Environmental Resistance

Every population has a biotic potential – the theoretical maximum rate at which it could grow if resources were unlimited and conditions were entirely favourable. In reality, this potential is almost never fully achieved.

Environmental Resistance: The combined effect of all unfavourable factors - such as adverse climate, food shortage, predators, and disease – that prevents a population from reaching its full biotic potential.

This type of growth occurs when a population lives in a confined space with limited resources.

• Early phase: Resources are abundant, death rate is low, and the population reproduces close to its intrinsic rate of increase, causing near-geometric growth.
• Later phase: As numbers rise, resources per individual shrink, so the growth rate steadily declines.
• Growth eventually stabilises once the population nears the environment's carrying capacity (K) – the maximum population size the environment can sustain.
• As density increases, intraspecific competition (competition among members of the same species for the same limited resources) intensifies, further slowing growth.
• This pattern produces a characteristic sigmoid (S-shaped) growth curve.

Logistic Growth Equation:

\[\frac{dN}{dt}=rN\left(-1-\frac{N}{K}\right)\]

As N approaches K, the term (1 − N / K) approaches zero, so the growth rate slows toward zero.

Example: A deer population introduced onto an island grows rapidly at first, then levels off once food and space become limiting.

Here, the factor's effect on the population is unrelated to how many individuals are present.

• Example: A toxic chemical entering a pond kills a fixed proportion of water fleas regardless of whether the population is large or small.
• Other examples: Sudden temperature changes, floods, storms, and forest fires.
• Without any resource ceiling, this produces an exponential (J-shaped) growth curve – a theoretical pattern rarely sustained for long in nature.

Exponential Growth Equation:

\[\frac{dN}{dt}=rN\]