proximity or contact. There is obviously something more than gravity at work in making this happen. If the entities are from divergent species, a lamb and a lion for instance, then the ultimate interaction may well consist of the lion eating the lamb; in some circumstances, the lion may lie down with the lamb, but that would be a low probability event.

The act of bringing two individual entities in close proximity such as to allow physical interaction will typically involve sensory capabilities of one or the other of the individuals to detect the presence of the other at a distance. Moreover, it will likely entail the motor functions of one or both of the entities to propel them together. Predators may use keen eyesight, sensitive hearing or a comprehensive sense of smell to detect the presence of prey through their forensic wakes in the physical world. Conversely, species that comprise typical prey may display physical characteristics or capabilities that make them difficult to detect or to directly attack. In essence, they learn to camouflage their forensic wakes. The arctic hare, which changes color between the summer and the winter, is a good example. Once a predator does locate its prey, it may use a variety of motor skills to achieve physical contact; motor skills that range from stealth to speed. Once in physical contact, the offensive and defensive facilities of the entities determine the end game of the interaction. The western coyote consumes the lamb, but retreats in pain from the defensive spray of a skunk.

For between-species interactions, extensions to purely physical system interactions fall into five major categories: coexistence, competitive coexistence, symbiotic, parasitic and predatory. These categories provide classification of the basic ground rules of interactions, specifically indicating how different species share the physical ecosystem in which they exist. The most benign form of sharing an ecosystem is coexistence. Through this style of interaction, inter-species contact is typically casual with little or no aggressive behavior exhibited by either species. In essence, coexistence defines the sharing of an ecosystem with no offensive or defensive interactions specifically initiated between species. In the most independent of cases, different species would be members of entirely different food chains. If one considered a highly constrained ecosystem of a lettuce patch populated by rabbits and earthworms, then the rabbits and the earthworms very probably live in a totally coexistent relationship. Their continuance as species is only tenuously related through the lettuce plants and excretion of body waste by the rabbits (and the earthworms and the lettuce for that matter), and it is quite plausible that either species could be removed without significantly impacting the other.

Perhaps, a more common variant of ecosystem sharing would be one in which species live in a state of competitive coexistence. Being contained or confined within the same ecosystem indicates that limitations imposed by the boundaries of the ecosystem are shared among the species it contains. For example, if two or more herbivore species share an ecosystem in which all members subsist on the vegetation within the ecosystem, then the ecosystem’s ability to support vegetation serves as a cumulative limit on the populations of all the herbivores. If food or water becomes in shorter supply due to the abundance of herbivores, whatever their species, then the numbers of each species will likely be limited. However, in this situation, if one species can preferentially consume the food supply, then that species may well have a natural selection advantage within that ecosystem. Consider a system of vegetation that both rabbits and cattle might consume. Rabbits reproduce more rapidly than cattle, but one cow eats more food than many rabbits. Following a period of competitive coexistence, it is quite plausible that the number of rabbits will increase to a level that can consume enough of the food supply to leave the cattle insufficient food for continued propagation. After that, the number of rabbits might well vary in some equilibrium determined by the growth rate of the lettuce.

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