Dr Theodore Evans, Associate Professor at the University of Western Australia in Perth, outlines the potential geographic growth for invasive termites and the subsequent impact on the wider environment.
Overview
Two Coptotermes species are among the world’s worst invaders, according to some. But just how much of the globe can C. gestroi and C. formosanus occupy? This depends on habitability of a region: the range of temperature, water supply and food resource that are suitable for the species. Also, the range of competitors and predators the species can endure. Most information about invasive termites comes from urban areas; little from forests.
However, due to human activities, urban areas are warmer and have a more consistent water supply than forests; these changes influence potential habitability. In their native ranges, C. gestroi is equatorial/tropical, whereas C. formosanus is subtropical/warm temperate. Both species are largely consistent in their invaded range, except C. gestroi in Brazil. Here, C. gestroi is found in the subtropics, but only in urban areas, thus showing the importance of human activity that allows habitability. In contrast to both these species, C. acinaciformis is found across all of mainland Australia, from the tropics to warm temperate latitudes. The potential distribution of C. acinaciformis is much wider than either C. gestroi or C. formosanus; so perhaps it is a good thing that this species has rarely spread beyond Australia.
Invasive Coptotermes species
Wood-eating termites in the Rhinotermitidae are among the worst invasive termite species. Two species of Coptotermes native to Asia are perceived to be particularly bad pests: C. gestroi and C. formosanus. These two species are now found far beyond their original, i.e. native, distributions in Asia. Both are found in many islands in the Indian, Pacific and even Atlantic (Caribbean Sea) Oceans. They are on continents also; C. gestroi is spread along the southeast coast of Brazil, and C. formosanus is spread across southeast USA.
Where these two species have invaded, they have typically become the major termite pest in urban areas — considered to be worst pests than species native to the location. Research comparing damage due to invasive and native termites in the same location is rare, however anecdotal information from pest control company websites and advertising provides some indication of their relative importance. Typically, the invasive species is featured in the invaded locations, and their greater destructive abilities relative to the local native termites is emphasised. Claims of efficacy for various pest control treatments are made or stressed against the invasive species rather than the local native species. In addition, the capacity of invasive Coptotermes species to attack and hollow living trees is highlighted, compared with a lack of capacity with the local native termites.
These two Asian Coptotermes species are so feared in their invaded ranges in part because of their more destructive capacity compared with local native species (perhaps especially Reticulitermes species). This greater destructive capacity is due in part to their ability to attack and hollow living trees — Coptotermes is almost unique in this ability. It is also due in part to their colonies of larger population sizes, which can cause greater damage in shorter time periods. There is some speculation that colonies in invaded locations are larger than colonies in native locations. If so, it is likely due in part to the lack of co-evolved competitors and predators. The invasive Coptotermes have left their old enemies behind in their native range, and so they are less hindered by competition and predation in the invaded locations.
There is another consideration, not so frequently addressed: the habitats in the original native geographic locations, and those in the invaded geographic locations. In essence, these Coptotermes evolved in natural, native forests, and moved into urban habitats as they were created by humans. However, almost all research and pest management of C. gestroi and C. formosanus in invaded locations is in urban habitats. Consequently, our understanding of these species and where they may live could be distorted by habitat.
Urban climate vs forest climate
Urban habitats differ enormously from the natural forest habitats that preceded them. Trees are cut and cleared. The soil changes with clearing and construction: in particular mineral-rich subsoil is exposed, and heavy machinery and urban construction compact the soil. These activities change the local climate. Without the tree canopy there is more variation in, but generally higher, temperatures. During the day there is greater sun exposure that heats buildings, roads and soil. Rainfall may decrease, as trees release volatile chemicals into the atmosphere that help to seed rain. When rain does fall, it runs off quickly due to the lack of roots and soil compaction. Thus soils are drier.
There are other changes. Humans heat their cities, either directly (heating systems, combustion engines and electric motors) or indirectly (building materials that absorb more heat from sunlight). Humans pipe water into cities, and pipes leak, thus providing water all year, regardless of rainfall. Much of this water is directed into buildings, thus water and wood food are found together. Thus, conditions for termites to attack buildings are much improved, as far as the termites are concerned.
In addition to these food and water resources, humans also change the ecological community. There are fewer native species adapted to the hotter and drier urban areas, and thus potential competitors and predators are reduced. And for those few species that can cope with urban conditions, humans are unwelcoming, and actively discourage them from entering their homes, and indeed kill them. So competitors (other termites or wood-eating insects) and predators (ants, centipedes, spiders etc) are removed, further improving conditions for termites in urban areas.
This all suggests the fear of C. gestroi and C. formosanus in those areas still free of these pests is warranted. But is this true for all uninvaded locations? Just how far can these two species spread around the globe? Are there inhabitable locations, locations in which either C. gestroi or C. formosanus (or both) can survive, and so people need not fear these pests?
Predicting potential Coptotermes distribution
Habitability of a region is determined largely by temperature, water supply and food resources. As Coptotermes eat a wide range of wood species, including most of the commercially important wood species used in construction, food resources are less important. Therefore, it is possible to use climate as a predictive tool.
The locations of C. gestroi and C. formosanus have been reported, in both their native range and invaded areas. These locations can be mapped, and climate data from those locations collected. Climate data are routinely collected by government meteorological agencies; the most common include daily minima and maxima temperatures and daily rainfall, and from these daily measurements, averages are calculated over months, quarters (i.e. seasons) or the year. These climate data can be compared statistically to determine the climate envelope that forms the habitable climate range of each of the two species.
Different sets of data can be used, alone or in combination. The first uses only climate data from the native distributions; a second uses climate data from the invaded distributions; a third uses only climate data from forest (or other natural habitat) — usually only available from native distributions; a fourth uses climate data from urban distributions. Once this process is completed, the habitable climate ranges are known, and then the potential global distributions of the two species can be estimated. This is done by mapping the habitable climate range onto the globe.
The potential global distribution of C. gestroi reflects its native distribution in Southeast Asia (Figure 1). This native distribution is equatorial-tropical, from Indonesia (south of the equator), to Singapore, Malaysia and up to Vietnam, then across to the Philippines. This is a range approximately 10° south to 20° north of the equator).
The invaded locations are similar, including tropical islands of the Indian Ocean (Mauritius and Reunion Islands), Pacific Ocean (Midway, Marquesas, Guam, Marshall, Hawaiian and Fijian Islands) and Atlantic Ocean (Caribbean Sea — Cuba, Jamaica, Cayman Islands, Puerto Rico and the Antilles Islands), and two continental distributions in Mexico (Manzanillo, Mexico City) and south-eastern Brazil (including Rio de Janeiro, Santos, and Sao Paulo). Brazil is interesting because it is at the edge of the habitable climate range even perhaps outside of this range. It is likely that C. gestroi can survive in urban areas here, because they are hotter than natural forest habitats; and it is notable that C. gestroi is not recorded in forested areas.
In a similar fashion, the potential global distribution of C. formosanus reflects its native distribution in east Asia (Figure 2). This native distribution is subtropical-warm temperate, from south-eastern China, across to Taiwan and perhaps the Ryukuo Islands of southern Japan. This is a range from 21° to 34°C north of the equator.
The invaded locations are similar, including islands of the Pacific Ocean (Japan, Midway, Marshall, and Hawaiian Islands), and a continental distribution in the USA. The Marshall Islands are interesting as they are well inside the tropics and might be considered outside the habitable climate range. However, it is possible the moderating influence of the ocean on a small island may keep the climate within the habitable range. Clearly there are more parts of the globe available to both species. This is particularly true for C. gestroi in South America, including much of the Amazon River basin, and patches from the Guianas in the east to northern Peru in the west (Figure 3).
There are areas of Africa, especially along the Ivory Coast of west Africa and patches in the Congo River basin. Closer to the native distribution, part of New Guinea is also habitable. Less of the globe is habitable for C. formosanus, with just the south of Brazil to north of Argentina highly suitable.
The source of the data also changes the habitable ranges. Comparing potential distributions based on data only from the native range and forested (non-urban) locations with distributions based on data from urban areas (more data from invaded locations than native locations) shows some differences. The most important difference for both species is distributions based on native range and forested locations is smaller than those based on urban locations. For C. gestroi, the distribution based on data from urban locations spreads further from the tropics, well south and west into Peru, Columbia and southern Brazil, and north into Central America. For C. formosanus, the distribution based on data from urban locations spreads further north into the USA, into Virginia on the coast and Tennessee inland, and along the coast of Baja California in the west (Figure 4).
These potential distributions may give some succour to people in less suitable (i.e. less likely habitable) locations and raise fears for people in more suitable (i.e. more likely habitable) locations. People in urban locations may have more to fear than those in rural locations, especially on the edges of the potentially habitable distributions.
However, the future spread of C. gestroi and C. formosanus may not be the most important spread of invasive termites. There are other invasive termite species, and some may have greater opportunity to spread than either of C. gestroi or C. formosanus.
Other more invasive Coptotermes?
There are other Coptotermes species found over greater climate gradients than C. gestroi (equatorial-tropical) or C. formosanus (subtropical-warm temperate). There are African species, such as C. amanii from east Africa and C. paradoxus from west Africa that have large ranges. However, as records are few, habitable climate ranges are difficult to calculate with any precision.
One Coptotermes species in Australia with a wide range, C. acinaciformis, is found over the entire mainland continent, from tropical to warm temperate latitudes (11° to 39 °S). This species is found from wet forests to deserts (along dry riverbeds), indicating a wider rainfall tolerance compared with the other invasive termites. Taken together, these wide ranges in climate suggests that C. acinaciformis may be able to invade a wider global distribution than either C. gestroi or C. formosanus.
Following the same process as used for C. gestroi or C. formosanus, the global distribution of C. acinaciformis can be mapped. Indeed, the distribution is wider, much wider (Figure 5).
C. acinaciformis not only covers much of the distributions for C. gestroi and C. formosanus, it may also cover new distributions in cooler and drier landscapes. Larger areas of North America, South America (particularly Argentina), and southern Africa are suitable. Even the coast around the Mediterranean Sea is suitable, including parts of Morocco, Algeria and Tunisia, Spain, Italy, Greece and Turkey. In fact, the only parts of the globe that are suitable for C. gestroi and/or C. formosanus that are not suitable for C. acinaciformis are the wettest tropical regions (such as the Amazon River and Congo River Basins).
Should we worry about C. acinaciformis invading much of the globe? The short answer is no. All invasive species must be moved from their native habitats by humans. This usually happens through trade, and the greater the volume of trade (especially with wood or wood products) the greater the risk of spreading the termite. That is why C. acinaciformis has invaded New Zealand and Fiji (through trade with Australia) — and it did not establish in either country (through careful pest management). As Australia does not trade large volumes of wood to South America, Africa or Europe, it is unlikely C. acinaciformis will spread beyond Australia.
Conclusion
Two of the most feared invasive termites, C. gestroi and C. formosanus, may have more potential suitable locations around the globe. Cities, i.e. urban habitats, offer more suitable conditions for these species, and so they may spread into cities with climates beyond their habitable climate range. Although worrying, there are other termites with wider climate ranges, and these may be more concerning as they can spread further than either C. gestroi and/or C. formosanus. Even though C. acinaciformis could be such a termite, it is unlikely as little wood is exported from Australia to suitable overseas locations. Nevertheless, the point is that there are other invasive termites, and other termite species that have not yet spread beyond their natural, native distributions, and among these species, there are some that may invade, and have climate ranges that are beyond those of either C. gestroi and/or C. formosanus. Thus, vigilance and quarantine controls are essential, to keep these potential invaders in their natural, native home ranges.
Dr Theodore Evans
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