Decomposer Insects

DECOMPOSER INSECTS Eduardo Galante and Mª Angeles Marcos-Garcia Centro Iberoamericano de la Biodiversidad Alicante, Spain In any natural or semi-natural that the decomposers operate at all the habitat, three types of organisms exist: levels. Thus, all energy not used by the producers, consumers and consumers and producers, as well as decomposers. Good functioning of the that accumulated in excretory products, ecosystem will depend on their suitable is used by the decomposers and action and interaction. Organisms recycled into the ecosystem. This capable of catching energy and process constitutes the energy cycle , synthesizing organic matter from and good functioning and subsistence of inorganic compounds constitute the the ecosystems depends on effective producers. Though chemosynthetic energy cycling. bacteria exist, most producers are In a terrestrial ecosystem, more plants. Green plants use solar energy than 90% of the organic matter and fix CO2, producing organic synthesized by the green plants remains compounds rich in energy. Most of this unconsumed, and it passes to the level accumulated energy will get used in the of decomposers as plant material ecosystem by respiratory processes and decaying on soil, together with the other vital functions, whereas some remains of animal corpses, and the organisms of the community such as the excretory products of all levels. consumers and decomposers will use The process of decomposition is the energy accumulated by other one of the most important events in the organisms. functioning of ecosystems. The consumers are heterotrophic Decomposition can be defined as the organisms that obtain food from process by means of which a dead producers or from other consumers, with organism or its remains are broken the possibility of several levels of down into the parts or elements that complexity inside an ecosystem. comprise it, and at the end of the Primary consumers feed directly on the process the animal or plant remains will producers. Secondary consumers feed have gradually disintegrated until their on primary consumers, etc. Finally, we structures cannot be recognized, and have the decomposers, saprophytic their complex organic molecules will organisms feeding on dead matter or have fragmented. This involves a decaying remains derived from complex process in which biotic and producers and consumers. Thus, the abiotic agents interact within the organic matter synthesized by ecosystem. To sum up, the producers goes on to other levels of decomposition processes cause the organisms across the trophic chains, liberation of energy and mineralization though much of the energy will be used of chemical nutrients, turning the in respiratory processes at all levels. organic matter into inorganic elements. In these trophic processes, an In the process of decomposition, important element to take into account is there are two phases, sometimes difficult to distinguish, that we could call centipedes (Diplopoda), woodlice ‘destruction’ and ‘degradation’ of the (Isopoda), etc. All these groups are organic matter. The process of responsible for the fragmentation of destruction refers to the initial phase of plant or animal remains, contributing to the decomposition cycle and is the destruction phase. They contribute characterized by breaking down the both to the redistribution of the organic organic remains by mechanical means remains and formation of soil elements. so that at the end of this process small- These groups of arthropod sized particles are obtained. During this decomposers are present in nearly all initial phase both abiotic (rain, wind, terrestrial habitats, and generally in very temperature, etc.) and biotic elements high numbers. In some cases, millions (decomposer fauna) play an important of individuals belonging to hundreds of role. In the second phase, degradation species have been identified in just one of the organic matter occurs, resulting in square meter. Especially in temperate the disintegration of the small particles areas, arthropods are the major into molecules, producing CO2, H2O and decomposers, playing a very important mineral salts as final products. role in degradation of waste. Thanks to The destruction phase also the action of the arthropods during the results in dispersion of the organic initial phase of fragmentation, organic matter, because the small resulting remains can be degraded, and particles can be taken by means of eliminated from the soil surface. diverse mechanisms out of the initial When a suitable community of source (dragging by wind and water, or decomposers exists, it prevents the direct action of animals such as burial, appearance of potential bottlenecks in ingestion, movement, etc.). It is the recycling of basic elements in the important to note that nutrients released ecosystem. Thus, when entomofauna by decomposers constitute a food capable of acting effectively on plant source for numerous animal species, and animal remains does not exist in a extending the cycle by means of terrestrial ecosystem, serious alterations incorporation of the nutrients into in the ecosystem arise, with a reduction tissues, and eventually constituting new in energy flow and great loss of corpses and excrements. biodiversity. The groups of arthropods involved in the processes of Decomposers of plant remains decomposition of animals and plants Arthropods play an important role remains belong to many taxa (Fig. 1). in degradation processes of plant They are considered to be mesofauna remains. Nevertheless, most lack the when their size ranges between 100 and ability to develop enzymatic processes 200 mm (such as mites (Acari), for degrading the fundamental springtails (Collembola) and small components of any plant: lignin and insects), or macrofauna when they are cellulose. Degradation of cellulose larger, such as some beetles (e.g., requires the presence of the enzyme Scarabaeidae, Geotrupidae or cellulase, which most arthropods lack. Silphidae), Diptera larvae (e.g., Many insects have solved this problem Muscidae, Sarcophagidae, by means of mutualistic relations with Scatophagidae or Calliphoridae), micro-organisms, having bacteria or symbiotic protozoa in the intestinal tract. (bacteria, fungi and protozoa) and plant Others take advantage the cellulase remains is very close, and therefore, in produced by external microflora. Among most cases it is inevitable that the the well-known insect decomposers are arthropods consume both resources termites (Isoptera) and cockroaches simultaneously. In some cases, the (Blattodea). The termites possess ingested biomass of micro-organisms is symbiotic bacteria and protozoa, and in more important nutritionally than the their absence wood cannot be ingested plant remains. assimilated by these insects. In many ecosystems millipedes The entomofauna of excrement and (Diplopoda) have special importance as corpses decomposers. These arthropods, which The studies of ecosystems have specialize in leaf litter consumption, generally paid preferential attention to sometimes are abundant, concentrated the decomposition processes of in relatively small areas, and active vegetative remains, due to the during a great part of the year. importance that litter has in the Another important group in the composition of the soil and the degradation of plant remains is woodlice contribution of its nutrients. The process (Isopoda), which also possess symbiotic of decomposition of carrion and dung, microorganisms in their intestine that though sometimes not well known and allows them to degrade cellulose. little studied in many ecosystems, is Not all arthropods assimilate also important. cellulose by means of symbiotic To understand the processes of bacteria, but they make use of woody degradation of corpses and excrements, materials that are pre-digested by and the attractiveness of this decaying extraintestinal microorganisms. Included matter to arthropods, we must consider in this group are some species of that they are very rich resources of springtails such as Tomocerus organic components that have very (Collembola), ambrosia beetles special microclimatic conditions. Dung (Coleoptera: Scolitydae), ants of the and carrion represent not only a rich genera Atta (Hymenoptera: Formicidae) source of energy, but also a very and termites (Isoptera) that cultivate specialized habitat that is exploited, in fungi. most cases, by very specific All plant remains do not always entomological fauna. This fauna obtains present the same difficulty of digestion food either directly, such as in the case for arthropods. Fruits are extensively of the coprophages and carrion-eating exploited by arthropods, thanks to the arthropods, or indirectly, such as in the existence of yeasts that enhance fruit case of the predators that feed on the decomposition, allowing many decomposers. arthropods such as flies (Drosophila is It is generally difficult to establish probably the best known example) and the limits of an arthropod community, wasps (Hymenoptera: Vespidae) to feed but when we study carrion or dung we into the product resulting from the meet a perfectly defined ecological unit, fermentation. Nevertheless, a fact that limited in both space and time. Carrion must be remembered is that the (Fig. 2) and dung present many special relationship between micro-organisms characteristics that influence the composition and dynamics of the set of taxonomic categories of carrion or dung species using them. These resources specialists are similar worldwide. constitute isolated

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