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

Abstract

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.

Keywords: biocrust; biological soil crust; climate.; definition; function; habitat; physical structure; taxonomy.

Fig. 1

Sites of biocrust studies in dryland and non‐dryland regions. Biocrusts occur in all drylands around the world, but also in non‐dryland regions if microclimatic conditions are suitable. Composition of biocrust types varies depending both on continent and climatic region. Dryland regions shown according to UNCCD (UNEP‐WCMC, 2007). Biocrust study sites marked according to list in Rodríguez‐Caballero et al. (2018a). Biocrust types occurring in the different dryland and non‐dryland regions are presented in inset bar charts, with the number of described sites of occurrence on the y‐axis. Bar charts are presented for the different continents, excluding Antarctica.

Fig. 2

Biological soil crust (biocrust) definition based on a decision tree approach.

Fig. 3

Biological soil crust (biocrust) definition illustrated in a Venn diagram. Ovals represent the four major elements of our biocrust definition. Biocrusts are consistent with the region where all four overlap. Other ‘outgroups’ are also mapped and labelled on the diagram, with the main reasons for their distinction from biocrusts listed. Parenthetical numbers indicate the relevant section of this review for each outgroup.

Fig. 4

Illustration of characteristics that define biocrusts (A, B) and of features that are not biocrusts (C–H). (A) Biocrusts aggregate surface soil particles, thus stabilizing soils; Sakaiká sclerophyllous shrubland, La Gran Sabana, Venezuela. (B) Filamentous biological structures (cyanobacterial filaments) cause a soil aggregation; Colorado Plateau, Southern Utah, USA. (C) Physical crust; Knersvlakte at Goedehoop farm, South Africa. (D) Microbial mat; Shannah, Oman. (E) Fungus of the genus Bovista with mycelium; Graz, Austria. (F) Vagrant Xanthoparmelia sp. (green) growing on top of a regular biocrust; Colorado Plateau, Southeast Utah (photographs: courtesy of Kyle Doherty). (G) Cyanobacterial macrocolonies; former limestone quarry, Aschfeld, Germany. (H) Lichen and bryophyte carpet, Illulisat, Greenland.

Fig. 5

Biological soil crusts (biocrusts) form miniature ecosystems. They are composed of photoautotrophic producers (i.e. cyanobacteria, algae, lichens, and bryophytes; shown in green), microfauna acting as consumers (i.e. protozoa, nematodes, tardigrades, rotifers, and microarthropods; shown in blue), and decomposers (i.e. fungi, bacteria, and archaea; shown in red). Colouration of drawing by Renate Klein‐Rödder, originally published in Belnap & Lange (2003); courtesy of Springer.

Fig. 6

Biocrusts varying in form and function within different climatic settings. (A) Smooth cyanobacteria‐dominated crust near Beersheba, Israel. (B) Lichen fields with Teloschistes sp. as the prominent genus; Namib Desert, Alexanderbay, South Africa. (C) Grit crust with weathered granite pebbles, enveloped and aggregated by several species of chlorolichens, accompanied by fungi and cyanobacteria; Atacama Desert, Chile. (D) Hypolithic biocrust with cyanobacteria growing on the sides and underside of translucent quartz pebbles; Goedehoop farm, Knersvlakte, South Africa. (E) Cyanobacteria‐dominated biocrust with cyano‐ and chlorolichens; Soebatsfontein region, Succulent Karoo, South Africa. (F) Cyanobacterially dominated pinnacled crust with mosses and lichens; Canyonlands National Park, Colorado Plateau region, USA; Cactacea occurring in‐between pinnacles. (G) ‘Wrinkled’ biocrust in semiarid regions; Great Basin region, northwest Utah, USA. (H) Cyanobacteria‐dominated biocrust with mosses and liverworts, impacted by trampling; Fazenda Brejo, Caatinga, Brazil.

Fig. 7

Biocrusts varying in form and function within different climatic, edaphic, and land‐use settings. (A) Polar region with low cover of vascular vegetation, but a dense cover of cyanobacteria‐dominated biocrusts with bryophytes; Zeppelinhamna, Ny Alesund, Spitsbergen. (B) Liverwort‐dominated biocrusts; because of extreme frost‐heaving, biocrusts are smoother overall than uncrusted soils; Icelandic Highlands. (C) ‘Peeling’ biocrust, dominated by cyanobacteria; Central Chihuahuan desert, Mexico. (D) Biocrust on gypsiferous soil, with particularly high coverage of chlorolichens; Tabernas Badlands, Spain. (E) Postglacial biocrust, dominated by mosses and cyanobacteria; Kangerlussuaq region, Greenland. (F) Biocrusts of temperate dry meadows (‘Trockenrasen’) with typical lichen community comprising Fulgensia fulgens, Toninia caerulionigricans, Cladonia convoluta, and Diploschistes muscorum; Ruine Homburg, Aschfeld, Germany. (G) Stora Alvaret (barren limestone terrace) on the island of Öland, Sweden; biocrusts with lichens of the genera Fulgensia, Psora, and Cladonia. (H) Biocrusts after fire in former forests; main photograph: Lolo Fire, western Montana, USA; insert: Cajete fire, northwestern New Mexico, USA (photographs courtesy of Henry Grover).