2005/11/03 | essay from NewScientist
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Global warming could trigger ant invasions
22:00 21 March 2005
NewScientist.com news service
Shaoni Bhattacharya
Global warming may lead to an unexpected threat from the insect world - swarming invasions of tiny ants - suggests new research.

The study of 665 ant colonies in environments ranging from tropical rainforests to frozen tundra suggests that in warmer environments the ants' body size shrinks, on average, while the number of individuals in the colony booms.

Global warming might shrink ant workers by as much as a third, says Michael Kaspari at the University of Oklahoma, US, and the Smithsonian Tropical Research Institute in Panama, who carried out the study: "And since ant species with small workers appear to be particularly successful at invading, ant invasions - already destructive - may become more common in a warming world."

Kaspari found that worker ant and colony size varied almost 100-fold in his survey of ant colonies in 49 ecosystems in the Americas. Average nest populations varied from 63 workers in a cold temperate pine forest, to over 9000 workers in a hot, temperate desert. "The tiniest colonies are not much bigger than the inside of a Cheerio while the largest colonies can fill up a garbage can," he told New Scientist.

Microscopic beasts
Worker ant size varied from the largest - the bala or bullet ant (see photo) which is about the size of the end segment of an adult human's thumb - to ants so small they can only be seen with a microscope. These dramatically different ants could be found co-existing in the tropics.

Kaspari says that theory suggested that body size should be bigger in the tropics, where there is more plentiful food and where the warmth enables cold-blooded creatures to perform better. But in fact, the average body size of worker ants was lower in those zones.

He suggests that higher temperature may be a "double-edged sword" when organisms are growing: "It allows you to forage, but a greater fraction of what you eat gets frittered away in metabolism." He adds that smaller organisms are also good at producing lots of offspring.

Energy constraints
Most exciting, says Kaspari, is that "we are the first people to show that the size of an organism varies in a predictable way, and not just with latitude but based on two fundamental properties of the ecosystem - temperature and productivity".

The study shows a "massive effort" in collecting data says Francis Ratnieks, a social insect expert at the University of Sheffield, UK. He notes that some large species, like leafcutter ants, eat different foods to other ant species, and therefore may respond differently to any warming. "Leafcutter ants have tapped into a rich food source, that of fresh leaves, and their energy constraints may well be very different to those of most other ants."

Kaspari now plans to see if other organisms vary in size in such a predictable way, starting with the "brown food web" of decomposing organisms in soil.

Journal reference: Proceedings of the National Academy of Sciences (DOI: 10.1073/pnas.0407827102)

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