Given the standard, though arbitrary, units of measure available to express measurements of ecological processes, the need often arises to seek an appropriate measurement scale (expressed in standard units) that best describes a process. Often standard units are inconvenient for use in describing particular processes, and it is often appropriate to 'rescale' the basic units of measurement to simplify the description of the process being studied. Often, rescaling the units of very different state variables can shed light on the unity of processes affecting them. The goal of rescaling is to seek a 'characteristic scale' that best describes a particular process.
Two types of measurements describe most ecological phenomena. The first is 'ecological kinetics', which describes changes in concentrations of substances over space and time. Familiar processes that can be described using this approach include population dynamics and nutrient cycles. The second is 'ecological energetics'. Phenomena such as primary productivity and respiration are characteristic of this set of processes. In any complex ecological system, both kinetic and energetic processes contribute to the overall system behavior. Much of the challenge of describing ecological processes involves sorting out which system components are best described as kinetic versus energetic processes. Indeed, the behavior of some system components may require both energetic and kinetic descriptions to capture the complexities of that behavior. For example, an organism in a population can be described both as an energetic system and as a component of a kinetic system. Seeking relationships between these two types of measurements can conceivably lead to new insights into complex ecological system behavior.
The basic unit of measure in a kinetic description is the quantity of a substance, expressed as moles (mol). Often, kinetics are described by state variables that express concentrations (or densities), expressed as moles per square meter (mol m~2). The goal of a kinetic description of an ecological system is to express system dynamics in space and time. Ecological rates can be described as changes in amounts of substances in time (mol s~ ) or space (mol m~ ), or as changes in concentrations (densities) in time (molm~2 s_1) or space (molm~3).
The standard units for ecological kinetics are appropriate for descriptions of some processes, but not others. On one hand, nutrient fluxes are best expressed using moles because the standard definition of a mole is the number of carbon-12 atoms per 0.012 kg of pure carbon-12. Since nutrient fluxes involve small elementary entities, such as molecules, ions, or particles, moles closely correspond to the purported processes that distribute these elementary entities in space and time. On the other hand, the primary entities involved in population dynamics are organisms, of which many fewer are measured than the primary entities of a nutrient flux. Using moles to express population densities would lead to measurements in units far removed from the primary entities. Hence, the units for population size are organisms and the units for population density are organisms per square meter. This arbitrary measurement scale, however, imposes complications on the description of populations of different types of organisms. Because different species of organisms undergo life history process on different spatial and temporal extents, the characteristic scale for population dynamics is a direct consequence of the average body size of each species. Thus, in order to seek descriptions of population dynamics that might demonstrate general properties of population change that are independent of species, it is desirable to rescale population kinetics to mass-independent units.
The basic unit of measurement for energetic processes is a joule (J). A joule is related to length, mass, and time by the relation J = kgm2s~2. In essence, a joule is a measure of inertia, that is, it describes acceleration of a unit of mass over a unit of distance. Hence, by definition, a joule is a measure of kinetic energy. Much of the energy in ecological systems, however, is stored as potential energy in the form of chemical bonds formed in the tissues of individual organisms. The rate of energy expenditure in an ecological system is measured as watts (W), which is the number of joules expended per second (J s~ ).
When potential energy is released to drive the biological processes that underlie ecological change, heat energy is generated. The faster or longer an energetic process operates the more heat is produced. When heat interacts with an object or substance, it increases temperature, the fundamental unit of temperature being kelvin (K). Temperature, then, is also a measure of energetic activity in an ecological system. The problem with using temperature to measure system energy use is that heat fluxes to and from the environment regulate temperature in an ecological system. In many studies of ecological energetics, descriptions of system energy use are formulated in terms of such heat fluxes. An additional complication is that energy use is intimately associated with changes in concentrations of molecules used to store and release energy (most notably oxygen and carbon dioxide). For example, oxygen consumption or carbon dioxide production are indirect measures of energy use. Because of this variety of ways of measuring energy use, it is often difficult to compare energetic processes that occur at different spatial or temporal extents.
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