One area of our inadequate knowledge is an understanding of the feedback loops, all of which are self-reinforcing. To fully appreciate exactly what a feedback loop is, let’s visit the whistling-thorn acacias of Kenya’s savanna in Africa. Unlike many acacias, the whistling-thorn does not deter herbivores through the production of toxic compounds. Instead, it recruits colonies of ants as bodyguards against hungry herbivores eager to chomp its leaves, such as giraffes and elephants. At the slightest movement of a branch the ants, which live only in these acacias, swarm out and deliver painful stings to munching giraffes, elephants, or other browsers.
The whistling-thorn acacia is a fair employer, however. In addition to having regular thorns, it also has modified pairs of thorns, which are joined at the base by a hollow, bulbous swelling (called a “domatia”) that is up to a little more than an inch in diameter. These thorns provide excellent nesting sites for the ants. In addition, special glands at the tips of their leaves produce a sweet secretion for the ants to eat.
Savage competition for the whistling-thorn exists among the four species of ants that attend to it. When the branches of one tree form a bridge to another, the ants invade their neighbors and battle violently until one colony wins control of the tree, after which the colony may grow to be one hundred thousand strong. The black-headed ant, which is the least warlike, comes out very badly in these battles, losing more of its population than any of the other three species.
To defend their trees against invasion, black-headed ants actively chew off all horizontal shoots, which causes the trees to grow tall and skinny and thereby avoid contact with trees that host enemy colonies. Pruning also causes the tree to allocate more energy to new shoots, healthier leaves, and larger nectaries, but unfortunately the ants also prune off all flower buds so the tree is effectively sterilized. Perhaps the tree trades reproduction for increased vigor and protection from browsing animals. As it turns out, however, the black-headed ant’s relationship with its acacia is more parasitic than mutually beneficial.
In comparison, the mimosa ant is not only the most antagonistic but also the most cooperative partner with its acacia. These ants rely heavily on the swollen domatia for shelter and are formidable protectors in return. But with no herbivores around to browse on the leaves, the ant’s services are not required, and the partnership begins to sour at both ends. The tree begins to evict the ants by shrinking its pro-ant services—namely, reducing the output of its nectaries. With less food and smaller homes, the ants are twice as likely to farm sap-sucking scale insects, whose waste fluid is a sugary liquid called honeydew, which the ants drink, but to make it, the scale insects must suck the juices of the tree. Consequently, the ants are less likely to marshal a defense against such marauding browsers as giraffes and elephants.
Conversely, Sjöstedt’s ants actually seem to benefit from a tree’s reduced investment in maintaining the aggressive mimosa ants. Less common than mimosa ants, Sjöstedt’s ants take a more relaxed attitude toward the partnership, one that could even be viewed as parasitic because it defends the tree less aggressively and ignores the swollen domatia. Instead, it occupies boreholes excavated by beetle larvae.
Because Sjöstedt’s ants are dependent on these beetle-created holes, they facilitate the beetle’s ability to feed on the trees. The ants don’t get upset with the suffering of their competitor, however. When the acacias reduce their provisions, Sjöstedt’s ants simply more than double the members of their colony.
Penzig’s ant, which is the only species that does not eat the nectar produced by its host acacia, actively destroys the nectar glands in order to make a tree less appealing to the other species. Consequently, the mutualistic feedback loops between whistling-thorn acacias and resident ants break down in various ways in the absence of large herbivores, and the acacias become less healthy as a result. Large herbivores are therefore critical components in the never-ending stories of these dynamic systems. For want of a giraffe or elephant to munch on the trees, the protective ants diminish and leave the whistling-thorn acacia in dire straits.1
Every system in the universe is governed by self-reinforcing feedback loops, which are little understood and thus virtually ignored in “managing” ecosystems for profit. Nevertheless, when our monetary expectations fall short of the predicted yield, we call on science.
Science, however, is ultimately a discipline of disproof, whereby something can be proven only by its actual, observable, quantifiable occurrence. Therefore, proof comes after the fact—not before it. Furthermore, science and technology have no sensitivity, make no judgments, and have no conscience. It is neither scientific endeavors nor technological advances that affect our collective environment, but rather the thoughts and values of the people who use the technology, who influence our overall respect for—or abuse of—the atmosphere, litho-hydrosphere, and biosphere, which in concert form the “commons” in which we live.
Building Blocks Of Sustainable Planning:
If you want to see another example of Nature’s feedback loops, click here
1. The preceding story of the whistling-thorn acacia is based on: (1) Truman P. Young, Cynthia H. Stubblefield, and Lynne A. Isbell. Ants on Swollen-Thorn Acacias: Species Coexistence in a Simple System. Oecologia 109 (1996):98–107 and (2) Todd Palmer, Maureen L. Stantan, Truman P. Young, and others. Breakdown of an Ant-Plant Mutualism Follows the Loss of Large Herbivores from an African Savanna. Science 319 (2008):192–195.
Text © by Chris Maser 2012. All rights reserved.
This essay is exerpted from my 2009 book, “Earth in Our Care: Ecology, Economy, and Sustainability,” Rutgers University Press, New Brunswick, NJ. 304 pp. If you want more information about this book or want to purchase it, visit “BOOKS” on my website.