The spatial distribution of soil-dwelling arthropods is a subject that continues to by and large remain a black box to researchers. This is due largely to the extraordinary complexity of soil ecology and the difficulty of studying creatures that, in a sense, are an inseparable part of the soil itself. Soils are often highly heterogeneous in terms of their mineral fractions and other physical properties, such as porosity and water retention, which in turn sets the stage for the intricacy of the invertebrate and microbial communities living within them.
While surface-dwelling species are capable of achieving relatively predictable or widespread distribution across a given area due to a greater ability for dispersal, soil-dwelling species are usually constricted in their movements by the structure of the porosphere, the space between soil particles. Imagine, if you would, a sprawling, multi-levelled maze, parts of which occasionally or are constantly flooded, and whose passages are subject to frequent rearrangement. The composition of the air may change around every bend, but in most instances, the CO2 concentration is much higher than what can be found in the air on the surface, and the concentration rises the deeper one goes. You may have a little bit of light in some of the top passages, but as you descend, you quickly find yourself plunged into a cavernous darkness. This is a simple picture of the porosphere, before one even begins to consider the effects fungi, predators, and soil engineers (like earthworms) have on further complicating the trophic and physical interior of this labyrinth.
Arthropods living within the porosphere are generally well-adapted to a variety of its environmental rigours, but still they are thought to be much more restricted in their movement and distribution than surface-dwelling species because of the tremendous variability of soil conditions and the microscopic size of many species. Soil-dwelling species of Collembola (springtails), for instance, have been found to be highly-constrained by the patchy distribution of resources and the microhabitats formed thereby (Da Silva et al., 2015).
Understanding how these cryptic species are distributed is critical in understanding what impact different soil invertebrate taxa play in contributing to the structure and functionality of ecosystem services provided by the soil, like carbon and nitrogen cycling. Knowing where and to what extent a species occurs lends insight into its resource utilization and ultimately how that utilization or consumption influences the part of the ecosystem it occupies. With a growing interest in the agroecological approach of conserving, regenerating, or rejuvenating soils by harnessing ecosystem services, understanding the lay of the labyrinth and the denizens living within that make these services possible is an area of research deserving of further inquiry.
Article referenced: Da Silva, P.M., et al. (2015). Soil fauna through the landscape window: factors shaping surface- and soil-dwelling communities across spatial scales in cork-oak mosaics. Landscape Ecology, 30(8), 1511-1526.