Intransitive Networks Research Articles

Nitrogen enrichment affects the competition network of aboveground species on the Inner Mongolia steppe

Nitrogen(N) enrichment changes the community structure of terrestrial ecosystems. However, the patterns and structural functions of species interaction networks under N-enrichment conditions are still unclear. We compared and analysed data from 13 years of N deposition experiments in the Inner Mongolia temperate steppe in China. We found that spatial association effectively detected intransitive networks, the structure of which was occasionally changed by species with low biomass but high-frequency occurrence, as an especially vital intermediary for forming a large network. The peak value of the network complexity was maintained at 2 gNm-2⁢year-1. In the third year after fertilization at 50 gNm-2⁢year-1, the species interaction changed to transitivity, which lasted for 8 years. Then, the environment completely replaced species interactions as the main factor affecting community composition. Surprisingly, the negative effect of N enrichment on the species network was counteracted by mowing and precipitation, which increased the probability of intransitive network formation. In the intransitive network, species diversity was not only maintained but also increased under low N-addition rates from 0 gNm-2⁢year-1 to 3 gNm-2⁢year-1. These studies indicated that N enrichment reduced the complexity of the species network and increased hierarchical differences among species competition. Intransitive competition is highly prevalent and a key driver of species coexistence and diversity maintenance in the typical steppe. Moreover, environmental factors directly affect plant intransitive competition, resulting in changes in species diversity.

Belowground feedbacks as drivers of spatial self-organization and community assembly.

Vegetation patterning in water-limited and other resource-limited ecosystems highlights spatial self-organization processes as potentially key drivers of community assembly. These processes provide insight into predictable landscape-level relationships between organisms and their abiotic environment in the form of regular and irregular patterns of biota and resources. However, two aspects have largely been overlooked; the roles played by plant - soil-biota feedbacks and allelopathy in spatial self-organization, and their potential contribution, along with plant-resource feedbacks, to community assembly through spatial self-organization. Here, we expand the drivers of spatial self-organization from a focus on plant-resource feedbacks to include plant - soil-biota feedbacks and allelopathy, and integrate concepts of nonlinear physics and community ecology to generate a new hypothesis. According to this hypothesis, below-ground processes can affect community assemblages through two types of spatial self-organization, global and local. The former occurs simultaneously across whole ecosystems, leading to self-organized patterns of biota, allelochemicals and resources, and niche partitioning. The latter occurs locally in ecotones, and determines ecotone structure and motion, invasion dynamics, and species coexistence. Studies of the two forms of spatial self-organization are important for understanding the organization of plant communities in drier climates which are likely to involve spatial patterning or re-patterning. Such studies are also important for developing new practices of ecosystem management, based on local manipulations at ecotones, to slow invasion dynamics or induce transitions from transitive to intransitive networks of interspecific interactions which increase species diversity.

Plant-soil feedbacks promote coexistence and resilience in multi-species communities.

Both ecological theory and empirical evidence suggest that negative frequency dependent feedbacks structure plant communities, but integration of these findings has been limited. Here we develop a generic model of frequency dependent feedback to analyze coexistence and invasibility in random theoretical and real communities for which frequency dependence through plant-soil feedbacks (PSFs) was determined empirically. We investigated community stability and invasibility by means of mechanistic analysis of invasion conditions and numerical simulations. We found that communities fall along a spectrum of coexistence types ranging from strict pair-wise negative feedback to strict intransitive networks. Intermediate community structures characterized by partial intransitivity may feature “keystone competitors” which disproportionately influence community stability. Real communities were characterized by stronger negative feedback and higher robustness to species loss than randomly assembled communities. Partial intransitivity became increasingly likely in more diverse communities. The results presented here theoretically explain why more diverse communities are characterized by stronger negative frequency dependent feedbacks, a pattern previously encountered in observational studies. Natural communities are more likely to be maintained by strict negative plant-soil feedback than expected by chance, but our results also show that community stability often depends on partial intransitivity. These results suggest that plant-soil feedbacks can facilitate coexistence in multi-species communities, but that these feedbacks may also initiate cascading effects on community diversity following from single-species loss.

Plant neighbour identity matters to belowground interactions under controlled conditions.

BackgroundRoot competition is an almost ubiquitous feature of plant communities with profound effects on their structure and composition. Far beyond the traditional view that plants interact mainly through resource depletion (exploitation competition), roots are known to be able to interact with their environment using a large variety of mechanisms that may inhibit or enhance access of other roots to the resource or affect plant growth (contest interactions). However, an extensive analysis on how these contest root interactions may affect species interaction abilities is almost lacking.Methodology/Principal FindingsIn a common garden experiment with ten perennial plant species we forced pairs of plants of the same or different species to overlap their roots and analyzed how belowground contest interactions affected plant performance, biomass allocation patterns, and competitive abilities under abundant resource supply. Our results showed that net interaction outcome ranged from negative to positive, affecting total plant mass and allocation patterns. A species could be a strong competitor against one species, weaker against another one, and even facilitator to a third species. This leads to sets of species where competitive hierarchies may be clear but also to groups where such rankings are not, suggesting that intransitive root interactions may be crucial for species coexistence.Conclusions/SignificanceThe outcome of belowground contest interactions is strongly dependent on neighbours' identity. In natural plant communities this conditional outcome may hypothetically help species to interact in non-hierarchical and intransitive networks, which in turn might promote coexistence.

Interaction rules affect species coexistence in intransitive networks

Interaction rules affect species coexistence in intransitive networks

Interaction rules affect species coexistence in intransitive networks

Intransitive communities, those in which species' abilities cannot be ranked in a hierarchy, have been the focus of theoretical and empirical research, as intransitivity could help explain the maintenance of biodiversity. Here we show that models for intransitive competition embedding slightly different interaction rules can produce opposite patterns. In particular, we find that interactions in which an individual can be outcompeted by its neighbors, but cannot outcompete its neighbors, produce negative frequency dependence that, in turn, promotes coexistence. Whenever the interaction rule is modified toward symmetry (the individual and the neighbors can outcompete each other) the negative frequency dependence vanishes, producing different coexistence levels. Macroscopically, we find that asymmetric interactions yield highest biodiversity if species compete globally, while symmetric interactions favor highest biodiversity if competition takes place locally.

DOES LOCAL COMPETITION INCREASE THE COEXISTENCE OF SPECIES IN INTRANSITIVE NETWORKS

Competitive intransitivity, a situation in which species' competitive ranks cannot be listed in a strict hierarchy, promotes species coexistence through "enemy's enemy indirect facilitation." Theory suggests that intransitivity-mediated coexistence is enhanced when competitive interactions occur at local spatial scales, although this hypothesis has not been thoroughly tested. Here, we use a lattice model to investigate the effect of local vs. global competition on intransitivity-mediated coexistence across a range of species richness values and levels of intransitivity. Our simulations show that local competition can enhance intransitivity-mediated coexistence in the short-term, yet hinder it in the long-term, when compared to global competition. This occurs because local competition slows species disaggregation, allowing weaker competitors to persist longer in the shifting spatial refuges of intransitive networks, enhancing short-term coexistence. Conversely, our simulations show that, in the long-term, local competition traps disaggregated species in unfavorable areas of the competitive arena, where they are excluded by superior competitors. As a result, in the long-term, global intransitive competition allows a greater number of species to coexist than local intransitive competition.

Who is the top dog in ant communities? Resources, parasitoids, and multiple competitive hierarchies

A wide variety of animal communities are organized into interspecific dominance hierarchies associated with the control and harvest of food resources. Interspecific dominance relationships are commonly found to be linear. However, dominance relations within communities can form a continuum ranging from intransitive networks to transitive, linear dominance hierarchies. How interference competition affects community structure depends on the configuration of the dominance interactions among the species. This study explores how resource size and the trait-mediated indirect effect (TMIE) specialist phorid fly parasitoids exert on interference competition, affect the transitive nature of competitive interactions in an assemblage of woodland ants. I quantify the linearity of networks of interactions associated with large and small food resources in the presence and absence of phorid parasitoids. Two distinct, significantly linear dominance hierarchies exist within the ant assemblage depending on the size of the disputed resource. However, the presence of phorid fly parasitoids eliminates the linearity of both dominance hierarchies. The host's phorid defense behaviors reduce the competitive asymmetries between the host and its subdominant competitors, increasing the indeterminacy in the outcome of competitive interactions. Thus, both resource size variation and phorid-induced TMIEs appear to facilitate coexistence in assemblages of scavenging ants.

Rock-scissors-paper and the survival of the weakest.

In the children's game of rock-scissors-paper, players each choose one of three strategies. A rock beats a pair of scissors, scissors beat a sheet of paper and paper beats a rock, so the strategies form a competitive cycle. Although cycles in competitive ability appear to be reasonably rare among terrestrial plants, they are common among marine sessile organisms and have been reported in other contexts. Here we consider a system with three species in a competitive loop and show that this simple ecology exhibits two counter-intuitive phenomena. First, the species that is least competitive is expected to have the largest population and, where there are oscillations in a finite population, to be the least likely to die out. As a consequence an apparent weakening of a species leads to an increase in its population. Second, evolution favours the most competitive individuals within a species, which leads to a decline in its population. This is analogous to the tragedy of the commons, but here, rather than leading to a collapse, the 'tragedy' acts to maintain diversity.

Competitive overgrowth in interactions among sessile colonial invertebrates: A comparison of stochastic and phenotypic variation

The outcome of interspecific competition has been shown to be variable in a variety of field and laboratory studies. This variability has been attributed to both stochastic and deterministic processes and, furthermore, has been used as evidence for deviation from at least three hypothetical competitive models. Here, I evaluate two sources of variation in the competitive interaction between two species in terms of their relative impact on the abundance of one competitor (a population response) and on species richness (a community response). Several levels of stochastic variation in the outcome of encounters between the highest and lowest ranked species in a multispecies assemblage provided a null, one-phenotype model for comparison against a two-phenotype model. Two competitive phenotypes of the lowest ranked species differed in terms of their ability to overgrow or be overgrown by the highest ranked species. Even low levels of phenotypic variation resulted in the persistence of the lowest ranked species and enhanced species richness. Comparable low levels of stochastic variation did not. Even competitive equivalency (Kay and Keough, 1981; Russ, 1982) in the stochastic model did not result in enhanced species richness. The question as to whether competitive intransitivity results from stochastic, environmental uncertainty or intrinsic determinism within real sessile assemblages remains unresolved at this time. The results of these simulations suggest that natural selection is more likely to favor competitive phenotypes which contribute to the formation of intransitive networks in assemblages with few rather than many species. Furthermore, the successful discrimination between phenotypic and stochastic variation will be much more difficult to attain in species-rich assemblages.

Indirect Mutualism: Variations on a Theme by Stephen Levine

Indirect Mutualism: Variations on a Theme by Stephen Levine