“Man — despite his artistic pretensions, his sophistication, and
his many accomplishments— owes his existence to a six inch layer of topsoil and the fact that it rains.”
Anon
The Environmental Importance of Termites
Rates of Soil Loss and Formation
Globally, soil is lost at an average of 906 kg/ha per year while it is only formed at an average rate of 700 kg/ha/year (Wakatuki and Rasyidin 1992). In parts of Europe, the rate of soil erosion can be 10 - 28 times the rate of formation (Verheijen et al. 2009). Soil formation rates in semi-arid Australia can be as low as one centimeter of soil created every 30,000 years, a rate far exceeded by erosion (Image 10) (Pillans 1997). Soil is a complex material that has undergone multiple physical, chemical and biological processes, often over an extensive period of time. An increasing global population will place even greater demands on land. Fortunately many invertebrates, including termites, facilitate soil development.
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Soil Development
Termites provide multiple ecosystem services including organic decomposition, increased soil organic matter, nutrient cycling (including nitrogen fixation), improved soil hydraulics (water filtration, water storage and reduced erosion), increased plant growth, increased plant, animal and microbial diversity and food for animals (including humans) (Jouquet et al. 2011). Termite mounds are nutrient- and microbial-rich ‘hotspots’ and they can leach nutrients and induce microbial growth and mineralization in the surrounding soil (Menichetti et al. 2014). Often termite mounds are leveled to provide a nutrient boost to soils but Menichetti et al. (2014) suggested leaving termite mounds in-situ might provide a longer-term benefit to surrounding soils as nutrients leach and improve soil microbiota over a longer period of time.
Organic Decomposition
Termites convert significant amounts of intractable organic materials into high carbon and nitrogen rich substrates using their digestive enzymes and microbiota (Verboom and Pate 2013). They can triple plant decomposition rates (Image 11) and double sheep dung decomposition (Noble et al 2009). They also remain active during heat or the dry months, one of the few invertebrates that remain so (Santana et al. 2015). |
Soil organic matter
Termite mounds are sources of organic matter from faeces, secretions and corpses. Brossard et al. (2007) found organic material content was three times that of surrounding soils (an average of 33.6 g/kg compared to 10.6 g/kg). There was also a high correlation with the proportion of sand in the soil, with organic matter being higher in sandy soils than silty soils, possibly because organic matter helps to bind particles in sandy soils.
Nutrient Cycling Termite mounds (Image 12) are generally higher than surrounding soils in potassium (Brossard et al. 2007), calcium and magnesium (Brossard et al 2007, Semhi et al. 2008) suggesting they are collecting materials from deeper soil layers. Termites are also the main vertical movers of phosphorous in soils (Belnap 2011). Semhi et al. (2008) suggested termites improved soil fertility by incorporating minerals from organic sources into the soil particles. Cation exchange capacity (the ability of soil to hold essential nutrients) values were 2 to 4 times higher in mounds (Brossard et al. 2007). |
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Nitrogen Fixation
Nitrogen is an essential plant nutrient and often the agricultural fertiliser in greatest demand. Though it makes up to 78% of air, it is very energy expensive to manufacture. Nitrogen fixating capability is found in bacteria associated with legumes (plants that can fix nitrogen) and in bacteria and archaea in the insect orders Blattodea (cockroaches and termites), Coleoptera (beetles) and Hymenoptera (ants, wasps and bees) (Ulyshen 2015). Evans et al. (2011) suggested termites might be fixing nitrogen at the rate of 9 mg/kg (the equivalent of 8 kg/ha). This represents 22 - 32% of normal nitrogen fertilizer use in a wheat-producing arid Australian ecosystem.
Mineralisation
Ji and Brune (2006) found the nests of soil-feeding Cubitermes (soil feeders) spp. showed ammonia concentrations 300% greater than the parent soil and very little (approximately 100g N/ha/year) of this is volatized (lost into the atmosphere) as nitrogen gas. This equates to mineralization of 12 - 18% of N ingested and produces 8.9 - 15.7 kg N/ha/year in a humid savannah (Ji and Brune 2006, Lavelle et al. (1997) cited in Ji and Brune (2006). This level of mineralisation is higher than that shown by earthworms (Edwards and Bohlen 1996 cited in Ji and Brune 2006). In addition, Holt (1987) estimated that termites might mineralise up to 20% of soil carbon yearly, yet produce no more than 3% of the total carbon respired by the soil.
Improved Soil Hydraulics
Dawes (2010) found termite mounds (Image 13) were associated with increased macroporosity (larger soil pores) and less soil density/compaction. Termite tunnelling increases water infiltration thereby lowering evaporation and decreasing runoff and erosion (Evans et al. 2011). Termites move clay particles into their mounds leading to clay content increases from 50-1100% higher than the surrounding soil (Brossard et al. 2007, Abe et al. 2012). Increased clay content is associated with increased water holding capability and Dawes (2010) found termite accessible areas had better year round water storage than termite-excluded areas. |
Increased Plant Growth
Evans and Dawes (2011) found termite and ant presence was associated with a 32% increase in wheat production, provided soil disturbance was minimal (i.e. no/low tillage and minimal traffic). Cadet et al (2004) found sugarcane yield was 5 times greater within levelled termitaria (termite nest) in sandy sugar cane fields in South Africa, despite greater nematode (animals that are often plant parasites) populations. Vegetation cover was better in termite-accessible plots in semi-arid Australia compared to termite-excluded plots (Dawes 2010). Andrianjaka et al. (2007) improved sorghum growth using a soil amendment of Cubitermes spp. (soil-feeder) termitaria, which decreased hemi-parasites and actinomycetes (bacteria) and increased arbuscular mycorrhizal (beneficial symbiotic fungi) presence. Soil with 10% termitaria amendment, showed improved growth (Duponnois et al. 2005).
Evans and Dawes (2011) found termite and ant presence was associated with a 32% increase in wheat production, provided soil disturbance was minimal (i.e. no/low tillage and minimal traffic). Cadet et al (2004) found sugarcane yield was 5 times greater within levelled termitaria (termite nest) in sandy sugar cane fields in South Africa, despite greater nematode (animals that are often plant parasites) populations. Vegetation cover was better in termite-accessible plots in semi-arid Australia compared to termite-excluded plots (Dawes 2010). Andrianjaka et al. (2007) improved sorghum growth using a soil amendment of Cubitermes spp. (soil-feeder) termitaria, which decreased hemi-parasites and actinomycetes (bacteria) and increased arbuscular mycorrhizal (beneficial symbiotic fungi) presence. Soil with 10% termitaria amendment, showed improved growth (Duponnois et al. 2005).