to plant diversity and identity remains largely unknown. In a biodiversity field experiment, we examined the effects of plant diversity and identity on the infectivity of entomopathogenic nematodes (EPNs, Heterorhabditis and Steinernema spp.), which prey on soil arthropods, and abundance of carnivorous non‐EPNs, which are predators of other nematode groups. To obtain a comprehensive view of the potential prey/food availability, we also quantified the abundance of soil insects and nonpredatory nematodes and the root biomass in the experimental plots. We used structural equation modeling (SEM) to investigate possible pathways by which plant diversity and identity may affect EPN infectivity and the abundance of carnivorous non‐EPNs. Heterorhabditis spp. infectivity and the abundance of carnivorous non‐EPNs were not directly related to plant diversity or the proportion of legumes, grasses and forbs in the plant community. However, Steinernema spp. infectivity was higher in monocultures of Festuca rubra and Trifolium pratense than in monocultures of the other six plant species. SEM revealed that legumes positively affected Steinernema infectivity, whereas plant diversity indirectly affected the infectivity of Heterorhabditis EPNs via effects on the abundance of soil insects. The abundance of prey (soil insects and root‐feeding, bacterivorous, and fungivorous nematodes) increased with higher plant diversity. The abundance of prey nematodes was also positively affected by legumes. These plant community effects could not be explained by changes in root biomass. Our results show that plant diversity and identity effects on belowground biota (particularly soil nematode community) can differ between organisms that belong to the same feeding guild and that generalizations about plant diversity effects on soil organisms should be made with great caution.