Detritus
In biology, detritus is non-living 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, detritus refers to leaf litter and other organic matter intermixed with soil, here known as humus.
In aquatic ecosystems, detritus refers to organic material suspended in water, here known as marine snow.
Dead plants or animals, material derived from animal tissues (such as skin cast off during moulting and excreta) gradually lose their form, due to both physical processes and the action of decomposers, including grazers, bacteria and fungi. Decomposition, the process through which organic matter is decomposed, takes place in many stages. Materials like proteins, lipids and sugars with low molecular weight are rapidly consumed and absorbed by micro-organisms and organisms that feed on dead matter. Other compounds, such as complex carbohydrates are broken down more slowly. In addition, the purpose of the various micro-organisms involved is not to break down these materials but to use them to gain the resources they require for their own survival and proliferation, and they are merely breaking them down as part of that process. Accordingly, at the same time that the materials of plants and animals are being broken down, the materials (biomass) making up the bodies of the micro-organisms are built up by a process of assimilation. When micro-organisms die, fine organic particles are produced, and if these are eaten by small animals which feed on micro-organisms, they will collect inside the intestine, and change shape into large pellets of dung. As a result of this process, most of the materials from dead organisms disappears from view and is not obviously present in any recognisable form, but is in fact present in the form of a combination of fine organic particles and the organisms using them as nutrients. This combination is detritus.
In ecosystems on land, detritus is deposited on the surface of the ground, taking forms such as the humic soil beneath a layer of fallen leaves. In aquatic ecosystems, most detritus is suspended in water, and gradually settles. In particular, many different types of material are collected together by currents, and much material settles in slowly-flowing areas.
Much detritus is used as a source of nutrition for animals. In particular, many bottom feeding animals (benthos) living in mud flats feed in this way. In particular, since excreta are materials which other animals do not need, whatever energy value they might have, they are often unbalanced as a source of nutrients, and are not suitable as a source of nutrition on their own. However, there are many micro-organisms which multiply in natural environments. These micro-organisms do not simply absorb nutrients from these particles, but also shape their own bodies so that they can take the resources they lack from the area around them, and this allows them to make use of excreta as a source of nutrients. In practical terms, the most important constituents of detritus are complex carbohydrates, which are persistent (difficult to break down), and the micro-organisms which multiply using these absorb carbon from the detritus, and materials such as nitrogen and phosphorus from the water in their environment to synthesise the components of their own cells.
A characteristic type of food chain called the detritus cycle takes place involving detritus feeders (detritivores), detritus and the micro-organisms that multiply on it. For example, mud flats are inhabited by many univalves which are detritus feeders, such as moon shells. When these detritus feeders take in detritus with micro-organisms multiplying on it, they mainly break down and absorb the micro-organisms, which are rich in proteins, and excrete the detritus, which is mostly complex carbohydrates, having hardly broken it down at all. At first this dung is a poor source of nutrition, and so univalves pay no attention to it, but after several days, micro-organisms begin to multiply on it again, its nutritional balance improves, and so they eat it again. Through this process of eating the detritus many times over and harvesting the micro-organisms from it, the detritus thins out, becomes fractured and becomes easier for the micro-organisms to use, and so the complex carbohydrates are also steadily broken down and disappear over time.
What is left behind by the detritivores is then further broken down and recycled by decomposers, such as bacteria and fungi.
This detritus cycle plays a large part in the so-called purification process, whereby organic materials carried in by rivers is broken down and disappears, and an extremely important part in the breeding and growth of marine resources. In ecosystems on land, far more essential material is broken down as dead material passing through the detritus chain than is broken down by being eaten by animals in a living state. In both land and aquatic ecosystems, the role played by detritus is too large to ignore.
