Effects of UVB radiation on freshwater biota: a meta‐analysis

Aim Stratospheric ozone depletion and simultaneous increases in UVB radiation due to human activities have the potential to affect freshwater biota. The goal of our study is to summarize the impacts of UVB on freshwater biota by comparing the differences in the general patterns, including the directions and the magnitudes of the impacts of UVB on four major freshwater taxa (phytoplankton, zooplankton, fish and amphibians). The potential driving forces for these differences are also explored. Location Global. Methods We performed a meta‐analysis on a database consisting of 146 studies including 127 species from four taxonomic groups. We tested for the effects of taxonomic group, experimental venue, developmental stage, UVB dosage and the latitude of organism provenance. Results UVB had significant negative effects on freshwater biota from the molecular–cellular to individual–population levels. However, these effects were highly variable among the taxonomic groups. In general, zooplankton was the most negatively affected group, whereas fish and amphibians were less affected. As direct fitness components, survival and reproduction were the two responses most affected by UVB. The sensitivities of individuals to UVB at different developmental stages were different for the same taxon, while the stage‐dependent sensitivity patterns also differed among different taxa. Additionally, effects of different experimental venues, UVB dosages and latitudes of organism provenance on the effects of UVB were detected. Main conclusions Our results suggest that UVB has significant negative effects on freshwater biota. We found that the effects of UVB varied among taxonomic groups, developmental stages, experimental venues, UVB dosages and latitudes of organism provenance. The variation in sensitivity among the different taxa has important implications for ecosystem responses. Given that stratospheric ozone is unlikely to recover to the levels of the 1980s in the upcoming decades, more conservation efforts should be taken to protect freshwater habitats from further damage by UVB.     

Soil fauna responses to invasive alien plants are determined by trophic groups and habitat structure: a global meta‐analysis

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.     

Interactions between exotic invasive plants and soil microbes in the rhizosphere suggest that 'everything is not everywhere'

BACKGROUND: The study of soil biota in the context of exotic plant invasions has led to an explosion in our understanding of the ecological roles of many different groups of microbes that function in roots or at the root-soil interface. Part of this progress has been the emergence of two biogeographic patterns involving invasive plants and soil microbes. First, in their non-native ranges invasive plants commonly interact differently with the same soil microbes than native plants. Second, in their native ranges, plants that are invasive elsewhere commonly interact functionally with soil microbes differently in their home ranges than they do in their non-native ranges. These studies pose a challenge to a long-held paradigm about microbial biogeography - the idea that microbes are not limited by dispersal and are thus free from the basic taxonomic, biogeographical and evolutionary framework that characterizes all other life on Earth. As an analogy, the global distribution of animals that function as carnivores does not negate the fascinating evolutionary biogeographic patterns of carnivores. Other challenges to this notion come from new measurements of genetic differences among microbes across geographic boundaries, which also suggest that meaningful biogeographic patterns exist for microorganisms. SCOPE AND CONCLUSIONS: We expand this discussion of whether or not 'everything is everywhere' by using the inherently biogeographic context of plant invasions by reviewing the literature on interactions among invasive plants and the microorganisms in the rhizosphere. We find that these interactions can be delineated at multiple scales: from individual plants to continents. Thus the microbes that regulate major aspects of plant biology do not appear to be exempt from the fundamental evolutionary processes of geographical isolation and natural selection. At the important scales of taxonomy, ecotype and ecosystem functions, the fundamental ecology of invaders and soil microbes indicates that everything might not be everywhere.     

Single introductions of soil biota and plants generate long‐term legacies in soil and plant community assembly

Recent demonstrations of the role of plant–soil biota interactions have challenged the conventional view that vegetation changes are mainly driven by changing abiotic conditions. However, while this concept has been validated under natural conditions, our understanding of the long‐term consequences of plant–soil interactions for above‐belowground community assembly is restricted to mathematical and conceptual model projections. Here, we demonstrate experimentally that one‐time additions of soil biota and plant seeds alter soil‐borne nematode and plant community composition in semi‐natural grassland for 20 years. Over time, aboveground and belowground community composition became increasingly correlated, suggesting an increasing connectedness of soil biota and plants. We conclude that the initial composition of not only plant communities, but also soil communities has a long‐lasting impact on the trajectory of community assembly.     

When and where plant‐soil feedback may promote plant coexistence: a meta‐analysis

Plant‐soil feedback (PSF) theory provides a powerful framework for understanding plant dynamics by integrating growth assays into predictions of whether soil communities stabilise plant–plant interactions. However, we lack a comprehensive view of the likelihood of feedback‐driven coexistence, partly because of a failure to analyse pairwise PSF, the metric directly linked to plant species coexistence. Here, we determine the relative importance of plant evolutionary history, traits, and environmental factors for coexistence through PSF using a meta‐analysis of 1038 pairwise PSF measures. Consistent with eco‐evolutionary predictions, feedback is more likely to mediate coexistence for pairs of plant species (1) associating with similar guilds of mycorrhizal fungi, (2) of increasing phylogenetic distance, and (3) interacting with native microbes. We also found evidence for a primary role of pathogens in feedback‐mediated coexistence. By combining results over several independent studies, our results confirm that PSF may play a key role in plant species coexistence, species invasion, and the phylogenetic diversification of plant communities.     

Soil acidification reduces the effects of short‐term nutrient enrichment on plant and soil biota and their interactions in grasslands

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.     

Low phosphorus supply constrains plant responses to elevated CO2: A meta‐analysis

Phosphorus (P) is an essential macro‐nutrient required for plant metabolism and growth. Low P availability could potentially limit plant responses to elevated carbon dioxide (eCO₂), but consensus has yet to be reached on the extent of this limitation. Here, based on data from experiments that manipulated both CO₂ and P for young individuals of woody and non‐woody species, we present a meta‐analysis of P limitation impacts on plant growth, physiological, and morphological response to eCO₂. We show that low P availability attenuated plant photosynthetic response to eCO₂ by approximately one‐quarter, leading to a reduced, but still positive photosynthetic response to eCO₂ compared to those under high P availability. Furthermore, low P limited plant aboveground, belowground, and total biomass responses to eCO₂, by 14.7%, 14.3%, and 12.4%, respectively, equivalent to an approximate halving of the eCO₂ responses observed under high P availability. In comparison, low P availability did not significantly alter the eCO₂‐induced changes in plant tissue nutrient concentration, suggesting tissue nutrient flexibility is an important mechanism allowing biomass response to eCO₂ under low P availability. Low P significantly reduced the eCO₂‐induced increase in leaf area by 14.3%, mirroring the aboveground biomass response, but low P did not affect the eCO₂‐induced increase in root length. Woody plants exhibited stronger attenuation effect of low P on aboveground biomass response to eCO₂ than non‐woody plants, while plants with different mycorrhizal associations showed similar responses to low P and eCO₂ interaction. This meta‐analysis highlights crucial data gaps in capturing plant responses to eCO₂ and low P availability. Field‐based experiments with longer‐term exposure of both CO₂ and P manipulations are critically needed to provide ecosystem‐scale understanding. Taken together, our results provide a quantitative baseline to constrain model‐based hypotheses of plant responses to eCO₂ under P limitation, thereby improving projections of future global change impacts.     

Invasive plants have different effects on trophic structure of green and brown food webs in terrestrial ecosystems: a meta‐analysis

Although invasive plants are a major source of terrestrial ecosystem degradation worldwide, it remains unclear which trophic levels above the base of the food web are most vulnerable to plant invasions. We performed a meta‐analysis of 38 independent studies from 32 papers to examine how invasive plants alter major groupings of primary and secondary consumers in three globally distributed ecosystems: wetlands, woodlands and grasslands. Within each ecosystem we examined if green (grazing) food webs are more sensitive to plant invasions compared to brown (detrital) food webs. Invasive plants have strong negative effects on primary consumers (detritivores, bacterivores, fungivores, and/or herbivores) in woodlands and wetlands, which become less abundant in both green and brown food webs in woodlands and green webs in wetlands. Plant invasions increased abundances of secondary consumers (predators and/or parasitoids) only in woodland brown food webs and green webs in wetlands. Effects of invasive plants on grazing and detrital food webs clearly differed between ecosystems. Overall, invasive plants had the most pronounced effects on the trophic structure of wetlands and woodlands, but caused no detectable changes to grassland trophic structure.     

Responses of belowground communities to large aboveground herbivores: Meta‐analysis reveals biome‐dependent patterns and critical research gaps

The 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.     

Infochemical use and dietary specialization in parasitoids: a meta‐analysis

Many parasitoid species use olfactory cues to locate their hosts. In tritrophic systems, parasitoids of herbivores can exploit the chemical blends emitted by plants in reaction to herbivore‐induced damage, known as herbivore‐induced plant volatiles (HIPVs). In this study, we explored the specificity and innateness of parasitoid responses to HIPVs using a meta‐analysis of data from the literature. Based on the concept of dietary specialization and infochemical use, we hypothesized that (i) specialist parasitoids (i.e., with narrow host ranges) should be attracted to specific HIPV signals, whereas generalist parasitoids (i.e., with broad host ranges) should be attracted to more generic HIPV signals and (ii) specialist parasitoids should innately respond to HIPVs, whereas generalist parasitoids should have to learn to associate HIPVs with host presence. We characterized the responses of 66 parasitoid species based on published studies of parasitoid behavior. Our meta‐analysis showed that (i) as predicted, specialist parasitoids were attracted to more specific signals than were generalist parasitoids but, (ii) contrary to expectations, response innateness depended on a parasitoid's target host life stage rather than on its degree of host specialization: parasitoids of larvae were more likely to show an innate response to HIPVs than were parasitoids of adults. This result changes our understanding of dietary specialization and highlights the need for further theoretical research that will help clarify infochemical use by parasitoids.     

Animal manure application and soil organic carbon stocks: a meta‐analysis

The impact of animal manure application on soil organic carbon (SOC) stock changes is of interest for both agronomic and environmental purposes. There is a specific need to quantify SOC change for use in national greenhouse gas (GHG) emission inventories. We quantified the response of SOC stocks to manure application from a large worldwide pool of individual studies and determined the impact of explanatory factors such as climate, soil properties, land use and manure characteristics. Our study is based on a meta‐analysis of 42 research articles totaling 49 sites and 130 observations in the world. A dominant effect of cumulative manure‐C input on SOC response was observed as this factor explained at least 53% of the variability in SOC stock differences compared to mineral fertilized or unfertilized reference treatments. However, the effects of other determining factors were not evident from our data set. From the linear regression relating cumulative C inputs and SOC stock difference, a global manure‐C retention coefficient of 12% ± 4 (95% Confidence Interval, CI) could be estimated for an average study duration of 18 years. Following an approach comparable to the Intergovernmental Panel on Climate Change, we estimated a relative SOC change factor of 1.26 ± 0.14 (95% CI) which was also related to cumulative manure‐C input. Our results offer some scope for the refinement of manure retention coefficients used in crop management guidelines and for the improvement of SOC change factors for national GHG inventories by taking into account manure‐C input. Finally, this study emphasizes the need to further document the long‐term impact of manure characteristics such as animal species, especially pig and poultry, and manure management systems, in particular liquid vs. solid storage.     

Volatile communication between plants that affects herbivory: a meta‐analysis

Volatile communication between plants causing enhanced defence has been controversial. Early studies were not replicated, and influential reviews questioned the validity of the phenomenon. We collected 48 well‐replicated studies and found overall support for the hypothesis that resistance increased for individuals with damaged neighbours. Laboratory or greenhouse studies and those conducted on agricultural crops showed stronger induced resistance than field studies on undomesticated species, presumably because other variation had been reduced. A cumulative analysis revealed that early, non‐replicated studies were more variable and showed less evidence for communication. Effects of habitat and plant growth form were undetectable. In most cases, the mechanisms of resistance and alternative hypotheses were not considered. There was no indication that some response variables were more likely to produce large effects. These results indicate that plants of diverse taxonomic affinities and ecological conditions become more resistant to herbivores when exposed to volatiles from damaged neighbours.     

Soil legacy effects of plants and drought on aboveground insects in native and range-expanding plant communities

Soils contain biotic and abiotic legacies of previous conditions that may influence plant community biomass and associated aboveground biodiversity. However, little is known about the relative strengths and interactions of the various belowground legacies on aboveground plant–insect interactions. We used an outdoor mesocosm experiment to investigate the belowground legacy effects of range-expanding versus native plants, extreme drought and their interactions on plants, aphids and pollinators. We show that plant biomass was influenced more strongly by the previous plant community than by the previous summer drought. Plant communities consisted of four congeneric pairs of natives and range expanders, and their responses were not unanimous. Legacy effects affected the abundance of aphids more strongly than pollinators. We conclude that legacies can be contained as soil ‘memories’ that influence aboveground plant community interactions in the next growing season. These soil-borne ‘memories’ can be altered by climate warming-induced plant range shifts and extreme drought.     

Transgenerational effects benefit offspring across diverse environments: a meta‐analysis in plants and animals

The adaptive value of transgenerational effects (the ancestor environmental effects on offspring) in changing environments has received much attention in recent years, but the related empirical evidence remains equivocal. Here, we conducted a meta‐analysis summarising 139 experimental studies in plants and animals with 1170 effect sizes to investigate the generality of transgenerational effects across taxa, traits, and environmental contexts. It was found that transgenerational effects generally enhanced offspring performance in response to both stressful and benign conditions. The strongest effects are in annual plants and invertebrates, whereas vertebrates appear to benefit mostly under benign conditions, and perennial plants show hardly any transgenerational responses at all. These differences among taxonomic/life‐history groups possibly reflect that vertebrates can avoid stressful conditions through their mobility, and longer‐lived plants have alternative strategies. In addition to environmental contexts and taxonomic/life‐history groups, transgenerational effects also varied among traits and developmental stages of ancestors and offspring, but the effects were similarly strong across three generations of offspring. By way of a more comprehensive data set and a different effect size, our results differ from those of a recent meta‐analysis, suggesting that transgenerational effects are widespread, strong and persistent and can substantially impact the responses of plants and animals to changing environments.     

Effects of abiotic factors and species interactions on estimates of male plant function: a meta‐analysis

The majority of angiosperms are hermaphroditic with total fitness comprised of both male and female components of reproduction. However, most studies examining the effects of abiotic factors and species interactions on fitness have focussed on female reproduction, potentially biasing our understanding of the consequences of environmental factors on total fitness. Here, we use meta‐analysis to test how environmental factors affect male function. We obtained 278 effect sizes from 96 studies that measured male function responses to manipulated environmental factors. We found significant effects of abiotic factors and species interactions on estimates of male function, with responses varying depending on environmental factor identity. Male and female responses were correlated for abiotic factor manipulations, but varied based on the type of species interaction (antagonistic or mutualistic). This suggests that measuring only female function may misrepresent whole‐plant reproduction depending on context. Finally, we found differences amongst components of male function in response to environmental factors, suggesting that some male function estimates may fail to capture the effects of environmental factors on male fitness. Our results demonstrate the importance of incorporating male function into ecological and evolutionary studies to provide a more accurate understanding of the effects of environmental factors on total fitness.     

Bringing function to structure: Root–soil interactions shaping phosphatase activity throughout a soil profile in Puerto Rico

1. Large areas of highly productive tropical forests occur on weathered soils with low concentrations of available phosphorus (P). In such forests, root and microbial production of acid phosphatase enzymes capable of mineralizing organic phosphorus is considered vital to increasing available P for plant uptake.
2. We measured both root and soil phosphatase throughout depth and alongside a variety of root and soil factors to better understand the potential of roots and soil biota to increase P availability and to constrain estimates of the biochemical mineralization within ecosystem models.
3. We measured soil phosphatase down to 1 m, root phosphatase to 30 cm, and collected data on fine‐root mass density, specific root length, soil P, bulk density, and soil texture using soil cores in four tropical forests within the Luquillo Experimental Forest in Puerto Rico.
4. We found that soil phosphatase decreased with soil depth, but not root phosphatase. Furthermore, when both soil and root phosphatase were expressed per soil volume, soil phosphatase was 100‐fold higher that root phosphatase.
5. Both root and soil factors influenced soil and root phosphatase. Soil phosphatase increased with fine‐root mass density and organic P, which together explained over 50% of the variation in soil phosphatase. Over 80% of the variation in root phosphatase per unit root mass was attributed to specific root length (positive correlation) and available (resin) P (negative correlation).
6. Synthesis: Fine‐root traits and soil P data are necessary to understand and represent soil and root phosphatase activity throughout the soil column and across sites with different soil conditions and tree species. These findings can be used to parameterize or benchmark estimates of biochemical mineralization in ecosystem models that contain fine‐root biomass and soil P distributions throughout depth.