Those of you whom work in forests, will, I am sure, be familiar with the term “green island”. To a forester or forest entomologist, a green island is a clump of trees that have, for some reason or other, survived the ravages of an insect outbreak. The earliest reference I can find to this phenomenon is in a 1927 paper by the German myrmecologist Hermann Eidmann (1897-1949), who described them as green oases, or, as the paper was written in German, more correctly, “grüne Oasen” (Eidmann, 1927).
Red wood ants helping maintain a “grüne Oasen” green oasis” in a German pine forest (Eidman, 1927).
As well as “farming aphids” to obtain sugar from their honeydew, ants also have a similar mutualistic relationship with plants that give them a sugary reward to protect them from herbivorous insects, except those that also provide the ants with sugar (Janzen 1966; Bentley, 1977). The mutualisms can be very sophisticated. In Michigan, the North American black cherry, Prunus serotina, times nectar production from its extra-floral nectaries to attract the ant Formica obscuripes when the larvae of its major herbivore, the eastern tent caterpillar, Malacosoma americanum are at their most vulnerable (Tilman, 1978). Trees that are protected have greatly reduced levels of herbivory. When more than one ant colony is involved, rather than single trees being protected, a group of trees can be saved from defoliation, and form a green island. The areas covered by these green islands can be quite extensive, for example two ant colonies of the ant Formica polyctena were enough to protect pine trees from the nun moth Lymantria monacha in Sweden within a 45 m diameter around the colonies (0.16 ha) (Wellenstein, 1980) and green islands of up to 3 ha have been reported (Eidmann, 1927).
Left – canopy of trees near ant nests, on the right, trees not close to ant nests Wellenstein (1980)
In Finland, one colony of the ant F. aquilonia is enough to create subarctic mountain birch (Betula pubescens), green islands of up to 0.12 ha in area (Laine & Niemelä, 1980).
Green islands attributed to the activity of the ant Formica aquilonia in subarctic Finland (Laine & Niemela, 1980).
It would seem that the case for the ants protecting the trees against defoliating herbivores and being the cause for the green islands is very convincing. Tom White, never one to avoid a controversy, disagreed. He suggested that it was the nest building activities of the ants that were the cause for the green islands, the refuse dumps provide higher concentrations of nutrients that the roots of surrounding trees can access and additionally soil moisture conditions are improved, both these factors encouraging more vigorous growth in those trees close to ant nests, making them less palatable to herbivores (White, 1985). The Finnish team responded to this with some additional data and arguments defending their hypothesis (Niemelä & Laine, 1986) and there the matter rested, for a while at least. Not satisfied with their post hoc response, the Finns came up with, to me at any rate, a very convincing field experiment where they showed that soil nitrogen did not vary significantly with distance from ant nests and that birch leaf nitrogen content and moth larval growth rates and survival were also not affected by distance from ant nests (Karhu, 1998; Karhu & Neuvonen, 1998), indicating that the green islands were indeed, due to predation by the ants and not improved tree nutrition.
Soil nitorgen in realtion to distance from ant colonies (Karhu & Neuvonen, 1998).
You might think that this would be the last word, but you would be wrong 🙂 The Karhu and Neuvonen paper, is, in the journal, followed by a “comment” paper by no less a person than Tom White (White 1998) in which he disputes in no uncertain terms, their interpretation of their new data. Matthias Schaefer, the then Editor of Oecologia, felt that some sort of explanation was needed and added a final note to the saga, which in itself makes very interesting reading. I get the feeling that there were some strong emotions involved 🙂
Pouring oil on troubled water – wise words from Editor-in-Chief Mathias Schaefer
Bentley, B.L. (1977) Extrafloral nectaries and protection by pugnacious bodyguards. Annual Review of Ecology & Systematics, 8, 407-427.
Eidmann, H. (1927) Weitre Beobachtungen über den Nutzen de roten Waldameise. Anzeiger für Schädlingskunde, 3, 49-51.
Janzen D.H. (1966) Coevolution of mutualism between ants and Acacias in Central America. Evolution, 20, 249-275.
Kaiser, W., Huguet, E., Casas, J., Commin, C. & Giron, D. (2010) Plant green-island phenotype induced by leaf-miners is mediated by bacterial symbionts. Proceedings of the Royal Society B, 277, 2311-2319.
Karhu, K.J. (1998) Effects of ant exclusion during outbreaks of a defoliator and a sap-sucker on birch. Ecological Entomology, 23, 185-194Kah.
Karhu, K.J. & Neuvonen, S. (1998) Wood ants and a geometrid defoliator of birch: predation outweighs beneficial effects through the host plant. Oecologia, 113, 509-516.
Laine, K.J. & Niemela, P. (1980) The influence of ants on the survival of mountain birches during an Oporinia autumnata (Lep., Geometridae) outbreak. Oecologia, 47, 39-42.
Niemela, P. & Laine, K.J. (1986) Green islands – predation not nutrition. Oecologia, 68, 476-478.
Tilman, D. (1978) Cherries, ants and tent caterpillars: timing of nectar production in relation in relation to susceptibility of caterpillars to ant predation. Ecology, 59, 686-692.
Wellenstein, G. (1980) Auswirkung hügelbauender Waldameisen der Formica rufa‐Gruppe auf forstschädliche Raupen und das Wachstum der Waldbäume. Zeitschrift für Angewandte Entomologie, 89, 145-157.
White, T.C.R. (1985) Green islands – nutrition not predation – an alternative hypothesis. Oecologia, 67, 455-456.
White, T.C.R. (1998) Green islands – still not explained. Oecologia, 113, 517-518.