Self-organization is the appearance of order and pattern in a system by internal processes, rather than through external constraints or forces. Plant distributions provide examples of both constraints and self-organization. On a mountainside, for instance, cold acts as an external constraint on the ecosystem by limiting the altitude at which a plant species can grow. Simultaneously, competition for growing sites and resources leads to self-organization within the community by truncating the range ofaltitudes where plant species do grow. Self-organization can also be seen among individuals within a population (e.g., within an ant colony or a flock of birds) and within individuals (e.g., among cells during development) (Figure 1).

A growing understanding of ways in which internal processes contribute to ecological organization has provided new perspectives on many phenomena familiar from traditional ecology. Self-organization usually involves interactions between components of a system, and is often closely identified with complexity. Also associated with self-organization is the idea of emergence: that is, features of the system emerge out of interactions, as captured by the popular saying, ''the whole is greater than the sum of its parts.'' It is necessary to distinguish between emergent features and other global properties of

Figure 1 Effect of competition on plant distributions on a gradient. The two plant species shown are adapted to different conditions, which are here found at either end of the slope. At left, there is no competition, so the distributions merge into one another. At right, competition truncates the distributions, leading to sharply defined altitudinal zones.

a system. For instance, although biomass production in a forest is a global property, it is simply the sum total of production by all the organisms within the forest. A stampede, on the other hand, is behavior that emerges when panic spreads from one animal to another within a herd.

Semantic and philosophical issues sometimes lead to confusion about self-organization. Self-organizing systems are usually open systems, that is, they share information, energy, or materials with their surroundings. However this does not necessarily mean that the external environment controls or determines the way they organize. A growing plant, for instance, absorbs water, light, and nutrient from its environment, but its shape and form are determined largely by its genes.

Also, in considering self-organization, it is important to clearly identify the system concerned, and in particular, what is external and what is internal? This issue arises in the difference between a community and an ecosystem. For a community, which consists of the biota of an area, the effect of (say) soil is an external constraint. However, for the corresponding ecosystem, which would include soils, the interactions between plants, microorganisms, and soil formation are internal processes. Defining the physical limits of an ecosystem poses similar problems. A lake, for instance, is not a closed ecosystem. Among other things, water birds come and go, removing some organisms and introducing others.

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