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opencc-zeroApr 2021View details →

Data from: Do plant-microbe interactions support the Stress Gradient Hypothesis?

The Stress Gradient Hypothesis (SGH), which predicts increasing ratios of facilitative:competitive interactions with increasing stress, has long been a guiding framework for conceptualizing plant-plant interactions. Recently, there has been a growing recognition of the roles of microbes in mitigating or exacerbating environmental stress for their plant hosts. As such, we might predict based on the SGH that beneficial microbial effects on plant performance should be positively associated with stress. We hypothesized that support for the SGH would also vary depending on the host plant's habitat specialization such that species that specialize in high stress habitats and thus likely coevolved with the resident microbes would exhibit stronger support for the SGH than non-specialist plant species. We further hypothesized that support for the SGH would vary with germination frequency, since boosting germination of low-frequency germinators is one effective means by which microbes can benefit plant species performance. Here, we explore whether plant-microbial interactions support the SGH, using 12 plant species native to the Florida rosemary scrub. We conducted factorial experiments that manipulated the presence of microbes in nine soils collected along an elevational stress gradient, and recorded germination frequency and biomass. Microbes increased the germination frequency of four species, all of which had relatively low germination rates. Furthermore, we found support for the SGH in nearly half of the species examined, with soil microbes facilitating germination with increasing stress for five of the 12 species tested, and none of the species exhibiting the opposite trend. Support for the SGH was not predicted by either the plant hosts' habitat specialization or germination frequency. In contrast to germination, biomass results showed little support for the SGH, with four of 12 species refuting and one species supporting SGH predictions. Taken together, our study documents that interactions between the soil microbial community and plant species along a stress gradient can support the SGH, but emphasizes that these effects are life history stage-dependent. This work also identifies a common mechanism (germination facilitation) by which microbes can benefit plant species in stressful habitats.

opencc-zeroSep 2020View details →

Contrasting biomass allocation responses across ontogeny and stress gradients reveal plant adaptations to drought and cold

How plants allocate their biomass to different organs is essential to understand plant adaptation and distribution. Overall, biomass allocation may follow fixed rules across taxa. They are also likely to exhibit substantial departure from these rules during ontogeny and in response to particular limiting factors to optimize their growth and maximize their survival. However, how plants adjust their allocation priorities depending on size and age across stress gradients remain largely unkown in wild populations. We examined ontogenetic variation in biomass allocation in Himalayan forb Potentilla pamirica across its 5250-5900 m elevation range, between populations from dry steppe, wet alpine and cold subnival zone. We tested whether biomass allocation followed optimal partitioning or fixed allometric rules using organ mass in 1019 individuals spanning 1-73 years. We found shifting biomass fractions with plant size and age, supporting the optimal partitioning theory. Young plants (<10 years) allocated similar proportions of biomass to leaves, stems, and roots, intermediate-aged plants (10-30 years) allocated more biomass to roots, while the oldest plants had 90% biomass in belowground stems. Major developmental processes including secondary thickening, branching and flowering begin 10-15 years earlier under more thermally favorable steppe conditions. Young steppe plants are larger than alpine and subnival plants, but these differences disappear in plants aged ~30, and the oldest alpine and subnival plants are larger than steppe plants. Plant age exerted significant control over biomass allocation after controlling for plant size. While in steppe plants the preference for stem biomass allocation increases with both size and age, for large alpine and subnival plants the stem prioritization decreases with age in favour of root and leaf mass fractions. We interpret the root and leaf prioritization in the oldest plants as a way to reduce carbon imbalances and the risk of frost damage to secure long life. Our analyses rejected ontogenetically fixed allometry and instead found high variation in biomass allocation depending on age, size and environment, supporting optimal partitioning theory. The uneven allocation of resources to different structures and functions during ontogenesis reflects plant adaptations to different levels of low-temperature and water stress across species elevation range.

opencc-zeroDec 2014View details →

Data from: Functional responses of multi-taxa communities to disturbance and stress gradients in a restored floodplain

1. Trait-based approaches can reveal the mechanisms through which disturbances or stress impact communities, allowing comparisons of the role of different mechanisms in shaping communities among taxonomic groups. Such information can lead to higher comparability, transferability and predictability of the outcome of restoration projects. However, multitaxa trait-based approaches were rarely used in the context of ecosystem restoration. 2. We investigated the responses to environmental gradients of seven taxa (vascular plants, staphylinid and carabid beetles, spiders, isopods, diplopods and earthworms) in a restored floodplain using a species traits approach. We assessed the impact of flood disturbances and soil hydric stress on the functional diversity (FD) and community-weighted mean (CWM) response of traits for each taxon. 3. Ordination of hydrological variables revealed two main gradients. The first was related to the spatiotemporal dynamics of flood disturbances and the second to the average changes in soil hydric conditions. 4. The analysis of CWM revealed that larger, poorly mobile species with narrow ecological tolerances were filtered by regular floods and/or changes in soil hydric conditions. 5. Functional diversity patterns differed between the two gradients: decreasing with increasing flood disturbance, but increasing along the soil hydric stress gradient. This suggests that the mechanisms shaping community composition differ between the two gradients with environmental filtering being dominant with increasing flood disturbances and competition decreasing with more soil hydric stress. 6. Synthesis and applications. Our study shows that the impact of restored flood disturbances and soil hydric stress on plant and invertebrate functional diversity and community-weighted mean can be positive, negative or more complex depending on the taxonomic group and environmental gradient considered. The patterns can to some extent be explained by the specific characteristics of each group. Larger, poorly mobile species with narrow ecological tolerances were particularly vulnerable to changes in disturbance and stress regime following floodplain restoration. These species may therefore be lost in the initial phases of restoration projects, but other more characteristic species of dynamic floodplains will be favoured. Understanding the consequences of these contrasted responses for biodiversity conservation and ecosystem functioning constitutes the next challenge for ecosystem restoration.

opencc-zeroDec 2016View details →

Data from: Biological invasion modifies the co-occurrence patterns of insects along a stress gradient

Biological invasions have become one of the most important drivers of biodiversity loss and ecosystem change world-wide. However, it is still unclear how invasions may interact with local abiotic stressors, which are expected to increase as global change intensifies. Furthermore, we know little about the response to biological invasions of insects, despite their disproportionate contribution to global animal biodiversity. The aim of the present work is to investigate the impact of an invasive aquatic insect on the co-occurrence patterns of native species of insects along a salinity gradient, and determine which assembly rules are driving these patterns. First, we characterised the habitat specialisation and functional niches of each species from physiological and biological traits, respectively, and their degree of overlap. Second, we used field data to compare the co-occurrence patterns of native and invasive species in invaded and non-invaded areas of southern Iberia and northern Morocco. Finally, we tested if habitat filtering or niche differentiation assembly rules mediate their co-occurrence. In non-invaded areas, habitat filtering drives habitat segregation of species along the salinity gradient, with a lower contribution of niche differentiation. The presence of the invasive insect modifies the distribution and co-occurrence patterns of native species. In invaded areas, niche differentiation seems to be the main mechanism to avoid competition among the invasive and native species, enabling coexistence and resource partitioning. The combined study of functional niche similarity and abiotic stressor tolerance of invasive and native species can improve our understanding of the effects of invasive species along abiotic stress gradients. This approach may increase our capacity to predict the outcomes of biological invasion in a global change context.

opencc-zeroNov 2021View details →

Environmental stress gradients regulate the relative importance of predator density- and trait-mediated indirect effects in oyster reef communities

Predators affect community structure by influencing prey density and traits, but the importance of these effects often is difficult to predict. We measured the strength of blue crab predator effects on mud crab prey consumption of juvenile oysters across a flow gradient that inflicts both physical and sensory stress to determine how the relative importance of top predator density-mediated indirect effects (DMIEs) and trait-mediated indirect effects (TMIEs) change within systems. Overall, TMIEs dominated in relatively benign flow conditions where blue crab predator cues increased oyster survivorship by reducing mud crab oyster consumption. Blue crab DMIEs became more important in high sensory stress conditions, which impaired mud crab perception of blue crab chemical cues. At high physical stress, the environment benefitted oyster survival by physically constraining mud crabs. Thus, factors that structure communities may be predicted based on an understanding of how physical and sensory performances change across environmental stress gradients.

FIG. 7 in Neutrophil: Lymphocyte Ratios as a Measure of Chronic Stress in Populations of the Hellbender (Cryptobranchus alleganiensis) across a Habitat Quality Gradient

FIG. 7. Hellbender neutrophil: lymphocyte ratio across 2017 and 2018 at Little River (LR; n ¼ 8), Tumbling Creek (TC; n ¼ 12), Hiwassee River (HR; n ¼ 11), Elk River (ER; n ¼ 3), North Fork Triplett Creek (NFTC; n ¼ 4), Little Buffalo River (LBR; n ¼ 4), Doe River (DR; n ¼ 1), and South Fork New River (SFNR; n ¼ 5). Letters indicate among-site differences, where sites that are significantly different have different letters, and error bars represent two standard deviations from the mean.

FIG. 5 in Neutrophil: Lymphocyte Ratios as a Measure of Chronic Stress in Populations of the Hellbender (Cryptobranchus alleganiensis) across a Habitat Quality Gradient

FIG. 5. Scatterplot showing regression between Hellbender neutrophil: lymphocyte ratio and water pH observed in 2018.

FIG. 4 in Neutrophil: Lymphocyte Ratios as a Measure of Chronic Stress in Populations of the Hellbender (Cryptobranchus alleganiensis) across a Habitat Quality Gradient

FIG. 4. Scatterplots showing multiple linear regression comparing Hellbender neutrophil: lymphocyte ratio and water characteristics for 2018. The first row of graphs depicts neutrophil: lymphocyte ratio on the y-axis and corresponding environmental variables on the x-axis; all other rows depict the relationship among water quality parameters assessed. R2 values are provided for each graph, the red line represents the overall trend line, and significant variables are indicated with an asterisk.

FIG. 3 in Neutrophil: Lymphocyte Ratios as a Measure of Chronic Stress in Populations of the Hellbender (Cryptobranchus alleganiensis) across a Habitat Quality Gradient

FIG. 3. Scatterplots showing regressions between Hellbender neutrophil: lymphocyte ratios and conductivity (A), pH (B), and dissolved oxygen (C), in 2017.

FIG. 1 in Neutrophil: Lymphocyte Ratios as a Measure of Chronic Stress in Populations of the Hellbender (Cryptobranchus alleganiensis) across a Habitat Quality Gradient

FIG. 1. Map of eight sites surveyed between 2017 and 2018 for Hellbenders, including North Fork Triplett Creek (NFTC) in Kentucky, Little Buffalo River (LBR), Hiwassee River (HR), Little River (LR), Doe River (DR), Tumbling Creek (TC), and Elk River (ER) in Tennessee, and South Fork New River (SFNR) in North Carolina.

opencc-zeroAug 2021View details →

Bird species co-occurrence patterns in an Alpine environment supports the stress gradient hypothesis

Understanding the relative contribution of different biotic interactions in shaping species assemblages constitutes a major goal in community ecology and consequently, multiple methods aimed at inferring the nature of these associations have emerged during the last decade. In this framework, the stress-gradient hypothesis (SGH) predicts that prevalent biotic interactions shift from competition to facilitation as abiotic stress increases (and productivity decreases). This hypothesis originally raised by plant ecologists has been barely applied to faunal communities. Here, we take advantage of 20 years of abundance data to investigate pairwise patterns in species co-occurrence in Alpine bird communities inhabiting two contrasting habitat types; forests (high-productivity) and mountain grasslands (low-productivity). We also integrate functional data with presence-absence and quantitative matrices in order to detect the signature of processes driving community assembly and test for limiting similarity. We employed a Bayesian approach, probabilistic pairwise association tests and joint Species Distribution Models; all methods revealed a higher frequency of positive interactions in mountain grasslands in agreement with what the SGH predicts. Both the frequency of positive and negative interactions remained moderately stable over the study period in both habitat types. There was no significant relationship between the degree of co-occurrence of species pairs and their functional distance in either habitat. However, when we only considered those combinations of species whose co-occurrence pattern deviated from that expected at random, we found that co-existing species are functionally more similar than those pairs that show segregated patterns in the forest assemblages. Such a relationship may arise via selective social information use and other processes including microhabitat preferences. Overall, our findings suggest that interspecific competition does not seem to be a major force driving the structure of bird assemblages in this mountain region.