Competitive Species Interactions Research Articles

Interconnection between density-regulation and stability in competitive ecological network

In natural ecosystems, species can be characterized by the nonlinear density-dependent self-regulation of their growth profile. Species of many taxa show a substantial density-dependent reduction for low population size. Nevertheless, many show the opposite trend; density regulation is minimal for small populations and increases significantly when the population size is near the carrying capacity. The theta-logistic growth equation can portray the intraspecific density regulation in the growth profile, theta being the density regulation parameter. In this study, we examine the role of these different growth profiles on the stability of a competitive ecological community with the help of a mathematical model of competitive species interactions. This manuscript deals with the random matrix theory to understand the stability of the classical theta-logistic models of competitive interactions. Our results suggest that having more species with strong density dependence, which self-regulate at low densities, leads to more stable communities. With this, stability also depends on the complexity of the ecological network. Species network connectance (link density) shows a consistent trend of increasing stability, whereas community size (species richness) shows a context-dependent effect. We also interpret our results from the aspect of two different life history strategies: r and K-selection. Our results show that the stability of a competitive network increases with the fraction of r-selected species in the community. Our result is robust, irrespective of different network architectures.

On the evolutionary emergence of predation

In models for the evolution of predation from initially purely competitive species interactions, the propensity of predation is most often assumed to be a direct consequence of the relative morphological and physiological traits of interacting species. Here we explore a model in which predation ability is an independently evolving phenotypic feature, so that even when the relative morphological or physiological traits allow for predation, predation only occurs if the predation ability of individuals has independently evolved to high enough values. In addition to delineating the conditions for the evolutionary emergence of predation, the model reproduces stationary and non-stationary multilevel food webs with the top predators not necessarily having size superiority.

Competitive species interactions constrain abiotic adaptation in a bacterial soil community.

Studies of abiotic adaptation often consider single species in isolation, yet natural communities contain many coexisting species which could limit or promote abiotic adaptation. Here we show, using soil bacterial communities, that evolving in the presence of a competitor constrained abiotic adaptation. Specifically, Pseudomonas fluorescens evolved alone was fitter than P. fluorescens evolved alongside Pseudomonas putida, when P. putida was absent. Genome analyses indicated this was due to mutation of the acetate scavenger actP, which occurred exclusively, and almost universally, in single‐species‐evolved clones. actP disruption was associated with increased growth in soil compared with wild‐type actP, but this benefit was abolished when P. putida was present, suggesting a role for carbon scavenging transporters in species interactions, possibly through nutrient competition. Our results show that competitive species interactions can limit the evolutionary response to abiotic selection, because the fitness benefits of abiotic adaptive mutations were negated in more complex communities.

Co-occurrence pattern of ground beetle (Coleoptera, Carabidae) assemblages along pollution gradient in scotch pine forest

Over the last 30 years there has been a great deal of interest in investigating patterns of species co-occurrence across space and time, which may be shaped by interspecific competition for shared resources. A good model of co-occurrence mechanisms is developed among predatory animals along a pollution gradient, where shared resources become more limited in more contaminated areas and the energy budget for detoxification is much higher. Community disassembly by heavy metal pollution may occur when the presence of toxic elements shifts patterns of species co-occurrence from structured to random. On the other hand, limited resources on a pollution gradient should lead to higher competition between dominant species. Disassembly may entail the loss of existing co-evolved interactions among species, which has ramifications for community dynamics and the quality of the functioning of polluted ecosystems. We expect an assemblage dominated by competitive species interactions to exhibit a significant segregation of taxa, whereas one dominated by mutualistic or syntrophic interactions would exhibit an aggregation of taxa. Responses of Carabidae co-occurrence patterns and changes in body size measures to heavy metal concentrations were investigated in a zinc contamination gradient in a Scots pine forest in the vicinity of Olkusz (southern Poland), at 12 study sites. The zinc concentration in the humus layer varied between 108 mg kg−1 dw to 6150 mg kg−1 dw. We used the C-score index, between all possible species pairs in a matrix. The ground beetle assemblages from the reference sites showed a significant segregation pattern. Community disassembly occurred only among assemblages in heavily polluted sites. The average value of skewness and kurtosis were significantly higher in the highly contaminated sites, indicating the greater proportion of small-bodied species in contaminated areas. The Gini coefficient was highest in the low contaminated sites, indicating the body-size inequality of carabid assemblages was greatest in the uncontaminated areas. Our data suggest that increased pollution contributes to the extinction of sensitive forest specialists with large body size and higher competitive abilities, leading to replacement by less sensitive generalists, with smaller body size and that the co-occurrence of species on heavily polluted sites is a result of unstable interactions between species in communities.

Spatial distribution of functional traits indicates small scale habitat filtering during early plant succession

Two major theories on community assembly (habitat filtering and limiting similarity) predict contrasting patterns in the spatial distribution of plant species. The respective distribution of species functional traits has received much less attention but is essential to understand facilitative and competitive species interactions. Here we apply a recently developed technique for phylogenetic analysis to study the spatial distribution of major functional traits during seven years of primary plant succession at a re-established catchment in a German lignite mine. Habitat filtering induced a significant temporal trend towards underdispersed functional trait diversity leading to patches of functionally similar vegetation. This functional convergence was not mirrored by a respective trend towards species aggregation. Differences in soil pH and nitrogen content were positively correlated with respective differences in trait expression. Separate analyses of trait distribution for aggregated and segregated species revealed trait convergence in co-occurring and random patterns in spatially segregated species pairs except for leaf size showing a strong temporal tendency for trait convergence in spatially segregated species. Contrasting patterns of species co-occurrence did not translate into respective contrasts in trait distribution. We interpret the prevalence of random trait associations in spatially segregated species as an indication that species occurrence patterns alone are weak predictors of assembly mechanisms. Mechanistic inference of species co-occurrence needs to be accompanied by a detailed assessment of the patterns of functional trait assembly.

Ecosystem Management Along Ephemeral Rivers: Trading Off Socio‐Economic Water Supply and Vegetation Conservation under Flood Regime Uncertainty

AbstractIn ecosystems driven by water availability, plant community dynamics depend on complex interactions between vegetation, hydrology, and human water resources use. Along ephemeral rivers—where water availability is erratic—vegetation and people are particularly vulnerable to changes in each other's water use. Sensible management requires that water supply be maintained for people, while preserving ecosystem health. Meeting such requirements is challenging because of the unpredictable water availability. We applied information gap decision theory to an ecohydrological system model of the Kuiseb River environment in Namibia. Our aim was to identify the robustness of ecosystem and water management strategies to uncertainties in future flood regimes along ephemeral rivers. We evaluated the trade‐offs between alternative performance criteria and their robustness to uncertainty to account for both (i) human demands for water supply and (ii) reducing the risk of species extinction caused by water mining. Increasing uncertainty of flood regime parameters reduced the performance under both objectives. Remarkably, the ecological objective (species coexistence) was more sensitive to uncertainty than the water supply objective. However, within each objective, the relative performance of different management strategies was insensitive to uncertainty. The ‘best’ management strategy was one that is tuned to the competitive species interactions in the Kuiseb environment. It regulates the biomass of the strongest competitor and, thus, at the same time decreases transpiration, thereby increasing groundwater storage and reducing pressure on less dominant species. This robust mutually acceptable strategy enables species persistence without markedly reducing the water supply for humans. This study emphasises the utility of ecohydrological models for resource management of water‐controlled ecosystems. Although trade‐offs were identified between alternative performance criteria and their robustness to uncertain future flood regimes, management strategies were identified that help to secure an ecologically sustainable water supply. Copyright © 2014 John Wiley & Sons, Ltd.

Subinhibitory Antibiotic Concentrations Mediate Nutrient Use and Competition among Soil Streptomyces

Though traditionally perceived as weapons, antibiotics are also hypothesized to act as microbial signals in natural habitats. However, while subinhibitory concentrations of antibiotics (SICA) are known to shift bacterial gene expression, specific hypotheses as to how SICA influence the ecology of natural populations are scarce. We explored whether antibiotic ‘signals’, or SICA, have the potential to alter nutrient utilization, niche overlap, and competitive species interactions among Streptomyces populations in soil. For nine diverse Streptomyces isolates, we evaluated nutrient utilization patterns on 95 different nutrient sources in the presence and absence of subinhibitory concentrations of five antibiotics. There were significant changes in nutrient use among Streptomyces isolates, including both increases and decreases in the capacity to use individual nutrients in the presence vs. in the absence of SICA. Isolates varied in their responses to SICA and antibiotics varied in their effects on isolates. Furthermore, for some isolate-isolate-antibiotic combinations, competition-free growth (growth for an isolate on all nutrients that were not utilized by a competing isolate), was increased in the presence of SICA, reducing the potential fitness cost of nutrient competition among those competitors. This suggests that antibiotics may provide a mechanism for bacteria to actively minimize niche overlap among competitors in soil. Thus, in contrast to antagonistic coevolutionary dynamics, antibiotics as signals may mediate coevolutionary displacement among coexisting Streptomyces, thereby hindering the emergence of antibiotic resistant phenotypes. These results contribute to our broad understanding of the ecology and evolutionary biology of antibiotics and microbial signals in nature.

Resource Use of Soilborne Streptomyces Varies with Location, Phylogeny, and Nitrogen Amendment

In this study, we explore variation in resource use among Streptomyces in prairie soils. Resource use patterns were highly variable among Streptomyces isolates and were significantly related to location, phylogeny, and nitrogen (N) amendment history. Streptomyces populations from soils less than 1 m apart differed significantly in their ability to use resources, indicating that drivers of resource use phenotypes in soil are highly localized. Variation in resource use within Streptomyces genetic groups was significantly associated with the location from which Streptomyces were isolated, suggesting that resource use is adapted to local environments. Streptomyces from soils under long-term N amendment used fewer resources and grew less efficiently than those from non-amended soils, demonstrating that N amendment selects for Streptomyces with more limited catabolic capacities. Finally, resource use among Streptomyces populations was correlated with soil carbon content and Streptomyces population densities. We hypothesize that variation in resource use among Streptomyces reflects adaptation to local resource availability and competitive species interactions in soil and that N amendments alter selection for resource use phenotypes.

Erosion of community diversity and stability by herbivore removal under warming

Climate change has the potential to influence the persistence of ecological communities by altering their stability properties. One of the major drivers of community stability is species diversity, which is itself expected to be altered by climate change in many systems. The extent to which climatic effects on community stability may be buffered by the influence of species interactions on diversity is, however, poorly understood because of a paucity of studies incorporating interactions between abiotic and biotic factors. Here, I report results of a 10-year field experiment, the past 7 years of which have focused on effects of ongoing warming and herbivore removal on diversity and stability within the plant community, where competitive species interactions are mediated by exploitation through herbivory. Across the entire plant community, stability increased with diversity, but both stability and diversity were reduced by herbivore removal, warming and their interaction. Within the most species-rich functional group in the community, forbs, warming reduced species diversity, and both warming and herbivore removal reduced the strength of the relationship between diversity and stability. Species interactions, such as exploitation, may thus buffer communities against destabilizing influences of climate change, and intact populations of large herbivores, in particular, may prove important in maintaining and promoting plant community diversity and stability in a changing climate.

Taylor's Law holds in experimental bacterial populations but competition does not influence the slope

Populations vary in time and in space, and temporal variation may differ from spatial variation. Yet, in the past half century, field data have confirmed both the temporal and spatial forms of Taylor's power Law, a linear relationship between log(variance) and log(mean) of population size. Recent theory predicted that competitive species interactions should reduce the slope of the temporal version of Taylor's Law. We tested whether this prediction applied to the spatial version of Taylor's Law using simple, well-controlled laboratory populations of two species of bacteria that were cultured either separately or together for 24 h in media of widely varying nutrient richness. Experimentally, the spatial form of Taylor's Law with a slope of 2 held for these simple bacterial communities, but competitive interactions between the two species did not reduce the spatial Taylor's Law slope. These results contribute to the widespread usefulness of Taylor's Law in population ecology, epidemiology and pest control.

Local‐scale spatial dynamics of ants in a temperate agroecosystem

AbstractAt the local scale, spatial aggregations in ant distribution are often thought to be driven by competitive interactions among dominant ant species, although niche preferences and habitat heterogeneity might also lead to patchiness. Nevertheless, competitive interactions might be particularly important in agroecosystems that are structurally more homogeneous than natural habitats. The spatial patterns of ants in two Australian vineyards were investigated by intensive pitfall trapping to examine if non‐random patterns occur and whether these might be the result of competitive species interactions as well as the influence of woody vegetation adjacent to the vineyards. Null model analyses suggested competitive species interactions within ant assemblages that might have been driven by numerically dominant species, even though both positive and negative associations between these were found. Consistent spatial aggregations indicated significant spatial overlap in distributions of some species. Such overlap suggests that potential coexistence might be attributed to temporal partitioning or differences in foraging strategies. The presence of woody vegetation had a marked influence on ant assemblage structure and competitive interactions, and might facilitate coexistence by increasing resource heterogeneity. The implications of these findings for sampling strategies and ecological processes within vineyards are discussed.

Quantifying Purported Competition with Individual- and Population-Level Metrics

Competitive species interactions may contribute to population declines. Purportedly, Red-bellied Woodpeckers (Melanerpes carolinus), a common species, and Red-cockaded Woodpeckers (Picoides borealis), an endangered species, compete for roosting and nesting cavities in living pine trees. To determine whether behavioral interactions measured at the individual level manifest themselves at the population level, we conducted field experiments designed to test whether the presence of Red-bellied Woodpeckers resulted in a decrease in fitness to Red-cockaded Woodpeckers. As part of a 4-year study examining the nature of interspecific interactions in two populations of Red-cockaded Woodpeckers (one stable, the Apalachicola Ranger District; one declining, the Wakulla Ranger District) in the Apalachicola National Forest, Florida, we conducted a set of Red-bellied Woodpecker removal experiments. Paradoxically, following the removal of Red-bellied Woodpeckers, we observed decreases in Red-cockaded Woodpecker group size, proportion of nests that were successful, and proportion of individuals remaining on territories. Removal of Red-bellied Woodpeckers may have exaggerated the immigration rate of Red-bellied Woodpeckers to Red-cockaded Woodpecker territories. The Red-cockaded Woodpeckers in the Apalachicola Ranger District likely can withstand pressure from immigrating Red-bellied Woodpeckers given that their population has remained relatively stable despite the presence of Red-bellied Woodpeckers. A major factor of population persistence in the Wakulla Ranger District was the high turnover rate of adult female Red-cockaded Woodpeckers, a phenomenon that was exacerbated by removal of Red-bellied Woodpeckers. Relying solely on observations of apparently competitive interactions between individuals may not necessarily provide information about population-level outcomes. Paradoxically, removing species that appear to be competitors may harm species of concern.

Conspecific Emotional Cooperation Biases Population Dynamics

In this paper, the authors evaluate the benefit of emotions in population dynamics and evolution. The authors enhance cellular automata (CA) simulating the interactions of competing populations with emotionally inspired rules in communication, interpretation, and action. While CAs have been investigated in studies of population dynamics due to their ability to capture spatial interactions, emotion-like interactions have yet to be considered. Our cellular stochastic system describes interacting foxes that feed on rabbits that feed on carrots. Emotions enable foxes and rabbits to improve their decisions and share their experiences with neighboring conspecifics. To improve the system’s biological relevance, it includes inter-species disease transmission, and emotions encode data pertaining to both survival and epidemic reduction. Results indicate that emotions increase adaptability, help control disease, and improve survival for the species that utilizes them. Simulations support the hypothesis that the acquisition of emotion may be an evolutionary result of competitive species interactions.

The relative importance of the species pool, productivity and disturbance in regulating grassland plant species richness: a field experiment

Summary Ecologists have generally focused on how species interactions and available niches control species richness. However, the number of species in the regional species pool may also control richness. Moreover, the relative influence of the species pool and species interactions on plant richness may change along productivity and disturbance gradients. We test the hypothesis that many species from the propagule pool will colonize into habitats of moderate productivity and moderate disturbance, and the number of species in the pool will, therefore, primarily control plant species richness in such habitats. The hypothesis also states that few species from the propagule pool will colonize into habitats of high productivity and minimal disturbance because competitive species interactions primarily control plant richness in such habitats. To test this hypothesis, we experimentally varied resource availability via fertilization and irrigation, the size of the available propagule pool via sowing the seeds of 49 species and disturbance via vegetation clipping. A larger propagule pool increased species richness 80% in the absence of fertilization and the presence of clipping but had no significant effect on richness in the presence of fertilization and the absence of clipping (significant fertilization × clipping × seed addition interaction). Irrigation increased species richness primarily in the absence of fertilization and the presence or clipping (significant fertilization × clipping × irrigation interaction). Synthesis. These results support the assertion that the control of plant species richness shifts from the number of species in the propagule pool to the intensity of species interactions as productivity due to soil fertility increases and disturbance decreases. However, increases in productivity due to irrigation caused an increase in species richness, probably because of reduced seedling desiccation. Other studies have found similar outcomes along natural productivity gradients where productivity is confounded with other variables, but this study is the first to find such an outcome along an experimental productivity gradient. These results suggest habitats with moderate soil fertility and disturbance may be more open to colonization from the propagule pool than other habitats, a finding that has implications for plant restoration and invasive species management.

Alternative models of vertebrate speciation in Amazonia: an overview

The main hypotheses proposed to explain barrier formation separating populations and causing the differentiation of vertebrate species in Amazonia are based on different (mostly historical) factors, as follows. (1) Changes in the distribution of land and sea or in the landscape due to tectonic movements or sea-level fluctuations (Paleogeography hypothesis). (2) The barrier effect of Amazonian rivers (River hypothesis). (3) A combination of the barrier effect of broad rivers and vegetational changes in Northern and Southern Amazonia (River-refuge hypothesis). (4) The isolation of forest blocks near areas of surface relief in the periphery of Amazonia during dry climatic periods of the Tertiary and Quaternary (Refuge theory). (5) Competitive species interactions and local species isolations in peripheral regions of Amazonia due to invasion and counterinvasion during cold/warm periods of the Pleistocene (Disturbance-vicariance hypothesis). (6) Parapatric speciation across steep environmental gradients without separation of the representative populations (Gradient hypothesis). Several of these hypotheses are probably relevant to a different degree for the speciation processes in different faunal groups or during different geological periods. The paleogeography hypothesis refers mainly to faunal differentiation during the Tertiary and in combination with the Refuge hypothesis; Milankovitch cycles leading to global climatic-vegetational changes affected the biomes of the world not only during the Pleistocene but also during the Tertiary and earlier geological periods. New geoscientific evidence for the effect of dry climatic periods in Amazonia supports the predictions of the Refuge theory.