Variation between individuals fosters regional species coexistence

Although individual‐level variation (IV) is ubiquitous in nature, it is not clear how it influences species coexistence. Theory predicts that IV will hinder coexistence but empirical studies have shown that it can facilitate, inhibit, or have a neutral effect. We use a theoretical model to explore the consequences of IV on local and regional species coexistence in the context of spatial environmental structure. Our results show that individual variation can have a positive effect on species coexistence and that this effect will critically depend on the spatial structure of such variation. IV facilitates coexistence when a negative, concave‐up relationship between individuals’ competitive response and population growth rates propagates to a disproportionate advantage for the inferior competitor, provided that each species specialises in a habitat. While greater variation in the preferred habitat generally fosters coexistence, the opposite is true for non‐preferred habitats. Our results reconcile theory with empirical findings.     

Generality in multispecies responses to ocean acidification revealed through multiple hypothesis testing

Decades of research have demonstrated that many calcifying species are negatively affected by ocean acidification, a major anthropogenic threat in marine ecosystems. However, even closely related species may exhibit different responses to ocean acidification and less is known about the drivers that shape such variation in different species. Here, we examine the drivers of physiological performance under ocean acidification in a group of five species of turf‐forming coralline algae. Specifically, quantitating the relative weight of evidence for each of ten hypotheses, we show that variation in coralline calcification and photosynthesis was best explained by allometric traits. Across ocean acidification conditions, larger individuals (measured as noncalcified mass) had higher net calcification and photosynthesis rates. Importantly, our approach was able to not only identify the aspect of size that drove the performance of coralline algae, but also determined that responses to ocean acidification were not dependent on species identity, evolutionary relatedness, habitat, shape, or structural composition. In fact, we found that failure to test multiple, alternative hypotheses would underestimate the generality of physiological performances, leading to the conclusion that each species had different baseline performance under ocean acidification. Testing among alternative hypotheses is an essential step toward determining the generalizability of experiments across taxa and identifying common drivers of species responses to global change.     

PHYSIOLOGICAL SNAPSHOTS REFLECT ECOLOGICAL PERFORMANCE OF THE SEA PALM,POSTELSIA PALMAEFORMIS (PHAEOPHYCAEA) ACROSS INTERTIDAL ELEVATION AND EXPOSURE GRADIENTS1

Postelsia palmaeformis Ruprecht is an intertidal kelp found only on very wave‐exposed rocky shores of the northeast Pacific. In areas dominated by mussels, Postelsia depends on wave‐induced disturbances to complete its life‐history cycle. Postelsia also recruits where mussels are absent, but not at less wave‐exposed shores. Thus, physical conditions related to wave exposure limit its horizontal distribution. It is not clear what limits the vertical distribution of Postelsia. We investigated factors contributing to Postelsia's limited distribution using transplant experiments, demographic monitoring, and field fluorometry to evaluate growth and performance across gradients of tidal elevation and wave exposure. Survivorship and growth were sharply reduced at upper and wave‐protected edges relative to mid‐level, wave‐exposed sporophytes. Reproductive output was reduced at upper and lower levels, and growth but not survivorship was lower at the lower level. Effects were independent of population of origin and were a manifestation of the environment. Maximum electron transport rates (ETR_m), light saturation parameters (E_k), and maximum quantum yields (ΔF/F_m) provided insight into physiological dynamics; all were lowest at the high edge, but increased when desiccation stress was alleviated by a mock sea‐spray treatment. The ETR_m and E_k values of low sporophytes were not as high as the values for mid‐sporophytes, despite higher or equivalent nitrogen content, chl a, and absorptance, suggesting a trade‐off between light‐capturing and carbon‐fixation capacity. Physiological limitations at upper and lower levels and deleterious desiccation effects at wave‐protected sites prevent establishment, thus constraining Postelsia to a mid‐zone, wave‐exposed distribution. Physical conditions related to wave exposure may limit the horizontal distribution of Postelsia because this kelp is also found in areas where mussels are lacking but not on less wave‐exposed shores.     

Cross-habitat effects shape the ecosystem consequences of co-invasion by a pelagic and a benthic consumer

Invasive species can have major impacts on ecosystems, yet little work has addressed the combined effects of multiple invaders that exploit different habitats. Two common invaders in aquatic systems are pelagic fishes and crayfishes. Pelagic-oriented fish effects are typically strong on the pelagic food web, whereas crayfish effects are strong on the benthic food web. Thus, co-invasion may generate strong ecological responses in both habitats. We tested the effects of co-invasion on experimental pond ecosystems using two widespread invasive species, one pelagic (western mosquitofish) and one benthic (red swamp crayfish). As expected, mosquitofish had strong effects on the pelagic food web, reducing the abundance of Daphnia and causing a strong trophic cascade (increase in phytoplankton). Crayfish had strong effects on the benthic food web, reducing the abundance of benthic filamentous algae. Yet, we also found evidence for important cross-habitat effects. Mosquitofish treatments reduced the biomass of benthic filamentous algae, and crayfish treatments increased Daphnia and phytoplankton abundance. Combined effects of mosquitofish and crayfish were primarily positively or negatively additive, and completely offsetting for some responses, including gross primary production (GPP). Though co-invasion did not affect GPP, it strongly shifted primary production from the benthos into the water column. Effects on snail abundance revealed an interaction; snail abundance decreased only in the presence of both invaders. These results suggest that cross-habitat effects of co-invaders may lead to a variety of ecological outcomes; some of which may be unpredictable based on an understanding of each invader alone.     

Maternally and Naturally Acquired Antibodies to Shiga Toxins in a Cohort of Calves Shedding Shiga-Toxigenic Escherichia coli

Calves become infected with Shiga toxin-producing Escherichia coli (STEC) early in life, which frequently results in long-term shedding of the zoonotic pathogen. Little is known about the animals'' immunological status at the time of infection. We assessed the quantity and dynamics of maternal and acquired antibodies to Shiga toxins (Stx1 and Stx2), the principal STEC virulence factors, in a cohort of 27 calves. Fecal and serum samples were taken repeatedly from birth until the 24th week of age. Sera, milk, and colostrums of dams were also assessed. STEC shedding was confirmed by detection of stx in fecal cultures. Stx1- and Stx2-specific antibodies were quantified by Vero cell neutralization assay and further analyzed by immunoblotting. By the eighth week of age, 13 and 15 calves had at least one stx₁-type and at least one stx₂-type positive culture, respectively. Eleven calves had first positive cultures only past that age. Sera and colostrums of all dams and postcolostral sera of all newborn calves contained Stx1-specific antibodies. Calf serum titers decreased rapidly within the first 6 weeks of age. Only five calves showed Stx1-specific seroconversion. Maternal and acquired Stx1-specific antibodies were mainly directed against the StxA1 subunit. Sparse Stx2-specific titers were detectable in sera and colostrums of three dams and in postcolostral sera of their calves. None of the calves developed Stx2-specific seroconversion. The results indicate that under natural conditions of exposure, first STEC infections frequently coincide with an absence of maternal and acquired Stx-specific antibodies in the animals'' sera.Shiga toxin-producing Escherichia coli (STEC), also known as enterohemorrhagic E. coli (EHEC), is a food-borne pathogen which can evoke life-threatening diseases, such as hemorrhagic colitis and hemolytic-uremic syndrome, in humans (26). Cattle and other ruminants are primary reservoirs for STEC serotypes that are typically associated with human disease, e.g., O157:H7. Calves become infected with STEC early in life via horizontal or vertical transmission (55) and do not develop clinical signs of infection but may shed the bacteria for several months and in great quantities (15, 64). Reduction of persistent STEC shedding in cattle would contribute greatly to preventing human STEC infections.Evidence that vaccination may be a sensible control option has come from studies in which cattle shed E. coli O157 less frequently following immunization with STEC O157:H7 antigens (48). However, several other studies deploying various STEC antigens produced conflicting data regarding the efficacy of vaccines to reduce or prevent STEC shedding by cattle (16, 61). Identification of candidate antigens is hampered by the limited knowledge of the immune responses occurring after bovine STEC infections, their kinetics, and their meaning for the control of STEC shedding. Serological responses against a variety of antigens following E. coli O157 colonization have repeatedly been reported. Infected animals frequently develop antibodies against STEC lipopolysaccharides (LPS), e.g., O157 LPS (25). Such antibodies inhibit STEC O157 adhesion to cells in vitro (45), but shedding is not affected by serum and mucosal O157 titers in vivo (25). Mucosal immune responses are directed mainly against membrane-associated and type III secreted STEC proteins (40). Type III secreted antigens are relatively conserved among non-O157 STEC serotypes and were assumed to be broadly cross-protective (48). Antibodies against Tir (translocated intimin receptor), intimin, and Esps (E. coli secreted proteins) A and B are detectable in calves and adult cattle after natural and experimental STEC infections or after vaccination based on these antigens (9, 16, 48, 60). Nevertheless, they do not limit the magnitude or duration of STEC shedding under field conditions (61), where cattle are confronted with a variety of different STEC strains (19, 55).Shiga toxins (Stx) are potent protein cytotoxins and represent the principal STEC virulence factors in the pathogenesis of human infections (49). Cumulating evidence shows that Stx act as immunomodulating agents during bovine STEC infections. Stx1 alters the cytokine expression pattern in mucosal macrophages (56) and intraepithelial lymphocytes (38) and suppresses the activation and proliferation of mucosal and peripheral lymphocytes in vitro (36, 37). The development of an adaptive cellular immune response is significantly delayed following experimental infection of calves with Stx2-producing STEC O157:H7 compared to that in animals inoculated with Stx-negative E. coli O157:H7 (22). In vitro and in vivo studies showed that Stx act during the early phases of immune activation rather than downregulating an established immunity (22, 57). Consequently, Stx may principally exhibit their immunomodulating activity upon first STEC infection of hitherto immunologically naïve animals.Antibodies against Stx may be essential to protect cattle from Stx-mediated immunosuppression, but only when they are present in sufficient amounts at the time of initial STEC infection. Stx-specific antibodies are detectable in sera and colostrums of naturally infected cows (6, 47). In contrast, naturally exposed calves mostly lack Stx-specific antibodies, and antibodies are barely inducible by repeated experimental STEC infections (22, 25). Maternal antibodies were considered to interfere with the development of an acquired anti-Stx immune response in calves (25), but mother-to-offspring transfer of such antibodies has not been confirmed to date. The objectives of this study were to investigate the dynamics of maternal Stx1- and Stx2-specific antibodies in calves held under conditions of natural exposure and to determine the age at the onset of acquired Stx immunity relative to the time of initial STEC infection.     

Phylogenetic Constraints Do Not Explain the Rarity of Nitrogen-Fixing Trees in Late-Successional Temperate Forests

Background

Symbiotic nitrogen (N)-fixing trees are rare in late-successional temperate forests, even though these forests are often N limited. Two hypotheses could explain this paradox. The ‘phylogenetic constraints hypothesis’ states that no late-successional tree taxa in temperate forests belong to clades that are predisposed to N fixation. Conversely, the ‘selective constraints hypothesis’ states that such taxa are present, but N-fixing symbioses would lower their fitness. Here we test the phylogenetic constraints hypothesis.

Methodology/Principal Findings

Using U.S. forest inventory data, we derived successional indices related to shade tolerance and stand age for N-fixing trees, non-fixing trees in the ‘potentially N-fixing clade’ (smallest angiosperm clade that includes all N fixers), and non-fixing trees outside this clade. We then used phylogenetically independent contrasts (PICs) to test for associations between these successional indices and N fixation. Four results stand out from our analysis of U.S. trees. First, N fixers are less shade-tolerant than non-fixers both inside and outside of the potentially N-fixing clade. Second, N fixers tend to occur in younger stands in a given geographical region than non-fixers both inside and outside of the potentially N-fixing clade. Third, the potentially N-fixing clade contains numerous late-successional non-fixers. Fourth, although the N fixation trait is evolutionarily conserved, the successional traits are relatively labile.

Conclusions/Significance

These results suggest that selective constraints, not phylogenetic constraints, explain the rarity of late-successional N-fixing trees in temperate forests. Because N-fixing trees could overcome N limitation to net primary production if they were abundant, this study helps to understand the maintenance of N limitation in temperate forests, and therefore the capacity of this biome to sequester carbon.     

Feedbacks between plant N demand and rhizosphere priming depend on type of mycorrhizal association

Ecosystem carbon (C) balance is hypothesised to be sensitive to the mycorrhizal strategies that plants use to acquire nutrients. To test this idea, we coupled an optimality‐based plant nitrogen (N) acquisition model with a microbe‐focused soil organic matter (SOM) model. The model accurately predicted rhizosphere processes and C–N dynamics across a gradient of stands varying in their relative abundance of arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) trees. When mycorrhizal dominance was switched – ECM trees dominating plots previously occupied by AM trees, and vice versa – legacy effects were apparent, with consequences for both C and N stocks in soil. Under elevated productivity, ECM trees enhanced decomposition more than AM trees via microbial priming of unprotected SOM. Collectively, our results show that ecosystem responses to global change may hinge on the balance between rhizosphere priming and SOM protection, and highlight the importance of dynamically linking plants and microbes in terrestrial biosphere models.     

Declining foliar and litter δ¹⁵N diverge from soil, epiphyte and input δ¹⁵N along a 120,000 yr temperate rainforest chronosequence

Patterns in the natural abundance of nitrogen (N) isotopes (1?N and 1?N) can help in the understanding of ecosystem processes along environmental gradients, because some processes fractionate against the heavier isotope. We measured δ1?N in many components of the Franz Josef soil chronosequence in New Zealand to see how each component varied along the sequence and within sites, and to see what this variation can tell us about how ecosystem processes such as N losses change with soil age. We analyzed δ1?N in foliage from 18 woody species, abscised leaves from seven woody species, three soil horizons, bryophytes, lichens, bulk deposition, and nodules from the N-fixing tree Coriaria arborea (Coriariaceae). Foliar δ1?N varied significantly across plant species. Foliage and bulk litter became 1?N-depleted as soil age increased. Soil N from organic and mineral horizons was significantly more 1?N-enriched than bulk litter N at each site. Increasing precipitation also decreased foliar and soil δ1?N. Comparing input and whole ecosystem δ1?N revealed limited evidence for net fractionation during N losses. These trends are consistent with some combination of increasing fractionation during plant N uptake, mycorrhizal transfer, within-plant processing, and soil decomposition as soils age.