20 April 2005
Australian Termites and Nutrient Recycling
Australia’s outback is characterized by minimal rainfall and nutrient deficient soils, which has led to slow nutrient cycling between plants and soils. Termites have adapted to this climate in astounding ways, particularly by their role as decomposers. Their symbiotic relationship with intestinal protozoa and bacteria allows them to digest grass, wood, and other debris. Termites are also capable of forming new soil.
Using their excrements, termites create structured mounds that regulate the microclimate for their survival. Depending on the species of termite and the surrounding environment, the mounds take on a variety of shapes and sizes. Along with the consumption of detritus materials, these mounds illustrate how termites are well suited for their environment. Nutrient recycling, soil formation, and their distinct methods in survival exemplify how termites are important contributors to outback ecology.
Australia is a captivating continent with a variety of environments like rainforests, dry grasslands, farmland, and aquatic reefs. Within the center of Australia, one finds an extremely arid setting with poor soil quality and slow cycling of nutrients between soil and plants. In this same area, termites thrive by consuming detritus material and building nests in which they can control their internal environment. These decomposers aid in nutrient cycling and soil formation by digesting debris and translocating soils. For centuries, termites have been an important factor in Australian ecology.
Australia’s soils are influenced by its geological history, current climate, and human impacts. Australia was initially part of the supercontinent, Pangaea, 400 – 600 million years ago. Pangaea later split into Laurasia and Gondwana with Australia being part of Gondwana. About 45 – 50 million years ago, Australia broke away to become a separate land mass. Throughout this time, the climates of Australia varied from tropical, to much colder, to arid. For about the past 35 million years, Australia has experienced a warming climate and increased aridity. There has been little formation of new fertile soil, because there has been very little volcanic activity or ice sheets moving over the continent. Australia’s soils are, therefore, underlain by salt and among the least fertile in the world (Aplin, 1998; Taylor, 2000).
Humans also influence soil quality. For the past 100 years, governments encouraged settlers and farmers to clear away native vegetation. This has led to water leaching through the soil and increased dryland salinity. Australian farmers’ use of traditional European farming methods also have led to “ . . . alarming levels of salinity, soil acidification, wind and water erosion, soil structural decline and loss of fertility, waterlogging, sodicity, non-wetting soils and so on . . . “ (Taylor, 2000).
Some vegetation has adapted to these harsh conditions by evolving to withstand the arid environment and infertile soil. Other than just evolutionary processes, plants are aided by other organisms to obtain sufficient nutrients for growth, namely, termites. These insects, too, have adapted to outback conditions and contribute to soil formation and biogeochemical (nutrient) recycling. The major biogeochemical cycles are: hydrologic, carbon, oxygen, nitrogen, sulfur, and phosphorus (Curtis, 1996).
There are close to 3,000 species of termites that belong to the order Isoptera. Isoptera consists of seven families that are phylogenetically separated into lower and higher termites. Mastotermitidae, Kalotermitidae, Hodotermitidae, Termopsidae, Rhinotermitidae, and Serritermitidae make up the lower termites. The Termitidae is the higher family of termites (Kambhampati and Eggleton, 2000).
Termites are further divided into either wood-dwelling or soil-dwelling. The majority of Australian termites are soil-dwellers that construct either above ground nests or dig underground galleries and are found in dry grasslands. Wood-dwelling termites do not create nests, but live in excavated galleries in the wood of trees in rainforests (Ratcliffe et al., 1952).
All known species of termites live in colonies that are composed of distinct castes: workers, soldiers, nymphs, larvae, and reproductives (king and queen). Each caste has a specific role that contributes to the productivity of the colony. The workers represent the majority of termites in a colony; they build the nest, take care of the eggs and young, and gather food. Soldiers protect the community from predators. Nymphs are immature termites that are developing into adults. Larvae are the immature young of the reproductives, and the reproductives are the actively reproducing males (kings) and females (queens) in the colony (Ratcliffe et al., 1952; Krishna and Weesner, 1969; Kambhampati and Eggleton, 2000).
Termite Diet and Digestion
Termites are consumers and detritivores that generally feed on grass, debris, and wood. The condition of the wood they eat is important; termites usually prefer dead or rotting wood to living wood. They lack the specific cellulases to break down cellulose, but are able to digest the cellulose and lignin found in their diet because of symbiotic relationships with flagellate protozoa, bacteria, and fungi (Ratcliffe et al., 1952; Krishna and Weesner, 1969).
There are unique species of oxymonad, trichomonda, and hypermastigote flagellates that are found in the families of the lower termites. These protozoa digest wood particles by hydrolyzing cellulose anaerobically, which produces glucose that can be absorbed by the termite (Figure 1). The higher termites, Termitidae, generally do not have protozoa in their intestines. Digestion of cellulose is accomplished by bacteria instead (Krishna and Weesner, 1970; Lee and Wood, 1971; Eutick et al., 1978).
Figure 1: Example of a large flagellate (Trichonympha) that resides in the gut of termites (Gillis and Haro, 2002).
Fungi affect termites by decomposing wood for termite consumption. Termites prefer wood that has been partially decayed by fungi. Decaying wood caused by certain fungi is able to “attract” termites by the chemical substances that result from the degradation. Wood-destroying fungi aid in breaking down toxic substances in the wood so termites can consume it safely (Rouland-LefŹvre, 2000). Fungi also are able to decompose lignin to simpler polysaccharides for termites to digest (Lee and Wood, 1971).
The most distinguishing behavior of termites is the avoidance of light and living within an enclosed environment. To accomplish this, termites construct a variety of nests that operate as a shield against predators and allow them to regulate the microclimate within. Depending upon the species and location, termite nests can take on a variety of forms: nests within wood, subterranean, arboreal, and mound (Ratcliffe et al., 1952; Abensperg-Traun and Perry, 1998).
The simplest and most primitive termite nest is that which is constructed within wood. Termites settle in a piece of wood and begin gnawing and enlarging chambers and galleries throughout. They are confined to this piece of wood which serves as a source of nutrition and shelter (Krishna and Weesner, 1970). The second type of nest is subterranean. These consist of a network of galleries and chambers that extend into the ground. Subterranean nests are developed by either digging directly from the ground, or there is an initial excavation of a large cavity and the termites form chambers and galleries around the central starting point (Krishna and Weesner, 1970; Lee and Wood, 1971).
Arboreal nests (Figure 3) are constructed in trees and connect with the soil through covered runways that travel down the surface of the trunk (Ratcliffe et al., 1952).
Figure 3: Photograph of an arboreal nest (Krishna and Weesner, 1970).
The last type of nest is perhaps the most interesting. Termite mounds are above ground nests and are built using carton, which is termite excreta, soil, and semi-digested wood. Termites pack carton together into a variety of structures that are resistant to erosion. “Termite mounds range in size from small domed or conical structure only a few centimeters in height and diameter to colossal mounds . . . [that] reach 9m or more in height and 20-30m, in diameter at the base.” (Lee and Wood, 1971). In Australia, these mounds can be found in both rainforests and dry grasslands (Figures 4 and 5).
Figure 4: Photograph of a termite mound in an Australian rainforest (http://discuss.foresight.org/~hibbert/Xmas00/TermiteMounds. html).
Figure 5: Photograph of termite mounds found in northern Australia (Abe et al., 2000).
Within these nests, termites are able to control the temperature, humidity, and internal atmosphere. The location and architecture of the nest determines thermal regulation; for example, if it is exposed to the sun or under forest canopy. The temperature inside the nest is always higher than the external temperature because of the metabolism of the termites. Of all microclimate factors measured, temperature is the most constant (Krishna and Weesner, 1970; Noirot and Darlington, 2000).
Maintaining the humidity within the nest is an important feature given that termites are extremely sensitive to changes in moisture. The moisture present within the nest is due to termite respiration, the surrounding environment, architecture, and material composition used in constructing the nest (Ratcliffe et al., 1952). Ventilation of the nests is essential in maintaining adequate oxygen and carbon dioxide concentrations. The internal atmosphere relies upon the structure of the galleries and porosity of nest material which allows for gaseous exchanges (Krishna and Weesner, 1970; Noirot and Darlington, 2000). Overall, the roles of termites as consumers, detritivores, and nest creators impact the surrounding ecosystem by contributing to nutrient recycling and formation of new soil.
Nutrient Recycling and Soil Formation
Termites affect the availability of nutrients by transporting organic matter and soil to mounds; which are later redistributed into the ground by natural decay and erosion. Termite mounds composed of carton contain a higher proportion of organic matter than the surrounding soils. The soil and nutrients are withheld from the biogeochemical cycles until the mounds are abandoned and erode (Lee and Wood, 1971).
A study in northern Australia revealed that nutrients may be contained in these structures for up to 30 years before being incorporated into the surrounding ecosystem. Soil materials and nutrients deposited by termites at the soil surface, however, “. . . was completely eroded and redistributed by 43mm of rain over a period of two days” (Holt and Lepage, 2000). Natural rainfall also causes leaching of nutrients from the mounds which increases the rate of nutrient recycling between soils and plants (Holt and Lepage, 2000).
Termites contribute to the exchange of many nutrients: carbon, nitrogen, oxygen, sulfur, phosphorus, calcium, magnesium, potassium, and sodium. Of these, termites affect the carbon and nitrogen cycles the greatest (Lee and Wood, 1971). Termites’ role as detritivores by decomposing wood cellulose is important to the carbon cycle. The decomposition process releases energy that would otherwise be unavailable to the ecosystem. When the nests that are constructed with termite excreta begin to erode, the organic carbon recycles back into the environment. Termites also contribute to the carbon cycle by serving as food for other animals, and exhaling carbon dioxide into the air (Lee and Wood, 1971; Curtis, 1996; Holt and Lepage, 2000).
The nitrogen cycle is affected in the same way. Once termite excreta are recycled back into the soil, the nitrogen accounts for increased fertility in the soil. There is another way, though, that nitrogen may be incorporated into the nitrogen cycle via termites – it can be fixed. Nitrogen fixing bacteria inhabit the gut of termites that fix atmospheric N2 to be integrated within termite tissues and excretions. These nitrogenous compounds are eventually dispersed to other termites, predators, or directly into the soil (Curtis, 1996).
Termites are essential to the recycling of nutrients in Australia and are also important in the formation of new soil. Lee and Wood in 1971 calculated that termites contribute 0.10mm/year to the accumulation of new soil. Termites also loosen and aerate soils and bring new particles from various soil horizons to the surface. A study calculated that over a period of 1,000 years, 37cm of subsurface soil could be translocated to the surface by mound-building termites. Overall, these processes contribute to Australia’s soil profile development and structure (Holt and Lepage, 2000).
Termites in the Future
Termites have been studied for decades for their ability to decompose decaying matter, aid in nutrient recycling, and form new soil. An interesting topic is that of humans using termites to their advantage globally. The term “decompiculture” is a term coined by Timothy Myles that refers to how decomposing organisms, like termites, could be grown for a variety of uses. Humans could live in symbiosis with termites by utilizing them in landfills to decompose waste, to improve soils by composting materials, to detoxify hazardous substances, and to produce biomass for animal feed and production of biochemicals. Decompiculture could eventually become a new biologic field that could have significant and important impacts on both humans and termites (Myles, 2005).
Australia has an array of biomes that are home to various plants, animals, and insects. The nutrient deficient soils and plants in the arid grasslands are aided by the nutrient recycling and soil forming capabilities of termites. Protozoa and bacteria found within the termite gut allow them to digest cellulose; therefore, termites are an important factor as decomposers within the biogeochemical cycles. Considering all these attributes, termites can be considered a significant contributor to enriching the ecology of Australia’s outback.
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