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Population Principles

Chapter 7

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Population Characteristics

• Population - Group of individuals of the same species inhabiting the same area simultaneously. (6.4 Billion – 2004)– Natality and Mortality

Natality - Number of individuals added through reproduction.

Birth Rate (Humans Born / 1,000)– Mortality - Number of individuals removed

via death.Death Rate (Humans Died / 1,000)

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Figure 7.1

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Population Characteristics

• Population Growth Rate - Birthrate minus the death rate. Often expressed as a percentage of the total population. (Geometrically – Population)(Arithmetically – Food Supply)

• Survivorship Curve - Shows proportion of individuals likely to survive to each age.– High mortality in young.– Mortality equitable among age classes.– Mortality high only in old age.

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Survivorship Curve (Fig.7.2)

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Sex Ratio and Age Distribution

• Sex Ratio - Age Distribution– Sex Ratio - Relative number of males and

females in a population.Females determine the number of

offspring produced in sexually reproducing populations.

– Age Distribution - Number of individuals of each age in the population.

Greatly influences population growth rate.

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Age Distribution in Human Populations (Fig.7.3)

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Population Density and Spatial Distribution

• Population Density - Number of individuals per unit area.– High population may lead to increased

competition for resources.Dispersal - Movement of individuals

from densely populated locations to new areas.

Emigration - Movement from an area. Immigration - Movement into an area.

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Population Growth Curve

• Biotic Potential - Inherent reproductive capacity. (Biological ability to produce offspring)– Generally, biotic potential is much above

replacement level.Natural tendency for increase.

All living populations follow an exponential growth curve.

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Exponential Growth Curve

• Lag Phase - First portion of the curve; slow population growth.

• Exponential Growth Phase (Log Phase)- More organisms reproducing causing accelerated growth; continues as long as birth rate exceeds death rate. *Currently**Currently*

• Stable Equilibrium Phase - Death rate and birth rate equilibrate; population stops growing.

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Biotic Potential (Fig.7.4)

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Typical Population Growth Curve (Fig.7.5)

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Carrying Capacity

• Carrying Capacity - Number of individuals of a species that can be indefinitely sustained in a given area without harming the habitat.

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Environmental Resistance

• Environmental Resistance - Any factor (limiting factor) in the environment limiting carrying capacity. – Four main factors:

Raw Material AvailabilityEnergy AvailabilityWaste Accumulation and DisposalOrganism Interactions

Disease, Predation, and Space

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Carrying Capacity (Fig.7.6)

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Bacterial Growth Curve (Fig.7.7)

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Fig. p.138

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Reproductive Strategies and Population Fluctuations

• Not all species reach a stable carrying capacity.– Species can be broadly lumped into two

categories:K-strategistsr-strategists

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K - Strategists

• Large organisms• Long-lived• Produce few offspring• Provide substantial parental care• Populations typically stabilize at a carrying

capacity.• Usually occupy relatively stable

environments.• Reproductive strategy is to invest in a few,

quality offspring.

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K - Strategists

• Controlled by density-dependent limiting factors.– Factors that become more severe as the

size of the population increases.Diseases

• Deer - Lions - Swans

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r - Strategists

• Small organisms• Short-lived• Produce many offspring• Little if any parental care• Usually do not reach carrying capacity

(boom-bust cycles).• Exploit unstable environments.• Reproductive strategy is to produce large

numbers of offspring to overcome high mortality.

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r - Strategists

• Controlled by density-independent limiting factors.– Population size is irrelevant to the limiting

factor.Weather Conditions

• Grasshoppers - Gypsy Moths - Mice

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Population Cycles (Fig.7.8)

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Human Population Growth

• Major reason for increasing human population growth rate is an increase in medical care, and a consequential decrease in death rates.

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Historical Human Population Growth (Fig.7.9)

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Doubling Time

• Doubling Time of a Population (years) :

• 70 / Population Growth Rate (%)

( 70 / 2.0% = 35 years )

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Doubling Time for the Human Population (Fig.7.10)

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Human Population Growth

• Interactions Affecting Carrying Capacity– Available Raw Materials– Available Energy– Waste Disposal– Interaction With Other Organisms

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Social Factors Influence Human Population

• Humans are social animals who have freedom of choice.– People make decisions based on history,

social situations, ethical and religious beliefs, and personal desires.

Biggest obstacles to controlling human population are not biological, but are the province of philosophers, theologians, politicians, and sociologists.

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Ultimate Size Limitation

• If the world continues to grow at current rate, population will surpass 12 billion by 2060.

• Human population subject to same biological constraints as other species.– Human population will ultimately reach a

carrying capacity and stabilize.Disagreement about exact size and

primary limiting factors.

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Elephant Trade

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Snow Goose (Pg.145)