Emergent properties really come into play when biological systems reach higher levels of complexity. This becomes evident already when cells or groups of cells communicate with each other as in the case of hormones and natural neural networks. Organs are composed of cells, their individual functions are important only to the organism as a whole. A heart, kidneys or lungs, are vital but their function is not existent when they are on their own. Organisms interacting as populations or societies provide properties which cannot be explained by properties of the individual organisms alone. They all go together in what we consider as ecosystems and thus are a part of the biosphere.
Cellular level. In regarding the outcome of interacting cells many studies have been concentrating on the organization of neuronal systems, which result in unexpected properties like the ability to move, to sense, to be intelligent, and to emote. The sensory systems, being connected to visual, auditory, or other communicative processes are all playing a major role in how successful living organisms are in performing specific life strategies. Reliable senses, and responding the right way to the received stimuli are crucial to the existence of many life forms, in processes like finding food, knowing when and where to escape, or creating bonds with other members of the species, for example, during reproduction.
Neural networks, like in our brains, consisting of a huge number of interconnected neurons, are so complex that unforeseen patterns in responses are bound to occur and have also been reported to exist. During the evolution of the brain, emergent properties, together with new cell types, local and large circuits have added up to the increasing complexity of brain function. Motor control, the control and coordination of motor activity are taken care of by our brain passing on signals to the limbs or organs involved.
Organ level. Numerous cells, often during morphogenesis differentiated in certain, specialized directions, form organs, take up a particular task of the organism, like for instance liver cells secreting enzymes, or kidney cells filtering and cleaning the coelom. Although the formal 'layout' for this functionality is existent in the genetic material of all cells, the eventual determination occurs during the development of the organism and the actual function of the organs may be viewed as emergent. The brain as an organ may serve as an example of this emergence: Here differentiated cells, with highly specialized physiological properties, go together and create activity patterns that are far more complex than expected from knowing the physiology of neural cells alone. The whole becomes more than the sum of the parts.
Organism level. Complex behavior occurs among the individual organisms that cannot be determined exclusively by internal factors. The sending, reception and interpretation of signals from interagent organisms, the relationship(s) to the outside, and thus semiotics play an important role, creating patterns impossible to foresee if only the subsystems are known. For example, in trees, the formation of branches and leaf mosaics have been studied in a number of recent investigations with modeling approaches as well as the allocation of resources between above- and belowground biomass and the related physiological mechanisms. A modeling study of this problem indicates a 'complex integrated growth pattern' which may only be understood as an emergent property as it is claimed to have no direct or indirect mechanistic basis related to subcellular activities. In a similar manner it was shown that whole-plant behavior is an emergent property arising from a rule-based model of the system. Communications between individuals, that is, their social interactions within a population, are important to the function of the organism as a whole and are indistinguishable from the emergence of ethological features. Stressing the importance of communication, may lead to an interpretation of the communicative process as an emergent interpretation ofsigns, which is described within the discipline ofsemiotics.
Population level. Populations are composed of individual organisms, interacting in various ways, differing in quantity and quality, throughout the biological system. The interactions may vary in character according to the complexity. At the one end of the spectrum, we find the single cell organisms interacting mostly on a material basis (matter fluxes). At the other end, there are colonial organisms forming complex societies, where brains, senses, memory, and thus informational interaction become dominating. Emergent properties as a result of individual level behavior and interactions in populations of social insects have been argued in several studies. For instance, the distribution of food to larvae of the fire ant has been argued as emerging from interactions between individuals, workers, and larvae. Cellular automata models were used to study the short time oscillations in ant colonies. The nonlinear dependencies describing the relationships between, and the movement of, individuals explain this behavior. The resulting oscillations were found to be emergent properties of the colony.
Ecosystem level. Ecosystems are inherently complex as they are composed of an embedded hierarchy of all the previously mentioned subsystems in close interaction with abiotic factors. Emergence is to be expected, but surprisingly few reports exist at this level, before all analyzing microcosms, forest ecosystems, predator-prey relationships, food webs, and the organization of aquatic communities.
Ecosystem behavior is often analyzed through modeling studies. The relation to emergent properties becomes clear when looking at recent efforts of structural dynamic modeling, where the changes in ecosystem composition and structure over time are analyzed. Another example is the work of B. C. Patten on the propagation of matter-energy through the ecosystem network, leading to the discovery of the importance of 'indirect effects, quantitative and qualitative utilities' of the system, results that are highly surprising and unexpected, and as such are emergent properties (see Ecological Network Analysis, Environ Analysis). Both examples link to higher-level information expressions such as ascendency, different kinds of entropy or information derived descriptors like exergy (see Exergy).
The ability of the ecosystem to perform with systematic directional changes in some macroscopic characters, not predictable from knowledge about the single ecosystem members alone, has been discussed since 1967 on the basis of the 24 principles of ecosystem development during succession in the second edition of E. P. Odum's Fundamentals of Ecology. Many other factors, known as indicators, orientors, or goal functions have been presented since then (Table 1).
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