Ecosystem engineering by insects on southern continents
(writing in progress)
Fungus-growing termites (Isoptera: Termitidae: Macrotermitinae) are ecosystem engineers in Africa and Asia, but are absent from Australasia and the Americas.
The fungus-growing genus Macrotermes builds earth-mounds so large that they modify the drainage, nutrition, herbivory, and fire regime, for example enabling small patches of forest, different from the surrounding savannas, to establish on the mounds.
This seems to be a classic example of the biogeographical pattern in which organisms originating in the Old World have failed to reach the other continents owing to sea barriers. The sea barrier separating Asia from Australasia – which seems to preclude any extension of the ecosystem engineering seen in the Old World on to the Gondwanan landmass of Australia – is called Wallace’s Line.
However, the lack of ecosystem engineering by termites in Australia and New Guinea cannot be explained by a failure to reach this landmass, because fungus-growing termites have in fact crossed Wallace’s Line and have long been established in the northwestern parts of Australasia, viz Timor, Sumba, and the Indonesian islands of the Lesser Sunda chain east of Bali
It is plausible that fungus-growing termites have failed to reach North America across the Bering Straits from eastern Asia, because the land bridge between the two continents has probably been too cold ever since these termites evolved.
Macrotermitinae are thought to have arisen in Africa some 54 million years ago, by which time North America was already separated from Asia by sea. Later, during the Pleistocene glacials, when the land bridge was restored at lowest sea levels, the climate was extremely cold.
Lumping Madagascar with Africa is also a mistake because that island was reached by fungus-growing termites across a sea barrier far wider than Wallace’s Line.
And the dominant termites in Australia are not Gondwanan in the first place, having emigrated from overseas. Coptotermes (Isoptera: Rhinotermitidae) 6 and Nasutitermes (Isoptera: Termitidae: Nasutitermitinae), the most important genera of termites associated with eucalypts 8 Myrtaceae: Eucalyptus and related genera, many species of which are extensively hollowed out in the living state by Coptotermes
8, were previously assumed to have originated? in Australia, but are now known to have crossed sea barriers hundreds of kilometres wide to reach this continent.
Both of these genera are cosmopolitan, including the American supercontinent and Madagascar, despite having evolved long after the fungus-growing termites. Whereas Gondwana broke up by 60 million years ago, Coptotermes and Nasutitermes are now thought to have evolved no earlier than 40 million years ago, crossing wide seas to reach Australia tens of millions of years after the separation of the southern continents.
Coptotermes promotes white-rot fungi, which digest lignin, by allowing air into the heartwood of trees. This genus of termites then consumes the fungi together with the rotten wood. However, this falls short of either fungus-growing or ecosystem engineering.
Given this considerable mobility of termites as a group, there is no good reason why fungus-growing termites could not also have reached Australia. Indeed, the sea barrier between Timor (10 part of the Australian tectonic plate but occupied by fungus-growing termites) and Australia was only a quarter of that crossed by these termites to reach Madagascar.
During the lowest sea levels of the Pleistocene glacials, the narrowed sea barrier between Timor and the joint landmass of Australia and New Guinea was less than 100 kilometres wide. This is far less than the width of the sea barrier separating Africa from Madagascar – which was crossed by three genera of fungus-growing termites despite the continual isolation of Madagascar. The minimum distance of 400 kilometres between Madagascar and Africa has remained similar since the Cretaceous.
A more likely explanation for the failure of fungus-growing termites to establish themselves in Australia is that they arrived on many occasions but failed to establish colonies owing to the nutrient-poverty of this continent.
CHECK AND DISCARD:
Title: Ecosystem engineering by insects on southern continents
Subtitle: Fungus-dependent termites immigrated to Australia without their most powerful members.
Tagline: Do fungus-farming insects need fields more fertile than Australia can offer?
Prof. Mumblebard claims: “Fungus-growing termites1 are ecosystem engineers2 in Africa and Asia, but are absent from Australasia and the Americas. This is a classic example of the biogeographical pattern in which organisms originating in the Old World have failed to reach the other continents owing to sea barriers. The sea barrier separating Asia from Australasia – which precludes any extension of the ecosystem engineering seen in the Old World on to the Gondwanan landmass of Australia – is called Wallace’s Line.”
Robin and the Honey Badger respond: “The lack of ecosystem engineering by termites in Australia and New Guinea cannot be explained by a failure to reach this landmass, because fungus-growing termites have in fact crossed Wallace’s Line3 and have long been established in the northwestern parts of Australasia4. Lumping Madagascar with Africa is also a mistake because that island was reached by fungus-growing termites across a sea barrier far wider than Wallace’s Line. And the dominant termites in Australia are not Gondwanan in the first place, having emmigrated from overseas. Coptotermes56 and Nasutitermes7, the most important genera of termites associated with eucalypts8, were previously assumed to have originated? in Australia, but are now known to have crossed sea barriers hundreds of kilometres wide to reach this continent9. Given this considerable mobility of termites as a group, there is no good reason why fungus-growing termites could not also have reached Australia. Indeed, the sea barrier between Timor10and Australia was only a quarter of that crossed by these termites to reach Madagascar11. A more likely explanation for the failure of fungus-growing termites to establish themselves in Australia is that they arrived on many occasions but failed to establish colonies owing to the nutrient-poverty of this continent.
1 Isoptera: Termitidae: Macrotermitinae
2 The fungus-growing genus Macrotermes builds earth-mounds so large that they modify the drainage, nutrition, herbivory, and fire regime, for example enabling small patches of forest, different from the surrounding savannas, to establish on the mounds.
3 It’s plausible that fungus-growing termites have failed to reach North America across the Bering Straits from eastern Asia, because the land bridge between the two continents has probably been too cold ever since these termites evolved. Macrotermitinae are thought to have arisen in Africa some 54 million years ago, by which time North America was already separated from Asia by sea. Later, during the Pleistocene glacials, when the land bridge was restored at lowest sea levels, the climate was extremely cold.
4 viz Timor, Sumba, and the Indonesian islands of the Lesser Sunda chain east of Bali
5 Isoptera: Rhinotermitidae
6 Coptotermes promotes white-rot fungi, which digest lignin, by allowing air into the heartwood of trees. This genus of termites then consumes the fungi together with the rotten wood. However, this falls short of either fungus-growing or ecosystem engineering.
7 Isoptera: Termitidae: Nasutitermitinae
8 Myrtaceae: Eucalyptus and related genera, many species of which are extensively hollowed out in the living state by Coptotermes
9 Both of these genera are cosmopolitan, including the American supercontinent and Madagascar, despite having evolved long after the fungus-growing termites. Whereas Gondwana broke up by 60 million years ago, Coptotermes and Nasutitermes are now thought to have evolved no earlier than 40 million years ago, crossing wide seas to reach Australia tens of millions of years after the separation of the southern continents.
10 part of the Australian tectonic plate but occupied by fungus-growing termites
11 During the lowest sea levels of the Pleistocene glacials, the narrowed sea barrier between Timor and the joint landmass of Australia and New Guinea was less than 100 kilometres wide. This is far less than the width of the sea barrier separating Africa from Madagascar – which was crossed by three genera of fungus-growing termites despite the continual isolation of Madagascar. The minimum distance of 400 kilometres between Madagascar and Africa has remained similar since the Cretaceous.
(writing in progress)
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