Unlike a nonliving resource, the genetic makeup of prey species will respond to exploitation through evolution, resulting in defensive adaptations. Defenses from exploitation can take a variety of forms, including behavioral, morphological, or biochemical defenses. Evolution can also result in the development of new attack strategies in consumers, resulting in a continual coevolutionary arms race between consumers and their prey. Exploitation pressure can be regulated by exploitation at higher trophic levels in a process called a trophic cascade. For example, if carnivores limit the population size of herbivores through heavy predation, then the pressure on plants from herbivory will be low. Consider in the equation above how an increase in death rate (d) for herbivores would affect the equilibrium concentration of the herbivore's resource (R*). The herbivore's resource is of course the plant population density. In this case, competitive interactions will be strong for plants and carnivores, but relatively weak for herbivores where response to predation will be important. A trophic cascade is often the basis of strategies in biocontrol. For example, Trichoderma herzianum, a mycoparasitic fungus that attacks the root pathogen Rhizoctonia solani, has been introduced to control Rhizoctonia density.
Exploitation can have a large influence on the outcome of competitive interactions between prey species. Exploitation can contribute to the coexistence of competing prey species by reducing the population size of the superior competitor. This results in increased resource abundance and ameliorates competition. Paine (1969) called predators that perform this function "keystone species." The term has since been used to describe any species whose effects on an ecosystem are more than would be expected based on their biomass in a given system. There is a tradeoff between competitive ability and anti-predator adaptations that allows keystone predation to take place. Ability to avoid exploitation is an alternative niche dimension that species can evolve to utilize.
When representative species are tested, most predatory soil fungi, protozoa, nematodes, and collembola will utilize multiple prey species. However, all also show feeding preferences for, or enhanced benefits from, particular prey species. It is therefore likely that predation regulates community composition (at the species level) by mediating competition between microbes or plant species. The former has been difficult to test because examining microbial community dynamics in situ is complicated; results from laboratory studies do not always provide adequate information about how natural systems operate. Klironomos and Kendrick
(1995) found that saprophytic fungi are often the preferred food source for arthropods, but arthropods will consume mycorrhizal fungi where saprophytic fungi are not available. These relationships can impact competition among plant species by altering the nutrients available from mycorrhizas to plants. Bever (2003) provides a comprehensive review of the mechanisms by which microbes mediate competition in plant communities.
In ecosystems, trophic relationships between organisms result in a complex web of interactions (a food web). A large number of organisms involved have multiple prey or multiple predators. Omnivorous predators utilize multiple prey from different trophic levels and may at times be in competition with potential prey. The study of how food web structure interacts with community composition and ecosystem processes is a field still in development. Experimental work has been performed largely with simple communities of protists and bacteria in microcosms. One preliminary conclusion from this work is that a greater number of trophic levels, or greater overall complexity, decreases the stability of constituent populations (Morin, 1999). However, this conclusion is in contrast to predictions of the constant connectance hypothesis, which is based on the observation that each species is less dependent on a single resource in more complex systems, providing more of a buffer to environmental fluctuations (Martinez and Dunne, 1998).
When microorganisms are included in soil food webs, the increase in complexity on the species level has been viewed as overwhelming. Microorganisms are normally represented by undifferentiated pools of biomass or are divided into very broad groups (e.g., fungi and bacteria). This is understandable because of the enormous diversity of soil microorganisms, the often unknown role of each taxon in a food web, and the fact that the focus of soil food web studies has typically been biogeochemical processes, not community structure. However, it also masks unique features of food webs arising when microbial species are included explicitly. There are no "top predators" in food webs containing microorganisms, because all organisms are exploited by parasites of varying lethality. Also, the presence of "three-species loops" has been the subject of controversy in food webs of macroscopic organisms and may be possible only when there is differential predation on species due to developmental stage. In microbial systems this food web structure has not been explicitly investigated, but, since many predators within the system are generalists, it seems likely that such loops can frequently occur due to random encounters.
Food webs including microorganisms must also account for the presence of decomposer organisms. These organisms are not predators because they do not directly impact population dynamics of a prey while obtaining their resources. Decomposer organisms obtain energy or nutrients from previously dead organisms or their by-products. This decomposition is critical to the recycling of nutrients that can be used in primary production. Decomposer organisms affect population dynamics of primary producers by supplying nutrients and often by competing with primary producers for the same resources ("immobilization").
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