How Many Animals Can Live There? The Number Moves

A population growing in a limited environment cannot grow indefinitely, because something runs short, and the level at which growth stops is what the term names. The standard model has growth slowing as the population rises and stopping when births equal deaths, producing a curve that rises steeply and then flattens. The limit is set by whatever resource runs out first, which may be food, water, nesting sites, shelter or space, and which may differ between seasons, so a population can be limited by summer food and by winter shelter in the same year.

Several things move it and the model is frequently used as though they do not:

• •Rainfall and temperature change how much the land produces
• •One severe winter can cut it for years
• •Populations alter their own environment, raising or lowering it
• •Overgrazing damages the vegetation and lowers it lastingly
• •Predators, disease and competitors change it
• •Human management changes it deliberately and constantly

Populations do not stop neatly at the limit and what happens instead is the part that matters practically. A population growing fast passes the level its environment can sustain before the shortage registers in births and deaths, so it overshoots, and the resource is then consumed faster than it regenerates. The correction is a crash rather than a levelling, and if the resource was damaged in the process the population settles below where it started. Introductions of deer and of reindeer to islands have produced exactly this sequence with documented figures, and the same shape appears wherever a population arrives somewhere with no predators and abundant food.

Wildlife and livestock management depend on the concept operationally, and the versions used differ from the textbook one in a way worth knowing. Ecological capacity is the number the land could support, and economic capacity is the lower number at which animals remain in good condition and the vegetation is not damaged, which is the figure a grazier actually manages to. Deer management sets a target density below the ecological limit precisely to protect tree regeneration. Fisheries aim at the population that produces the greatest sustainable catch, which is roughly half the unexploited level rather than the maximum. In each case the managed target is deliberately below the limit.

Extending the idea to human populations has a long history and runs into a specific objection. Every historical estimate of the limit for humans has been exceeded, because the limit for a species that changes its own productive capacity is not a property of the land alone. Agriculture, fertiliser, trade and energy have all raised it enormously, and there is no accepted method of calculating a figure that accounts for future technology. Against that, the underlying point survives, since consumption of soil, water and fish stocks faster than they regenerate is overshoot by definition, whatever the total population turns out to be able to reach.

The level at which a population stops growing is set by whichever resource runs short first, and that level moves with weather, season, damage to the vegetation and the population's own effects. Fast-growing populations overshoot before the shortage registers, so the correction is a crash and can settle below the starting point. Human estimates have all been exceeded, because people change their own productive capacity.