Effects Of Species Identity Research Articles

Altitude and species identity drive leaf litter decomposition rates of ten species on a 2950 m altitudinal gradient in Neotropical rain forests

AbstractIdentifying the environmental factors controlling litter decomposition is key to understanding the magnitude and rates of nutrient cycling in tropical forests, and how they may be influenced by climate variability and environmental change. We carried out a leaf litter translocation experiment in mature rain forest over a 2,520 m altitudinal gradient in Costa Rica. Leaf litter decomposition rates (k) of ten tree species, two dominant species from each ecosystem, plus two standard species, were calculated over 540 days in four life zones. k was lowest in montane with 0.83 per year and lower montane forests with 2.21 per year. k did not differ between lowland and premontane forests at 3.12 per year, in spite of the 3℃ difference of mean annual temperature between these life zones. k varied fourfold among species. Species decomposition rates ranked as follows, and were predictably related to leaf economic spectrum traits of the species: Acalypha communis (standard, fast decomposer)» Hyeronima oblonga > Alchornea latifolia, Quercus bumelioides, Jarava ichu (standard, slow decomposer)> Minquartia guianensis > Magnolia sororum > Vochysia allenii > Pourouma bicolor, Carapa guianensis. These two slowest‐decomposing species were native premontane and lowland forest dominants, respectively, with tough, low‐nutrient leaves. The ranking of species by k varied very little among life zones suggesting that decomposer organisms in very different ecosystems and environments react in similar ways to the litter quality in general. We conclude that while k decreases with temperature in rain forests on tropical mountains, bioclimatic zones defined as premontane may be “functionally lowland.” The effects of species identity on decomposition rates on tropical mountains are consistent and independent of environment for both standard and native species. Under climate change on these mountains, if moisture regimes do not change, decomposition rates will increase due to rising temperatures. Soil carbon storage may therefore decrease. Changes in the altitudinal distributions of currently dominant species will also affect this critically important biogeochemical process.

Ontogenetic variation in the diet of the anuran community from a semi-arid environment in the southeastern Chihuahuan Desert.

Although ontogeny influences dietary composition and trophic niche breadth in many anurans, its effects on diet have been little analyzed in sympatric species. In this study, we analyzed interspecific and ontogenetic variation in dietary composition and trophic niche width in an anuran community from a semi-arid environment. We found a more profound effect of species identity than body size on dietary composition, with the diet of four species dominated by formicids, that of two others by coleopterans and formicids, and that of the remaining species not dominated by specific prey types. We found ontogenetic changes in dietary composition in three of four species analyzed, in which consumption of some small insects decreased as predator size increased, regardless of species. Additionally, we did not find ontogenetic change in prey number consumed in any of the four species, but prey size increased with increasing predator size in all of them. Most species exhibited a narrow trophic niche, which was even narrower in adults in three of the four species analyzed. Costello’s modified plots revealed a high variation among individuals in termite consumption in Anaxyrus punctatus, and in more prey types in Spea multiplicata. Our results suggest that this community is not size-structured, and that ontogenetic diet shifts are mainly caused by passive sampling toward prey of different sizes. Finally, comparisons with previous data revealed an interpopulation pattern, in which trophic niche width contracts as aridity increases, possibly because of an increase in interspecific competition for trophic resources.

Ecosystem functioning in relation to species identity, density, and biomass in two tunneller dung beetles

1. Species abundance, biomass, and identity are the main factors that influence ecosystem functioning. Previous studies have shown that community attributes and species identity help to maintain natural ecosystem functioning.2. This study examined how species identity, biomass, and abundance in dung pats (i.e. density) of dung beetles affect multiple ecological functions: dung removal, seed dispersal, and germination. Specifically, two species of tunnellers were targeted: Onthophagus illyricus (Scopoli, 1763) and Copris lunaris (Linnaeus, 1758). In accordance with their natural abundance, densities ranging from 10 to 80 individuals were considered for O. illyricus, and those from two to eight were considered for C. lunaris, spanning the total biomass per treatment from 0.22 to 1.76 g.3. Results showed that, even at higher abundance, O. illyricus is not as efficient as C. lunaris. These results show that species identity, biomass, and density are crucial factors for maintaining ecosystem functioning. The combined effect of species identity and density/biomass facilitated dung removal and seed dispersal. Conversely, species identity is the only relevant factor for germination. Moreover, relationships among functions depend on the species investigated: C. lunaris showed a positive correlation between dung removal and seed dispersal, whereas O. illyricus showed a positive correlation between germination and dung removal.4. In conclusion, optimal ecosystem functioning depends on multiple factors, such as density and species identity, and thus also on body size, nesting strategies and ecological functions investigated. Moreover, the loss of larger and efficient species cannot be compensated by higher abundances of small species.

Consequences of consumer origin and omnivory on stability in experimental food web modules

Abstract Food web stability, a fundamental characteristic of ecosystems, is influenced by the nature and strength of species interactions. Theory posits that food webs are stabilised by omnivory and disrupted by novel consumers. To test the effects of secondary consumer origin and trophic level on basal resource stability, we constructed crayfish–snail–algae modules using four congeneric species of crayfish (Faxonius spp.), two from native populations (Faxonius propinquus and Faxonius virilis) and two from non‐native populations (Faxonius limosus and Faxonius rusticus). We performed surgical manipulations of crayfish feeding structures to create omnivore food web and predator food chain modules. We compared the temporal stability of these modules using measures of the coefficient of variation of the basal resource (benthic algae). Consistent with theoretical and empirical predictions, food web modules with omnivory had the lowest coefficient of variation. However, contrary to prediction, we did not find consistently higher coefficients of variation in modules with non‐native species. Rather, across species, we found the lowest coefficient of variation in modules with one of the non‐native species (F. rusticus) and one native species (F. virilis), owing to stronger interactions between these crayfish species and their snail and algal food resources. The results suggest that omnivory is indeed stabilising and that very weak interactions or very low attack rates of the consumer on the basal resource can be unstable. Thus, we demonstrate that omnivores may have different impacts than predators when introduced into a novel ecosystem, differences that can supersede the effect of species identity.

Effect of species identity and diversity on biomass production and its stability in cover crop mixtures

Thanks to positive interactions between species, growing mixtures of cover crops allows improving the ecosystem services provided by cover crop cultivation. In this study, the influence of species diversity but also of species identity and mixture composition on cover crop biomass production and its stability in diverse growing conditions was studied. Several field experiments (varying soil type, preceding crop, soil tillage, sowing density, nitrogen fertilization and spatial replication) were set up in Switzerland during the period 2013−2016. In these experiments, the performance of cover crop species grown as sole crops was compared to that of multispecific mixtures. Part of these experiments followed a simplex design in which four cover crop species were combined together with different proportions, producing a total of 25 mixtures of varied diversity. The other experiments compared sole crop and mixture biomass production in standard randomised block or split plot experiments.Globally, mixtures tended to produce slightly more biomass than the sole crops, with an average between 2 t/ha and 3.2 t/ha for sole crops and of about 3.5 t/ha for mixtures. Overyielding as well as transgressive overyielding were observed, in 81% and 37% of the cases on average, respectively. However no effect of the level of species diversity within mixtures could be found. Biomass production of cover crops was highly influenced by their growing conditions and by the identity of the species involved, especially for sole crops and bispecific mixtures. The analyses of the simplex experiments allowed to show that species interactions played an important role in biomass production in 7 out of 15 growing conditions, even for a short growing period of about three months. Most of the cover crop mixtures with the highest biomass production had a rather low diversity, i.e. about two species on average, but the identity of the species involved in these mixtures depended on the growing conditions. Our results do not show a strong diversity effect on the biomass production of cover crop mixtures cultivated for a short growing period, but a stronger effect of species identity and of the growing conditions. Mixtures with low diversity generally outcompete more diverse mixtures, but more diverse mixtures offer an insurance effect given the unpredictability of growing conditions during cover crop cultivation.

Plant species identity and soil characteristics determine rhizosphere soil bacteria community composition in European temperate forests.

Soil bacteria and understorey plants interact and drive forest ecosystem functioning. Yet, knowledge about biotic and abiotic factors that affect the composition of the bacterial community in the rhizosphere of understorey plants is largely lacking. Here, we assessed the effects of plant species identity (Milium effusum vs. Stachys sylvatica), rhizospheric soil characteristics, large-scale environmental conditions (temperature, precipitation and nitrogen (N) deposition), and land-use history (ancient vs. recent forests) on bacterial community composition in rhizosphere soil in temperate forests along a 1700 km latitudinal gradient in Europe. The dominant bacterial phyla in the rhizosphere soil of both plant species were Acidobacteria, Actinobacteria and Proteobacteria. Bacterial community composition differed significantly between the two plant species. Within plant species, soil chemistry was the most important factor determining soil bacterial community composition. More precisely, soil acidity correlated with the presence of multiple phyla, e.g. Acidobacteria (negatively), Chlamydiae (negatively) and Nitrospirae (positively), in both plant species. Large-scale environmental conditions were only important in S. sylvatica and land-use history was not important in either of the plant species. The observed role of understorey plant species identity and rhizosphere soil characteristics in determining soil bacterial community composition extends our understanding of plant-soil bacteria interactions in forest ecosystem functioning.

Trophic consistency of supraspecific taxa in below‐ground invertebrate communities: Comparison across lineages and taxonomic ranks

AbstractAnimals that have similar morphological traits are expected to share similar ecological niches. This statement applies to individual animals within a species and thus species often serve as the functional units in ecological studies. Species are further grouped into higher‐ranked taxonomic units based on their morphological similarity and thus are also expected to be ecologically similar. On the other hand, theory predicts that strong competition between closely related species may result in differentiation of ecological niches. Due to high diversity and limited taxonomic expertise, soil food webs are often resolved using supraspecific taxa such as families, orders or even classes as functional units.Here, we tested the trophic differentiation and consistency of supraspecific taxa across major lineages of temperate forest soil invertebrates: Annelida, Chelicerata, Myriapoda, Crustacea and Hexapoda. Published data on stable isotope compositions of carbon and nitrogen were used to infer basal resources and trophic level, and explore the relationship between taxonomic and trophic dissimilarity of local populations.Genera and families had normal and unimodal distributions of isotopic niches, suggesting that supraspecific taxa are trophically consistent. The isotopic niche of local populations varied considerably resulting in large overlap of niches among species. Within the same genus, the effect of species identity on stable isotope composition of populations was not significant in 92% of cases. More than 50% of the variability in Δ15N values (trophic level) across lineages was explained by classes and orders, while the variability in Δ13C values (basal resources) was explained mostly by families and genera. The variability in stable isotope composition in Chelicerata and Hexapoda was explained by lower taxonomic ranks than in Myriapoda.We compiled a comprehensive list of mean Δ13C and Δ15N values of invertebrate taxa from temperate forest soils allowing to refine soil food web models when measurements of trophic niches of local populations are not feasible. Supraspecific taxa are meaningful as trophic nodes in food web studies, but the consistency varies among taxa and the choice of taxonomic resolution depends on the research question; generally, identification of taxa should be more detailed in more diverse taxonomic groups.Aplain language summaryis available for this article.

Pollination by wild bees yields larger strawberries than pollination by honey bees

Abstract A diverse array of wild bee species may provide more effective pollination than the widely employed European honey bee (Apis mellifera L.). High species richness within crop pollinator assemblages has been linked to enhanced fruit and seed yields, but species richness is often confounded with abundance in studies of pollinator communities. We investigated the effects of bee diversity and species identity on pollen deposition and crop yield in the strawberry (Fragaria × ananassa) variety Jewel through a field experiment that independently manipulated the species richness and abundance of flower visitors. We used a new pollen deposition measurement technique to determine the pollen contribution of individual bees in an assemblage of flower visits. We compared the performance of wild bee species and managed honey bees, as pollinators of strawberry. We also calculated the influence of species richness, visit frequency and visitor identity on fruit mass, using the fruit that developed from each sampled flower. Species richness of flower visitors did not influence floral pollen loads or strawberry mass. Honey bees and wild bees deposited the same amount of pollen per visited flower. However, strawberries that developed from flowers visited by wild bees were heavier than flowers visited by honey bees. In addition, flowers visited by a combination of wild and honey bees produced strawberries that weighed less than flowers receiving purely WB visits. Synthesis and applications. Our findings show that honey bee pollination results in lower yields than wild bee pollination in a strawberry crop. Consequently, if managed honey bees in strawberry fields displace wild pollinators, growers may obtain suboptimal yields. Management efforts aimed at the maintenance or enhancement of wild pollinator populations may therefore be a cost‐effective way to increase both crop yield and biodiversity on strawberry farms.

Analyzing effects of intra- and interspecific competition on timber quality attributes of Fagus sylvatica L.—from quality assessments on standing trees to sawn boards

Timber quality is the main driver of timber prices and is strongly influenced by the competition a tree experiences until its day of harvest. Regulating competition is an integral part of silviculture, and therefore, deeper understanding of the competitor’s influence on timber quality is important. Since mixed forest stands and the share of broadleaved tree species have increased in the recent past because of a changed forest policy in several countries, effects of mixture types on timber quality are of increasing importance. In this study, we investigated the effects of intra- and interspecific competition on the internal timber quality of European beech (Fagus sylvatica L.). To analyze the effects of competition intensity and competitor species identity on the timber quality of 82 target beech trees, three different approaches were used: terrestrial laser scanning (TLS), a quality assessment on the standing tree by local district foresters, and a quality assessment of the sawn wood (boards) after harvesting. We investigated the relationship between external and internal quality features and additionally compared the different approaches to assess quality. We found that the present competitive situation was partly related to internal timber quality, with increasing competition leading to increased internal timber quality. We further observed more discoloration in timber of beech trees growing in mixture with other broadleaved tree species. We also showed that predicting discoloration is possible through the number of bark anomalies on the stem surface. Also, the external quality assessment of local foresters on standing trees predicted the internal timber features well. Finally, TLS appeared to be a valuable addition for assessing timber quality in situ.

Soil organic carbon stability in forests: Distinct effects of tree species identity and traits.

Rising atmospheric CO2 concentrations have increased interest in the potential for forest ecosystems and soils to act as carbon (C) sinks. While soil organic C contents often vary with tree species identity, little is known about if, and how, tree species influence the stability of C in soil. Using a 40year old common garden experiment with replicated plots of eleven temperate tree species, we investigated relationships between soil organic matter (SOM) stability in mineral soils and 17 ecological factors (including tree tissue chemistry, magnitude of organic matter inputs to the soil and their turnover, microbial community descriptors, and soil physicochemical properties). We measured five SOM stability indices, including heterotrophic respiration, C in aggregate occluded particulate organic matter (POM) and mineral associated SOM, and bulk SOM δ15 N and ∆14 C. The stability of SOM varied substantially among tree species, and this variability was independent of the amount of organic C in soils. Thus, when considering forest soils as C sinks, the stability of C stocks must be considered in addition to their size. Further, our results suggest tree species regulate soil C stability via the composition of their tissues, especially roots. Stability of SOM appeared to be greater (as indicated by higher δ15 N and reduced respiration) beneath species with higher concentrations of nitrogen and lower amounts of acid insoluble compounds in their roots, while SOM stability appeared to be lower (as indicated by higher respiration and lower proportions of C in aggregate occluded POM) beneath species with higher tissue calcium contents. The proportion of C in mineral associated SOM and bulk soil ∆14 C, though, were negligibly dependent on tree species traits, likely reflecting an insensitivity of some SOM pools to decadal scale shifts in ecological factors. Strategies aiming to increase soil C stocks may thus focus on particulate C pools, which can more easily be manipulated and are most sensitive to climate change.

Identifying the tree species compositions that maximize ecosystem functioning in European forests

Abstract Forest ecosystem functioning generally benefits from higher tree species richness, but variation within richness levels is typically large. This is mostly due to the contrasting performances of communities with different compositions. Evidence‐based understanding of composition effects on forest productivity, as well as on multiple other functions will enable forest managers to focus on the selection of species that maximize functioning, rather than on diversity per se. We used a dataset of 30 ecosystem functions measured in stands with different species richness and composition in six European forest types. First, we quantified whether the compositions that maximize annual above‐ground wood production (productivity) generally also fulfil the multiple other ecosystem functions (multifunctionality). Then, we quantified the species identity effects and strength of interspecific interactions to identify the “best” and “worst” species composition for multifunctionality. Finally, we evaluated the real‐world frequency of occurrence of best and worst mixtures, using harmonized data from multiple national forest inventories. The most productive tree species combinations also tended to express relatively high multifunctionality, although we found a relatively wide range of compositions with high‐ or low‐average multifunctionality for the same level of productivity. Monocultures were distributed among the highest as well as the lowest performing compositions. The variation in functioning between compositions was generally driven by differences in the performance of the component species and, to a lesser extent, by particular interspecific interactions. Finally, we found that the most frequent species compositions in inventory data were monospecific stands and that the most common compositions showed below‐average multifunctionality and productivity. Synthesis and applications. Species identity and composition effects are essential to the development of high‐performing production systems, for instance in forestry and agriculture. They therefore deserve great attention in the analysis and design of functional biodiversity studies if the aim is to inform ecosystem management. A management focus on tree productivity does not necessarily trade‐off against other ecosystem functions; high productivity and multifunctionality can be combined with an informed selection of tree species and species combinations.

Resource stoichiometry shapes community invasion resistance via productivity-mediated species identity effects.

Diversity-invasion resistance relationships are often variable and sensitive to environmental conditions such as resource availability. Resource stoichiometry, the relative concentration of different elements in the environment, has been shown to have strong effects on the physiology and interactions between different species. Yet, its role for diversity-invasion resistance relationships is still poorly understood. Here, we explored how the ratio of nitrogen (N) and phosphorus affects the productivity and invasion resistance of constructed microbial communities by a plant pathogenic bacterium, Ralstonia solanacearum. We found that resource stoichiometry and species identity effects affected the invasion resistance of communities. Both high N concentration and resident community diversity constrained invasions, and two resident species, in particular, had strong negative effects on the relative density of the invader and the resident community productivity. While resource stoichiometry did not affect the mean productivity of the resident community, it favoured the growth of two species that strongly constrained invasions turning the slope of productivity-invasion resistance relationship more negative. Together our findings suggest that alterations in resource stoichiometry can change the community resistance to invasions by having disproportionate effects on species growth, potentially explaining changes in microbial community composition under eutrophication.

Tree species identity surpasses richness in affecting soil microbial richness and community composition in subtropical forests

Plant interactions and feedbacks with soil microorganisms play an important role in sustaining the functions and stability of terrestrial ecosystems, yet the effects of tree species diversity on soil microbial community in forest ecosystems are still not well understood. Here, we examined the effects of tree species richness (1–12 species) and the presence of certain influential tree species (sampling effect) on soil bacterial and fungal communities in Chinese subtropical forests, using high-throughput Illumina sequencing for microbial identification. We observed that beta rather than alpha diversities of tree species and soil microorganisms were strong coupled. Multivariate regression and redundancy analyses revealed that the effects of tree species identity dominated over tree species richness on the diversity and composition of bacterial and fungal communities in both organic and top mineral soil horizons. Soil pH, nutrients and topography were always identified as significant predictors in the best multivariate models. Tree species have stronger effect on fungi than bacteria in organic soil, and on ectomycorrhizal fungi than saprotrophic fungi in mineral topsoil. Concluding, tree species identity, along with abiotic soil and topographical conditions, were more important factors determining the soil microbial communities in subtropical forests than tree diversity per se.

The independent and coupled effects of feedstock characteristics and reaction conditions on biocrude production by hydrothermal liquefaction

We examined the independent and coupled effects of temperature (150–350 °C), reaction time (1–100 min), slurry concentration (30 and 120 g Lrxn-1), biochemical composition (5.2–28.5 wt% lipid, 14.7–50.9 wt% protein), and species identity (Nannochloropsis, Chlorella, and Spirulina) on the yield and composition of biocrude oil produced by hydrothermal liquefaction. Measured properties included gravimetric yield, elemental (C, H, N, S, O, and P) composition and recovery, higher-heating value and energy recovery, and fatty-acid profile, content, and recovery. All examined factors affect the yield and composition of the biocrude, with biochemical composition and temperature exhibiting the greatest impacts. We probed the effects of slurry concentration and species identity over numerous combinations of temperature, reaction time, and biochemical composition that were previously unexamined, demonstrating the effects of both slurry concentration and species identity to be of the same order of magnitude as reaction time. Increased slurry concentration appears to promote Maillard reactions that result in increased biocrude yield, C content, and N content and decreased O content. Moreover, the extent of these Maillard reactions may be affected by the ratio of proteins to carbohydrates, with carbohydrates serving as the limiting reactant. High-lipid, 30 g Lrxn-1 slurries reacted at 300 °C for 3.2 min (including 1 min heat-up) generally yielded more biocrude with higher C and H content and lower N, S, and O content than did their high-protein, 120 g Lrxn-1, 200 °C, or 31.6 min counterparts. This condition also provided recoveries of saturated, monounsaturated, and polyunsaturated fatty acids in the biocrude of up to 89.3, 80.1, and 64.7 wt%, respectively, demonstrating for the first time that fast hydrothermal liquefaction can be an effective means of recovering high-value unsaturated fatty acids. The results and expansive experimental data herein provide a deeper level of understanding for microalgal hydrothermal liquefaction, enabling a greater extent of reaction engineering for the process than previously possible.

The Effect of nest Size and Species Identity on Plant Selection in Acromyrmex Leaf-Cutting Ants

Leaf-cutting ants are key organisms because their role as primary consumers and potential agricultural pests. However, their foraging ecology was mostly studied as response of extrinsic factors such as climate and plant species traits. We evaluated the effects of intrinsic factors (i. e., nest size and species identity) on the foraging behavior in two species of Acromyrmex Mayr leaf-cutting ants. While large and small nests of A. lobicornis Emery showed similar feeding behavior, small nests of A. striatus Roger harvested a greater percent of the plant species available in their foraging area and showed a higher level of selectivity than larger nests. We discussed some probable reasons for this pattern. Our results highlight the relevance of intrinsic factors and species characteristics to explain changes in the foraging ecology of leaf-cutting ants as colony growth.

Species interactions between forbs and grass-clover contribute to yield gains and weed suppression in forage grassland mixtures

Grass-legume mixtures characterized by high productivity, efficient nitrogen (N) use and strong weed suppression were proposed to increase sustainability of grassland production. Adding competitive forbs to grass-clover mixtures can be expected to further enhance the high productivity of grass-clover mixtures. This study investigates whether and to what extent adding forbs to a grass-clover mixture results in yield gains, and estimates the contributions of forb species (identity effect) and interactions with the grass-clover mixture (diversity effect) to these gains. It further examines the effectiveness of weed suppression in forb-containing grass-clover mixtures. Fifteen pure and mixed stands, all comprising the grass Lolium perenne L. and the legume Trifolium pratense L., and one or three competitive forb species (Cichorium intybus L., Carum carvi L., Plantago lanceolata L.) were established in a two-year field experiment and fertilised at two levels of N application. The diversity effect between P. lanceolata and the L. perenne-T. pratense mixture and a strong identity effect of P. lanceolata jointly contributed to yield gains of 10–21% over the binary L. perenne-T. pratense reference mixture for a wide range of P. lanceolata proportions across years and N fertilisation levels. In contrast, comparably smaller diversity and/or identity effects of C. intybus and C. carvi resulted in maximal yield gains of 5% and 7%, respectively, occurring at a narrower proportional range, but also in yield losses at high forb proportion. Including up to 80% forb in the L. perenne-T. pratense mixture effectively maintained weed suppression, but the degree of weed suppression decreased with increasing forb proportion in the following year. Including selected forbs, especially P. lanceolata, in productive grass-clover mixtures is promising for further enhancing productivity, meanwhile effectively suppressing short-term weed growth in intensively managed grasslands.

Rhizobacterial communities of five co-occurring desert halophytes.

BackgroundRecently, researches have begun to investigate the microbial communities associated with halophytes. Both rhizobacterial community composition and the environmental drivers of community assembly have been addressed. However, few studies have explored the structure of rhizobacterial communities associated with halophytic plants that are co-occurring in arid, salinized areas.MethodsFive halophytes were selected for study: these co-occurred in saline soils in the Ebinur Lake Nature Reserve, located at the western margin of the Gurbantunggut Desert of Northwestern China. Halophyte-associated bacterial communities were sampled, and the bacterial 16S rDNA V3–V4 region amplified and sequenced using the Illumina Miseq platform. The bacterial community diversity and structure were compared between the rhizosphere and bulk soils, as well as among the rhizosphere samples. The effects of plant species identity and soil properties on the bacterial communities were also analyzed.ResultsSignificant differences were observed between the rhizosphere and bulk soil bacterial communities. Diversity was higher in the rhizosphere than in the bulk soils. Abundant taxonomic groups (from phylum to genus) in the rhizosphere were much more diverse than in bulk soils. Proteobacteria, Firmicutes, Actinobacteria, Bacteroidetes and Planctomycetes were the most abundant phyla in the rhizosphere, while Proteobacteria and Firmicutes were common in bulk soils. Overall, the bacterial community composition were not significantly differentiated between the bulk soils of the five plants, but community diversity and structure differed significantly in the rhizosphere. The diversity of Halostachys caspica, Halocnemum strobilaceum and Kalidium foliatum associated bacterial communities was lower than that of Limonium gmelinii and Lycium ruthenicum communities. Furthermore, the composition of the bacterial communities of Halostachys caspica and Halocnemum strobilaceum was very different from those of Limonium gmelinii and Lycium ruthenicum. The diversity and community structure were influenced by soil EC, pH and nutrient content (TOC, SOM, TON and AP); of these, the effects of EC on bacterial community composition were less important than those of soil nutrients.DiscussionHalophytic plant species played an important role in shaping associated rhizosphere bacterial communities. When salinity levels were constant, soil nutrients emerged as key factors structuring bacterial communities, while EC played only a minor role. Pairwise differences among the rhizobacterial communities associated with different plant species were not significant, despite some evidence of differentiation. Further studies involving more halophyte species, and individuals per species, are necessary to elucidate plant species identity effects on the rhizosphere for co-occurring halophytes.

Active and species-specific dispersal behaviour in a marine nematode cryptic species complex

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 600:71-83 (2018) - DOI: https://doi.org/10.3354/meps12658 Active and species-specific dispersal behaviour in a marine nematode cryptic species complex N. De Meester1,2,*, T. Van Daele1, J. Van Malderen1, L. Monteiro1, C. Van Colen1, S. Derycke1,2,3, T. Moens1 1Marine Biology Unit, Department of Biology, Ghent University, Gent 9000, Belgium 2Center for Molecular Phylogeny and Evolution, Ghent University, Gent 9000, Belgium 3Aquatic Environment and Quality, Research Institute for Agriculture, Fisheries and Food, Oostende 8400, Belgium *Corresponding author: nele.demeester@ugent.be ABSTRACT: Dispersal is a 3-step process, consisting of a departure, transience and settlement step. In marine nematodes, dispersal is expected to be passive because nematodes lack an active planktonic phase. The transoceanic distribution of several species suggests effective large-scale dispersal. Nevertheless, it remains unclear whether nematodes can actively influence their dispersal and whether species-specific differences exist. Such differences could contribute to the commonly observed co-existence of ecologically similar species. Three separate experiments were conducted on 4 species of the cryptic species complex of Litoditis marina (Pm I, Pm II, Pm III and Pm IV) to test the effect of species identity and behaviour in the dispersal steps. A flume experiment demonstrated that nematode emergence increased with increasing flow speed and that they could also actively emerge from decomposing algae (departure step). Movement analysis of nematodes in seawater highlighted behaviour which contributes to the transience step. The settlement experiment demonstrated that L. marina can choose on which substratum to settle in still water. Species-specific differences were found in the departure and transience steps. In the departure step, Pm IV exhibited more active behaviour than Pm I. In the transience step, Pm II and Pm IV showed the highest body bend frequency. These species-specific differences correspond with geographical distribution patterns, which are smaller in the species with weaker dispersal behaviour, and lend support to the idea that dispersal can facilitate coexistence of closely related species through a trade-off between competition and dispersal. KEY WORDS: Active dispersal · Cryptic species · Nematodes Full text in pdf format PreviousNextCite this article as: De Meester N, Van Daele T, Van Malderen J, Monteiro L, Van Colen C, Derycke S, Moens T (2018) Active and species-specific dispersal behaviour in a marine nematode cryptic species complex. Mar Ecol Prog Ser 600:71-83. https://doi.org/10.3354/meps12658 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 600. Online publication date: July 30, 2018 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2018 Inter-Research.

Grass competition overwhelms effects of herbivores and precipitation on early tree establishment in Serengeti

Abstract Savanna ecosystems span a diverse range of climates, edaphic conditions, and disturbance regimes, the complexity of which has stimulated long‐standing interest in the mechanisms that maintain tree–grass coexistence. One hypothesis suggests that tree establishment is strongly limited by one or several demographic bottlenecks at early stages of the tree life cycle. A major impediment to testing this hypothesis is the lack of data on the relative strengths of different bottlenecks across key environmental gradients. To identify demographic bottlenecks that limit early tree establishment (0–18 months), we conducted a series of transplant experiments with two savanna trees species (Acacia robusta and Acacia tortilis) across a natural rainfall and soil fertility gradient in the Serengeti ecosystem, Tanzania. We tested the interactive effects of precipitation, herbivory, seed scarification, grass competition, water limitation, and tree species identity on two key life stages: germination and early seedling survival (0–2 months), and juvenile seedling survival (2–18 months). Germination and early seedling survival increased as a function of rainfall, in the absence of herbivores and when seeds were scarified. Juvenile seedling survival, in contrast, decreased with rainfall but increased in the absence of herbivores. Grass removal had the single strongest (positive) effect on juvenile seedling survival of any treatment. Soil moisture monitoring and grass‐addition treatments revealed that grasses negatively affected seedlings in ways that were not necessarily linked to soil moisture. A demographic model combining all effects across early life stages showed that the strength of grass competition on juvenile seedling survival was the key factor limiting early tree establishment. While rainfall had an unexpected opposing effect on the two life stages, the net effect of mean annual precipitation on early tree establishment was positive. Synthesis. Successful tree establishment in Serengeti is maximized by a seemingly unlikely sequence of events: (a) scarification of seeds by browsers, (b) heavy rainfall to promote germination, (c) intensive grazing (but absence of browsers), and (d) dry conditions during juvenile seedling growth (>2 months) to reduce competition with grasses. By considering a wide suite of conditions and their interactions, our experimental results are relevant to ongoing debates about savanna vegetation dynamics and structural shifts in tree:grass ratios.

Stochastic and deterministic effects on interactions between canopy and recruiting species in forest communities

Abstract Interactions between established (canopy) and recruiting individuals (recruits) are pervasive in plant communities. Studies on recruitment in forests have mainly focused on negative density‐dependent conspecific interactions, while the outcomes of heterospecific canopy–recruit interactions have received much less attention and are generally assumed to be driven by stochastic processes. Herein, we explore the relative influence of stochastic (abundance) and deterministic (species identity and phylogenetic distance) effects on the frequency of canopy–recruit interactions and characterize the interactions in terms of their spatial consistency and effect on recruitment (depressing, neutral or enhancing). In 12 plots (50 × 50 m) of mixed pine–oak forests in southern Spain, we identified all saplings recruiting beneath 56 shrub and tree species, and in open areas not covered by woody plants. We used generalized linear mixed models to investigate the influence of stochastic and deterministic processes on the frequency of canopy–recruit interactions, on their spatial consistency and their effects on recruitment, and applied neutral null models to evaluate the spatial consistency in the occurrence of interactions across plots. Deterministic and stochastic interactions were equally common, emphasizing the prevalence of non‐neutral effects. Among the realized interactions, 36.8% enhanced recruitment, 49.05% were neutral, and 14.1% depressed recruitment. Many potential interactions (42.08%) were not observed in any study sites, presumably due to the scarcity of the interacting species. Moreover, the probability that two species formed a canopy–recruit interaction, the frequency of their interaction and the probability that the interaction had an enhancing effect on recruitment, all increased with the phylogenetic distance between the interacting species. However, the prevalence of these effects depended on the recruitment environment (heterospecific, conspecific or open). Recruitment‐enhancing interactions between heterospecifics were more consistently realized in different sites than neutral or depressing interactions. The establishment of canopy–recruit interactions (which species recruits beneath which others, and how often) is not simply determined by stochastic events. Indeed, due to their prevalence, we argue that deterministic canopy–recruit interactions are important drivers of plant community dynamics. A plain language summary is available for this article.