The soil food web is the community of organisms that are
interdependent for sources of carbon and energy.
Communities are variously defined but a common thread in the
definitions is interaction for resources, including food and space:
|"...(communities are) not mere assemblages of species
living together, but form closely-knit communities or societies similar
to our own." C. Elton (1927)
"an assemblage of populations of plants, animals, bacteria and fungi
that live in an environment and interact with one another, forming
together a distinctive living system with its own composition,
structure, environmental relations, development and function" R.
"A collection of organisms in an environment" J. Emlen (1977)
"Organisms that interact in a given area" P. Price (1984)
"Associations of plants and animals that are spatially delimited and
that are dominated by one or more prominent species or by a physical
characterisitic" R. Ricklefs (1990)
"Community: the species that occur together in space and time" Begon,
Harper, and Townsend (1996)
The Nematodes' Picnic
Lyrics: Kathy Merrifield
Vocals: Pointless Sisters
From Lewis Carroll:
Big fleas have little fleas
Upon their backs to bite'em
And little fleas have smaller fleas
And so ad infinitum
Resources and Roles in Foodwebs
- Organisms can be classified as
autotrophs or heterotrophs.
- Autotrophs obtain their
carbon and energy by fixing atmospheric CO2 using light as an
energy source (photosynthesis). Green plants are autotrophs. Some
autotrophic bacteria use chemical reactions as the energy source, e.g.
S-reducing bacteria use electrons from sulphur and reduce it to H2S.
- Heterotrophs obtain their
carbon and energy from other organisms. They consume them while still alive
(parasites), or kill them in the act of consumption (predators), or feed on
their waste products, excretions, secretions, or remains (detritivores).
Most organisms other than green plants are heterotrophs.
- The food supply to soil organisms
originates through two main channels:
- The photosynthetic activities of
plants. Plant roots leak - root exudates; root tissues are sloughed during
growth, when damaged, or when no longer useful to the plant. Herbivores
graze on or parasitize plant roots. All the organisms exploiting the primary
producer in this manner become food and energy sources for other organisms
in the soil foodweb.
- Plant residues, manure and other
organic material falling on or applied to the soil surface, or incorporated
into the soil. All the organisms decomposing this material also become food
and energy sources for other organisms in the soil foodweb.
- Bacteria, fungi, plant-parasitic
nematodes, root-grazing insects, gophers, etc. feed directly on plant roots
as primary consumers. They are subject to parasitism and predation by other
organisms; they die, defecate, excrete, etc. and provide food for other
- Bacteria and fungi assimilate plant
secretions, and plant debris. They decompose dead organic matter of
intrinsic or extrinsic origin. In general, more resistant substrates and
substrates with higher C:N ratios are more likely to be exploited by fungi
than by bacteria, while more labile substrates and those with lower C:N
ratios may be predominated by bacteria.
- Nematodes (e.g. Tylenchida: Aphelenchina
and arthropods (Collembola and mites) feed on fungi. Nematodes (e.g. Rhabditida), and Protozoa (flagellates and amoebae) feed on bacteria.
Predaceous nematodes (e.g. Mononchus), omnivorous nematodes (e.g.
arthropods (e.g. mites, Collembola), Protozoa (e.g. amoebae),
etc. feed on nematodes. Arthropods (e.g. predaceous mites) feed on
mites; Collembola and mites are parasitized by bacteria and fungi.
- At every trophic interaction, the
debris and leakage become substrate for other organisms; the wastes and
secretions of the new owners of the carbon molecules originally fixed by the
autotroph are substrate for other organisms.
- Carbon, Nitrogen, and other
molecules are mineralized during the metabolic processes of all organisms in
the web through
osmoregulation, etc. The mineralized molecules
(CO2, NH4, NO3, etc) are available to
plants, bacteria and fungi in the soil, or are returned to the atmosphere.
- Organisms in the soil tend to be
aggregated in areas where carbon and energy originally enters the foodweb,
e.g., in the plant rhizosphere, close to the soil surface, or in the tillage
- The size of the foodweb is limited
by the amount of carbon and energy entering it. As much as 90% of the
remaining resources may be lost at each trophic interchange. That limits the
length of chains or channels running through the web to four or five
interchanges. In other words, a carbon molecule entering the web is unlikely
to pass through more than five different organisms without being respired.
In foodwebs limited in size by minimal carbon input the number of
interchanges will be fewer as there are insufficient resources to support
organisms at higher trophic levels. In that case, turnover of the microbial
biomass may be small and minerals will be immobilized.
- Organisms at higher trophic levels
in the soil foodweb are often more susceptible to disturbance than those
smaller-bodied organisms at lower trophic levels. Further, the organisms at
lower trophic levels are often opportunists, responding rapidly to
availability of resources. Consequently, foodwebs in disturbed systems tend
to be predominated by primary decomposers and direct herbivores since their
predators are absent. That again leads to immobilization of minerals by the opportunists and to a lack of biological regulation of their abundance and
- Decomposition of organic matter
- Cycling of minerals and nutrients
- Redistribution of minerals and
nutrients in space and time
- Reservoirs of minerals and nutrients
- Sequestration of carbon
- Detoxification of pollutants
- Modification of soil structure
- Community self-regulation
- Biological regulation
or suppression of pest
proportion of the potential links in the foodweb that are actually realized.
the number of links in the food web that perform the same "function";
complex foodwebs should have greater redundancy.
lack of change in the "function" of the food web when a link is broken
or a node removed; greater redundancy leads to greater functional resilience.
Suppressive: when there are sufficient
predators of various kinds in the food web that populations of opportunistic
species are actually reduced, i.e. the integral effect of the food web is
suppressive to the opportunistic organisms.
Conducive: there are few higher trophic layers
in the food web so that there is little predation on opportunists. The structure
of the soil community is such that it will "allow", or at least not prevent,
increase of opportunists.
Regulated: somewhere between conducive and
suppressive. Opportunists may not decline in number but will also not increase
exponentially. Their populations are regulated at relatively constant levels by
the combined effect of various predators in the food web.
Remember, the predators of nematodes are not only other nematodes but also
certain fungi, mites, collembola, protozoa, some bacteria, etc. The more
abundant these various guilds of organisms, the more likely that opportunistic
prey species will be regulated or suppressed.
Assessment and Monitoring of Soil
- Structural Analysis
- Physical: Sample,
extract, identify, enumerate each organism group - bacteria, fungi, protozoa,
nematodes, arthropods, annelids, rodents…….., etc. (An enormous job!)
- Biochemical Analysis:
PLFAs, DNA profiles, etc. indicate the presence of key organism groups or taxa.
(Probably access relatively few taxa)
- Indicator Guilds:
Monitor the presence and abundance of key taxa that are indicative of the
presence of specific trophic guilds that perform critical or desired
functions. (Soil nematode guilds occur at all trophic levels in food webs,
they can be monitored by simple standardized techniques and are proving to be
useful indicators of foodweb status).
Confirm that key functions are occurring at desired rates; measure rates of
suppressiveness, decomposition, mineralization, respiration……etc. (Useful, but difficult to
interpret in terms of the key players in the system).
Bongers, T. and H. Ferris. 1999. Nematode community structure as a
bioindicator in environmental monitoring. Trends in Evolution and Ecology
Ferris, H., T. Bongers, and R. G. M. de Goede. 2001. A framework for soil
food web diagnostics: extension of the nematode faunal analysis concept. Applied
Soil Ecology 18:13-29.
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