Population Dynamics

Objectives

• How do populations change in size, density, and makeup in response to environmental stress?

• What is the role of predators in controlling population size?

• What different reproductive patterns do species use to enhance their survival?

Population Dynamics:How can populations change?

1. Size (number of individuals)2. Density (population size in a certain space at a given time)3. Dispersion (spatial pattern in habitat)– Clumped– Uniform– Random

4. Age distribution (proportion of individuals in each age group in population)

When do populations change?

• In response to…– Environmental stress– Changes in environmental conditions

Population Size• Total number of individual organisms in a population (# of

organisms of the same species in the same place at the same time)

• Natality- Number of individuals added through reproduction– Crude Birth Rate: Births per 1000 individuals per year

• Mortality - Number of individuals removed through death– Crude Death Rate: Deaths per 1000 individuals per year

Population Density• Population Density :the amount of individuals in

a population per unit area (or volume if aquatic)– Some species exist in high densities - Mice– Some species exist in low densities - Mountain lions

• Density can depend upon– social/population structure– mating relationships– time of year

Population Distribution (Dispersion)

Dispersion is the spatial pattern of distribution

clumped uniform

random

Age Structure

• Shows the % of the population in age groups (e.g. 0-4 years, 70-79 years, etc.)

• The age structure of a population dictates whether is will grow, shrink, or stay the same size

Age Structure DiagramsPositive Growth Zero Growth Negative Growth Pyramid Shape Vertical Edges Inverted Pyramid

What’s going on in Qatar?

www.cia.gov (CIA factbook)

Population Growth• Population growth depends upon – birth rates– death rates– immigration rates (into area)– emigration rates (exit area)

Pop = Pop0 + (b + i) - (d + e)

Zero Population Growth (ZPG) (b + i) = (d + e)

Population Growth

• Populations show two types of growth– Exponential • J-shaped curve• Growth is independent of population

density– Logistic • S-shaped curve• Growth is dependent on population density

Exponential Growth Graph

Logistic Growth

Exponential Growth?• One female elephant will produce 6 young over her 100

year life span. In a population, this amounts to a growth rate of 2%

• how many elephants could result from one male and one female in 750 years

• N = 2e0.02 x750 = 2e15 = 6.5 million elephants! • Even counting the deaths, there would be TOO MANY

ELEPHANTS for their habitat to support!

CLEARLY elephants do

NOT grow exponentially!

They must follow logistic

growth!

Carrying Capacity

• Basic Concept: Over a long period of time, populations of species in an ecosystem are usually in a state of equilibrium (balance between births and deaths)

Carrying Capacity (K)• Exponential curve is not realistic due to

carrying capacity of area• K = maximum number of individuals a habitat

can support over a given period of time• Not a fixed quantity – can change based on

several factors

Logistic GrowthReproductive time lag causes population overshoot• Could lead to a

die-off • Population will

not be steady curve due to resources (prey) and predators

Reproductive Strategies• “Goal” of every species is to produce as many offspring as

possible

• Each individual has a limited amount of energy to put towards life and reproduction

• This leads to a trade-off of long life or high reproductive rate

• Natural Selection has lead to two strategies for species: r - strategists and K - strategists

r – Strategists (rate)• Opportunists – reproduce

and disperse rapidly if conditions are favorable or after a disturbance

• Spend most of their time in exponential growth

• Maximize reproductive life

• Minimum life expectancy• Populations crash as new

species move in and outcompete

K

K - Strategists

• Maintain population at carrying capacity (K)

• Thrive with fairly stable environmental conditions

• Maximize lifespan

K

R Strategists

R-strategists• Many small offspring• Little or no parental care and protection of offspring• Early reproductive age• Most offspring die before reaching reproductive age• Small adults• Adapted to unstable climate and environmental conditions• High population growth rate – (r)• Population size fluctuates wildly above and below carrying

capacity – (K)• Generalist niche• Low ability to compete• Early successional species

K- Strategist• Fewer, larger offspring• High parental care and protection of offspring• Later reproductive age• Most offspring survive to reproductive age• Larger adults• Adapted to stable climate and environmental

conditions• Lower population growth rate (r)• Population size fairly stable and usually close to

carrying capacity (K)• Specialist niche• High ability to compete• Late successional species