Lisette Lenoir

Bottom–up or top–down control in forest soil microcosms? Effects of soil fauna on fungal biomass and C/N mineralisation

A major question in soil ecology is whether soil food webs are regulated by resources or by predators, i.e. bottom–up (donor) or top–down controlled. We tested the hypothesis that meso- and macrofaunal soil predators can regulate fungivore populations and, thereby cause a top–down cascade effect on fungal biomass and decomposition/mineralisation processes in boreal forest soils. The study was performed as a microcosm experiment with two contrasting soils (humus layers), one poor and one rich in N, and with different combinations of fungivore and predator soil fauna added to “defaunated” soil. In comparison with control microcosms lacking mesofauna (but with nematodes and protozoans), the presence of a diverse Collembola and Oribatida fungivore community significantly reduced the FDA-active fungal biomass or tended to reduce the ergosterol fraction of the fungal biomass in the N-poor humus, but no clear effect could be detected in the N-rich humus. Fungivores as well as fungivores plus predators (a predator community consisting of gamasids, spiders and beetles or a subset thereof) reduced C mineralisation and increased net N mineralisation in both soils. The presence of predators (particularly gamasid mites) reduced collembolan numbers and alleviated the negative effect of fungivores on fungal biomass in the N-poor soil. In the N-rich soil, the presence of predators increased fungal biomass (ergosterol) in relation to the “defaunated” soil. Therefore, a top–down trophic cascade could be detected in the N-poor humus but not in the N-rich humus. Our results suggest that the degree of top–down control in soil fauna communities depends on resource quality and soil fertility.

Effects of Formica ants on soil fauna-results from a short-term exclusion and a long-term natural experiment

Wood ants (Formica spp.) were hypothesised to affect the composition and greatly reduce the abundance of large-sized soil fauna by predation. This was tested in two ways. Firstly, a 4-year-long experimental study was carried out in a mixed forest. Five ant-free 1.3-m2 plots were created by fenced exclosures within an ant territory. Five nearby plots had fences with entrances for the ants. In addition, five non-fenced control plots were selected. Soil fauna (e.g. Coleoptera, Diptera larvae, Collembola and Araneae) was sampled during the summers of 1997–2000. The soil fauna was affected by the exclosures but there was no detectable effect of ants on the soil fauna. Secondly, soil fauna was studied within a large-scale natural experiment in which the long-term (30 years) effects of red wood ants could be assessed inside and outside ant territories. This long-term natural experiment revealed no significant effects of ants on the abundance or composition of soil fauna. The results from the two studies indicate that the effects of wood ants on soil fauna are fairly small. The hypothesis that wood ants are key-stone predators on soil fauna could, thus, not be supported.

Response of the foraging behaviour of red wood ants (Formica rufa group) to exclusion from trees

Abstract 1 Because of the large numbers within a colony and their aggressive nature, red wood ants (Formica rufa group) have a potential to greatly influence the cold‐temperate forest ecosystem. Wood ants are omnivorous and hunt in trees as well as on the forest floor.

Can wood ants distinguish between good and bad food patches on the forest floor

Abstract Wood ants ( Formica rufa group) normally feed on secretions of aphids to obtain carbohydrates, and on free-living invertebrates to obtain proteins. The availability of protein resources is usually unpredictable, and the demand for proteins is high during the period when ant larvae are developing. Thus, ants should select high quality food patches, i.e. patches with plenty of prey, when these are available. Foraging on the forest floor is seldom observed, but should be an alternative behaviour during periods of scarce food supply in the trees. To study the hunting behaviour of wood ants on soil invertebrates, ants were offered fly larvae (maggots) in two different quantities (six or two per pah) at two distances from an ant trail. Maggots exposed on the forest floor were found by randomly patrolling ant scouts regardless of bait quality. However, scouts that found the baits with six larvae recruited workers faster and, on average, four times as many workers were recruited to the six-larvae bait than to the bait with only two larvae. This indicates that ants can distinguish between poor and rich patches and that they are able to use this information to recruit more workers. Also, more workers were recruited and more maggots were carried away from patches nearby trails than from those far away. The results indicate that, during the warm season, ants explore and exhibit adaptive ‘foraging behaviour’ on the forest floor in the whole territory, not only close to trails. Consequently, wood ant feeding on soil invertebrates may be a common way for obtaining a large amounts of protein.

Effects of coniferous resin on fungal biomass and mineralisation processes in wood ant nest materials

Abstract Wood ants (Formica rufa group) often bring large quantities of conifer resin to their mounds. The aim of this study was to test the hypothesis that the resin acts as a fungicide and thereby reduces C and N mineralisation. Two laboratory incubation experiments were carried out using two different materials: F/H layer from a Scots pine (Pinus sylvestris) stand and mixed litter from Scots pine and Norway spruce (Picea abies) stands. We estimated the effects of resin addition on fungal biomass and on the rates of C and N mineralisation. Addition of resin to the F/H material caused an increase in fungal biomass and C mineralisation, whereas N mineralisation decreased. Addition of resin to litter material did not significantly affect fungal biomass or C and N mineralisation. The results indicate that rather than having a fungicidal effect, resin acts as a C source that increases C mineralisation (mainly from the resin itself) and decreases net N mineralisation. The latter factor might be important in preventing plants dependent on inorganic N from invading and covering the ant mounds.

Effects of Timing of Grazing on Arthropod Communities in Semi-Natural Grasslands

Abstract Arthropod communities were investigated in two Swedish semi-natural grasslands, each subject to two types of grazing regime: conventional grazing from May to September (continuous grazing) and traditional late management from mid-July (late grazing). Pitfall traps were used to investigate abundance of carabids, spiders, and ants over the grazing season. Ant abundance was also measured by mapping nest density during three successive years. Small spiders, carabids and ants (Myrmica spp.) were more abundant in continuous grazing than in late grazing while larger spiders, carabids, and ants (Formica spp.) were more abundant in late grazing. The overall abundance of carabids was higher in continuous grazing in the early summer but higher in late grazing in the late summer. The switch of preference from continuous to late grazing coincided with the time for larvae hibernating species replacing adult hibernating. We discuss possible explanations for the observed responses in terms of effects of grazing season on a number of habitat variables for example temperature, food resources, structure of vegetation, litter layer, competition, and disturbance.

Underdispersion and overdispersion of traits in terrestrial snail communities on islands

Understanding and disentangling different processes underlying the assembly and diversity of communities remains a key challenge in ecology. Species can assemble into communities either randomly or due to deterministic processes. Deterministic assembly leads to species being more similar (underdispersed) or more different (overdispersed) in certain traits than would be expected by chance. However, the relative importance of those processes is not well understood for many organisms, including terrestrial invertebrates. Based on knowledge of a broad range of species traits, we tested for the presence of trait underdispersion (indicating dispersal or environmental filtering) and trait overdispersion (indicating niche partitioning) and their relative importance in explaining land snail community composition on lake islands. The analysis of community assembly was performed using a functional diversity index (Raos quadratic entropy) in combination with a null model approach. Regression analysis with the effect sizes of the assembly tests and environmental variables gave information on the strength of under- and overdispersion along environmental gradients. Additionally, we examined the link between community weighted mean trait values and environmental variables using a CWM-RDA. We found both trait underdispersion and trait overdispersion, but underdispersion (eight traits) was more frequently detected than overdispersion (two traits). Underdispersion was related to four environmental variables (tree cover, habitat diversity, productivity of ground vegetation, and location on an esker ridge). Our results show clear evidence for underdispersion in traits driven by environmental filtering, but no clear evidence for dispersal filtering. We did not find evidence for overdispersion of traits due to diet or body size, but overdispersion in shell shape may indicate niche differentiation between snail species driven by small-scale habitat heterogeneity. The use of species traits enabled us to identify key traits involved in snail community assembly and to detect the simultaneous occurrence of trait underdispersion and overdispersion.

Ants protect conifer seedlings from feeding damage by the pine weevil Hylobius abietis

1 Ants that protect food resources on plants may prey on (or deter) herbivores and thereby reduce damage. Red wood ants (of the Formica rufa group) are dominant ants in boreal forests of Eurasia and affect the local abundance of several herbivorous species. 2 The pine weevil Hylobius abietis (L.) is a herbivore that causes severe damage by feeding on the bark of coniferous seedlings within areas of forest regeneration. 3 We investigated whether ants can protect conifer seedlings from pine weevil feeding. In a manipulative experiment, ants were attracted to sugar baits attached to spruce seedlings and the damage caused by pine weevils was compared with control seedlings without ant‐baits. 4 The feeding‐scar area was approximately one‐third lower on the seedlings with ant‐baits compared with the controls. Besides red wood ants, Myrmica ants were also attracted in high numbers to the ant baits and the relative effects of these species are discussed. 5 The results obtained in the present study support the trophic cascade hypothesis (i.e. damage to herbivores is suppressed in the presence of predators). The decreased pine weevil feeding on the baited seedlings was probably a result of nonconsumptive interactions [i.e. the presence of (or harassment by) ants distracting pine weevils from feeding]. 6 Understanding the role of ants may have important implications for future strategies aiming to control pine weevil damage. For example, maintaining suitable conditions for ants after harvesting stands may be an environmentally friendly but currently unexploited method of for decreasing weevil damage.

Effects of grazing and ant/beetle interaction on seed production in the legume Vicia sepium in a seminatural grassland

Abstract 1. Seed production can be affected by cattle grazing, insect herbivores, and seed predators. Bush vetch, Vicia sepium L. (Fabaceae), possess extra floral nectaries (EFNs), which can attract ants that act as plant defenders.

Long-term effects of stump harvesting and site preparation on pools and fluxes of soil carbon and nitrogen in central Sweden

ABSTRACT Clear-cutting and subsequent site preparation is a common forest management practice in the Nordic countries. Slash and stumps have been suggested to replace fossil fuels in district heating plants. Stump/root removal from clear-cuts will reduce the amount of decomposable substrate in the soil and thus decrease soil CO2 emission, but stump lifting is also supposed to stimulate CO2 efflux and counteract CO2 reduction. To determine the effects of stump/root harvesting in relation to site preparation (stumps retained) on pools and fluxes of carbon (C) and nitrogen (N) after a period of 20–30 years, soil sampling was performed in four pairs of stump/root harvesting (S) and patch scarification (P) stands in central Sweden in 2009. Stump/root harvesting resulted in a significantly lower soil C pool in the humus layer. Annual heterotrophic respiration (RH) in the whole soil profile was significantly lower after the S than after the P treatment, the mean difference being 38 ± 23 g CO2–C m−2 year−1 (95% CI). This difference was larger than corresponding data from stump/root decomposition models (11–18 g CO2–C m−2 year−1), but both empirical and model data showed that stump/root harvesting will reduce heterotrophic soil CO2 emissions to the atmosphere during the actual period.