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8.3 Population Ecology

Syllabus
2025
Topic
8.3
Level
—

Learning objectives

8.3A—Describe factors that influence growth dynamics of populationsDescribe factors that influence growth dynamics of populations.• Populations comprise individual organisms of the same species that interact with one another and with the environment in complex ways.• Many adaptations in organisms are related to obtaining and using energy and matter in a particular environment.- i. Population growth dynamics depend on birth rate, death rate, and population size. Relevant equation: Population Growth— dN dt BD=− where dt = chage in time B = birth rate D = death rate N = population size dN = change in population size

Calculating Population Growth

A population is a group of organisms of the same species whose members interact with one another and with their environment. Its growth dynamics depend on population size and on how births and deaths change that size over time.

\frac{dN}{dt}=B-D

NN = population size; dNdN = change in population size; dtdt = change in time; BB = birth rate; DD = death rate. Therefore dN/dtdN/dt is population change per unit time. Positive means growth, zero means no net change, and negative means decline.

Illustrative calculation: if a population has B=120B=120 births per year and D=80D=80 deaths per year, then dN/dt=120−80=40dN/dt=120-80=40 individuals per year. The result describes the net rate of increase under those stated rates.

\frac{dN}{dt}=r_{\max}N

For reproduction without constraints, rmax⁡r_{\max} is the maximum per-capita growth rate and NN is the current population size. If N=500N=500 individuals and rmax⁡=0.08 year−1r_{\max}=0.08\,\text{year}^{-1}, then dN/dt=(0.08)(500)=40dN/dt=(0.08)(500)=40 individuals per year. As NN increases, the absolute increase becomes larger, producing exponential growth.

Adaptations that improve access to energy or matter in a particular environment can change survival or reproduction and therefore alter BB, DD, or the realized growth rate.

The exponential equation describes unconstrained reproduction; it is not a claim that real populations can grow exponentially forever. Keep rates and time units consistent when substituting values.