Aboveground Food Webs Research Articles

NUTRIENT SUBSIDIES TO BELOWGROUND MICROBES IMPACT ABOVEGROUND FOOD WEB INTERACTIONS

Historically, terrestrial food web theory has been compartmentalized into interactions among aboveground or belowground communities. In this study we took a more synthetic approach to understanding food web interactions by simultaneously examining four trophic levels and investigating how nutrient (nitrogen and carbon) and detrital subsidies impact the ability of the belowground microbial community to alter the abundance of aboveground arthropods (herbivores and predators) associated with the intertidal cord grass Spartina alterniflora. We manipulated carbon, nitrogen, and detrital resources in a field experiment and measured decomposition rate, soil nitrogen pools, plant biomass and quality, herbivore density, and arthropod predator abundance. Because carbon subsidies impact plant growth only indirectly (microbial pathways), whereas nitrogen additions both directly (plant uptake) and indirectly (microbial pathways) impact plant primary productivity, we were able to assess the effect of both belowground soil microbes and nutrient availability on aboveground herbivores and their predators. Herbivore density in the field was suppressed by carbon supplements. Carbon addition altered soil microbial dynamics (net potential ammonification, litter decomposition rate, DON [dissolved organic N] concentration), which limited inorganic soil nitrogen availability and reduced plant size as well as predator abundance. Nitrogen addition enhanced herbivore density by increasing plant size and quality directly by increasing inorganic soil nitrogen pools, and indirectly by enhancing microbial nitrification. Detritus adversely affected aboveground herbivores mainly by promoting predator aggregation. To date, the effects of carbon and nitrogen subsidies on salt marshes have been examined as isolated effects on either the aboveground or the belowground community. Our results emphasize the importance of directly addressing the soil microbial community as a factor that influences aboveground food web structure by affecting plant size and aboveground plant nitrogen.

Trickle‐down effects of aboveground trophic cascades on the soil food web

Trophic cascades are increasingly being regarded as important features of aboveground and belowground food webs, but the effects of aboveground cascades on soil food webs, and vice versa, remains essentially unexplored. We conducted an experiment consisting of model synthesised communities containing grassland plant and invertebrate species, in which treatments included soil only, soil+plants, soil+plants+aphids, and soil+plants+aphids+predators; predator treatments consisted of the lacewing Micromus tasmaniae and ladybird beetle Coccinella undecimpunctata added either singly or in combination. Addition of Micromus largely reversed the negative effects of aphids on plant biomass, while both of the predator species caused large changes in the relative abundances of dominant plant species. Predators of aphids also affected several components of the belowground subsystem. Micromus had positive indirect effects on the primary consumer of the soil decomposer food web (microflora), probably through promoting greater input of basal resources to the decomposer subsystem. Predator treatments also influenced densities of the tertiary consumers of the soil food web (top predatory nematodes), most likely through inducing changes in plant community composition and therefore the quality of resource input to the soil. The secondary consumers of the soil food web (microbe‐feeding nematodes) were, however, unresponsive. The fact that some trophic levels of the soil food web but not others responded to aboveground manipulations is explicable in terms of top‐down and bottom‐up forces differentially regulating different belowground trophic levels. Addition of aphids also influenced microbial community structure, promoted soil bacteria at the expense of fungi, and enhanced the diversity of herbivorous nematodes; in all cases these effects were at least partially reversed by addition of Micromus. These results in tandem point to upper level consumers in aboveground food webs as a potential driver of the belowground subsystem, and provide evidence that predator‐induced trophic cascades aboveground can have effects that trickle through soil food webs.

Variable dependence on detrital and grazing food webs by generalist predators: aerial insects and web spiders

Recent studies have shown that organisms from the detritus food web subsidize generalist predators in aboveground food webs, but its significance in space and time is largely unknown. Here we report seasonal dynamics of aerial insects from grazing and detritus food webs in both forest and grassland habitats, and show how these patterns influence the dependence of web spiders on the detritus food web. Detrital insects were more abundant in spring, decreased in summer, and then increased slightly in autumn. This pattern was most conspicuous in Nematocera. Due to different seasonal activity patterns of grazing and detrital insects, the proportion of detrital insects was greater in spring and autumn. Detrital insects were relatively more abundant in the forest than in the grassland. Prey captured by web spiders generally reflected seasonal and spatial patterns of aerial insect abundance. In particular, Leucauge spiders reversed their dependence on the two food webs seasonally. Body size of spiders was negatively correlated with the proportion of detrital prey, suggesting that the detrital subsidy is essential for relatively small predators. This size effect probably resulted from interaction of the following two factors: 1) the maximum body size of prey that can be caught increased with spider body size, 2) larger body size classes of aerial insects included a higher proportion of insects from the grazing food web.

Effects of earthworms on plant growth: patterns and perspectives: The 7th international symposium on earthworm ecology · Cardiff · Wales · 2002

Summary A total of 67 studies were located in which the response of plants to presence of earthworms was investigated. The number of studies increased strongly during the last decade. In most of the studies (79 %) shoot biomass of plants significanty increased in the presence of earthworms. However, knowledge on effects of earthworms on plant growth is very biased; most studies investigated crop plants, particularly cereals, and pastures; very little is known on plant species in more natural communities. Recently, interest in tropical plant species has increased considerably, however, the studies have considered almost exclusively agricultural plant species. Generally, experiments focused on the response of plant shoots but 45 % of the studies also considered roots. Most of the studies investigated European earthworms (Lumbricidae); very little is known on other earthworm species. Some early studies indicated that earthworms affect the composition of plant communities but only very recently has it been documented that earthworms affect plant competition. However, there is virtually no information on how earthworms affect plant performance in detail including fitness parameters such as flowering and seed production. It has been realized recently that earthworms not only modify plant growth and vegetation structure but also the susceptability of plants to herbivores. Herbivore performance might be stimulated but also reduced due to the presence of earthworms. Furthermore, earthworms function as subsidiary food resources to generalist predators when herbivore prey is scarce. The complex indirect interactions between earthworms and the aboveground system deserve further investigation in both natural and agricultural ecosystems. The imperative for future research is adopting an ecological rather than an agricultural perspective in studying earthworm-plant interrelationships and viewing earthworms as driving factors of the aboveground food web. It is suggested that studies on earthworm-plant interactions may contribute significantly to a more comprehensive understanding of terrestrial ecosystems and to the development of more environmentally friendly agricultural practices.

Effects of earthworms on plant growth: patterns and perspectivesThe 7th international symposium on earthworm ecology · Cardiff · Wales · 2002

Summary A total of 67 studies were located in which the response of plants to presence of earthworms was investigated. The number of studies increased strongly during the last decade. In most of the studies (79%) shoot biomass of plants significanty increased in the presence of earthworms. However, knowledge on effects of earthworms on plant growth is very biased; most studies investigated crop plants, particularly cereals, and pastures; very little is known on plant species in more natural communities. Recently, interest in tropical plant species has increased considerably, however, the studies have considered almost exclusively agricultural plant species. Generally, experiments focused on the response of plant shoots but 45% of the studies also considered roots. Most of the studies investigated European earthworms (Lumbricidae); very little is known on other earthworm species. Some early studies indicated that earthworms affect the composition of plant communities but only very recently has it been documented that earthworms affect plant competition. However, there is virtually no information on how earthworms affect plant performance in detail including fitness parameters such as flowering and seed production. It has been realized recently that earthworms not only modify plant growth and vegetation structure but also the susceptability of plants to herbivores. Herbivore performance might be stimulated but also reduced due to the presence of earthworms. Furthermore, earthworms function as subsidiary food resources to generalist predators when herbivore prey is scarce. The complex indirect interactions between earthworms and the aboveground system deserve further investigation in both natural and agricultural ecosystems. The imperative for future research is adopting an ecological rather than an agricultural perspective in studying earthworm-plant interrelationships and viewing earthworms as driving factors of the aboveground food web. It is suggested that studies on earthworm-plant interactions may contribute significantly to a more comprehensive understanding of terrestrial ecosystems and to the development of more environmentally friendly agricultural practices.

Sensitivity of ecosystem functioning to changes in trophic structure, functional group composition and species diversity in belowground food webs

The objective of the present paper, using decomposer food webs as a tool, is to explore the levels of the ecological hierarchy (trophic groups, feeding guilds, species populations) at which reduction in complexity brings about significant changes in ecosystem performance. A review is given of various mini‐ecosystem studies that have recently been conducted at the University of Jyväskylä. It is hypothesized that the typical features of soils as a habitat, and the peculiarities of belowground food webs, such as the commonness of indirect interactions (mediated through abiotic resources) among the biota, together with the high frequency of polyphagy/omnivory among soil organisms, produce a diversity‐ecosystem functioning relationship that is likely to differ from those of aquatic and aboveground food webs. Experiments showed that alterations in ‘trophic levels’ were reflected in significant changes in decomposition processes, which, in turn, had substantial impact on primary production. Similarly, heterogeneity within ‘trophic levels’ was shown to be associated with increased growth of birch and pine seedlings. In contrast, species diversity within a feeding guild had little or no influence on ecosystem‐level processes. However, the species‐specific properties of individual taxa were shown to be more influential in affecting plant growth than species number per se. For example, the presence of an omnivorous enchytraeid species in the mini‐ecosystems was observed to consistently be associated with high biomass production of tree seedlings. It is concluded that the so‐called trophic dynamic models based on direct feeding interactions are of limited value in predicting the outcomes of interactions taking place belowground.

Decomposer animals and bioremediation of soils

Although microorganisms are degrading the contaminants in bioremediation processes, soil animals can also have important — while usually an indirect — role in these processes. Soil animals are useful indicators of soil contamination, both before and after the bioremediation. Many toxicity and bioavailability assessment methods utilizing soil animals have been developed for hazard and risk-assessment procedures. Not only the survival of the animals, but also more sensitive parameters like growth, reproduction and community structure have often been taken into account in the assessment. The use of bioassays together with chemical analyses gives the most reliable results for risk analyses. This is because physical, chemical and biological properties of the remediated soil may be changed during the process, and it is possible that transformation rather than mineralization of the contaminants has taken place. In addition, the soil may contain other harmful substances than those searched in chemical analyses. Finally, because the ultimate goal of the bioremediation should be — together with mineralization of the harmful substances — the ecological recovery of the soil, development of diverse decomposer community as a basis of the functioning ecosystem should be ensured. Soil animals, especially the large ones, can also actively take part in the ecological recovery processes through their own activity. The potential risk of transfer of contaminants accumulated in soil animals to the above-ground food webs should be borne in mind.

The Contribution of Microarthropods to Aboveground Food Webs: A Review and Model of Belowground Transfer in a Coniferous Forest

Although belowground food webs have received much attention, studies concerning microarthropods in nondetrital food webs are scarce. Because adult oribatid mites often number between 250,000–500,000/m2 in coniferous forests, microarthropods are a potential food resource for macroarthropod and vertebrate predators of the forest floor. Although the contribution of microarthropods to aboveground food webs has received little attention, sufficient data concerning macroarthropods and vertebrate predators were available at the Savannah River Site (SRS, Aiken, South Carolina) to construct a food web model of the various trophic interactions. To supplement this analysis, literature of microarthropod predation by arthropods and vertebrates was reviewed. This information was incorporated with the existing data to produce a model for taxa occurring in coniferous forests at the SRS. Because of the diversity and natural history of microarthropod predators, both vertebrate and invertebrate, the resulting web is quite connected and includes transfers to many trophic levels. The diets of arthropods and vertebrates are variable; yet feeding patterns reflect the relative abundance of prey at a place and time. Also, many predators feed on members of their own group. These factors suggest that belowground transfers are deserved of more attention in these and other forest food webs where substantial numbers of detritus feeding invertebrates inhabit the soil/litter interface and are available as prey items. Moreover, this model can be generalized to describe the dynamics of arthropod and vertebrate communities in other coniferous forests. The functioning of terrestrial ecosystems is dependent upon the interrelationships between aboveground and belowground food webs, and transfers of biotic components of the decomposer subsystem to aboveground consumers connect the two subsystems. It is hoped that those consumers traditionally associated with foliage-based food webs be reconsidered, as they may be gaining a proportion of their nutrition from organisms in the detrital pathway.