Juha Mikola

No evidence of trophic cascades in an experimental microbial-based soil food web

Trophic-dynamic theories predict the biomass and productivity of trophic levels to be partially top-down regulated in food webs, and that the top-down regulation will manifest itself as cascading trophic interactions. We tested the two principal predictions deduced from these theories: trophic cascades of (1) biomass regulation and (2) productivity regulation occur in food webs. n nWe created three food webs with either one, two, or three trophic levels in soil microcosms containing a sterilized mixture of leaf litter and humus. Twenty species of bacteria and fungi formed the first trophic level, a bacterivorous nematode (Caenorhabditis elegans) and a fungivorous nematode (Aphelenchoides sp.) the second level, and a predatory nematode (Prionchulus punctatus) the third level. We sampled the microcosms destructively four times during a 5-mo experiment for estimations of the biomass of each of the trophic levels. CO2 evolution was analyzed once or twice a week, and NH4+-N concentration in the soil was measured at the end of the experiment. Glucose was added to the microcosms every second week to provide energy for the microbes. The biomass of microbivores was clearly regulated by the predator. The abundance of bacteria was not affected by the food chain length, and the abundance of fungi was higher in the presence of nematodes than in the pure microbial community. Net mineralization of N and C was highest in the food chains with two trophic levels, at an intermediate level in the presence of predators, and lowest in the pure microbial communities. Microbial production (estimated on the basis of microbial respiration) was higher in the food webs with two and three trophic levels than when the microbes were growing alone. Whether the biomass of the second trophic level was reduced by the predator or not had no effect on microbial biomass or microbial productivity. n nTherefore, although the microbivore biomass and mineralization of both C and N were regulated by the predator, our experiment did not provide evidence of cascading trophic interactions regulating the microbial biomass and productivity in decomposer food webs. The facts that microbes were able to compensate totally for the consumed biomass by increasing their turnover rate and that the microbes did not behave as a uniform trophic level prevented a trophic cascade of biomass regulation from occurring in our soil food web. Similarly, since microbial productivity did not depend on the biomass at the second trophic level, neither did a trophic cascade of productivity regulation take place.

Defoliation and patchy nutrient return drive grazing effects on plant and soil properties in a dairy cow pasture

Large herbivores can influence plant and soil properties in grassland ecosystems, but especially for belowground biota and processes, the mechanisms that explain these effects are not fully understood. Here, we examine the capability of three grazing mechanisms - plant defoliation, dung and urine return, and physical presence of animals (causing trampling and excreta return in patches) - to explain grazing effects in Phleum pratense-F estuca pratensis dairy cow pasture in Finland. Comparison of control plots and plots grazed by cows showed that grazing maintained original plant-community structure, decreased shoot mass and root N and P concentrations, increased shoot N and P concentrations, and had an inconsistent effect on root mass. Among soil fauna, grazing increased the abundance of fungivorous nematodes and Aporrectodea earthworms and decreased the abundance of detritivorous enchytraeids and Lumbricus earthworms. Grazing also increased soil density and pH but did not affect average soil inorganic-N concentration. To reveal the mechanisms behind these effects, we analyzed results from mowed plots and plots that were both mowed and treated with a dung and urine mixture. This comparison revealed that grazing effects on plant attributes were almost entirely explained by defoliation, with only one partly explained by excreta return. Among belowground attributes, however, the mechanisms were more mixed, with effects explained by defoliation, patchy excreta return, and cow trampling. Average soil inorganic-N concentration was not affected by grazing because it was simultaneously decreased by defoliation and increased by cow presence. Presence of cows created great spatial heterogeneity in soil N availability and abundance of fungivorous nematodes. A greenhouse trial revealed a grazing-induced soil feedback on plant growth, which was explained by patchiness in N availability rather than changes in soil biota. Our results show that grazing effects on plant attributes can be satisfactorily predicted using the effects of defoliation, whereas those on soil fauna and soil N availability need understanding of other mechanisms as well. The results indicate that defoliation-induced changes in plant ecophysiology and the great spatial variation in N availability created by grazers are the two key mechanisms through which large herbivores can control grassland ecosystems.

Functional diversity of decomposer organisms in relation to primary production

Abstract The term `biodiversity is claimed to lack connections to a serious scientific background. In this work, we approached the concept of biodiversity from a functional point of view by asking: “At what level of the ecological organization (species, trophic species/feeding guilds, trophic levels etc.) should reduction in biodiversity matter to bring about visible changes in ecosystem performance?” We investigate the concepts of `functional diversity and `ecosystem performance in relation to feeding habits (such as fungivory, detritivory etc.) of soil fauna and plant growth. After analysing the results of a number of microcosm studies, we came into the following conclusions: (i) trophic level diversity has clear impacts on primary production – more so when the number of trophic levels is low; (ii) manipulation of large predators at the top of the food chain had no influence on plant growth, whereas smaller mesostigmatid predators generally showed a negative impact on plant growth and nutrient uptake; (iii) declining species diversity alters carbon mineralization in heterotrophic systems due to changing interactions between trophic levels; (iv) species composition within a functional group can affect biomass production of plants; (v) inclusion of ectomycorrhizal fungi in model ecosystems may be of fundamental importance to understanding the most significant interactions between primary producers and organisms in detrital food webs.

Studying the effects of plant species richness on ecosystem functioning: does the choice of experimental design matter?

Abstract. We established two parallel greenhouse experiments to empirically test the effect of experimental design on the conclusions made of the effects of plant species richness on ecosystem functioning. The experiments included an identical group of six grassland plants and were performed under identical environmental conditions, but were set up according to two different designs. In the richness design (RD), which specifically aims at testing the effect of species richness, each replicate at each richness level was chosen by a separate random draw from the total pool of species, whereas in the richness and composition design (RCD), which aims at testing the effect of both richness and composition, richness levels included deliberately replicated monocultures (at the one-species level) and mixtures (at other levels) of constituent species. When regression analysis was applied, both experimental designs found a positive effect of plant species richness on primary production (estimated using shoot mass accumulation), with species richness explaining 34% and 16% of the total variation in production in RD and RCD, respectively. Based on an overyielding analysis, this positive overall effect of species richness on primary production was in both experiments caused by the sampling effect rather than complementarity. When analysis of variance was applied, RCD also found that shoot production was greatly affected by the presence of one species, the legume Trifolium hybridum, and that 98–99% of variation in primary production within richness levels, i.e. the variation not explained by species richness, was explained by species composition. It appears that while the RD experiment suggests that plant species richness was a significant controller of primary production in our experimental grassland community, the RCD experiment suggests that primary production was mainly determined by plant species composition, i.e. the identity of the species present. Our results are consistent with earlier mathematical simulations in that experimental designs differ in their ability to discriminate the effects of species richness and composition on ecosystem functioning and may therefore lead to different conclusions of the role of species richness in functioning. We propose that this may partly explain the different relative role of plant species richness in ecosystem functioning in earlier investigations using RD and RCD.

Responses of microbial-feeding nematodes to organic matter distribution and predation in experimental soil habitat

Abstract Soils are spatially heterogeneous environments, a condition which is likely to affect the structure and function of soil food webs. To study the influence of soil organic matter distribution on decomposer food webs, we established microcosms that contained either: (1) large patches of initially sterile humus-litter mixture placed within sterile sand matrix (creating a large-patch habitat; LPH); or (2) blend of sand and humus-litter mixture (creating a small-patch habitat; SPH). Ten species of bacteria and ten species of fungi, and two bacterial-feeding ( Acrobeloides tricornis , Caenorhabditis elegans ) and two fungal-feeding ( Aphelenchus avenae , Aphelenchoides sp.) nematodes were added to both habitats to form the two lowest trophic levels of the food web. Microbivore predation was manipulated by adding a predatory nematode ( Prionchulus punctatus ) to half of the replicates of both habitats. The microcosms were destructively sampled three times during the 15-week experiment to estimate nematode biomass. The biomasses of A. tricornis and C. elegans were on average 35% and 21% lower, respectively, in LPH than in SPH, whereas the biomass of the two fungivores was not affected by organic matter distribution. Top predator biomass was higher in LPH than in SPH at the first and second samplings. The top predator had both negative and positive effects on its prey species, and the effects varied with sampling. The biomass ratio between fungivores and bacterivores was marginally higher ( P =0.080) in LPH (average 1:14) than in SPH (1:20), and significantly higher in the presence (1:13) than in the absence (1:25) of the top predator. In LPH, 69% of A. tricornis and 84% of top predator biomass was found in sand, while the majority (83%) of C. elegans lived in patches. At the end of the experiment a higher proportion of A. tricornis and A. avenae lived in patches in the presence than in the absence of the top predator. The results infer that organic matter distribution may affect the biomass of microbial-feeding and predatory nematodes in soil and produce a shift between the bacterial and fungal energy channels. Mechanisms mediating the effects of organic matter distribution on microbivores and on the top predator seem to differ, however, and therefore a change in organic matter distribution may increase biomass at one trophic level while simultaneously decreasing biomass at another.

Effects of microbivore species composition and basal resource enrichment on trophic-level biomasses in an experimental microbial-based soil food web

Abstract Previous theoretical and empirical evidence suggests that species composition within trophic levels may profoundly affect the response of trophic-level biomasses to enhanced basal resources. To test whether species composition of microbivorous nematodes has such an effect in microbial-based soil food webs, I created three microcosm food webs, consisting of bacteria, fungi, bacterial-feeding nematodes (Acrobeloides tricornus, Caenorhabditis elegans), fungal-feeding nematodes (Aphelenchus avenae, Aphelenchoides sp.) and a predatory nematode (Prionchulus punctatus). The food webs differed in species composition at the second trophic level: food web A included A. tricornus and Aph. avenae, food web B included C. elegans and Aphelenchoides sp., and food web AB included all four species. I increased basal resources by adding glucose to half of the replicates of each food web, and sampled microcosms destructively four times during a 22-week experiment to estimate the biomass of organisms at each trophic level. Microbivore species composition significantly affected bacterivore and fungivore biomass but not bacterial, fungal or predator biomass. Greatest bacterivore and fungivore biomass was found in food web A, intermediate biomass in food web AB, and smallest biomass in food web B. Basal resource addition increased the biomass of microbes and microbivores but did not affect predator biomass. Importantly, microbivore species composition did not significantly modify the effect of additional resources on trophic-level biomasses. The presence of a competitor reduced the biomass of A. tricornus and Aph. avenae, in that the biomass of these species was less in food web AB than in food web A, whereas the biomass of C. elegans and Aphelenchoides sp. was not affected by their potential competitors. The biomass of Aph. avenae increased with additional resources in the absence of the competitor only, while the biomass of A. tricornus and Aphelenchoides sp. increased also in the presence of their competitors. The results imply that microbivore species composition may determine the second-level biomass inu2009simple microbe-nematode food webs, but may not significantly affect biomass at other levels or modify the response of trophic-level biomasses to enhanced basal resources. The study also shows that even if the role of predation in a food web is diminished, the positive response of organisms to increased resource availability may still be hindered by competition.

Defoliation and the availability of currently assimilated carbon in the Phleum pratense rhizosphere

Abstract It has been hypothesised that defoliation and aboveground herbivory increase the availability of currently assimilated C to organisms living in plant rhizospheres. We established a growth chamber experiment consisting of Phleum pratense individuals growing in sand culture to examine the short- and long-term effects of defoliation on the availability of current C assimilates in the P. pratense rhizosphere. Using 14CO2 pulse labelling, we followed partitioning of currently assimilated C between shoots, roots and rhizosphere-derived organic matter (RDOM). The experiment constituted of two treatments, defoliation history and recent defoliation, in a fully factorial design. Defoliation history had three levels: (1) no past defoliation, (2) defoliation 1 week before labelling, and (3) defoliation 1, 2 and 3 weeks before labelling, while recent defoliation had two levels: (1) no defoliation and (2) defoliation immediately before labelling. Recent defoliation reduced the amount of 14C radioactivity in shoots, roots and RDOM, while defoliation history did not have a significant effect. Neither treatment affected the proportions of shoot, root and RDOM radioactivity-to-total recovered radioactivity or the ratio of RDOM radioactivity-to-root radioactivity. The results suggest that the amount of current C assimilates found in the P. pratense rhizosphere decreases shortly after defoliation, but is not affected by defoliations that have occurred at least 1 week earlier. The results further suggest that the defoliation treatments, despite affecting the quantity of assimilated current C, do not affect the allocation pattern of assimilated C within P. pratense individuals or the ratio of current assimilates found in P. pratense roots-to-those available in its rhizosphere. On the whole, our results do not support the hypothesis that defoliation increases the availability of current photosynthate to soil decomposer food webs.