Microbiotic crust habitats offer an excellent system in which to examine small scale biodiversity. These crusts are themselves composed of an assortment of cyanobacteria, eukaryotic algae, mosses, and lichenized fungi. They remain poorly understood and understudied considering their dominance in arid habitats. They are particularly important in western North America where they can make up as much as 42% of the soil cover (Ladyman and Muldavin 1996). Some argue that biotic crusts are of no importance to ecosystem functioning (West 1990) and some contend that they compete with more important plants such as the grasses required for livestock grazing (Savory 1988). Others have gathered evidence that these communities are ecologically important, particularly with reference to retarding soil erosion, increasing soil fertility, increasing water infiltration, and interacting with vascular plant germination and nutrient uptake (Harper and Marble 1988, Johansen 1993, Metting 1991, West 1990). With such extensive cover and the ability to modify their microhabitat, it is quite possible that crusts provide important resources to a large segment of undiscovered arid land soil biota.
There are several reasons to expect that micro arthropods make use of these habitats. Both mites (Acari), and springtails (Collembola) are known to eat lichen, mosses and fungi in other systems (Lawrey 1987). Many mites and collembolans are also capable of withstanding extremes of temperature and moisture (Somme and Conradi-Larsen 1977), both of which are common characteristics of the arid western U.S. and associated crust habitats. These traits make both arthropod groups prime candidates for investigation in these microhabitats.
Essentially no information exists about the influence of the identity of microbiotic crusts on the diversity of the faunas that may be associated with them. A few single species studies do show some relationship between a particular macro arthropod group and a crustal habitat. The most detailed of these elucidated the energy budget for a desert isopod known to regularly feed on soil crusts (among other plant material) (Shachak et al. 1976, Steinberger 1989). Other macro arthropods known to feed on crusts include harvester ants (Loria and Hernstadt 1980); tenebrionid beetles (Rogers et al. 1988) and weevils, (Chown and Scholtz 1989).
An awareness of the number and identities of the organisms that frequent, and perhaps depend on biotic crusts, is important for a number of reasons. If we are to understand how important these systems are to healthy maintenance of arid and semi-arid systems, we need to know what animal and microbial species use them. One current argument about crusts is that they should be broken up to free the nutrients they are sequestering (Savory 1988). Whether this concern is realistic depends, in part, on the decomposers that frequent crusts. Mites and Collembola are among the most important decomposers in habitats around the world. They are primary decomposers in that they can digest organic matter itself. They are better known as secondary decomposers, feeding on the fungi and microbial biomass in decaying plant matter. They may also disperse spores and redistribute organic matter (Behan and Hill 1978). In arid regions mite activity depends on immediate past weather condition, but even in dry soils there is often some activity. The influence of tydeids on the early stages of litter decomposition in the Chihuahuan Desert led Whitford (1996) to consider the family a keystone group. Tydeids are predators that consume soil nematode worms, which in turn consume microbial biomass. In litter sacs without tydeids, the nematodes increased to such an extent that microbial biomass became overgrazed and overall decomposition rates slowed. Collembola grazing at intermediate levels has also been shown to stimulate fungal growth on oak leaf litter (Ineson et al. 1982).
The western U.S. is characterized by variability of topography, soil types and climate regimes, and hence patchy environments. These factors produce both isolated habitats and high levels of endemism across such diverse taxa as the vertebrates and vascular plants. Bryophytes are present in all semiarid crust sites, but become dominant over lichens in more northern sites (Johansen et al. 1993) while crusts in hot deserts are often less developed with fewer lichens and mosses. The vast differences reported among soil and crust types (i.e., dominated by algae, lichen, or bryophytes) suggest that we might expect quite different arthropod associations on these different crust types. The patchy distribution of these habitats suggests that the faunas may differ dramatically across locations. It is also possible that these habitats support a high level of micro arthropod endemism.
In 1998, National Science Foundation's Division of Biotic Surveys and Inventories provided funding to Johansen, Flechtner, Lewis, and St. Clair to carry out an extensive taxonomic study of microbiotic crust communities over the entire western U.S. They are currently characterizing the cyanobacteria, eukaryotic algae, mosses, and lichenized fungi from each of thirty distinct locations between approximately 49oN and 32oN latitude. Their study will provide exceptional new information on the identities of the associated groups that make up these multi-kingdom systems. In addition, it will provide an important new level of information on the biotic diversity of arid and semi-arid lands in North America. More importantly for this project, their results and the accompanying database will provide an exceptional template almost never available, on which to overlay an examination of the diversity of arthropods that may be associated with these micro-communities.
The Shepherd lab is currently investigating the biodiversity of microarthropod groups in microbiotic crusts in pinyon-juniper woodland in New Mexico. This preliminary work will prepare us to establish a database of all arthropods found on or in crusts. In addition, we hope to obtain funding to sample from the sites identified by Johansen et al. and to link their database with our own to provide information on the taxonomic relationships between particular crust types and arthropods.