Indirect interactions and plant community structure

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Morphological variability in tree root architecture indirectly affects coexistence among competitors in the understory

Interactions between plants can have strong effects on community structure and function. Variability in the morphological, developmental, physiological, and biochemical traits of plants can influence the outcome of plant interactions and thus have important ecological consequences. However, the ecological ramifications of trait variability in plants are poorly understood and have rarely been tested in the field. We experimentally tested the effects of morphological variation in root architecture of Quercus douglasii trees in the field on interactions between understory plants and community composition. Our results indicate that variability among Q. douglasii tree root systems initiates a striking reversal in the competitive effects of dominant understory grass species on a less common species. Trees with a deeprooted morphology facilitated exotic annual grasses and these annual grasses, in turn, competitively excluded the native perennial bunchgrass, Stipa pulchra. In contrast, Q. douglasii trees with shallow-rooted morphologies directly suppressed the growth of exotic annual grasses and indirectly released S. pulchra individuals from competition with these annual grasses. Morphological variation in the root architecture of Q. douglasii created substantial conditionality in the outcomes of competition among species which enhanced the potential for indirect interactions to sustain coexistence and increase community diversity

Models of Experimental Competitive Intensities Predict Home and Away Differences in Invasive Impact and the Effects of an Endophytic Mutualist

Understanding the role of competition in the organization of communities is limited in part by the difficulty of extrapolating the outcomes of small-scale experiments to how such outcomes might affect the distribution and abundance of species. We modeled the community-level outcomes of competition, using experimentally derived competitive effects and responses between an exotic invasive plant, Centaurea stoebe, and species from both its native and nonnative ranges and using changes in these effects and responses elicited by experimentally establishing symbioses between C. stoebe and fungal endophytes. Using relative interaction intensities (RIIs) and holding other life-history factors constant, individual-based and spatially explicit models predicted competitive exclusion of all but one North American species but none of the European species, regardless of the endophyte status of C. stoebe. Concomitantly, C. stoebe was eliminated from the models with European natives but was codominant in models with North American natives. Endophyte symbiosis predicted increased dominance of C. stoebe in North American communities but not in European communities. However, when experimental variation was included, some of the model outcomes changed slightly. Our results are consistent with the idea that the effects of competitive intensity and mutualisms measured at small scales have the potential to play important roles in determining the larger-scale outcomes of invasion and that the stabilizing indirect effects of competition may promote species coexistence

Fungal Endophytes Directly Increase the Competitive Effects of an Invasive Forb

Competitive outcomes among plants can vary in different abiotic and biotic conditions. Here we tested the effects of two phylotypes of Alternaria endophytes on the growth, competitive effects, and competitive responses of the exotic invasive forb Centaurea stoebe. Centaurea stoebe was a better competitor against North American grass species than grasses from its European home range in the absence of endophytes. However, one endophyte both increased the biomass of C. stoebe and reduced the competitive effect of North American grasses on C. stoebe. The competitive effects of C. stoebe on grass species native to North America were enhanced by both fungal endophytes, but not for native European grasses. We do not know the mechanism by which endophytes increased C. stoebe\u27s competitive ability, and particularly against biogeographically new neighbors, but one endophyte increased the competitive ability of C. stoebe without increasing its size, suggesting mechanisms unrelated to increased growth. We tested only a fraction of the different endophytic fungi that have been found in C. stoebe, only scratching the surface of understanding their indirect effects. However, our results are the first to demonstrate such effects of a fungal endophyte infecting an invasive forb, and one of the few to show that endophyte effects on competition do not have to be mediated through herbivory

Inhibitory effects of Eucalyptus globulus on understorey plant growth and species richness are greater in non-native regions

Aim: We studied the novel weapons hypothesis in the context of the broadly distributed tree species Eucalyptus globulus. We evaluated the hypothesis that this Australian species would produce stronger inhibitory effects on species from its non-native range than on species from its native range. Location: We worked in four countries where this species is exotic (U.S.A., Chile, India, Portugal) and one country where it is native (Australia). Time period: 2009–2012. Major taxa studied: Plants. Methods: We compared species composition, richness and height of plant communities in 20 paired plots underneath E. globulus individuals and open areas in two sites within its native range and each non-native region. We also compared effects of litter leachates of E. globulus on root growth of seedlings in species from Australia, Chile, the U.S.A. and India. Results: In all sites and countries, the plant community under E. globulus canopies had lower species richness than did the plant community in open areas. However, the reduction was much greater in the non-native ranges: species richness declined by an average of 51% in the eight non-native sites versus 8% in the two native Australian sites. The root growth of 15 out of 21 species from the non-native range were highly suppressed by E. globulus litter leachates, whereas the effect of litter leachate varied from facilitation to suppression for six species native to Australia. The mean reduction in root growth for Australian plants was significantly lower than for plants from the U.S.A., Chile and India. Main conclusions: Our results show biogeographical differences in the impact of an exotic species on understorey plant communities. Consistent with the novel weapons hypothesis, our findings suggest that different adaptations of species from the native and non-native ranges to biochemical compounds produced by an exotic species may play a role in these biogeographical differences.NSF EPSCoR Track-1, Grant/Award Number: EPS-1101342; Australian Research Council, Grant/Award Number: DE12010222

Vaktbikkja i Brussel? -en studie av norske journalisters forutsetninger for å utfylle sitt samfunnsoppdrag

Norges EU-tilpasning, gjennom blant annet EØS-avtalen, utfordrer det norske demokratiet fordi norske borgere har begrenset innflytelse i de politiske prosessene i EU. Dette forsterker behovet for medias deltakelse for å sørge for informasjon om, og drive kontroll av, den politiske sfære. Spørsmålet som stilles i denne studien er om forutsetningene ligger til rette for at norsk presse i Brussel kan oppfylle sin demokratiske rolle i forbindelse med norsk EU-integrasjon. For å belyse dette har jeg, for det første, undersøkt hvordan vi kan forstå de norske journalistene i Brussel som kritiske. For det andre har jeg undersøkt journalistenes syn på EU-politikk, under forutsetning av at dette har betydning for medievinklingene deres. Sentrale funn er, for det første, at pressen forstås som kritisk i den forstand den formidler korrekt informasjon, men i mindre grad tolker den politiske makten og driver kontroll av politikerne. For det andre at pressen i hovedsak anser EU-politikk for å være maktkamp. På bakgrunn av funnene har jeg konkludert med at norske presse i Brussel har dårlige forutsetninger til å drive kritisk journalistikk og gi et totalt bilde av EU-politikkens kompleksitet. I lys av norsk EU-integrasjon, tyder disse funnene på, at norsk presse i Brussel ikke har tilstrekkelig med forutsetninger for å kunne utfylle sin samfunnsrolle, og bøte på det demokratiske underskudd EØS-avtalen skaper

Raw individual biomass data

Raw individual biomass dat

Data from: Diversity increases indirect interactions, attenuates the intensity of competition, and promotes coexistence

A fundamental assumption of coexistence theory is that competition inevitably decreases species diversity. Consequently, in the quest to understand the ecological regulators of diversity, there has been a great deal of focus on processes with the potential to reduce competitive exclusion. However, the notion that competition must decrease diversity is largely based on the outcome of two-species interaction experiments and models, despite the fact that species rarely interact only in pairs in natural systems. In a field experiment, we found that competition among native perennial plants in multispecies assemblages was far weaker than competition between those same species in pairwise arrangements and that indirect interactions appeared to weaken direct competitive effects. These results suggest that community assembly theory based on pairwise approaches may overestimate the strength of competition and likelihood of competitive exclusion in species-rich communities. We also found that Centaurea stoebe, a North American invader, retained strong competitive effects when competing against North American natives in both pairwise and multispecies assemblages. Our experimental results support an emerging body of theory suggesting that complex networks of competing species may generate strong indirect interactions that can maintain diversity and that ecological differentiation may not be necessary to attenuate competition

Appendix A. Detailed results of two-way ANOVA model and figure of reproductive output results.

Detailed results of two-way ANOVA model and figure of reproductive output results