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| Global gradients in ant size: how it happens, how we know, and what it means to you... | ||||||||||||||||||||||
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| In the April 5 issue of Proceedings of the National Academy of Science, our article "Global energy gradients and size in colonial organisms: Worker mass and worker number in ant colonies" demonstrates that ant colonies vary ten-fold in size as you move from warm deserts and tundra habitats to tropical rainforests. Two changing parts of the global environment, temperature and plant production, predict much of this variation in creatures E. O. Wilson calls, "the little things that run the world". | ||||||||||||||||||||||
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Dr. Mike Kaspari |
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| What are colonial organisms? What makes them special?--An organism is colonial if it is made up of individuals that are, to varying degrees, coordinated. Many of earth's largest and most important organisms, from coral reefs to social insect colonies, are colonial. A coral reef is made up of millions of polyps; the reefs they form are homes to much of the sea's biodiversity. An ant colony is made of up of ten to a few million individuals; ants are one of the most important and conspicuous land animals. | ||||||||||||||||||||||
| mkaspari@ou.edu | ||||||||||||||||||||||
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| How does an ant colony grow?--Ant colonies are "superorganisms". The mass of an ant colony thus is the sum of the weights of all the individuals in that colony. Ants, like many wasps, bees, termites and aphids, are eusocial, meaning that a queen lays the eggs and her offspring raise them to adulthood, where they too join the workforce. Thus a queen can grow her colony by adding more workers or making sure the workers are, individually, bigger. | ||||||||||||||||||||||
| Do ant colonies vary much in size?--Enormously. The 400+ species from a single patch of tropical rainforest in Costa Rica or Panama vary widely in both worker mass and worker number. This in turn creates colonies that are so small that they can be squeezed into the space within a capital "O" and colonies so big that they would fill to the brim your standard outdoor garbage can.
For example, consider the photograph to the left. Pictured is Paraponera clavata, also known as bala, or the bullet ant for its powerful sting. This is one of the largest worker ants found anywhere. If you look closely you will find, perched on its antenna to the left, a worker of Carebara reina, a tiny and inconspicuous part of the rainforest soil fauna. Both species can be found in the forests of Central America; together they represent the sometimes dizzying extremes in size that a single taxon may span (picture terrestrial mammals from a shrew to a hippo and you get the idea). |
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| This photo a composite from the website of Jack Longino, Evergreen College, USA | ||||||||||||||||||||||
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| Likewise the number of workers in an ant colony can vary from less than 10 to well over a million. Here a former student of mine, Mary Kay Johnston (now at the University of Texas) is taking a gander at the hanging nest of a tropical Azteca colony (so named for after the ancient warrior nation). Azteca colonies may support millions of workers which you disturb at your peril as they come flooding out biting and spraying a noxious compound redolant of old blue cheese. | ||||||||||||||||||||||
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| But for all the conspicuous grandeur of Azteca, there are thousands of little colonies peppering the tropical forest floor at densities of around 5 per square meter. This is where most of the biodiversity is. These colonies have only around 100 or so workers and function, we think, to regulate the decomposition that releases nutrients locked up in dead leaves back to soil. These tiny litter ants are the unsung heros of the ant world, and are increasingly common as one moves toward the tropics. | ||||||||||||||||||||||
| So how did you find out that colonies of ants have smaller workers, but more of them, in warmer environments?--In the mid-90s, our lab, including Dr. Leeanne Alonso of Conservation International, Dr. Alfonso Alonso of the Smithsonian Institution, and Mike Weiser, a doctoral candidate now at the University of Arizona, visited 49 ecosystems in the New World. At each site we sampled ants in exactly the same way--laying square frames on the groundand counting and collecting every colony in those frames. It took literally years to identify all the species, many new to science. Then we simply (!) counted the number of ants per colony and measured them under a microscope to estimate their mass.
We found that the average size of a colony across those communities tended to increase with temperature, even as the size of the workers in the colony decreased. Since our work was partially funded to explore the effects of climate change--particularly global warming--on ecological communities, were were amazed that such a clear link existed at the global scale. |
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| And...why the pattern?--We don't know for sure, nor do we know how universal this pattern is. We are the first that we know of to take on such a global survey, and are looking forward to finding out what other ecologists discover when they do similar work for their favorite taxon. These results are consistent with the notion that when ectotherms (i.e. "cold-blooded" organisms that get their body heat from their environment) grow up in a warmer environment, their metabolic engines run at a faster clip. One result is that a smaller fraction of every calorie they consume goes into building their body and more is returned, wasted if you will, back to the environment. As a result, ants in warmer environments tend mature at a smaller size. However, ectotherms also tend perform better when its warm. So we think that once they mature, all that extra energy is harnessed to grow the colony. This yields the pattern of larger colonies with smaller ants in warmer climates.
Now something else is also going on. Warm environments, when there is plenty of water, also tend to grow their vegetation lush--the engines of plant growth are best stoked by plenty of water and plenty of sunshine (compare the tropics with warm deserts: both warm, only one with plenty of water) . And its the plants that provide food for the entire ecosystem. So where there is more plant growth, there is more fuel to build ants and other living things. But we found something that stunned us, even though we knew from the work of colleagues that it could theoretically happen. More food translated into smaller colonies. In the most productive environments, like the tropical forests and the subtropical hardwood forests of the American southeast, the number of ants in a an average colony was smaller than in comparably warm deserts. How could this be? Small colonies may have an achilles heel--the need to have more food on a consistent basis than large colonies, which can store up food against lean times. Rich, productive communities shod that achilles heel with a nice padded diet of plenty, allowing small things to flourish. |
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| Why is this important to the environment, and to us?-- The size of an organism is probably the best predictor of its ecology. Small things just have a faster clock-they live shorter lives, tend to be more active, and require more meals to stay alive. For example, a human can go over 100 days without eating; a shrew will starve to death in a day. Small things also perceive the world differently than large things. Small things see a world full of structure--shrews walk through an environment that we humans walk over.
One way to think about the impact of changing size distribution is to picture an American Football league that did not limit the number players on the roster but did limit its total mass. You could have one team made up of lots of wiry wide receiver-types; another team could be made up of thick, burley offensive linemen. Now one would imagine that the playing styles of those two teams, their "niches", would be radically different. The team with smaller players would race around and substitute often, all the time taking advantage of their speed and agility. The team with the big brutes would play power ball and just roll over opponents by marching up and down the field (my favorite team, the Nebraska Cornhuskers, are actually making the transition from the latter to the former so I think about this a lot!). Ant species of differing body sizes would act in similar ways, fast, tiny and frenetic, vs. big lumbering bad-asses (see Paraponera, above: getting stung by one of those is like slamming your hand in a car door). So an ecosystem full of small organisms will likely grow them faster, kill them faster, rot them faster, recycle them faster, and, quite possibly, evolve faster. A warmer world could just operate at a faster tempo, and ecologists are only beginning to imagine (with and without sports analogies!) how that may shape our future environment. A second, ant-specific way that global warming may shape our lives comes from the fact that they can be pernicious pests when introduced to an environment where they have no natural enemies. This seems to be the story with the invasive fire ant and argentine ant in North America, and is repeated around the world. Such invasives can lower an ecosystem's biodiversity, interfere with the growth of its populations, attack agricultural plants and livestock, and in the case of the fire ant with its nasty sting, cause a a real concern for the young, old, and those with compromised immune systems. Unfortunately, some evidence suggests that it is large colonies of ants with small workers that are often particularly good invasives. And growing global temperatures, by fostering just such colonies, may be making invasions that much easier. |
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| Author: Mike Kaspari Last Updated: 9 April 2005 |
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This page was built and maintained with support from the National Science Foundation | ||||||||||||||||||||