An ecological individual is a physiological and ecological unit that responds independently, (or potentially independently) of other individuals to its environment. Such individuals may be:

A genetic individual (genet) consists of all the tissues derived from a single zygote. In solitary organisms, the ecological individual is the same as the genet. In colonial and modular organisms, the genet is divided into genetically identical ramets (a subset of the tissues derived from a zygote). The ramets may be morphologically, physiologically and ecologically similar (monomorphic e.g. banana plants, coral polyps), or they may differ greatly in form and function (polymorphic e.g. reproductive, defensive, feeding polyps in Hydrozoa; leaves, sepals, petals, carpels, stamens in Angiosperms). Colonial and (to a lesser extent) modular forms may be chimeras, containing ramets from two or more genets (e.g. grafted plants, ascidians).

To survive, grow and complete its life cycle, an individual's environment must include necessary:

Conditions - usually physical and chemical factors remaining within certain limits (e.g. temperature, humidity, pH, salinity …)

Resources - physical, chemical or biological materials that are used or consumed by the individual (e.g. substrates, food, shelter, nesting sites…)

Necessary conditions and resources may vary greatly during an individual’s life as its physiological and developmental states change. These changes may be permanent (irreversible) or reversible; if reversible, they may be predictable (e.g. seasonal) or erratic responses (to other factors).

These may be physiological, behavioral, developmental, anatomical, morphological, or ecological. Many physiological responses exist as differences between internal and external environments:

The progression from conforming è regulating è homeostasis leads to increasing physiological and ecological independence of the individual from the stresses of external environmental changes. But regulation and (especially) homeostasis are increasingly "expensive" in terms of the energy, food and other resources needed to maintain the differences between internal and external environments.

As metabolic "costs" rise, organisms increasingly have internal mechanisms to assign priorities to particular functions, and then to allocate energy etc. to the functions. The basic priority is:

Conformers tend to have little flexibility in how they use energy and resources: allocation is usually sequential and additive - excess energy/resources are used for the next higher function.

In general, conformers use little energy for survival and maintenance (they simply shut down); instead most resources are devoted to rapid growth and reproduction. Conformers tend to be small-bodied, short-lived, rapidly developing organisms.

Regulators and (especially) homeostatic organisms tend to be larger-bodied, longer-lived and slower developers. They tend to devote most energy and resources to survival and maintenance, and much smaller proportions to growth and reproduction. However they often accumulate extensive food and energy reserves (as fat, protein or carbohydrate) that allow them to continue "normal" physiological and ecological activity through prolonged periods of variable and/or adverse conditions. They also have considerable flexibility in how energy is allocated to functions: changing priorities can maximize benefits during favorable conditions, and minimize effects of adverse conditions.

Physiological tolerances and responses are measured in the laboratory by exposing individuals to an array of conditions along an environmental gradient (usually one factor, e.g. temperature) and measuring a physiological or ecological index of the individual's performance under that condition (e.g. metabolic rate, fat content, growth rate, number of young). Plotting the index of performance (y-axis) against the intensity of the environmental gradient (x-axis) gives a Response Curve. The range of conditions under which the organism can carry out a specified function defines the Zone of Tolerance for that function along that gradient. A typical response curve is bell-shaped, and the four functions (listed above) tend to have increasingly narrower zones of tolerance that are "nested" within one another, and clustered about the conditions (Optimal or Preferred) where performance is greatest:

Dominant mechanisms for responding to the environment and for allocating resources are often correlated with the shapes of the response curves. Conformers tend to have tall, narrow response curves with acute peaks and spanning only a small part of the gradient. Homeostatic organisms tend to have low, broad, flat-topped response curves spanning a large part of the environmental gradient. Regulators lie somewhere on a continuum between conformer and homeostasis.

Grain is an index of the scale of environmental variation (in space and/or time), relative to the scale on which the organism detects ("perceives") and responds to environmental variation. Grain is a continuum with endpoints defined by analogy with sandpaper:

Dominant response modes are also often correlated with environmental grain. Conformers tend to be limited to relatively coarse-grained situations - they do very well as long as conditions are close to optimal. Conversely, homeostatic organisms are often successful in fine-grained situations, performing well under a broad range of conditions. Regulators are usually intermediate.

 The concept of "niche" has been very important in the development of many ecological ideas. The concept itself has evolved over the decades, encompassing at least three major ideas. The development of the niche concept is presented below as a historical progression

  A. The Concept of Niche as "Ecological Place"

It is currently fashionable to avoid using the term "niche", but the concepts and thinking associated with this term permeates much modern ecological thinking. Niche may include any or all of the three concepts (space, resources, role), so it is the responsibility of the user to define clearly how "niche" is being used. It may be measured by aspects of any or all of the four functions (survival, maintenance, growth, reproduction).

No matter what concept or definition of niche is used, each niche can be measured in two ways:

 The Realized niche is almost always SMALLER than the Fundamental niche, for many reasons:

The Realized niche is sometimes LARGER than the Fundamental niche, for other reasons: