Soil food web

The soil food web is the community of organisms living all or part of their lives in the soil. It describes a complex living system in the soil and how it interacts with the environment, plants, and animals.

Food webs describe the transfer of energy between species in an ecosystem. While a food chain examines one, linear, energy pathway through an ecosystem, a food web is more complex and illustrates all of the potential pathways. Much of this transferred energy comes from the sun. Plants use the sun’s energy to convert inorganic compounds into energy-rich, organic compounds, turning carbon dioxide and minerals into plant material by photosynthesis. Plant flowers exude energy-rich nectar above ground and plant roots exude acids, sugars, and ectoenzymes into the rhizosphere, adjusting the pH and feeding the food web underground.[2][3][4]

Plants are called autotrophs because they make their own energy; they are also called producers because they produce energy available for other organisms to eat. Heterotrophs are consumers that cannot make their own food. In order to obtain energy they eat plants or other heterotrophs.

Soil food webUSDA
An example of a topological food web (image courtesy of USDA)[1]

Above ground food webs

In above ground food webs, energy moves from producers (plants) to primary consumers (herbivores) and then to secondary consumers (predators). The phrase, trophic level, refers to the different levels or steps in the energy pathway. In other words, the producers, consumers, and decomposers are the main trophic levels. This chain of energy transferring from one species to another can continue several more times, but eventually ends. At the end of the food chain, decomposers such as bacteria and fungi break down dead plant and animal material into simple nutrients.

Methodology

The nature of soil makes direct observation of food webs difficult. Since soil organisms range in size from less than 0.1 mm (nematodes) to greater than 2 mm (earthworms) there are many different ways to extract them. Soil samples are often taken using a metal core. Larger macrofauna such as earthworms and insect larva can be removed by hand, but this is impossible for smaller nematodes and soil arthropods. Most methods to extract small organisms are dynamic; they depend on the ability of the organisms to move out of the soil. For example, a Berlese funnel, used to collect small arthropods, creates a light/heat gradient in the soil sample. As the microarthropods move down, away from the light and heat, they fall through a funnel and into a collection vial. A similar method, the Baermann funnel, is used for nematodes. The Baerman funnel is wet, however (while the Berlese funnel is dry) and does not depend on a light/heat gradient. Nematodes move out of the soil and to the bottom of the funnel because, as they move, they are denser than water and are unable to swim. Soil microbial communities are characterized in many different ways. The activity of microbes can be measured by their respiration and carbon dioxide release. The cellular components of microbes can be extracted from soil and genetically profiled, or microbial biomass can be calculated by weighing the soil before and after fumigation.

Types of food webs

Indirect interaction
An example of a soil interaction web. Image courtesy of Kalessin11.

There are three different types of food web representations: topological (or traditional) food webs, flow webs and interaction webs. These webs can describe systems both above and below ground.

Topological webs

Early food webs were topological; they were descriptive and provided a nonquantitative picture of consumers, resources and the links between them. Pimm et al. (1991) described these webs as a map of which organisms in a community eat which other kinds. The earliest topological food web, made in 1912, examined the predators and parasites of cotton boll weevil (reviewed by Pimm et al. 1991). Researchers analyzed and compared topological webs between ecosystems by measuring the web’s interaction chain lengths and connectivity.[5] One problem faced in standardizing such measurements is that there are often too many species for each to have a separate box. Depending on the author, the number of species aggregated or separated into functional groups may be different.[6] Authors may even eliminate some organisms. By convention, the dead material flowing back to detritus is not shown, as it would complicate the figure, but it is taken account in any calculations.[6]

Flow webs

Miosis build on interconnected food chains , adding quantitative information on the movement of carbon or other nutrients from producers to consumers. Hunt et al. (1987) published the first flow web for soil, describing the short grass prairie in Colorado, USA. The authors estimated nitrogen transferral rates through the soil food web and calculated nitrogen mineralization rates for a range of soil organisms. In another landmark study, researchers from the Lovinkhoeve Experimental Farm in the Netherlands examined the flow of carbon and illustrated transfer rates with arrows of different thicknesses.[7]

In order to create a flow web, a topological web is first constructed. After the members of the web are decided, the biomass of each functional group is calculated, usually in kg carbon/hectare. In order to calculate feeding rates, researchers assume that the population of the functional group is in equilibrium. At equilibrium, the reproduction of the group balances the rate at which members are lost through natural death and predation[8] When feeding rate is known, the efficiency with which nutrients are converted into organism biomass can be calculated. This energy stored in the organism represents the amount available to be passed on to the next trophic level.

After constructing the first soil flow webs, researchers discovered that nutrients and energy flowed from lower resources to higher trophic levels through three main channels.[7][8] The bacterial and fungal channels had the largest energy flow, while the herbivory channel, in which organisms directly consumed plant roots, was smaller. It is now widely recognized that bacteria and fungi are critical to the decomposition of carbon and nitrogen and play important roles in both the carbon cycle and nitrogen cycle.

Interaction web

An interaction web, shown above right,[9] is similar to a topological web, but instead of showing the movement of energy or materials, the arrows show how one group influences another. In interaction food web models, every link has two direct effects, one of the resource on the consumer and one of the consumer on the resource.[10] The effect of the resource on the consumer is positive, (the consumer gets to eat) and the effect on the resource by the consumer is negative (it is eaten). These direct, trophic, effects can lead to indirect effects. Indirect effects, represented by dashed lines, show the effect of one element on another to which it is not directly linked.[10] For example, in the simple interaction web below, when the predator eats the root herbivore, the plant eaten by the herbivore may increase in biomass. We would then say that the predator has a beneficial indirect effect on the plant roots.

Food web control

Bottom-up effects

Bottom-up effects occur when the density of a resource affects the density of its consumer.[11] For example, in the figure above, an increase in root density causes an increase in herbivore density that causes a corresponding increase in predator density. Correlations in abundance or biomass between consumers and their resources give evidence for bottom-up control.[11] An often-cited example of a bottom-up effect is the relationship between herbivores and the primary productivity of plants. In terrestrial ecosystems, the biomass of herbivores and detritivores increases with primary productivity. An increase in primary productivity will result in a larger influx of leaf litter into the soil ecosystem, which will provide more resources for bacterial and fungal populations to grow. More microbes will allow an increase in bacterial and fungal feeding nematodes, which are eaten by mites and other predatory nematodes. Thus, the entire food web swells as more resources are added to the base.[11] When ecologists use the term, bottom-up control, they are indicating that the biomass, abundance, or diversity of higher trophic levels depend on resources from lower trophic levels.[10]

Top-down effects

Ideas about top-down control are much more difficult to evaluate. Top-down effects occur when the population density of a consumer affects that of its resource;[10] for example, a predator affects the density of its prey. Top-down control, therefore, refers to situations where the abundance, diversity or biomass of lower trophic levels depends on effects from consumers at higher trophic levels.[10] A trophic cascade is a type of top-down interaction that describes the indirect effects of predators. In a trophic cascade, predators induce effects that cascade down food chain and affect biomass of organisms at least two links away.[10]

The importance of trophic cascades and top-down control in terrestrial ecosystems is actively debated in ecology (reviewed in Shurin et al. 2006) and the issue of whether trophic cascades occur in soils is no less complex[12] Trophic cascades do occur in both the bacterial and fungal energy channels.[13][14][15] However, cascades may be infrequent, because many other studies show no top-down effects of predators.[16][17] In Mikola and Setälä’s study, microbes eaten by nematodes grew faster when they were grazed upon frequently. This compensatory growth slowed when the microbe feeding nematodes were removed. Therefore, although top predators reduced the number of microbe feeding nematodes, there was no overall change in microbial biomass.

Besides the grazing effect, another barrier to top down control in soil ecosystems is widespread omnivory, which by increasing the number of trophic interactions, dampens effects from the top. The soil environment is also a matrix of different temperatures, moistures and nutrient levels, and many organisms are able to become dormant to withstand difficult times. Depending on conditions, predators may be separated from their potential prey by an insurmountable amount of space and time.

Any top-down effects that do occur will be limited in strength because soil food webs are donor controlled. Donor control means that consumers have little or no effect on the renewal or input of their resources.[10] For example, aboveground herbivores can overgraze an area and decrease the grass population, but decomposers cannot directly influence the rate of falling plant litter. They can only indirectly influence the rate of input into their system through nutrient recycling which, by helping plants to grow, eventually creates more litter and detritus to fall.[18] If the entire soil food web were completely donor controlled, however, bacterivores and fungivores would never greatly affect the bacteria and fungi they consume.

While bottom-up effects are no doubt important, many soil ecologists suspect that top-down effects are also sometimes significant. Certain predators or parasites, when added to the soil, can have a large effect on root herbivores and thereby indirectly affect plant fitness. For example, in a coastal shrubland food chain the native entomopathogenic nematode, Heterorhabditis marelatus, parasitized ghost moth caterpillars, and ghost moth caterpillars consumed the roots of bush lupine. The presence of H. marelatus correlated with lower caterpillar numbers and healthier plants. In addition, the researchers observed high mortality of bush lupine in the absence of entomopathogenic nematodes. These results implied that the nematode, as a natural enemy of the ghost moth caterpillar, protected the plant from damage. The authors even suggested that the interaction was strong enough to affect the population dynamics of bush lupine;[19] this was supported in later experimental work with naturally-growing populations of bush lupine.[20]

Top down control has applications in agriculture and is the principle behind biological control, the idea that plants can benefit from the application of their herbivore’s enemies. While wasps and ladybugs are commonly associated with biological control, parasitic nematodes and predatory mites are also added to the soil to suppress pest populations and preserve crop plants. In order to use such biological control agents effectively, a knowledge of the local soil food web is important.

Community matrix models

A community matrix model is a type of interaction web that uses differential equations to describe every link in the topological web. Using Lotka-Volterra equations, that describe predator-prey interactions, and food web energetics data such as biomass and feeding rate, the strength of interactions between groups is calculated.[21] Community matrix models can also show how small changes affect the overall stability of the web.

Stability of food webs

Mathematical modeling in food webs has raised the question of whether complex or simple food webs are more stable. Until the last decade, it was believed that soil food webs were relatively simple, with low degrees of connectance and omnivory.[12] These ideas stemmed from the mathematical models of May which predicted that complexity destabilized food webs. May used community matrices in which species were randomly linked with random interaction strength to show that local stability decreases with complexity (measured as connectance), diversity, and average interaction strength among species.[22]

The use of such random community matrices attracted much criticism. In other areas of ecology, it was realized that the food webs used to make these models were grossly oversimplified[23] and did not represent the complexity of real ecosystems. It also became clear that soil food webs did not conform to these predictions. Soil ecologists discovered that omnivory in food webs was common,[24] and that food chains could be long and complex[8] and still remain resistant to disturbance by drying, freezing, and fumigation.[12]

But why are complex food webs more stable? Many of the barriers to top-down trophic cascades also promote stability. Complex food webs may be more stable if the interaction strengths are weak[22] and soil food webs appear to consist of many weak interactions and a few strong ones.[21] Donor controlled food webs may be inherently more stable, because it is difficult for primary consumers to overtax their resources.[25] The structure of the soil also acts as a buffer, separating organisms and preventing strong interactions.[12] Many soil organisms, for example bacteria, can remain dormant through difficult times and reproduce quickly once conditions improve, making them resilient to disturbance.

Stability of the system is reduced by the use of nitrogen-containing inorganic and organic fertilizers, which cause soil acidification.

Interactions not included in food webs

Despite their complexity, some interactions between species in the soil are not easily classified by food webs. Litter transformers, mutualists, and ecosystem engineers all have strong impacts on their communities that cannot be characterized as either top-down or bottom-up.

Litter transformers, such as isopods, consume dead plants and excrete fecal pellets. While on the surface this may not seem impressive, the fecal pellets are moister and higher in nutrients than the surrounding soil, which favors colonization by bacteria and fungi. Decomposition of the fecal pellet by the microbes increases its nutrient value and the isopod is able to re-ingest the pellets. When the isopods consume nutrient-poor litter, the microbes enrich it for them and isopods prevented from eating their own feces can die.[26] This mutualistic relationship has been called an “external rumen”, similar to the mutualistic relationship between bacteria and cows. While the bacterial symbionts of cows live inside the rumen of their stomach, isopods depend on microbes outside their body.

Ecosystems engineers, such as earthworms, modify their environment and create habitat for other smaller organisms. Earthworms also stimulate microbial activity by increasing soil aeration and moisture, and transporting litter into the ground where it becomes available to other soil fauna.[12] Fungi create nutritional niche for other organisms by enriching nutritionally extremelly scarce food - the dead wood.[27] This allows xylophages to develop and in turn affect dead wood, contributing to wood decomposition and nutrient cycling in the forest floor.[28] In aboveground and aquatic food webs, the literature assumes that the most important interactions are competition and predation. While soil food webs fit these sorts of interactions well, future research needs to include more complex interactions such as mutualisms and habitat modification.

While they cannot characterize all interactions, soil food webs remain a useful tool for describing ecosystems. The interactions between species in the soil and their effect on decomposition continue to be well studied. Much remains unknown, however, about soil food webs stability and how food webs change over time.[12] This knowledge is critical to understanding how food webs affect important qualities such as soil fertility.

See also

References

  1. ^ "Soil Biology Primer Photo Gallery". Natural Resources Conservation Service - Soils. Soil and Water Conservation Society, U.S. Department of Agriculture. Retrieved 14 August 2016.
  2. ^ Marschner, Horst (1995). Mineral Nutrition of Higher Plants. ISBN 978-0124735439.
  3. ^ Walker, T. S.; Bais, H. P.; Grotewold, E.; Vivanco, J. M. (2003). "Root Exudation and Rhizosphere Biology". Plant Physiology. 132 (1): 44–51. doi:10.1104/pp.102.019661. PMC 1540314. PMID 12746510.
  4. ^ Power, Michael L. (2010). Anne M. Burrows; Leanne T. Nash (eds.). The Evolution of Exudativory in Primates / Nutritional and Digestive Challenges to Being a Gum-feeding Primate. Springer. p. 28. ISBN 9781441966612. Retrieved 2 October 2012.
  5. ^ Pimm S.L., Lawton J.H. & Cohen J.E. (1991), "Food web patterns and their consequences", Nature, 350 (6320): 669–674, Bibcode:1991Natur.350..669P, doi:10.1038/350669a0
  6. ^ a b de Ruiter P.C., A.M. Neutel and J.C. Moore (1996), "Energetics and stability in below ground food webs", in G. Polis and K.O Winemiller (eds.), Food webs: integration of patterns and dynamics, Chapman & HallCS1 maint: Uses editors parameter (link)
  7. ^ a b Brussaard, L.J., A. van Veen, M.J. Kooistra, and G. Lebbink (1988), "The Dutch Programme on soil ecology of arable farming systems I. Objectives, approach, and preliminary results", Ecological Bulletins, 39: 35–40CS1 maint: Multiple names: authors list (link)
  8. ^ a b c Hunt, H.W., D.C. Coleman, E.R. Ingham, R.E. Ingham, E.T. Elliott, J.C. Moore, S.L. Rose, C.P.P. Reid, and C.R. Morley (1987), "The detrital food web in a shortgrass prairie", Biology and Fertility of Soils, 3: 57–68CS1 maint: Multiple names: authors list (link)
  9. ^ USDA-NRCS, 2004, "The Soil Food web" in The Soil Biology Primer. Url accessed 2006–04-11
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  11. ^ a b c Shurin, J.B., D.S. Gruner, and H. Hillebrand (2006), "All wet or dried up? Real differences between aquatic and terrestrial food webs", Proceedings of the Royal Society, 273 (1582): 1–9, doi:10.1098/rspb.2005.3377, PMC 1560001, PMID 16519227CS1 maint: Multiple names: authors list (link)
  12. ^ a b c d e f Wardle, D.A (2002), Communities and Ecosystems: Linking the aboveground and belowground components Monographs in population biology, 31, Princeton University Press. New Jersey
  13. ^ Santos, P.F., J. Phillips, and W.G. Whitford (1981), "The role of mites and nematodes in early stages of buried litter decomposition in a desert", Ecology (Washington DC), 62 (3): 664–669, doi:10.2307/1937734, JSTOR 1937733CS1 maint: Multiple names: authors list (link)
  14. ^ Allen-Morley, C.R. & D.C. Coleman (1989), "Reliance of soil biota in various food webs to freezing perturbations", Ecology, 70 (4): 1127–1141, doi:10.2307/1941381, JSTOR 1941381
  15. ^ Katarina Hedlund & Maria Sjögren Öhrn (2000), "Tritrophic interactions in a soil community enhance decomposition rates", Oikos, 88 (3): 585–591, doi:10.1034/j.1600-0706.2000.880315.x, archived from the original on 2013-01-05
  16. ^ Mikola J. & H. Setälä (1998), "No evidence of tropic cascades in an experimental microbial-based food web", Ecology, 79: 153–164, doi:10.1890/0012-9658(1998)079[0153:NEOTCI]2.0.CO;2
  17. ^ Laakso J. & H. Setälä (1999), "Population- and ecosystem-effects of predation on microbial-feeding nematodes", Oecologia, 120 (2): 279–286, Bibcode:1999Oecol.120..279L, doi:10.1007/s004420050859, PMID 28308090
  18. ^ Moore, J.C., K. McCann, H. Setälä, and P.C. de Ruiter (2003), "Top down is bottom up: does predation in the rhizosphere regulate aboveground dynamics?", Ecology, 84 (4): 846–857, doi:10.1890/0012-9658(2003)084[0846:TIBDPI]2.0.CO;2CS1 maint: Multiple names: authors list (link)
  19. ^ Strong, D. R., H.K. Kaya, A.V. Whipple, A.L, Child, S. Kraig, M. Bondonno, K. Dyer, and J.L. Maron (1996), "Entomopathogenic nematodes: natural enemies of root-feeding caterpillars on bush lupine", Oecologia (Berlin), 108 (1): 167–173, Bibcode:1996Oecol.108..167S, doi:10.1007/BF00333228, PMID 28307747CS1 maint: Multiple names: authors list (link)
  20. ^ Evan L. Preisser & Donald R. Strong (2004), "Climate affects predator control of an herbivore outbreak", American Naturalist, 163 (5): 754–762, doi:10.1086/383620, PMID 15122492
  21. ^ a b de Ruiter, P.C., Neutel, A.-M., & Moore, J.C.; Neutel; Moore (1995), "Energetics, patterns of interaction strengths, and stability in real ecosystems", Science, 269 (5228): 1257–1260, Bibcode:1995Sci...269.1257D, doi:10.1126/science.269.5228.1257, PMID 17732112CS1 maint: Multiple names: authors list (link)
  22. ^ a b May, R.M (1973), Stability and complexity in model ecosystems, Princeton: Princeton University Press. New Jersey
  23. ^ Polis, G.A. (1991), "Complex trophic interactions in deserts: an empirical critique of food web theory", American Naturalist, 138: 123–155, doi:10.1086/285208
  24. ^ Walter, D.E. D.T. Kaplan & T.A. Permar (1991), "Missing links: a review of methods used to estimate trophic links in food webs" (Submitted manuscript), Agriculture, Ecosystems and Environment, 34: 399–405, doi:10.1016/0167-8809(91)90123-F
  25. ^ De Angelis, D.L. (1992), Dynamics of nutrient cycling and food webs, Chapman and Hall. London. England, ISBN 978-0-12-088458-2
  26. ^ Hassall, M., S.P. Rushton (1982), "The role of coprophagy in the feeding strategies of terrestrial isopods", Oecologia, 53 (3): 374–381, Bibcode:1982Oecol..53..374H, doi:10.1007/BF00389017, PMID 28311744CS1 maint: Multiple names: authors list (link)
  27. ^ Filipiak, Michał; Sobczyk, Łukasz; Weiner, January (2016-04-09). "Fungal Transformation of Tree Stumps into a Suitable Resource for Xylophagous Beetles via Changes in Elemental Ratios". Insects. 7 (2): 13. doi:10.3390/insects7020013. PMC 4931425.
  28. ^ Filipiak, Michał; Weiner, January (2016-09-01). "Nutritional dynamics during the development of xylophagous beetles related to changes in the stoichiometry of 11 elements". Physiological Entomology. 42: 73–84. doi:10.1111/phen.12168. ISSN 1365-3032.

External links

Albert Bates

Albert Kealiinui Bates (born January 1, 1947) is an influential figure in the intentional community and ecovillage movements.A lawyer, author and teacher, he has been director of the Global Village Institute for Appropriate Technology since 1984 and of the Ecovillage Training Center at The Farm in Summertown, Tennessee, since 1994.

Bates has been a resident of The Farm since 1972. A former attorney, he argued environmental and civil rights cases before the U.S. Supreme Court and drafted a number of legislative Acts during a 26-year legal career. The holder of a number of design patents, Bates invented the concentrating photovoltaic arrays and solar-powered automobile displayed at the 1982 World's Fair. He served on the steering committee of Plenty International for 18 years, focussing on relief and development work with indigenous peoples, human rights and the environment. An emergency medical technician (EMT), he was a founding member of The Farm Ambulance Service. He was also a licensed Amateur Radio operator.

Bokashi (horticulture)

Bokashi is a process that converts food waste and similar organic matter into a soil amendment which adds nutrients and improves soil texture. It differs from traditional composting methods in several respects. The most important are:

The input matter is fermented by specialist bacteria, not decomposed.

The fermented matter is fed directly to field or garden soil, without requiring further time to mature.

As a result virtually all input carbon, energy and nutrients enter the soil food web, having been neither emitted in greenhouse gases and heat nor leached out.Other names attributed to this process include bokashi composting, bokashi fermentation and fermented composting.

Compost

Compost ( or ) is organic matter that has been decomposed in a process called composting. This process recycles various organic materials otherwise regarded as waste products and produces a soil conditioner (the compost).

Compost is rich in nutrients. It is used, for example, in gardens, landscaping, horticulture, urban agriculture and organic farming. The compost itself is beneficial for the land in many ways, including as a soil conditioner, a fertilizer, addition of vital humus or humic acids, and as a natural pesticide for soil. In ecosystems, compost is useful for erosion control, land and stream reclamation, wetland construction, and as landfill cover (see compost uses).

At the simplest level, the process of composting requires making a heap of wet organic matter (also called green waste), such as leaves, grass, and food scraps, and waiting for the materials to break down into humus after a period of months. However, composting also can take place as a multi-step, closely monitored process with measured inputs of water, air, and carbon- and nitrogen-rich materials. The decomposition process is aided by shredding the plant matter, adding water and ensuring proper aeration by regularly turning the mixture when open piles or "windrows" are used. Earthworms and fungi further break up the material. Bacteria requiring oxygen to function (aerobic bacteria) and fungi manage the chemical process by converting the inputs into heat, carbon dioxide, and ammonium.

Detritus

In biology, detritus () is dead particulate organic material (as opposed to dissolved organic material). It typically includes the bodies or fragments of dead organisms as well as fecal material. Detritus is typically colonized by communities of microorganisms which act to decompose (or remineralize) the material. In terrestrial ecosystems, it is encountered as leaf litter and other organic matter intermixed with soil, which is denominated "soil organic matter". Detritus of aquatic ecosystems is organic material suspended in water and piling up on seabed floors, which is referred to as marine snow.

Elaine Ingham

Elaine Ingham is an American microbiologist and soil biology researcher and founder of Soil Foodweb Inc. She is known as a leader in soil microbiology and research of the soil food web. She is an author of the USDA's Soil Biology Primer. In 2011, Ingham was named as The Rodale Institute's chief scientist.

Food web

A food web (or food cycle) is the natural interconnection of food chains and a graphical representation (usually an image) of what-eats-what in an ecological community. Another name for food web is consumer-resource system. Ecologists can broadly lump all life forms into one of two categories called trophic levels: 1) the autotrophs, and 2) the heterotrophs. To maintain their bodies, grow, develop, and to reproduce, autotrophs produce organic matter from inorganic substances, including both minerals and gases such as carbon dioxide. These chemical reactions require energy, which mainly comes from the Sun and largely by photosynthesis, although a very small amount comes from hydrothermal vents and hot springs. A gradient exists between trophic levels running from complete autotrophs that obtain their sole source of carbon from the atmosphere, to mixotrophs (such as carnivorous plants) that are autotrophic organisms that partially obtain organic matter from sources other than the atmosphere, and complete heterotrophs that must feed to obtain organic matter. The linkages in a food web illustrate the feeding pathways, such as where heterotrophs obtain organic matter by feeding on autotrophs and other heterotrophs. The food web is a simplified illustration of the various methods of feeding that links an ecosystem into a unified system of exchange. There are different kinds of feeding relations that can be roughly divided into herbivory, carnivory, scavenging and parasitism. Some of the organic matter eaten by heterotrophs, such as sugars, provides energy. Autotrophs and heterotrophs come in all sizes, from microscopic to many tonnes - from cyanobacteria to giant redwoods, and from viruses and bdellovibrio to blue whales.

Charles Elton pioneered the concept of food cycles, food chains, and food size in his classical 1927 book "Animal Ecology"; Elton's 'food cycle' was replaced by 'food web' in a subsequent ecological text. Elton organized species into functional groups, which was the basis for Raymond Lindeman's classic and landmark paper in 1942 on trophic dynamics. Lindeman emphasized the important role of decomposer organisms in a trophic system of classification. The notion of a food web has a historical foothold in the writings of Charles Darwin and his terminology, including an "entangled bank", "web of life", "web of complex relations", and in reference to the decomposition actions of earthworms he talked about "the continued movement of the particles of earth". Even earlier, in 1768 John Bruckner described nature as "one continued web of life".

Food webs are limited representations of real ecosystems as they necessarily aggregate many species into trophic species, which are functional groups of species that have the same predators and prey in a food web. Ecologists use these simplifications in quantitative (or mathematical representation) models of trophic or consumer-resource systems dynamics. Using these models they can measure and test for generalized patterns in the structure of real food web networks. Ecologists have identified non-random properties in the topographic structure of food webs. Published examples that are used in meta analysis are of variable quality with omissions. However, the number of empirical studies on community webs is on the rise and the mathematical treatment of food webs using network theory had identified patterns that are common to all. Scaling laws, for example, predict a relationship between the topology of food web predator-prey linkages and levels of species richness.

Index of soil-related articles

This is an index of articles relating to soil.

John D. Hamaker

John D. Hamaker (1914–1994), was an American mechanical engineer, ecologist, agronomist and science writer in the fields of soil regeneration, rock dusting, mineral cycles, climate cycles and glaciology.

Manure

Manure is organic matter, mostly derived from animal feces except in the case of green manure, which can be used as organic fertilizer in agriculture. Manures contribute to the fertility of the soil by adding organic matter and nutrients, such as nitrogen, that are utilised by bacteria, fungi and other organisms in the soil. Higher organisms then feed on the fungi and bacteria in a chain of life that comprises the soil food web.

In the past, the term "manure" included inorganic fertilizers, but this usage is now very rare.

Microbivory

Microbivory (adj. microbivorous, microbivore) is a feeding behavior consisting of eating microbes (especially bacteria) practiced by animals of the mesofauna, microfauna and meiofauna.Microbivorous animals include some soil nematodes, springtails or flies such as Drosophila sharpi. A well known example of microbivorous nematodes is the model roundworm Caenorhabditis elegans which is maintained in culture in labs on agar plates, fed with the 'OP50' Escherichia coli strain of bacteria.

In food webs of ecosystems, microbivores can be distinguished from detritivores, generally thought playing the roles of decomposers, as they don't consume decaying dead matter but only living microorganisms.

Outline of sustainable agriculture

The following outline is provided as an overview of and topical guide to sustainable agriculture:

Sustainable agriculture – applied science that integrates three main goals, environmental health, economic profitability, and social and economic equity. These goals have been defined by a variety of philosophies, policies and practices, from the vision of farmers and consumers. Perspectives and approaches are very diverse, the following topics intend to help understanding what sustainable agriculture is.

Regenerative agriculture

Regenerative agriculture is a conservation and rehabilitation approach to food and farming systems. It focuses on topsoil regeneration, increasing biodiversity, improving the water cycle, enhancing ecosystem services, supporting biosequestration, increasing resilience to climate change, and strengthening the health and vitality of farm soil. Practices include, recycling as much farm waste as possible, and adding composted material from sources outside the farm.Regenerative agriculture on small farms and gardens is often based on ideologies like permaculture, agroecology, agroforestry, restoration ecology, keyline design and holistic management. Large farms tend to be less ideology driven, and often use "no-till" and/or "reduced till" practices.

On a regenerative farm, yield should increase over time. As the topsoil deepens, production may increase and less external compost inputs are required. Actual output is dependent on the nutritional value of the composting materials, and the structure and content of the soil.

Rhizosphere

The rhizosphere is the narrow region of soil that is directly influenced by root secretions, and associated soil microorganisms known as the root microbiome. The rhizosphere contains many bacteria and other microorganisms that feed on sloughed-off plant cells, termed rhizodeposition, and the proteins and sugars released by roots. This symbiosis leads to more complex interactions, influencing plant growth and competition for resources. Much of the nutrient cycling and disease suppression needed by plants occurs immediately adjacent to roots due to root exudants and communities of microorganisms. The rhizosphere also provides space to produce allelochemicals to control neighbours and relatives. The plant-soil feedback loop and other physical factors are important selective pressures for the communities and growth in the rhizosphere.

Soil biology

Soil biology is the study of microbial and faunal activity and ecology in soil.

Soil life, soil biota, soil fauna, or edaphon is a collective term that encompasses all organisms that spend a significant portion of their life cycle within a soil profile, or at the soil-litter interface.

These organisms include earthworms, nematodes, protozoa, fungi, bacteria, different arthropods, as well as some reptiles (such as snakes), and species of burrowing mammals like gophers, moles and prairie dogs. Soil biology plays a vital role in determining many soil characteristics. The decomposition of organic matter by soil organisms has an immense influence on soil fertility, plant growth, soil structure, and carbon storage. As a relatively new science, much remains unknown about soil biology and its effect on soil ecosystems.

Soil carbon

Soil carbon includes both inorganic carbon as carbonate minerals, and as soil organic matter. Soil carbon plays a key role in the carbon cycle, and thus it is important in global climate models.

Soil ecology

Soil ecology is the study of the interactions among soil biology, and between biotic and abiotic aspects of the soil environment. It is particularly concerned with the cycling of nutrients, formation and stabilization of the pore structure, the spread and vitality of pathogens, and the biodiversity of this rich biological community.

Trophic cascade

Trophic cascades are powerful indirect interactions that can control entire ecosystems, occurring when a trophic level in a food web is suppressed. For example, a top-down cascade will occur if predators are effective enough in predation to reduce the abundance, or alter the behavior, of their prey, thereby releasing the next lower trophic level from predation (or herbivory if the intermediate trophic level is a herbivore).

The trophic cascade is an ecological concept which has stimulated new research in many areas of ecology. For example, it can be important for understanding the knock-on effects of removing top predators from food webs, as humans have done in many places through hunting and fishing.

A top-down cascade is a trophic cascade where the top consumer/predator controls the primary consumer population. In turn, the primary producer population thrives. The removal of the top predator can alter the food web dynamics. In this case, the primary consumers would overpopulate and exploit the primary producers. Eventually there would not be enough primary producers to sustain the consumer population. Top-down food web stability depends on competition and predation in the higher trophic levels. Invasive species can also alter this cascade by removing or becoming a top predator. This interaction may not always be negative. Studies have shown that certain invasive species have begun to shift cascades; and as a consequence, ecosystem degradation has been repaired.For example, if the abundance of large piscivorous fish is increased in a lake, the abundance of their prey, smaller fish that eat zooplankton, should decrease. The resulting increase in zooplankton should, in turn, cause the biomass of its prey, phytoplankton, to decrease.

In a bottom-up cascade, the population of primary producers will always control the increase/decrease of the energy in the higher trophic levels. Primary producers are plants, phytoplankton and zooplankton that require photosynthesis. Although light is important, primary producer populations are altered by the amount of nutrients in the system. This food web relies on the availability and limitation of resources. All populations will experience growth if there is initially a large amount of nutrients.In a subsidy cascade, species populations at one trophic level can be supplemented by external food. For example, native animals can forage on resources that don't originate in their same habitat, such a native predators eating livestock. This may increase their local abundances thereby affecting other species in the ecosystem and causing an ecological cascade. For example, Luskin et al (2017) found that native animals living in protected primary rainforest in Malaysia found food subsidies in neighboring oil palm plantations. This subsidy allowed native animal populations to increase, which then triggered powerful secondary ‘cascading’ effects on forest tree community. Specifically, crop-raiding wild boar (Sus scrofa) built thousands of nests from the forest understory vegetation and this caused a 62% decline in forest tree sapling density over a 24-year study period. Such cross-boundary subsidy cascades may be widespread in both terrestrial and marine ecosystems and present significant conservation challenges.

These trophic interactions shape patterns of biodiversity globally. Humans and climate change have affected these cascades drastically. One example can be seen with sea otters (Enhydra lutris) on the Pacific coast of the United States of America. Over time, human interactions caused a removal of sea otters. One of their main prey, the pacific purple sea urchin (Strongylocentrotus purpuratus) eventually began to overpopulate. The overpopulation caused increased predation of giant kelp (Macrocystis pyrifera). As a result, there was extreme deterioration of the kelp forests along the California coast. This is why it is important for countries to regulate marine and terrestrial ecosystems.Predator-induced interactions could heavily influence the flux of atmospheric carbon if managed on a global scale. For example, a study was conducted to determine the cost of potential stored carbon in living kelp biomass in Sea Otter enhanced ecosystems. The study valued the potential storage between $205 million and $408 million dollars (US) on the European Carbon Exchange (2012).

Urban agriculture

Urban agriculture, urban farming, or urban gardening is the practice of cultivating, processing and distributing food in or around urban areas. Urban agriculture can also involve animal husbandry, aquaculture, agroforestry, urban beekeeping, and horticulture. These activities occur in peri-urban areas as well, and peri-urban agriculture may have different characteristics.Urban agriculture can reflect varying levels of economic and social development. It may be a social movement for sustainable communities, where organic growers, "foodies," and "locavores" form social networks founded on a shared ethos of nature and community holism. These networks can evolve when receiving formal institutional support, becoming integrated into local town planning as a "transition town" movement for sustainable urban development. For others, food security, nutrition, and income generation are key motivations for the practice. In either case, more direct access to fresh vegetables, fruits, and meat products through urban agriculture can improve food security and food safety.

General
Producers
Consumers
Decomposers
Microorganisms
Food webs
Example webs
Processes
Defense,
counter
Ecology: Modelling ecosystems: Other components
Population
ecology
Species
Species
interaction
Spatial
ecology
Niche
Other
networks
Other
Topics in soil science
Main fields
Soil topics
Soil type
Applications
Related fields
Societies, Initiatives
Scientific journals
See also

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