The growth or decline of a population over a period of time can be expressed in terms of the changes in biomass (ΔB) or of numbers (ΔN). The changes that occur are the result of the sum of the individuals born (R) minus those dying (M) during the interval, the growth (G) of the individuals in the population, and of the net difference between emigration (E) and immigration (I). Thus we can write for biomass $$\Delta B = R - M + G + I - E,$$ (3–1) all expressed in units of biomass. The change in the number of individuals (ΔN) is expressed by $$\Delta N = R - M + I - E.$$ (3–2) In this case the growth of each individual is disregarded. The two equations are interchangeable by simply accounting for the biomass of the individuals in the population. We will return to biomass expressions in Chapter 7, where we deal with secondary production. In this chapter we concentrate on numbers of individuals, since most of the work on growth of populations is done on this basis.