Interactions between organisms have been discussed in terms of resource use. However, environmental modulators (as shown in Table 8.1) can also be affected by organisms. The activities of both nitrifying bacteria and plant roots decrease soil pH, and soil temperature is affected by plant and litter cover. This can have positive or negative impacts on the growth of another species, depending on the species' niche requirements.
Some organisms alter the spatial arrangement of components of the environment or serve as new habitat themselves. These organisms are called "ecosystem engineers" and have widespread effects on an ecosystem beyond their own resource use (Jones et al., 1994). Large, competitively dominant organisms such as trees are obvious examples of ecosystem engineers. Earthworms are ecosystem engineers because they bury plant litter and create macropores in soil.
Many species are commonly found together because they have similar modulator niche requirements or they are adapted to rely on the presence of a common ecosystem engineer. One use of the term "guild" in ecology is to describe such a group of species. Unfortunately, another use of "guild" is to describe species with similar resource requirements, resulting in competition and potentially separated habitats. Wilson (1999) clarified use of the term by designating the former groups ^-guilds (organisms that commonly occur together because of similar modulator or habitat niche requirements) and the latter group a-guilds (organisms that have similar resource requirements and therefore could potentially exclude each other through competition).
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