What is a biocrust? A refined, contemporary definition for a broadening research community.

Studies of biological soil crusts (biocrusts) have proliferated over the last few decades. The biocrust literature has broadened, with more studies assessing and describing the function of a variety of biocrust communities in a broad range of biomes and habitats and across a large spectrum of disciplines, and also by the incorporation of biocrusts into global perspectives and biogeochemical models. As the number of biocrust researchers increases, along with the scope of soil communities defined as 'biocrust', it is worth asking whether we all share a clear, universal, and fully articulated definition of what constitutes a biocrust. In this review, we synthesize the literature with the views of new and experienced biocrust researchers, to provide a refined and fully elaborated definition of biocrusts. In doing so, we illustrate the ecological relevance and ecosystem services provided by them. We demonstrate that biocrusts are defined by four distinct elements: physical structure, functional characteristics, habitat, and taxonomic composition. We describe outgroups, which have some, but not all, of the characteristics necessary to be fully consistent with our definition and thus would not be considered biocrusts. We also summarize the wide variety of different types of communities that fall under our definition of biocrusts, in the process of highlighting their global distribution. Finally, we suggest the universal use of the Belnap, Büdel & Lange definition, with minor modifications: Biological soil crusts (biocrusts) result from an intimate association between soil particles and differing proportions of photoautotrophic (e.g. cyanobacteria, algae, lichens, bryophytes) and heterotrophic (e.g. bacteria, fungi, archaea) organisms, which live within, or immediately on top of, the uppermost millimetres of soil. Soil particles are aggregated through the presence and activity of these often extremotolerant biota that desiccate regularly, and the resultant living crust covers the surface of the ground as a coherent layer. With this detailed definition of biocrusts, illustrating their ecological functions and widespread distribution, we hope to stimulate interest in biocrust research and inform various stakeholders (e.g. land managers, land users) on their overall importance to ecosystem and Earth system functioning.

Broader Impacts for Ecologists: Biological Soil Crust as a Model System for Education.

Biological soil crusts (biocrusts) are a complex community of algae, cyanobacteria, lichens, bryophytes, and assorted bacteria, fungi, archaea, and bacteriophages that colonize the soil surface. Biocrusts are particularly common in drylands and are found in arid and semiarid ecosystems worldwide. While diminutive in size, biocrusts often cover large terrestrial areas, provide numerous ecosystem benefits, enhance biodiversity, and are found in multiple configurations and assemblages across different climate and disturbance regimes. Biocrusts have been a focus of many ecologists, especially those working in semiarid and arid lands, as biocrusts are foundational community members, play fundamental roles in ecosystem processes, and offer rare opportunities to study biological interactions at small and large spatial scales. Due to these same characteristics, biocrusts have the potential to serve as an excellent teaching tool. The purpose of this paper is to demonstrate the utility of biocrust communities as a model system in science education. Functioning as portable, dynamic mini ecosystems, biocrusts can be used to teach about organisms, biodiversity, biotic interactions, abiotic controls, ecosystem processes, and even global change, and can be easy to use in nearly every classroom setup. For example, education principles, such as evolution and adaptation to stress, or structure and function (patterns and processes) can be applied by bringing biocrusts into the classroom as a teaching tool. In addition, discussing the utility of biocrusts in the classroom - including theory, hypothesis testing, experimentation, and hands-on learning - this document also provides tips and resources for developing education tools and activities geared toward impactful learning.

Applying community ecological theory to maximize productivity of cultivated biocrusts.

Degraded rangelands around the world may benefit from the reestablishment of lost biological soil crusts (biocrusts, soil surface cryptogamic-microbial communities). Cultivation of biocrust organisms is the first step in this process, and may benefit from harnessing species interactions. Species interactions are a dominant force structuring ecological communities. One key element of community structure, species richness, is itself important because it can promote the productivity of the entire community. Here, we use biological soil crusts as a model to test the effects of species interactions on production of biocrust materials for use in ecosystem rehabilitation. We screened eight different moss and lichen species from semiarid rangelands of Montana, USA, for growth potential under two watering regimes. Mosses generally grew well, but we were unable to cultivate the selected lichen species. We produced a >400% increase in the biomass of one species (Ceratodon purpureus). We tested whether a parasite-host relationship between two lichens could be used to enhance productivity of the parasite species, but this also resulted in no net gain of lichen productivity. Finally, we constructed all possible community combinations from a pool of five moss species to test for overyielding (community productivity exceeding that expected from the growth of community members in monoculture), and to determine both if, and the mode in which, species richness increases productivity. Polycultures yielded more than would be expected based upon the production of community constituents in monoculture. Using structural equation models, we determined that there was a modest effect of species richness on community productivity (r = 0.24-0.25), which was independent of a stronger effect of the identity of species in the community (r = 0.41-0.50). These results will contribute to the optimization of biocrust cultivation, promoting the development of this emerging ecological rehabilitation technology.