The term “b” in population growth represents:
In the standard continuous population-growth notation, b denotes the per-capita birth rate: the expected number of births contributed per individual per unit time. Multiplying b by population size gives the total birth input B = bN under a homogeneous model. Similarly, d is the per-capita death rate and D = dN. Their difference gives the instantaneous per-capita growth rate r = b − d, leading to dN/dt = (b − d)N for a closed population. A raw birth number is an absolute count and is usually written B, so distinguishing uppercase totals from lowercase per-capita rates prevents dimensional error
Ref: Ecology: Concepts and Applications, Molles, 9th Ed., Ch. 11
