• Energy Research

Abstract Body size is a central functional trait in ecological communities. Despite recognition of the importance of above ground–below ground interactions, effects of above‐ground herbivores on size and abundance–size relationships in soil fauna are almost uncharted. Depending on climate and soil properties, herbivores may increase basal resources of soil food webs, or reduce pore space, mechanisms expected to have contrasting effects on soil animal body size. We investigated how body size and shape of soil nematodes responded to mammalian grazers in three semi‐arid grassland sites, along a gradient of soil texture and organic matter (OM) in a long‐term herbivore removal study. We analysed nematode mass, length, diameter, body size distribution and biomass distribution. We formulated two mechanistic hypotheses to assess whether resource availability or pore space was the dominant abiotic control and modulated the effects of grazing. In ungrazed soils, average and maximum nematode size, as well as abundance and biomass of large nematodes, were greater in the high‐OM than in the low‐OM soil, and intermediate in the medium‐OM soil. Grazing promoted larger sizes in the low‐OM soil, where it had been shown to increase OM and microbial biomass, and led to more homogeneous average size and body size distribution across sites. The results support the hypothesis that nematode size was controlled by basal resource availability rather than by pore space. However, body shape might have been constrained by small pores in the fine‐texture, high‐OM soil, where nematodes were more elongated. Grazing may facilitate larger sizes in soil nematode communities by boosting basal resources where these are limiting, with important implications for estimations of nematode biomass and contribution to carbon and nutrient cycling. These findings contribute to the insofar‐limited mechanistic understanding of how herbivores can shape functional traits of soil fauna and demonstrate that animals at one trophic level may control patterns in body size and abundance–size relationships in other trophic levels without a direct predator prey or competitive linkage between them.

Nature, 572 (7768) ISSN:0028-0836 ISSN:1476-4687

AbstractThe importance of herbivore–plant and soil biota–plant interactions in terrestrial ecosystems is amply recognized, but the effects of aboveground herbivores on soil biota remain challenging to predict. To find global patterns in belowground responses to vertebrate herbivores, we performed a meta‐analysis of studies that had measured abundance or activity of soil organisms inside and outside field exclosures (areas that excluded herbivores). Responses were often controlled by climate, ecosystem type, and dominant herbivore identity. Soil microfauna and especially root‐feeding nematodes were negatively affected by herbivores in subarctic sites. In arid ecosystems, herbivore presence tended to reduce microbial biomass and nitrogen mineralization. Herbivores decreased soil respiration in subarctic ecosystems and increased it in temperate ecosystems, but had no net effect on microbial biomass or nitrogen mineralization in those ecosystems. Responses of soil fauna, microbial biomass, and nitrogen mineralization shifted from neutral to negative with increasing herbivore body size. Responses of animal decomposers tended to switch from negative to positive with increasing precipitation, but also differed among taxa, for instance Oribatida responded negatively to herbivores, whereas Collembola did not. Our findings imply that losses and gains of aboveground herbivores will interact with climate and land use changes, inducing functional shifts in soil communities. To conceptualize the mechanisms behind our findings and link them with previous theoretical frameworks, we propose two complementary approaches to predict soil biological responses to vertebrate herbivores, one focused on an herbivore body size gradient, and the other on a climate severity gradient. Major research gaps were revealed, with tropical biomes, protists, and soil macrofauna being especially overlooked.

AbstractLong‐term observations of ecological communities are necessary for generating and testing predictions of ecosystem responses to climate change. We investigated temporal trends and spatial patterns of soil fauna along similar environmental gradients in three sites of the McMurdo Dry Valleys, Antarctica, spanning two distinct climatic phases: a decadal cooling trend from the early 1990s through the austral summer of February 2001, followed by a shift to the current trend of warming summers and more frequent discrete warming events. After February 2001, we observed a decline in the dominant species (the nematodeScottnema lindsayae) and increased abundance and expanded distribution of less common taxa (rotifers, tardigrades, and other nematode species). Such diverging responses have resulted in slightly greater evenness and spatial homogeneity of taxa. However, total abundance of soil fauna appears to be declining, as positive trends of the less common species so far have not compensated for the declining numbers of the dominant species. Interannual variation in the proportion of juveniles in the dominant species was consistent across sites, whereas trends in abundance varied more. Structural equation modeling supports the hypothesis that the observed biological trends arose from dissimilar responses by dominant and less common species to pulses of water availability resulting from enhanced ice melt. No direct effects of mean summer temperature were found, but there is evidence of indirect effects via its weak but significant positive relationship with soil moisture. Our findings show that combining an understanding of species responses to environmental change with long‐term observations in the field can provide a context for validating and refining predictions of ecological trends in the abundance and diversity of soil fauna.

Background and aims - Intense rains are becoming more frequent. By causing waterlogging, they may increase soil erosion and soil surface compaction, hamper seedling establishment, and reduce plant growth. Since anecic earthworms make vertical burrows that improve water infiltration, we hypothesised that they can counteract such disturbance. Methods - In a field experiment, intact soil mesocosms with ryegrass (Lolium multiflorum), with or without introduced adult Lumbricus terrestris, underwent either a precipitation regime with two intense rain events (36 mm, at beginning and end of spring), or a control regime with the same cumulative rainfall but no intense events. Short-term response of soil moisture and lagged response of plant growth were measured, and soil macroporosity was quantified. Results - Intense rains reduced ryegrass shoot biomass (by 16–21 % on average) only in the absence of earthworms. Waterlogging duration aboveground was not affected, whereas soil moisture contents after intense rainfall tended to drop faster with earthworms present. Continuous vertical macropores were found only in the mesocosms to which earthworms had been added. The number of such macropores was 2.4 times higher under the intense precipitation regime, despite similar earthworm survival. Conclusions - We found that anecic earthworms can offset negative effects of intense rainfall on plant growth aboveground. Underlying mechanisms, such as macropore formation and enhanced nutrient cycling, are discussed. We also observed that altered precipitation patterns can modify earthworm burrowing behaviour, as earthworms had produced more burrows under the intense regime