Any soil sample contains active individuals from many species, representing a diversity of phyla and functional groups (Fig. 5.5). It will also include numerous inactive species that do not grow under the habitat conditions at the time of sampling. In the sections below, we will consider examples of the distribution pattern in the soil and how that changes through time. Species occurrence in the soil is spatially restricted by abiotic and biotic parameters. Some species are adapted
for low-oxygen microhabitats, others prefer high moisture content, or others may be limited by size to large pore spaces. Species within functional groups (such as bacterivores) also have different food preference, developmental and dispersal strategies. On short time scales (minutes to days), individuals respond to changes in aeration, moisture, temperature, and resource abundance and quality. On longer time scales (days to months), predation, parasitism, and seasonal changes in abiotic and food resources affect which species are active or quiescent. On even longer time scales (years to centuries), changes in soil physical structure and above-ground succession affect the below-ground decomposition food web. Above-ground species succession changes root and litter input, and chemistry of the litter. Therefore, both species composition and number of active individuals are continuously changing and adjusting to new conditions. Within a guild, samples represents species that are decreasing and increasing in abundance with time. That is because there are species that were adjusted to past conditions (decreasing in number of individuals) and those that are adjusted to current conditions (increasing in number of individuals). As a result, samples contain a small number of species that are abundant, and others that are rare. The latter consist of species that are adjusted to past conditions, as well as those newly active in the microhabitat. This activity pattern can be observed at different time scales, from diurnal responses to weather changes, to suc-cessional trends over decades.
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