Trophic interactions refer to the transfer of energy or nutrients from one organism to another. The stored energy or nutrients of the prey serve as a resource for the consumer. Evolution has invented an array of methods for consumers to steal stored resources. In studying energy flow through an ecosystem, it may be useful to categorize consumers into trophic levels (e.g., herbivores and carnivores). To understand the impact of exploitation on population dynamics it is important to know if the consumer is a predator, causing the immediate death of prey so that prey resources can be consumed in a single event, or is a parasite, obtaining only a portion of the prey's resources without killing it, so that the same organism can be used in the future. Another important distinction is between a generalist, who consumes many different prey species, and a specialist, which consumes very few. As with all neat categories in ecology, there is actually a gradient of lifestyles that fall between the extremes.
Exploitation in soil biota is widespread. Most soil animals, including protozoa, nematodes, collembola, mites, earthworms, etc., obtain their resources through exploitation of bacteria, fungi, or plant roots. Invertebrates that ingest plant detritus normally get most of their energy and nutrients from microorganisms residing on the detritus (their "prey") rather than directly from the detritus. Many species of fungi have been shown to attack bacterial colonies and other fungi, and there are also fungi that attack soil animals. Bdellovibrio is a bacterial predator that attacks other bacteria. All organisms also appear to serve as a habitat for an assemblage of smaller organisms, many of which are parasitic.
A predator's effect on prey population dynamics is to increase the death rate. This effect can be modeled, along with predator population dynamics, using the Lotka-Volterra equations shown here (adapted to the notation used in previous sections):
The subscripts R and C denote properties of the prey and predator populations, respectively. Death rate of the prey is a function of predator population density, and reproduction rate of the predator is a function of prey population density. The number of attacks per unit time per predator, a, is assumed to be constant, but the total number of prey killed per unit time (calculated aNCNR) increases due to either increased prey or predator abundance. Predator reproduction rate is proportional to the number of prey killed, but also depends on e, the efficiency with which the predator can use prey to reproduce. Values of e less than 1 imply that a single predator must kill multiple prey organisms to reproduce, while values greater than 1 imply that the predator can reproduce using the resources from a single prey organism.
The attack rate of predators on prey is normally constant over only a limited range of prey and predator population densities. The total number of attacks per unit time per predator (aNR) will reach an asymptote as prey population continues to rise, because of predator satiation, the minimum handling time required for each kill, and time spent performing other activities. Attack rates may also decline when population density of a particular prey species is below a threshold level, if the predator does not invest energy in pursuing prey that is too scarce. These phenomena can be incorporated into the Lotka-Volterra model using a nonlinear "functional response" in place of a.
Predators will aggregate in patches of high prey density. At very high predator population densities, attack rates may decline because other resources become limiting. Predators will then disperse into less resource-rich, but also less competitive, habitats. This is a consequence of intraspecific competition. Predatory pressure is also a factor in habitat quality for the prey. Predator-free patches serve as refuges for the prey population and can significantly impact metapopulation dynamics. Elliott et al. (1980) found that a finer-textured soil contained more bacteria protected from predation by nematodes. The finer-textured soil contained a larger proportion of pores too small for the nematodes to utilize. Amoebae were able to use these pores, and there was a greater increase in growth of nematodes that preyed on both bacteria and nematodes when amoebae were added to the fine-compared to a coarse-textured soil.
Parasitism is a considerably more complicated phenomenon to model than predation because prey are weakened by parasites, which impacts reproduction and death rates. Parasitism can decrease the accumulation of biomass or rate of development. In terms of the Monod model discussed above, parasitism may cause the infected subpopulation to have a decreased maximum growth rate (u0) or to waste resources through increased maintenance utilization (m). Parasitism also typically increases the death rate, either through prolonged exposure to the parasite or by making the prey more sensitive to other causes of mortality.
The details of the route of transmission of a parasite between hosts are critical to understanding how the parasite is spread. Some parasites are able to colonize new hosts from dead tissue. For example, the plant root pathogens in the genera Gaeumannomyces, Rhizoctonia, and Pythium are able to live saprophytically within plant residue and colonize new roots from these habitats. Higher quality habitat patches allow pathogenic hyphae to grow farther through the soil (to at least 15 cm) to colonize new roots. The probability distribution of colonization of a root from a particular inoculum source would also depend on a variety of other factors such as the species involved, temperature, moisture, and soil texture. Planting crops at wider distances apart (i.e., reducing host density) is known to reduce the spread of root diseases because the practice limits dispersal. Some parasites are transferred by other species or other components of the environment (vectors), and their spread is tightly linked to dynamics of these factors. In soil, fungal spores, bacteria, and viruses are transported passively in water and can be transferred by invertebrate vectors such as plant-parasitic nematodes and mites.
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