Nov 11 2009
Population Biology: Growth Curves
Every Env. Sci. text presents two prototypical growth curves for a population:
- The “J” Curve (for exponential growth)
- The “S” Curve (for logistic growth)
Each curve is drawn from a few base assumptions:
- The “J” Curve assumes growth rate is constant, and resources are unlimited. Growth occurs at the intrinsic rate of growth (r). So, no limiting factors are restraining the population of that species. This is unlikely to happen as no organism exists outside a community (outside relationships or niches).
- The “S” Curve assumes there is a numerical limit to the number of individuals that the population can sustain in that area, known as the carrying capacity (K).
Then, I introduced and we discussed a mathematical approximation known as the Rule of 70. This “rule” accounts for the growth you see in a “J” curve graph (the shape is due to doubling time):
70 (yes, the number) = Doubling Time
Annual Growth Rate (%)
*This “rule” is useful for any estimation of when a population will double–but, remember that the assumption is that growth rate remains constant.
Then, we tried to link r-selected and K-selected species to these growth curves.
- In general, we would expect r-selected species to display a “J” curve
- In general, we would expect K-selected species to display a “S” curve
*Of course, there are exceptions. K- or r-selected species can display either! All “J” curves eventually transition to “S” due to environmental resistance (competition for resources, species interactions, disease, etc…).
Finally, we linked r-selected and K-selected species to survivorship curves:
So, do you remember which curve would be most typical of an r-selected species?