Andrea P. Castillo-Monroy

Do biotic interactions modulate ecosystem functioning along stress gradients? Insights from semi-arid plant and biological soil crust communities

Climate change will exacerbate the degree of abiotic stress experienced by semi-arid ecosystems. While abiotic stress profoundly affects biotic interactions, their potential role as modulators of ecosystem responses to climate change is largely unknown. Using plants and biological soil crusts, we tested the relative importance of facilitative–competitive interactions and other community attributes (cover, species richness and species evenness) as drivers of ecosystem functioning along stress gradients in semi-arid Mediterranean ecosystems. Biotic interactions shifted from facilitation to competition along stress gradients driven by water availability and temperature. These changes were, however, dependent on the spatial scale and the community considered. We found little evidence to suggest that biotic interactions are a major direct influence upon indicators of ecosystem functioning (soil respiration, organic carbon, water-holding capacity, compaction and the activity of enzymes related to the carbon, nitrogen and phosphorus cycles) along stress gradients. However, attributes such as cover and species richness showed a direct effect on ecosystem functioning. Our results do not agree with predictions emphasizing that the importance of plant–plant interactions will be increased under climate change in dry environments, and indicate that reductions in the cover of plant and biological soil crust communities will negatively impact ecosystems under future climatic conditions.

Biological soil crusts modulate nitrogen availability in semi-arid ecosystems: insights from a Mediterranean grassland

Biological soil crusts (BSCs) greatly influence the N cycle of semi-arid ecosystems, as some organisms forming them are able to fix atmospheric N. However, BSCs are not always taken into account when studying biotic controls on N cycling and transformations. Our main objective was to understand how BSCs modulate the availability of N in a semi-arid Mediterranean ecosystem dominated by the tussock grass Stipa tenacissima. We selected the six most frequent soil cover types in the study area: S. tenacissima tussocks (ST), Retama sphaerocarpa shrubs (RS), and open areas with very low (BS), low (LC) medium (MC) and high (HC) cover of well developed and lichen-dominated BSCs. The temporal dynamics of available N dynamics followed changes in soil moisture. Available NH4+-N did not differ between microsites, while available NO3--N was substantially higher in the RS than in any other microsite. No significant differences in the amount of available NO3--N were found between ST and BS microsites, but these microsites had more NO3--N than those dominated by BSCs (LC, MC and HC). Our results suggest that BSCs may be inhibiting nitrification, and highlight the importance of this biotic community as a modulator of the availability of N in semi-arid ecosystems.

Biological soil crusts (biocrusts) as a model system in community, landscape and ecosystem ecology

Model systems have had a profound influence on the development of ecological theory and general principles. Compared to alternatives, the most effective models share some combination of the following characteristics: simpler, smaller, faster, general, idiosyncratic or manipulable. We argue that biological soil crusts (biocrusts) have unique combinations of these features that should be more widely exploited in community, landscape and ecosystem ecology. In community ecology, biocrusts are elucidating the importance of biodiversity and spatial pattern for maintaining ecosystem multifunctionality due to their manipulability in experiments. Due to idiosyncrasies in their modes of facilitation and competition, biocrusts have led to new models on the interplay between environmental stress and biotic interactions and on the maintenance of biodiversity by competitive processes. Biocrusts are perhaps one of the best examples of micro-landscapes—real landscapes that are small in size. Although they exhibit varying patch heterogeneity, aggregation, connectivity and fragmentation, like macro-landscapes, they are also compatible with well-replicated experiments (unlike macro-landscapes). In ecosystem ecology, a number of studies are imposing small-scale, low cost manipulations of global change or state factors in biocrust micro-landscapes. The versatility of biocrusts to inform such disparate lines of inquiry suggests that they are an especially useful model system that can enable researchers to see ecological principles more clearly and quickly.

La costra biológica del suelo: Avances recientes en el conocimiento de su estructura y función ecológica

La costra biologica del suelo (CBS) es una comunidad biotica formada por la intima asociacion entre particulas de suelo, cianobacterias, algas, hongos, liquenes, hepaticas y briofitos. La CBS esta ampliamente distribuida en muchos tipos de suelo y en casi todas las comunidades vegetales donde la luz alcanza la superficie del suelo, si bien predomina en zonas aridas, semiaridas, alpinas y polares. Este conjunto de organismos proporciona un aporte importante de carbono y nitrogeno al suelo, incrementa su estabilidad y lo protege frente a la accion erosiva de la lluvia y el viento. Tambien modula la infiltracion y afecta de manera directa a las plantas vasculares y otros organismos. El conocimiento de la biologia, ecologia y fisiologia de la CBS ha aumentado ampliamente en los ultimos anos. No obstante, existe un vacio de informacion importante respecto a la influencia de la CBS en los ciclos biogeoquimicos, especialmente del fosforo y carbono, asi como a todo lo referente a las interacciones bioticas, tanto entre los componentes de la CBS entre si como entre la CBS y los microorganismos, plantas vasculares e invertebrados. Es necesario descentralizar la informacion y extenderla a otras areas del mundo, ya que la mayoria de los estudios se han realizado en zonas aridas y semiaridas de los Estados Unidos de America, Australia, Israel y China. Es especialmente preocupante la falta de estudios en toda America Central y del Sur, pese a que la CBS debe ser un componente biotico de primera magnitud en paises como Chile, Argentina, Peru y Mexico, por citar solo unos ejemplos. Con el fin de despertar el interes de la comunidad cientifica de habla hispana sobre este importante conjunto de organismos, y de identificar los principales avances y lagunas existentes en nuestro conocimiento sobre su ecologia, en el presente trabajo revisamos las caracteristicas y distribucion de la CBS, asi como sus efectos en la estructura y funcionamiento de los ecosistemas en los que se encuentran.

Aspects of soil lichen biodiversity and aggregation interact to influence subsurface microbial function

Background and aimsMany previous studies have evaluated aboveground–heterotrophic belowground interactions such as plant-soil feedbacks, plant-mycorrhizal fungi associations or plant-actinorhizal symbioses. However, few studies have used biocrusts, which are specialized soil communities of autotrophic cyanobacteria, mosses, lichens and non-photosynthetic fungi and bacteria that are prevalent in drylands worldwide. These communities largely influence ecosystem functioning, and can be used as a model system for studying above-belowground interactions. In this study, we evaluated how biocrusts affect the functional diversity and biomass of microbial diversities beneath biocrusts.MethodsWe performed two microcosm experiments using biocrust-forming lichens where we manipulated their biotic attributes to test independently the effects of species richness (from two to eight species), composition, evenness (maximal and low evenness) and spatial pattern (clumped and random distribution) on the microbial catabolic profile and microbial functional diversity.ResultsMicrocosms with a random pattern had a higher microbial catabolic profile than those with a clumped pattern. Significant richness × evenness × pattern and richness × evenness interactions were found when analyzing microbial catabolic profile and biomass, respectively. Microcosms with a random pattern, intermediate number of species, and maximal evenness level had higher microbial catabolic profile. At the maximal evenness level, assemblages had higher microbial catabolic profile and microbial biomass when they contained four species. The richness × evenness × pattern interaction was the most informative predictor of variations in microbial catabolic profile.ConclusionsOur results indicate that soil microorganisms are influenced by biocrusts, just as they are influenced by plants, and highlight the importance of higher order interactions among species richness, evenness, and spatial pattern as drivers of microbial communities. The results also emphasize the importance of studying several biotic attributes simultaneously when studying biocrust-soil microorganism interactions, as in nature, community properties do not exert their influence in isolation.

Biological Soil Crusts as a Model System in Ecology

We explore in this chapter how biological soil crusts (biocrusts) may serve as a useful model system for studying multiple questions of interest in ecology, including biodiversity–ecosystem function relationships, positive and negative species interactions along environmental gradients, the source–sink hydrological dynamics in drylands, and ecosystem resistance and resilience. To illustrate our views, we synthesize recent and ongoing studies that are employing biocrusts as model systems to tackle these and other related questions, emphasizing the main features of biocrusts that make them special and well suited to advance ecological theory and our understanding of many important topics in community and ecosystem ecology. We complete the synthesis of the studies conducted so far with recommendations aiming to promote the use of biocrusts by community and ecosystem ecologists.