Alien Invasive Plant Effect on Soil Fauna Is Habitat Dependent

Shifts in densities of apex predators may indirectly affect fundamental ecosystem processes, such as decomposition, by altering patterns of cascading effects propagating through lower trophic levels. These top-down effects may interact with anthropogenic impacts, such as climate change, in largely unknown ways. We investigated how changes in densities of large predatory arthropods in forest leaf-litter communities altered lower trophic levels and litter decomposition. We conducted our experiment in soil communities that had experienced different levels of long-term average precipitation. We hypothesized that altering abundances of apex predators would have stronger effects on soil communities inhabiting dry forests, due to lower secondary productivity and greater resource overexploitation by lower trophic levels compared to wet forests. We experimentally manipulated abundances of the largest arthropod predators (apex predators) in field mesocosms replicated in the leaf-litter community of Iberian beech forests that differed in long-term mean annual precipitation by 25% (three dry forests with MAP<1,250mm and four wet forests with MAP>1,400mm). After one year, we assessed abundances of soil fauna in lower trophic levels and indirect impacts on leaf-litter decomposition using litter of understorey hazel, Corylus avellana. Reducing densities of large predators had a consistently negative effect on final abundances of the different trophic groups and several taxa within each group. Moreover, large predatory arthropods strongly impacted litter decomposition, and their effect interacted with the long-term annual rainfall experienced by the soil community. In the dry forests, a 50% reduction in the densities of apex predators was associated with a 50% reduction in decomposition. In wet forests, the same reduction in densities of apex soil predators did not alter the rate of litter decomposition. Our results suggest that predators may facilitate lower trophic levels by indirectly reducing competition and resource overexploitation, cascading effects that may be more pronounced in drier forests where conditions have selected for greater competitive ability and more rapid resource utilization. These findings thus provide insights into the functioning of soil invertebrate communities and their role in decomposition, as well as potential consequences of soil community responses to climate change.

Soil food web stability in response to grazing in a semi-arid prairie: The importance of soil textural heterogeneity

Despite increasing frequency of invasions by alien plant species with widespread ecological and economic consequences, it remains unclear how belowground compartments of ecosystems are impacted. In order to synthetize current knowledge and provide future directions for research we performed a meta‐analysis assessing the impact of invasive alien plant species on soil fauna abundance. Compared to previous synthesis on this topic, we included together in our model the trophic group of each soil faunal taxa (from herbivores to predators) and habitat structure, namely open and closed habitats (i.e. grass and shrub dominated areas versus forested areas). In doing so, we highlighted that both moderators strongly interact to determine the response of soil fauna to the presence of invasive alien plants. Soil fauna abundance increase in the presence of invasive species only in closed habitats (+18.2%). This pattern of habitat‐dependent response (positive effect in closed habitats) was only found for primary consumers (i.e. herbivores +29.1% and detritivores +66.7%) within both detritus‐based and live root‐based trophic pathways. Abundances of predators and microbivores did not respond to the presence of IAS irrespective of habitat structure. For several groups, the habitat structure (open or closed) significantly drove their responses to the presence of invasive alien species. In addition, we carefully considered potential sources of bias (e.g. geographic, taxonomic and functional) within the collected data in an attempt to highlight gaps in available knowledge on the subject. Our findings support the conclusions of previous studies on the subject by demonstrating 1) that soil fauna abundance is impacted by biological invasions, 2) that initial habitat structure has a strong influence on the outcome and 3) that responses within the soil fauna differ between trophic levels with a stronger response of primary consumers.

Plant functional trait identity and diversity effects on soil meso- and macrofauna in an experimental grassland

Engineered biochar effects on soil physicochemical properties and biota communities: A critical review

Question Does a reduction in size class of soil biota aggravate the impacts of soil compaction on plant communities? Location Controlled mesocosm study with a European ruderal plant community exposed to soil compaction and soil biota communities of different size classes. Methods We evaluated the individual and combined effects of soil compaction and soil biota communities of different size classes (<2000, <212, <20 μm, natural control) on the structure and productivity of a ruderal plant community, and the establishment of a neophyte (Lupinus polyphyllus) in a full-factorial mesocosm study. Results Soil compaction reduced the productivity of the plant community, but the negative effect decreased in magnitude as the size class of the soil biota community was reduced. Consistently, shoot growth of the dominant grasses was most reduced by soil compaction when bigger soil biota were present; the shoot growth of forbs showed an opposite pattern. In contrast, legumes seem to profit from soil compaction independent of the size class of the soil biota community, and were detrimentally affected only in the natural control soil. Establishment of the neophyte L. polyphyllus was best under natural soil conditions and was not affected by soil compaction. Conclusions A reduction in soil biota size due to intensified management may change the impacts of soil compaction on plant communities. The results indicate that soil compaction has the most detrimental effect on primary productivity in soil communities where a range of size classes of soil biota is present and the functional complexity of the soil biota community is therefore high.

Soil nitrogen (N) and phosphorus (P) contents, and soil acidification have greatly increased in grassland ecosystems due to increased industrial and agricultural activities. As major environmental and economic concerns worldwide, nutrient enrichment and soil acidification can lead to substantial changes in the diversity and structure of plant and soil communities. Although the separate effects of N and P enrichment on soil food webs have been assessed across different ecosystems, the combined effects of N and P enrichment on multiple trophic levels in soil food webs have not been studied in semiarid grasslands experiencing soil acidification. Here we conducted a short-term N and P enrichment experiment in non-acidified and acidified soil in a semiarid grassland on the Mongolian Plateau. We found that net primary productivity was not affected by N or P enrichment alone in either non-acidified or acidified soil, but was increased by combined N and P enrichment in both non-acidified and acidified soil. Nutrient enrichment decreased the biomass of most microbial groups in non-acidified soil (the decrease tended to be greatest with combined N and P enrichment) but not in acidified soil, and did not affect most soil nematode variables in non-acidified or acidified soil. Nutrient enrichment also changed plant and microbial community structure in non-acidified but not in acidified soil, and had no effect on nematode community structure in non-acidified or acidified soil. These results indicate that the responses to short-term nutrient enrichment were weaker for higher trophic groups (nematodes) than for lower trophic groups (microorganisms) and primary producers (plants). The findings increase our understanding of the effects of nutrient enrichment on multiple trophic levels of soil food webs, and highlight that soil acidification, as an anthropogenic stressor, reduced the responses of plants and soil food webs to nutrient enrichment and weakened plant-soil interactions.

Effect of Good Agricultural Practices under no-till on litter and soil invertebrates in areas with different soil types

Agricultural intensification is one of the greatest threats to soil biota and function. In contrast, set-aside still remains a management practice in certain agri-environmental schemes. In Hungary, the establishment of sown set-aside fields is a requirement of agri-environmental schemes in High Nature Value Areas. We tested the effects of set-aside management on soil biota (bacteria, microarthropods, woodlice and millipedes), soil properties and organic matter decomposition after an initial establishment period of two years. Cereal – set-aside field pairs, semi-natural grasslands and cereal fields were sampled in the Heves Plain High Nature Value Area in Eastern Hungary, in May 2014. Topsoil samples were taken from each site for physical, chemical, microbial analyses and for extraction of soil microarthropods. Macrodecomposers were sampled by pitfall traps for two weeks. The biological quality of soil was estimated by the integrated QBS index (‘‘Qualità Biologica del Suolo’’, meaning ‘‘Biological Quality of Soil’’) based on diversity of soil microarthropods. To follow early stage organic matter decomposition, we used tea bags filled with a site-independent, universal plant material (Aspalathuslinearis, average mass 1.26 ± 0.03 g). Tea bags were retrieved after 1 month to estimate the rate of mass loss. We found significant differences between habitat types regarding several soil physical and chemical parameters (soil pH, K and Na content). The study showed positive effects of set-aside management on soil biodiversity, especially for microarthropods and isopods. However, we did not experience similar trends in relation to soil bacteria and millipedes. There was higher intensity of organic matter decomposition in soils of set-aside fields and semi-natural grasslands (remaining mass on average: 74.17% and 76.6%, respectively) compared to cereal fields (average remaining mass: 81.3%). Out of the biotic components, only the biological quality of soil significantly influenced (even if marginally) plant tissue decomposition. Our results highlight the importance of set-aside fields as shelter habitats for soil biota, especially for arthropods. Set-aside fields that are out of a crop rotation for 2 years could be a valuable option for maintaining soil biodiversity, as these fields may simultaneously conserve elements of above- and below-ground diversity.

Stable isotope analysis is one of the most widely used techniques to estimate trophic position and provides fundamental insight into the structure and management of ecological communities. To account for the effects of geographic variation in isotope levels, trophic position is typically estimated relative to an isotope “baseline” (i.e., material representing geographic variation) using a methodology such as a formula or statistical analysis. There is, however, remarkable variation in the baselines and methodologies used to estimate trophic position from stable isotopes. The consequences of this lack of standardization are unknown but could result in biased or erroneous conclusions. We conducted a literature review to quantify the variation in baselines and methodologies used to estimate trophic position from stable isotopes. Next, we assessed the consequences of this variation on individual species estimates and food web structure by extracting published trophic positions and applying various baselines and methodologies to existing data sets. We identified 10 baselines and eight methodologies, the use of which varied by ecosystem studied. Moreover, we found that different baselines and methodologies yield significantly different trophic position estimates for individual species, as well as different conclusions about food web structure. Authors should avoid biological interpretations of absolute, stand‐alone trophic positions (as these are prone to a number of biases). We recommend pairing stable isotope analysis with other techniques for more robust conclusions. Increased sample size may mitigate some of the variation caused by different baselines and methodologies; however, an alarmingly large proportion of studies collected only one sample in at least one trophic group (41% of all reviewed studies). Authors should collect a minimum of five samples per trophic group (but ten for best practices) from as many trophic groups as possible to increase statistical power and redundancy in comparisons. When sample size is unavoidably constrained, we recommend using compound‐specific isotope analysis with a taxon‐specific trophic discrimination factor, because it may be more accurate and require fewer samples to maintain appropriate statistical power. Implementing our recommendations will increase the robustness and accuracy of conclusions based on stable isotopes, resulting in better management decisions and a more accurate understanding of ecological communities.

Linking above-ground and below-ground interactions: how plant responses to foliar herbivory influence soil organisms

BackgroundOver the past two decades many studies have demonstrated that plant species diversity promotes primary productivity and stability in grassland ecosystems. Additionally, soil community characteristics have also been shown to influence the productivity and composition of plant communities, yet little is known about whether soil communities also play a role in stabilizing the productivity of an ecosystem.Methodology/Principal FindingsHere we use microcosms to assess the effects of the presence of soil communities on plant community dynamics and stability over a one-year time span. Microcosms were filled with sterilized soil and inoculated with either unaltered field soil or field soil sterilized to eliminate the naturally occurring soil biota. Eliminating the naturally occurring soil biota not only resulted in lower plant productivity, and reduced plant species diversity, and evenness, but also destabilized the net aboveground productivity of the plant communities over time, which was largely driven by changes in abundance of the dominant grass Lolium perenne. In contrast, the grass and legumes contributed more to net aboveground productivity of the plant communities in microcosms where soil biota had been inoculated. Additionally, the forbs exhibited compensatory dynamics with grasses and legumes, thus lowering temporal variation in productivity in microcosms that received the unaltered soil inocula. Overall, asynchrony among plant species was higher in microcosms where an unaltered soil community had been inoculated, which lead to higher temporal stability in community productivity.Conclusions/SignificanceOur results suggest that soil communities increase plant species asynchrony and stabilize plant community productivity by equalizing the performance among competing plant species through potential antagonistic and facilitative effects on individual plant species.

Habitat loss and degradation are key drivers of the current biodiversity crisis. Most research focuses on the question of which traits allow species to persist in degraded habitats. We asked whether a species' trophic position or niche width influences the resilience of species in degraded habitats and to what extent habitat degradation affects trophic interactions between species. We used nitrogen isotope ratios (15N:14N, expressed as δ15N value) to quantify and compare trophic positions and niche widths of understory birds inhabiting old‐growth and young secondary forests in the Pacific lowlands of Costa Rica. We found that a species' trophic position rather than its trophic niche width determined its persistence in secondary forests. Species feeding at lower trophic levels in old‐growth forests were less likely to persist in secondary forests than those occupying a higher trophic position in old‐growth forests. This pattern is likely induced by the occurrence of relatively large‐bodied habitat specialists with a flexible and high‐trophic level diet in secondary forests. These habitat specialists likely caused generalist bird species to lower their trophic position relative to conspecifics in old‐growth forests. Regarding trophic niche widths, species in secondary forests tend to have larger niche widths than old‐growth forest species. However, as old‐growth forest specialists and generalists did not differ in their niche widths, no systematic effect of trophic niche width on species persistence after forest disturbance was found. This is the first study that shows a systematic effect of trophic position on the persistence of a wide range of bird species in a disturbed forest ecosystem. It therefore provides important insights into species' responses to habitat degradation and the conservation value of secondary forests. To improve habitat quality for old‐growth forest birds and facilitate avian seed dispersal, the creation of large contiguous forest patches should be prioritised when implementing reforestation measures.

Omnivores are generally believed to be flexible in their diet and trophic position: seasonal, ontogenetic and site‐based differences in trophic position have been observed. We compared consumed and assimilated diet among four species within a group of omnivorous freshwater crayfish, to determine whether species that occur together at a site occupy different trophic positions. Diets of Geocharax falcata, Gramastacus insolitus, Cherax destructor and Euastacus bispinosus (Decapoda: Parastacidae) were compared using stable isotopes (δ13C and δ15N) and gut content analysis across nine sites that varied in their species composition. Gramastacus insolitus consumed mainly plant material across all sites. Geocharax falcata consumed either plants or animals or both at different sites. Its trophic level was consistently similar to G. insolitus, despite differences in gut contents and source for dietary carbon. Cherax destructor consumed animals and had a relatively stable trophic position among sites. Relative trophic position of these three species was consistent across sites and regardless of food consumed, they were positioned as omnivores at a lower trophic level than predators but higher than primary producers and herbivores. Euastacus bispinosus occupied a higher trophic level than other invertebrate species but δ13C levels did not differ among sites. Cherax destructor and G. falcata may show flexibility in food sources and in the assimilation of food that determines their trophic position relative to other crayfish species. In contrast, G. insolitus and E. bispinosus are likely to show both a more fixed diet and less flexible trophic position. Therefore, not all omnivores show the flexible diet and trophic position generally reported in the literature. Some species of omnivorous crayfish may maintain a relatively constant trophic position across sites, seasons or changes in food availability regardless of whether their consumed diet alters or not.

False indigo (Amorpha fruticosa L.) is an invasive exotic plant introduced to Europe in the early eighteenth century. Its spread has been rapid, particularly in disturbed wetland habitats, where it forms dense impermeable monospecific stands and modifies habitat conditions. The impact of A. fruticosa on native plant communities has been well analyzed, however knowledge concerning the possible effects on soil invertebrates and particularly carabid beetles is completely lacking. This study analyzed the impact of an A. fruticosa invasion on carabid beetles and other soil invertebrates. Soil fauna was sampled by pitfall traps at natural habitats, initially colonized by A. fruticosa, and habitats largely invaded by A. fruticosa. In total 2,613 carabid beetles belonging to 50 species and 72,166 soil invertebrates were collected. The invasion of A. fruticosa strongly affected the carabid beetle species composition, which clearly differed between all studied sites. Widespread euritopic carabid beetle species showed positive responses to A. fruticosa invasion, while the activity density of open habitat species strongly declined. Mean individual biomass was significantly higher at invaded sites due to increased incidence of large carabids (genus Carabus Linne, 1758). Carabid beetle activity density and abundance of soil invertebrates were considerably higher at invaded sites than in natural sites. Conversely, the impact of A. fruticosa on carabid beetle species richness and diversity was less pronounced, most likely due to immigration from adjacent habitats. Changes in carabid beetle species composition and abundance of soil invertebrates were most likely due to changes in vegetation structure and microclimate. The results suggest that A. fruticosa invasion considerably affected carabid beetles, an insect group that is only indirectly related to plant composition. Therefore, severe future changes can be expected in invertebrate groups that are closely related to plant composition, since A. fruticosa cannot be completely removed from the habitat and covers relatively large areas.