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Population Ecology

Terms in this set (93)

A populations growth rate is the change in the number of individuals in the population (ΔN) per unit time (Δt).

If no immigration or emigration is occurring, then a population's growth rate is equal to the number of individuals (N) in the population times the difference between the birth rate per individual (b) and the death rate per individual (d). The difference between the birth rate and death rate per individual is called the per capita rate of increase (r). Per capita means for each individual.

ΔN/Δt = N(b-d) or ΔN/Δt = N= N*r when there is no emigration or immigration

If the per capita birth rate is greater than the per capita death rate, then r is positive and the population is growing. But if the per capita death rate is greater than the per capita birth rate, then r is negative and the population is decreasing. If r is zero then the population is stable


When conditions are optimal for a particular species - meaning birth rates per individual are as low as possible - then r reaches a maximal value called the intrinsic rate on increase, rmax. When this happens, growth rate is expressed as ΔN/Δt = rmax*N

Examples of rmax in populations:
- In fruit flies, which breed at a young age and produce many offspring each year, rmax is high.
- rmax is low in species such as giant pandas and coconut palms, which take years to mature and produce few offspring each year.

Each species has a characteristic rmax that does not change. But at any specific time, a population has an instantaneous growth rate, or per capita rate of increase, symbolized by r. Whereas rmax tells you what the max growth rate is, r tells you what it is at a particular time. So r is always less than or equal to rmax.

A population's r is likely to be different from r values of other populations of the same species (such as upland and lowland populations of L. vivipara), and to change over time (such as in wet years versus dry years). The instantaneous growth rate is dynamic.