Zoology · Ch 11 — Organisms and Population
Growth Models / Curves
Growth Models / Curves
Populations do not grow in an arbitrary way over time; instead, they tend to follow one of two characteristic, recognisable growth patterns when population size is plotted against time. The J-shaped growth form describes explosive, near-exponential increase that then halts abruptly once a limiting factor appears or environmental resistance sets in — many insect populations show this pattern, surging during the rainy season and then collapsing at the season's end. The S-shaped (sigmoid) growth form, by contrast, describes growth that starts slowly, accelerates, and then gradually decelerates as environmental resistance mounts, eventually settling into a stable equilibrium — a pattern typical of populations of small mammals. Both forms begin from the same underlying biotic potential (the maximum reproductive capacity of the organism under ideal conditions) but are ultimately capped, in different ways, by the environment's carrying capacity (the largest population size the habitat can …
What this figure shows. A graph with population size on the vertical axis and time on the horizontal axis, superimposing two growth trajectories starting from the same origin. The J curve rises steeply and continuously along what is labelled the biotic potential, climbing without bending until it terminates abruptly, illustrating unchecked exponential increase suddenly halted by a limiting factor. The S curve initially tracks close to the J curve at low population size but then bends over as it is pulled down by environmental resistance, flattening out as it approaches a horizontal line labelled the carrying capacity, illustrating growth that self-limits as resources become scarce. The two curves sharing one graph make clear that both trace the same initial bioti …