Within-trophic group interactions of bacterivorous nematode species and their effects on the bacterial community and nitrogen mineralization

Knowledge of the interactions between organisms within trophic groups is important for an understanding of the role of biodiversity in ecosystem functioning. We hypothesised that interactions between bacterivorous nematodes of different life history strategies would affect nematode population development, bacterial community composition and activity, resulting in increased N mineralization. A microcosm experiment was conducted using three nematode species (Bursilla monhystera, Acrobeloides nanus and Plectus parvus). All the nematode species interacted with each other, but the nature and effects of these interactions depended on the specific species combination. The interaction between B. monhystera and A. nanus was asymmetrically competitive (0,-), whereas that between B. monhystera and P. parvus, and also A. nanus and P. parvus was contramensal (+, -). The interaction that affected microcosm properties the most was the interaction between B. monhystera and P. parvus. This interaction affected the bacterial community composition, increased the bacterial biomass and increased soil N mineralization. B. monhystera and P. parvus have the most different life history strategies, whereas A. nanus has a life history strategy intermediate to those of B. monhystera and P. parvus. We suggest that the difference in life history strategies between species of the same trophic group is of importance for their communal effect on soil ecosystem processes. Our results support the idiosyncrasy hypothesis on the role of biodiversity in ecosystem functioning.

Nutrient enrichment and food chains: can evolution buffer top-down control?

We show how evolutionary dynamics can alter the predictions of classical models of the effects of nutrient enrichment on food webs. We compare an ecological nutrient-plant-herbivore food-chain model without evolution with the same model, including herbivore evolution, plant evolution, or both. When only herbivores are allowed to evolve, the predictions are similar to those of the ecological model without evolution, i.e., plant biomass does not change with nutrient addition. When only plants evolve, nutrient enrichment leads to an increase in the biomass of all compartments. In contrast, when plants and herbivores are allowed to coevolve, although these two classical patterns are common, a wide variety of other responses is possible. The form of the trade-offs that constrain evolution of the two protagonists is then critical. This stresses the need for experimental data on phenotypic traits, their costs and their influence on the interactions between organisms and the rest of the community.

Effects of fungal food quality and starvation on the fatty acid composition of Protaphorura fimata (Collembola)

The lipid pattern of animals is influenced by species, life stage, environmental conditions and diet. We investigated the effects of food quality and starvation on the phospholipid (PLFA) and neutral lipid (NLFA) fatty acid pattern of the collembolan Protaphorura fimata. Collembolans were fed with two common soil fungi, Agrocybe gibberosa and Chaetomium globosum, of which the cellular lipid composition was analysed. A. gibberosa was grown on agar with different nitrogen contents, resulting in altered fatty acid patterns and C:N ratios, i.e. fungi of different food quality. Collembolans did not mirror the lipid composition of the fungal diet as the pattern of major NLFAs in P. fimata was vice versa. Presumably, altered food quality of fungi caused compensatory responses by the collembolans, thereby diminishing the fungal signal. In a further experiment P. fimata (previously maintained with C. globosum) was kept without food for up to 4 weeks. Starvation resulted in a decline in the total amount of NLFAs; however, it did not affect the fatty acid pattern, indicating that NLFAs were degraded indiscriminately. Generally, the PLFA profile of the collembolans changed only slightly due to variations in diet quality or starvation.

Diversity and dynamics of microbial communities in soils from agro-ecosystems

Soil microbial communities are integrally involved in biogeochemical cycles and their activities are crucial to the productivity of terrestrial ecosystems. Despite the importance of soil microorganisms, little is known about the distribution of microorganisms in the soil or the manner in which microbial community structure responds to changes in land management. We investigated the structure of microbial communities in the soil over two years in a series of replicated plots, that included, cultivated fields, fields abandoned from cultivation and fields with no history of cultivation. Microbial community structure was examined by monitoring the relative abundance of ribosomal RNA (rRNA) from seven of the most common bacterial groups in soil (the Alpha and Beta Proteobacteria, Actinobacteria, Cytophagales, Planctomycetes, Verrucomicrobia and the Acidobacteria) and the Eukarya. These data reveal that soil microbial communities are dynamic, capable of significant change at temporal scales relative to seasonal events. However, despite temporal change in microbial community structure, the rRNA relative abundance of particular microbial groups is affected by the local environment such that recognizable patterns of community structure exist in relation to field management.

Fatty acid composition and dynamics of selected fungal-feeding nematodes and fungi

Fatty acid profiles of fungal-feeding nematodes, Aphelenchus avenae and Aphelenchoides composticola, and selected fungi were determined in microcosm cultures of agar, broth, or sand amended with organic matter. Fatty acids of A. avenae and A. composticola included 16:0 18:0, 18:1omega7, 18:1omega9, 18:2, 20:0, 20:1, 20:2, 20:3 and 20:4 phospholipid fatty acids (PLFAs) and neutral lipid fatty acids (NLFAs). The nematodes differed in relative amounts of saturated and C(18) fatty acids. Similar C(16) and C(18) PLFAs and whole-cell fatty acids were found in Rhizoctonia solani, Fusarium oxysporum and Trichoderma sp. with 18:2omega6 as the major component. The C(20) fatty acids were not found in these fungi. Although only present in the nematodes, C(20) PLFAs were only detected when nematode population levels were > or =22 per gram of sand, suggesting that there is a detection threshold that might limit their use as biomarkers in the soil community. After removal of nematodes from a food source, the relative amount of C(20) PLFAs (structural components of nematode cell membranes) decreased more slowly than the C(16) and C(18) PLFAs, which may have reflected ingested fungal cytoplasm in the nematode intestine. In the early stage of organic matter decomposition, total and fungal PLFAs were lower in the presence of A. composticola then in its absence at C:N ratios > or =30:1.

The Logic and Realism of the Hypothesis of Exploitation Ecosystems

Hypotheses on trophic dynamics in terrestrial ecosystems fall into two major categories: those in which plants are assumed to be invulnerable to their consumers and those in which the build-up of plant biomass is assumed to require top-down control of folivores. The hypothesis of exploitation ecosystems (EEH) belongs to the latter category and focuses particularly on the consequences of the high energetic costs of maintenance of endotherms. Carnivorous endotherms require relatively high prey densities in order to break even. Moreover, they are dependent on folivorous prey during the limiting season, at least at high latitudes. The endotherm branch of the grazing web is thus predicted to collapse from three-link trophic dynamics (carnivores → folivores → plants → inorganic resources) to two-link dynamics (folivores → plants → inorganic resources) along gradients of decreasing primary productivity. Consequently, the vegetation of cold and unproductive areas is predicted to be under intense winter grazing pressure, which prevents the accumulation of aboveground plant biomass and excludes erect woody plants. In the most extreme habitats (e.g., polar deserts and their high alpine counterparts), even folivorous endotherms are predicted to be absent, and the scanty vegetation is predicted to be structured by preemptive competition. Within temperature-determined productivity gradients, EEH is corroborated by biomass patterns, by patterns in the structure and dynamics of carnivore, folivore, and plant communities, and by experimental results. The general idea of top-down trophic dynamics is supported for other autotroph-based systems, too, but the relevance and sufficiency of the energy constraint in explaining patterns in trophic dynamics appears to be variable. Moreover, critical empirical evidence for or against the capacity of folivorous insects to regulate plant biomass has not yet been obtained. Another open question is the ability of boreal and temperate browsers, evolved in productive environments with intense predation pressure and abundance of forage, to prevent the regeneration of the least palatable tree species. There are, thus, many open questions waiting to be answered and many exciting experiments waiting to be conducted.

Effects of Enrichment on Three-Level Food Chains with Omnivory

Although omnivory (the consumption of resources from more than one trophic level) is widespread, this fundamental limitation to the applicability of food chain theory to real communities has received only limited treatment. We investigated effects of enrichment (increasing carrying capacity, K, of the resource) on a system consisting of a resource (R), an intermediate consumer (N), and an omnivore (P) using a general mathematical model and tested the relevance of some of its predictions to a laboratory system of mixed bacteria (=R) and the ciliates Tetrahymena (=N) and Blepharisma (=P). The model produced six major predictions. First, N may facilitate or inhibit P. Enrichment may revert the net effect of N on P from facilitation to inhibition. Second, along a gradient of K, up to four regions of invasibility and stable coexistence of N and P may exist. At the lowest K, only R is present. At somewhat higher K, N can coexist with R. At intermediate K, either N and P coexist, or either consumer excludes the other depending on initial conditions. At the highest K, N may be excluded through apparent competition and only R and P can coexist. The pattern of persistence of Tetrahymena and Blepharisma along an enrichment gradient conformed fairly well to the scenario allowing coexistence at intermediate K. Third, for stable equilibria of the omnivory system, R always increases and N always decreases with K. The abundances of bacteria and Tetrahymena were suggestive of such a pattern but did not allow a strict test because coexistence occurred at only one level of enrichment. Fourth, an omnivore can invade an R-N system at a lower K than an otherwise identical specialist predator of N. Fifth, an omnivore can always invade a food chain with such a specialist predator. Sixth, over ranges of K where both omnivory systems and otherwise identical three-level food chains are feasible, N is always less abundant in the omnivory system, whereas the relative abundances of R and P in omnivory systems compared to food chains may change with K. It is thus possible that total community biomass at a given K is lower in an omnivory system than in a food chain. Both the model and the experimental results caution that patterns of trophic-level abundances in response to enrichment predicted by food chain theory are not to be expected in systems with significant omnivory.

Trophic cascades revealed in diverse ecosystems

New studies are documenting trophic cascades in theoretically unlikely systems such as tropical forests and the open ocean. Together with increasing evidence of cascades, there is a deepening understanding of the conditions that promote and inhibit the transmission of predatory effects. These conditions include the relative productivity of ecosystems, presence of refuges and the potential for compensation. However, trophic cascades are also altered by humans. Analyses of the extirpation of large animals reveal loss of cascades, and the potential of conservation to restore not only predator populations but also the ecosystem-level effects that ramify from their presence.