Role of co-occurring competition and facilitation in plant spacing hydrodynamics in water-limited environments

Effects of competition and predation operating at individual and population levels: an overview of results from a long-term field experiment

After an overview of the discussion about the existence of intra- and interspecific competition that illustrates the contradictory opinions I conclude that long-term field experiments are needed for firm conclusions. I discuss in some detail the role of two factors that limit population size of secondary cavity nesting birds e.g. territorial behavior and adequate cavities. This is followed by an overview of experimental long-term field studies in Belgium showing that intra- and interspecific competition in a great tit-blue tit system exists. By using nestbox configurations with high densities of nestboxes that differ in the diameter of their entrance hole in replicate study plots it is possible to manipulate the breeding densities of great tit Parus major and blue tit Cyanistes caeruleus independently, thereby varying the intensity of intra- and interspecific competition between these two coexisting species. When blue tit densities are experimentally increased local recruitment of great tits increases, and adult great tit post-breeding dispersal to other study plots decreases, implying that great tits use blue tit density to evaluate habitat quality and that high blue tit density results in heterospecific attraction. The reverse is not true. An experimental increase in great tit density leading to an increase in interspecific competition in a plot where blue tit density was already high leads to a decrease in blue tit nestling mass (illustrating interspecific competition for food), but to a gradual increase in blue tit body size. Both are primarily caused by an increase in the body size of immigrants (caused by intraspecific competition for protected roosting holes) in contrast to the control plot, where neither is observed. I also summarize behavioral, ecological and possible evolutionary effects of sparrowhawks on blue tits after sparrowhawks settled in an isolated study plot halfway through the study: adult survival substantially decreased for both sexes, but more for females that laid large clutches, leading to selection for females that laid a smaller clutch. This led to a change in the reproduction/survival life-history trade-off. Adult winter weights and nestling weights decreased, and the heaviest fledglings were selected against. Furthermore the frequency of polygyny increased. The long-term experiments also document the role of the use of public information and that species that compete can be attracted to sites in which competitor density is high.

Consumer pressure and supplemental pollination mediate shrub facilitation of a native annual desert plant

Shrubs are important factors in the assembly of desert plant and animal communities. By providing shelter and resources to other plants and animals, shrubs can change plant-animal interactions including those with consumers and pollinators. Here, we test the hypothesis that shrubs facilitate the reproduction of other desert plants by influencing pollination and compensation for consumer pressure. We used the known benefactor Larrea tridentata as our focal shrub species and the flowering annual Malacothrix glabrata as a potential protege in the Mojave Desert. We tested the effects of facilitation (shrub microsite), consumer pressure (both artificial folivory and florivory), and pollination (ambient or supplemented) on flower and seed production of the annual M. glabrata. We found that floral production and seed mass were similar between microsites but that pollen was limited under shrubs in the absence of any other manipulation. Plants under shrubs produced more flowers and seeds than in the open when folivory and florivory treatments were applied. Malacothrix glabrata experienced a cost to association with L. tridentata in terms of pollen limitation but plants were better able to compensate for consumer pressure under shrubs through increased flower and seed production when damaged. Therefore, association with shrubs involves a reproductive trade-off between costs to pollination and benefits to compensation for consumer pressure.

Community plant height modulated by aridity promotes spatial vegetation patterns in Alxa plateau in Northwest China

Spatial vegetation patterns are associated with ecosystem stability and multifunctionality in drylands. Changes in patch size distributions (PSDs) are generally driven by both environmental and biological factors. However, the relationships between these factors in driving PSDs are not fully understood. We investigated 80 vegetation plots along an aridity gradient in the Alxa plateau, Northwest China. The sizes of vegetation patches were obtained from aerial images, and the heights of patch-forming species were measured in the field. Soil samples were collected on the bare ground between patches for determination of physiochemical properties. Point pattern analysis was used to infer plant-plant interactions. A model selection procedure was employed to select the best predictors for the shape of PSDs and biological factors (vegetation total cover, community plant height, and plant-plant interactions). We then used structural equation modeling to evaluate the direct and indirect effects of environmental and biological factors on the shape of PSDs. In our study area, two types of PSDs coexisted, namely those that best fit to power law distributions and those that best fit to lognormal distributions. Aridity was the main environmental factor, while community mean height and competition between plants were the main biological factors for the shape of PSDs. As aridity and community mean height increased, power law-like PSDs were exhibited, whereas competition led to deviations of PSDs from power laws. Aridity affected the shape of PSDs indirectly through changes in community mean height. Community mean height was correlated with competition, thereby indirectly affecting the shape of PSDs. Our results suggest the use of community functional traits as a link between the environment and plant-plant interactions, which may improve the understanding of the underlying mechanisms of PSD dynamics.

Uncovering patterns of freshwater positive interactions using meta-analysis: Identifying the roles of common participants, invasive species and environmental context

Positive interactions are sensitive to human activities, necessitating synthetic approaches to elucidate broad patterns and predict future changes if these interactions are altered or lost. General understanding of freshwater positive interactions has been far outpaced by knowledge of these important relationships in terrestrial and marine ecosystems. We conducted a global meta-analysis to evaluate the magnitude of positive interactions across freshwater habitats. In 340 studies, we found substantial positive effects, with facilitators increasing beneficiaries by, on average, 81% across all taxa and response variables. Mollusks in particular were commonly studied as both facilitators and beneficiaries. Amphibians were one group benefiting the most from positive interactions, yet few studies investigated amphibians. Invasive facilitators had stronger positive effects on beneficiaries than non-invasive facilitators. We compared positive effects between high- and low-stress conditions and found no difference in the magnitude of benefit in the subset of studies that manipulated stressors. Future areas of research include understudied facilitators and beneficiaries, the stress gradient hypothesis, patterns across space or time and the influence of declining taxa whose elimination would jeopardise fragile positive interaction networks. Freshwater positive interactions occur among a wide range of taxa, influence populations, communities and ecosystem processes and deserve further exploration.

Toward spatio-temporal delineation of positive interactions in ecology

Given unprecedented rates of biodiversity loss, there is an urgency to better understand the ecological consequences of interactions among organisms that may lost or altered. Positive interactions among organisms of the same or different species that directly or indirectly improve performance of at least one participant can structure populations and communities and control ecosystem process. However, we are still in need of synthetic approaches to better understand how positive interactions scale spatio-temporally across a range of taxa and ecosystems. Here, we synthesize two complementary approaches to more rigorously describe positive interactions and their consequences among organisms, across taxa, and over spatio-temporal scales. In the first approach, which we call the mechanistic approach, we make a distinction between two principal mechanisms of facilitation-habitat modification and resource modification. Considering the differences in these two mechanisms is critical because it delineates the potential spatio-temporal bounds over which a positive interaction can occur. We offer guidance on improved sampling regimes for quantification of these mechanistic interactions and their consequences. Second, we present a trait-based approach in which traits of facilitators or traits of beneficiaries can modulate their magnitude of effect or how they respond to either of the positive interaction mechanisms, respectively. Therefore, both approaches can be integrated together by quantifying the degree to which a focal facilitator's or beneficiary's traits explain the magnitude of a positive effect in space and time. Furthermore, we demonstrate how field measurements and analytical techniques can be used to collect and analyze data to test the predictions presented herein. We conclude by discussing how these approaches can be applied to contemporary challenges in ecology, such as conservation and restoration and suggest avenues for future research.

Understanding neighborhood effects to increase restoration success of woody plant communities

Revegetation is the most common procedure in the restoration of disturbed areas; this practice usually aims at reconstructing plant communities that can last without further management. A low-cost strategy to assist these efforts is the application of ecological knowledge in the design of the restoration. Promoting ecological processes that enhance the functioning of the restored community could result in higher restoration success. Among these processes, plant-plant interactions, e.g., facilitation and competition, can play an important role, both facilitating and impeding the development of a self-sustaining plant community. Although these processes have been well-studied in nature, we rarely have sufficient knowledge about the whole plant community. To develop that knowledge, we leverage on a restoration experiment that took place after a mine toxic spill, where ~15,000 woody plants from 13 species were planted and geolocated. Species were planted in three mixtures mimicking natural communities found along soil moisture gradients (xerophyte, intermediate, and hydrophyte). Plantings also varied in density. Approximately 2,600 plants were monitored for damage status, survival, and growth, for 4 yr. We analyzed growth performance of six targeted species as a function of their damage status, planted mixture, and density. Growth was also assessed on the basis of neighboring plants, accounting for the species identity and distance to the focal plant. Results show that survival among planted species was relatively high and was mostly unaffected by mixture or density of the plantings. Only very damaged plants in one species experienced a decrease in survival with increasing density. Neighborhood effects on growth show positive, neutral, and negative interactions among the tested species; these also varied depending on the type of growth performance considered (height, crown area, diameter). The species-specific results ranged from positive to negative, varying between pair of species and growth performance metric. Results gathered from our neighborhood analyses on plant growth provide valuable information for the design of planting schemes that could enhance the performance of the target species. The methods developed can be applied to other systems and species. Given the potential impacts that facilitation and competition may have during revegetation, these interactions could be considered in restoration operations.

SERINC as a Restriction Factor to Inhibit Viral Infectivity and the Interaction with HIV

The serine incorporator 5 (SERINC5) is a recently discovered restriction factor that inhibits viral infectivity by preventing fusion. Retroviruses have developed strategies to counteract the action of SERINC5, such as the expression of proteins like negative regulatory factor (Nef), S2, and glycosylated Gag (glycoGag). These accessory proteins downregulate SERINC5 from the plasma membrane for subsequent degradation in the lysosomes. The observed variability in the action of SERINC5 suggests the participation of other elements like the envelope glycoprotein (Env) that modulates susceptibility of the virus towards SERINC5. The exact mechanism by which SERINC5 inhibits viral fusion has not yet been determined, although it has been proposed that it increases the sensitivity of the Env by exposing regions which are recognized by neutralizing antibodies. More studies are needed to understand the role of SERINC5 and to assess its utility as a therapeutic strategy.