It is useful to present briefly some concepts from ecology in order to understand the nature of agriculture as a relationship between an exploiting species and one or several exploited species existing in a cultivated, human-made ecosystem.
Limiting Factor and Ecological Valence
All living beings find in the environment the resources necessary for their material existence: space, habitat, food, and the possibility to throw away waste materials derived from their life functions. Resources in any given environment are limited. Thus, there necessarily appears at one time or other conflict between the growing needs of a species that is multiplying within a given environment and the limited resources of this environment. When the population density becomes too great, when the quantities of water, minerals, pastures, or prey available at a particular critical period are totally consumed or become too scarce to remain easily accessible, then the growth of this population is blocked. The same thing happens when the waste material thrown away by a particular species encumbers the sites they occupy, diminishing or polluting its sources of provisions. The element of the environment that determines the maximum density the population of a species can attain over the long term at a given site is called the limiting factor. Of course, limiting factors vary from one species to another and vary from one environment to another for the same species.
In certain environments, a particular limiting factor for the development of a species (temperature, rainfall, food) can be found below a threshold of minimum tolerance or above a threshold of maximum tolerance, on the basis of which the development of this species becomes impossible. The level of this threshold varies according to species and their tolerance with respect to characteristics of the environment. The higher animals, humans, and certain domestic animals in particular, are very tolerant in relation to their environment. Their capacity to populate varied environments, that is, their ecological valence, is higher and their area of geographical extension is vast. On the other hand, some species demand very narrowly defined and rarely realized environmental conditions.
Hence they are not widely dispersed and their ecological valence is weak. The term “ecological valence” will be used here in a larger sense. It will designate not only the ability of a species to occupy varied environments, but also its capacity to populate them more or less densely. In this sense, the ecological valence of a species designates its potential for development: it is measured not only by the geographical extent of the species’ distribution but also by the maximum population density it can attain at the peak of its development.
Competition, Exploitation, Symbiosis
Often, two or more species struggle over the same resources. The opposition between the population of each species and the limitations of the environment is coupled with an opposition between the populations of each species in competition for the same resources. This competition, whether or not it involves an open struggle between competing populations, leads to their coexistence, within certain parameters, or to the elimination of one or several species.
One species can also exploit another, which acts as support, pasture, or prey for it. This exploitation can harm the development of the exploited species but, conversely, the development of the exploiting species can be conditioned by that of the exploited species. Such is the case when the latter forms an irreplaceable resource for the former. For example, a population of pandas is limited by the population of bamboos upon which it feeds exclusively.
Sometimes there exists between two species a reciprocal and necessary relation of exploitation, a relation that can be considered mutually beneficial to those species. Such a situation is called mutualism or symbiosis. For example, the nitrogen binding bacteria lodged in the bulges (or nodules) of the roots of leguminous plants contribute to supplying those plants with nitrogen. Ruminants and horses harbor bacteria in their intestines that facilitate the digestion of the cellulose materials essential in their dietary regime. Certain plants can only be pollinated by insects that gather the pollen.
Labor, Fabrication of the Environment, Agriculture, and Breeding
Some species transform the environment where they live to make it more accommodating and increase the available resources for their own use. They thereby increase their own ecological valence. Numerous animals build nests, shelters, and even an artificial environment (e.g., the collective urbanism of beavers, bees, termites, ants) that is necessary for their development. This transformation, this fabrication of the environment, is the product of a labor that is not, as is sometimes said, unique to the human species.
Moreover, some animal species go beyond the exploitation of other species by simple predation. They are devoted to transforming the environment in such a way as to create fabricated conditions of life that favor the development of the species they exploit. These exploited species, which could hardly develop without the support of the exploiting species, are called domestic. Some species of ants and termites cultivate mushrooms, which they eat. Other species of ants raise aphids whose honeydew they consume. In order better to understand the nature of the relations between cultivating or breeding species and domestic species, a quick analysis of the manner in which some ants manage their environment and organize the life of the species they exploit is not without interest.
The origin of ants goes back some 180 million years and evolution has produced around 18,000 species with different anatomies and modes of life. The oldest forms are generally insectivores, the forms from the middle period of evolution are omnivores, and the later forms practice specialized dietary regimes. By forcing the analysis a little, one could say that after the hunter nomadism of the early forms, a sedentary mode of life with the gathering of food appeared. Developing this metaphor, one could say that about a hundred of these species practice agriculture and breeding.
The Cultivator Ants
Several species of tropical American ants live in association with a particular species of domestic fungus. These ants manage the environment by constructing nests, galleries, and caves for the fungi. Among some species, the galleries go down several meters in depth and emerge into rooms with flat floors and vaulted roofs, sometimes as long as 1 meter and as wide as 30 centimeters, where the mushroom gardens are set up. In the heart of this layout, the immense central nest is sometimes linked up with several dozen small satellite nests within a radius of 200 meters. These ants also build a transportation infrastructure, a radiating network of trails made of built-up earth, several dozen meters long, 1 to 2 centimeters wide and set up for double circulation: one column of ants leaves for the harvest, while another returns to the nest with its cargo.
In order to multiply the mushrooms they eat, these ants methodically practice a whole series of cultivation processes. They prepare a bed for cultivation by collecting diverse organic debris (pieces of leaves, wood, roots, or tubers) from the outside which they tear up, grind, and fashion into mushroom beds. They plant fragments of cultivated mushrooms in these beds and systematically eliminate any other species of mushroom that begins to develop. Finally they regularly cut the filaments of mycelium, which prevents the fructification of the mushrooms and causes the formation of bulges, the mycotetes, which is what they exclusively eat. The social division of labor is well defined. The largest individuals guard the entrances to the nest and rarely leave those positions. The midsized individuals go outside the nest to harvest the plant debris, which they break up and mix into pellets. The smallest individuals maintain the mushroom gardens, feed the young larva and leave the nest only at the end of their lives. But this apparently well-regulated division of labor does not prevent some individuals from being undisciplined or even lazy. In exchange for all of the work involved in fabricating the environment and caring for the mushrooms in order to facilitate their multiplication, the ants receive abundant food, which can support the needs of hundreds of millions of them.
The Breeder Ants
Other species of ants live in association with a species of aphid, or mealybug. This partnership is a true form of breeding. In order to protect the aphids that they exploit, the breeder ants dig caves and lay out shelters in the ground or in a sort of carton, which are eventually linked up by galleries. The individuals in charge of guarding the shelters ward off the aphids’ predators and tear the wings of those that attempt to escape.
Among some species, the breeding is done by permanent underground stabling. The aphids are placed in chambers dug out around the roots of plants, where they can directly take the sap they feed on. Among other species, the breeding is done in the open air and the ants organize the food for the aphids by transporting them to better pasturage, namely to still-growing, young shoots. The reproduction of the aphids is carried out in good conditions, because the reproducing females are kept in underground chambers where the eggs are sheltered is, in turn, conditioned by that of the exploited species.d during the winter. The ants eat the aphids’ honeydew, their excrement, which is rich in sugars and other organic molecules derived from the sap of the plants they have ingested. To accomplish this, the ants rub the abdomens of the aphids with their antennae, stimulating them to excrete their honeydew.
The species of aphids raised by the ants are different from wild species. These are true domestic species whose wild ancestors are unknown. But one can assume that each species of domestic aphid is the result of a coevolution that simultaneously produced the species of breeder ant with which it is associated.
The relationship between these ants and the mushrooms or aphids is not a pure and simple one of exploitation. The ants act upon the environment and on the mode of life of the domestic species they eat. They work to favor their development and protect them. They thus increase the ecological valence of the species they exploit and, as a result, extend the nutritional limits of their own development.
Increasing the ecological valence of the exploited species in order to increase that of the exploiting species is the basic logic governing the particular relations between species that characterize agriculture and breeding. Cultivating or breeding a species, far from marking the end of its exploitation, is only, on the contrary, the extension and intensification of this exploitation by other means. Agriculture and breeding are thus elaborated forms of mutualism, but a dissymmetric mutualism in which the development of the exploited species is controlled by the labor of the exploiting species and the development of the exploiting species is, in turn, conditioned by that of the exploited species.
Written by Marcel Mazoyer and Laurence Roudart (translated by James H. Membrez) in "A History of World Agriculture", Earthscan, UK, 2006, excerpts pp. 29-33. Digitized, adapted and illustrated to be posted by Leopoldo Costa.