Unravelling the feeding habits of fungivores

The aim of this project was to investigate and clarify the feeding habits of fungivorous soil fauna, with the overall objective of improving our understanding of their functional roles in the ecosystem. Special focus was given to symbiotic ectomycorrhizal (EM) fungi, that, owing to their abundance and diversity, are considered to constitute an important food source for fungivorous soil fauna and may be a factor regulating these faunal communities. Previous studies on soil animal feeding habits have been strongly dependent on the methodology used; therefore an additional aim was to find the technique most suitable for studying fungivorous feeding. The results in this thesis confirm that it is necessary to combine a number of different methods in order to determine the feeding habits of fungivorous soil fauna. The total abundance of oribatid mites was significantly reduced in spruce forest stands that were girdled to restrict flow of photoassimilates to roots and ectomycorrhizal fungi, and Oppiella nova was the species that showed the most marked reduction in abundance, especially in spruce forest soils. This reduction in abundance could be explained by the results from a microcosm study, in which O. nova was clearly favoured by the presence of certain EM fungi and increased its abundance of both adults and juveniles in microcosms with the two EM species Suillus variegatus and Paxillus involutus. In the same study, Cognettia sphagnetorum was suppressed by the presence of EM fungi, which partly explains why this species reach high populations after clear-cutting. PCR in combination with washing of the body surface and dissection was a successful method for analysing diets of fungal feeding oribatid mites. This method will be considered for future analyses of field collected animals. We could also demonstrate that the isotopic composition of fungivorous animals should be interpreted with great care, when used as a method for studying fungivore feeding habits. The range of values recorded from field collected Diptera larvae utilising a single food source was large enough to span across several trophic levels when these were determined using fixed enrichment values from literature data. The δ¹⁵N of the animals was higher than that of the ingested fungal tissue, but similar to the δ¹⁵N of the protein and amino acids of the food

Associated learning of odour and colour in the seven-spotted ladybird Coccinella septempunctata (L.)

The ability of the seven-spotted ladybird Coccinella septempunctata (L.) to learn to associate a combined stimulus of colour and odour with food was investigated. Four odours (menthol, 1,8-cineole, methyl salicylate and perfume) and three colours (red, yellow and green) formed twelve treatments that were combined with aphids and sweetened butter. For testing the attraction of the given treatment, the ladybirds were tested in a two-way olfactometer in which the ladybirds chose between the treatment and a control. The ladybirds were more attracted to three combinations (1,8-cineole and yellow, menthol and yellow and perfume and green) following the treatment. This could indicate conditioning or learning. They also showed a strong attraction to the combinations of 1,8-cineole and green and methyl salicylate and green even without prior treatment, which could indicate innate behaviour. The results contribute to the understanding of ladybird foraging behaviour and learning abilities. The potential to increase the effectiveness of ladybirds as a biological control agent is discussed. Sammanfattning " Inlärning av färg och doft hos sjuprickiga nyckelpigor Coccinella septempunctata (L.) – ett olfaktormeter experiment" I denna undersökning studerades sjuprickiga nyckelpigors Coccinella septempunctata (L.) förmåga att associera en kombinerad stimuli av färg och doft med mat. Fyra dofter (mentol, 1,8-cineol, metyl salicylat, och parfym) och tre färger (rött, gult och grönt) formade tillsammans med mat tolv behandlingar. Nyckelpigornas attraktion till den givna behandlingen testades i tvåarmade olfactometrar där de valde mellan behandlingen och en kontroll. Nyckelpigorna var mer attraherade till tre kombinationer (1,8-cineol och gult, mentol och gult och parfym och grönt) efter behandlingen. Detta kan indikera någon typ av betingat beteende eller inlärning. Nyckelpigorna var också starkt attraherade till kombinationen av 1,8-cineol och grönt och methyl salicylate och grönt även utan tidigare kontakt med behandlingen, vilket kan indikera någon typ av medfött beteende. Resultaten bidrar till ökad kunskap om nyckelpigors födosök och inlärningsmöjligheter. I studien diskuteras även om resultaten kan bidra till förbättrad användning av nyckelpigor inom biologisk bekämpning.I denna undersökning studerades sjuprickiga nyckelpigors Coccinella septempunctata (L.) förmåga att associera en kombinerad stimuli av färg och doft med mat. Fyra dofter (mentol, 1,8-cineol, metyl salicylat, och parfym) och tre färger (rött, gult och grönt) formade tillsammans med mat tolv behandlingar. Nyckelpigornas attraktion till den givna behandlingen testades i tvåarmade olfactometrar där de valde mellan behandlingen och en kontroll. Nyckelpigorna var mer attraherade till tre kombinationer (1,8-cineol och gult, mentol och gult och parfym och grönt) efter behandlingen. Detta kan indikera någon typ av betingat beteende eller inlärning. Nyckelpigorna var också starkt attraherade till kombinationen av 1,8-cineol och grönt och methyl salicylate och grönt även utan tidigare kontakt med behandlingen, vilket kan indikera någon typ av medfött beteende. Resultaten bidrar till ökad kunskap om nyckelpigors födosök och inlärningsmöjligheter. I studien diskuteras även om resultaten kan bidra till förbättrad användning av nyckelpigor inom biologisk bekämpning

Tamm Review: On the nature of the nitrogen limitation to plant growth in Fennoscandian boreal forests

The supply of nitrogen commonly limits plant production in boreal forests and also affects species composition and ecosystem functions other than plant growth. These interrelations vary across the landscapes, with the highest N availability, plant growth and plant species richness in ground-water discharge areas (GDAs), typically in toe-slope positions, which receive solutes leaching from the much larger groundwater recharge areas (GRAs) uphill. Plant N sources include not only inorganic N, but, as heightened more recently, also organic N species. In general, also the ratio inorganic N over organic N sources increase down hillslopes. Here, we review recent evidence about the nature of the N limitation and its variations in Fennoscandian boreal forests and discuss its implications for forest ecology and management. The rate of litter decomposition has traditionally been seen as the determinant of the rate of N supply. However, while N-rich litter decomposes faster than N-poor litter initially, N-rich litter then decomposes more slowly, which means that the relation between N % of litter and its decomposability is complex. Moreover, in the lower part of the mor-layer, where the most superficial mycorrhizal roots first appear, and N availability matters for plants, the ratio of microbial N over total soil N is remarkably constant over the wide range in litter and soil C/N ratios of between 15 and 40 for N-rich and N-poor sites, respectively. Nitrogen-rich and -poor sites thus differ in the sizes of the total N pool and the microbial N pool, but not in the ratio between them. A more important difference is that the soil microbial N pool turns over faster in N-rich systems because the microbes are more limited by C, while microbes in N-poor systems are a stronger sink for available N. Furthermore, litter decomposition in the most superficial soil horizon (as studied by the so-called litter-bag method) is associated with a dominance of saprotrophic fungi, and absence of mycorrhizal fungi. The focal zone in the context of plant N supply in N-limited forests is further down the soil profile, where ectomycorrhizal (ECM) roots become abundant. Molecular evidence and stable isotope data indicate that in the typical N-poor boreal forests, nitrogen is retained in saprotrophic fungi, likely until they run out of energy (available C-compounds). Then, as heightened by recent research, ECM fungi, which are supplied by photosynthate from the trees, become the superior competitors for N. In N-poor boreal soils strong N retention by microorganisms keeps levels of available N very low. This is exacerbated by an increase in tree C allocation to mycorrhizal fungi (TCAM) relative to net primary production (NPP) with decreasing soil N supply, which causes ECM fungi to retain much of the available soil N for their own growth and transfer little to their tree hosts. The transfer of N through the ECM fungi, and not the rate of litter decomposition, is likely limiting the rate of tree N supply under such conditions. All but a few stress-tolerant less N-demanding plant species, like the ECM trees themselves and ericaceous dwarf shrubs, are excluded. With increasing N supply, a weakening of ECM symbiosis caused by the relative decline in TCAM contributes to shifts in soil microbial community composition from fungal dominance to bacterial dominance. Thus, bacteria, which are less C-demanding, but more likely to release N than fungi, take over. This, and the relatively high pH in GDA, allow autotrophic nitrifying bacteria to compete successfully for the NH4+ released by C-limited organisms and causes the N cycle to open up with leaching of nitrate (NO3−) and gaseous N losses through denitrification. These N-rich conditions allow species-rich communities of N-demanding plant species. Meanwhile, ECM fungi have a smaller biomass, are supplied with N in excess of their demand and will export more N to their host trees. Hence, the gradient from low to high N supply is characterized by profound variations in plant and soil microbial physiologies, especially their relations to the C-to-N supply ratio. We propose how interactions among functional groups can be understood and modelled (the plant-microbe carbon-nitrogen model). With regard to forest management these perspectives explain why the creation of larger tree-free gaps favors the regeneration of tree seedlings under N-limited conditions through reduced belowground competition for N, and why such gaps are less important under high N supply (but when light might be limiting). We also discuss perspectives on the relations between N supply, biodiversity, and eutrophication of boreal forests from N deposition or forest fertilization

Production and turnover of ectomycorrhizal extramatrical mycelial biomass and necromass under elevated CO2 and nitrogen fertilization

Extramatrical mycelia (EMM) of ectomycorrhizal fungi are important in carbon (C) and nitrogen (N) cycling in forests, but poor knowledge about EMM biomass and necromass turnovers makes the quantification of their role problematic. We studied the impacts of elevated CO2 and N fertilization on EMM production and turnover in a Pinus taeda forest. EMM C was determined by the analysis of ergosterol (biomass), chitin (total bio- and necromass) and total organic C (TOC) of sand-filled mycelium in-growth bags. The production and turnover of EMM bio- and necromass and total C were estimated by modelling. N fertilization reduced the standing EMM biomass C to 57% and its production to 51% of the control (from 238 to 122 kg C ha-1 yr-1), whereas elevated CO2 had no detectable effects. Biomass turnover was high (~13 yr-1) and unchanged by the treatments. Necromass turnover was slow and was reduced from 1.5 yr-1 in the control to 0.65 yr-1 in the N-fertilized treatment. However, TOC data did not support an N effect on necromass turnover. An estimated EMM production ranging from 2.5 to 6% of net primary production stresses the importance of its inclusion in C models. A slow EMM necromass turnover indicates an importance in building up forest humus

Dining local: the microbial diet of a snail that grazes microbial communities is geographically structured

Achatinella mustelina is a critically endangered tree snail that subsists entirely by grazing microbes from leaf surfaces of native trees. Little is known about the fundamental aspects of these microbe assemblages: not taxonomic composition, how this varies with host plant or location, nor whether snails selectively consume microbes. To address these questions, we collected 102 snail faecal samples as a proxy for diet, and 102 matched-leaf samples from four locations. We used Illumina amplicon sequencing to determine bacterial and fungal community composition. Microbial community structure was significantly distinct between snail faeces and leaf samples, but the same microbes occurred in both. We conclude that snails are not ‘picky’ eaters at the microbial level, but graze the surface of whatever plant they are on. In a second experiment, the gut was dissected from nonendangered native tree snails in the same family as Achatinella to confirm that faecal samples reflect gut contents. Over 60% of fungal reads were shared between faeces, gut and leaf samples. Overall, location, sample type (faeces or leaf) and host plant identity all significantly explained the community composition and variation among samples. Understanding the microbial ecology of microbes grazed by tree snails enables effective management when conservation requires captive breeding or field relocation