Changes in Population Size

Changes in the population size of a species occur when individuals are added to or removed from a population.

4 factors that affect population size:

  • natality (birth rate)
  • mortality (death rate)
  • immigration (organisms migrate into population)
  • emigration (organisms migrate out of population)

Population Growth = organisms (in) – organisms (out)

Ex. The following changes occurred in a deer population over one year:

natality = 3; immigration = 7

mortality = 5; emigration = 6

Calculate the population growth.

The more mature an ecosystem (i.e. the longer it has been around), the more stable its size: population growth = 0.

  • steady state is when natality = mortality and immigration = emigration
  • open population (natural) – all four factors are acting on the population
  • closed pop (lab) – no immigration or emigration

Studying Changes in Population

To better study population growth, scientists can collect data on the age structure of the population. This can be graphed against population size at each age group to produce a population histogram.

A double histogram also depicts the proportions of males and females.

The shape of the histogram allows you to predict changes in the population:

  • wide base
  • young population (MEXICO)
  • rapidly growing (lots of births)
/
  • base slightly wider than rest
  • stable population
  • little to no growth

  • wider above than at base (UNITED STATES)
  • declining population
/

Carrying Capacity

The population size of a species is continuously adjusted by its interaction with both its food supply, its predators, and competitors in its own trophic level.

Usually a population of organisms cannot grow forever. Population size is limited by many things:

Carrying capacity – the largest population of a species that an ecosystem can support

  • Carrying capacity for a species is determined by the following factors:
  1. Materials and Energy (abiotic factors)
  • amount of energy from the sun
  • supply of water
  • supply of nutrients needed for growth (C, N)
  1. Food chains (biotic factors)
  • the pop’n size at any trophic level is determined by the pop’n sizes in all the levels below it (availability of food), and size of pop’n in the level above it (predators)
  • ex. grass  rabbit  fox
  1. competition (biotic factors)
  • The demand for resources (water, food, mates, space) results in competition among individuals for these resources
  • intraspecific  wolf vs. wolf(INTRA-INSIDE)
  • interspecific  wolf vs. coyote(INTER-BETWEEN)
  1. Density (biotic and abiotic)
  • refers to a large number of individuals within a given area – draw boxes to illustrate
  • if density of pop’n increases beyond a suitable level, conditions become such that the pop’n will decrease
  • i.e stress, spread of disease, parasites
  • Density Dependent Factors (ddf) - factors that affect a pop’n as the pop’n grows in size – ex. disease, stress, competition for food, mates…
  • Density Independent Factors (dif) – factors that affect a pop’n regardless of its size – ex. flood, drought, fire, spraying of pesticides,