For many of us, an ant colony is something that grows between two glass plates when you “just add ants” to a little bit of dirt. Glass and little red plastic frames being in short supply out in nature, wild ants have to make do with other materials to build their homes. Some ants do house their colonies in dirt, but many others can make a whole colony in a rolled-up leaf or inside an acorn. These items being abundant in nature, the question arises, “what determines how many ant colonies are out there?” Certainly not every acorn or leaf could have an ants cozied up inside.
This is the old ‘abundance’ question in ecology, which is usually answered in the following way: the number of animals living in an area is a balance between how many are born and how many die. But this isn’t very satisfactory- we want to know the ultimate cause, not the proximate mechanism. A more satisfactory answer may come from energy theory.
One of the first things we learn in our elementary science classes (other than that ants live between glass plates) is that all animals need food to survive. Although animals each eat different things, the light energy captured by plants was the original source of the energy in their food (except for some crazy creatures that live around hydrothermal vents deep under the ocean). A simplistic answer to the abundance question is that the number of animals living in an area is determined by how much energy plants make available to them. Simple answers rarely capture the all the details that make ecosystems interesting to ecologists, but sometimes they are insightfully accurate for biogeographers who study the whole planet.
Michael Kaspari and Michael Weiser recently published research showing that, from
’s alpine tundra to the Peruvian tropical forest, the overall number of ant colonies in a particular place is strongly related to the amount that plants grow (NPP, net primary productivity) relative to the average energy needs of a single colony. Although the ants were studied 15 years ago, they combined this older data with recent experiments on ant metabolism (how they burn energy) to estimate how much energy a colony needs based on how large the worker ants are. These energetic requirements were then used to figure out how many colonies the total energy available from plants could support (total plant energy / energy needed for one colony). Colorado
What’s particularly cool is that on a species-by-species basis, plant production didn’t do a great job of predicting how many colonies would be found for a particular species; some species had very few colonies even in place with lots of plant production, like tropical forests. Even the most abundant ant species at each location sometimes had fewer colonies than amount of energy available to them would predict. It was only at the level of the whole community, when colonies from all ant species were counted together, that the energetic calculations did a good job of predicting the number of colonies.
The authors think that this could be because each ant species has a specific diet that is different from most other ant species and is only consuming a small fraction of all the energy available out there in the ecosystem. But, collectively ants eat a wide variety of things (spider eggs, farmed fungus, etc…) and together consume a larger fraction of what’s out there. This means that the whole community is more limited by the energy available than any one species.
Another fascinating pattern also emerged from this research- even though the number of ant colonies increased in places with more plant production, the total biomass (how much all the ants weigh) did not change. This seems paradoxical at first since more ants should equal more weight; ten people usually weigh more than one person. But, ants vary in size to a much greater extent than humans do. The world smallest ant could set up a colony inside the head of the world’s largest ant (antArk). What’s more, ants are generally smaller in tropics where there is greater plant production. So, colonies become more abundant, but ant’s get smaller, leading to overall ant biomass staying about the same from desert shrublands to highly productive tropical forests. Sometimes, in biogeography, why things don’t change from place to place is far more interesting than why they do.
You can find this article at:
Kaspari, M., & Weiser, M. (2012). Energy, taxonomic aggregation, and the geography of ant abundance Ecography, 35 (1), 65-72 DOI: 10.1111/j.1600-0587.2011.06971.x