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Dynamics of Population Growth

Factors that Increase or Decrease Populations

Factors that Regulate Population Growth

Conservation Biology

Chapter 6 Outline

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Biotic potential refers to unrestrained biological reproduction. Biological organisms can produce enormous numbers of offspring if their reproduction is unrestrained.

Constraints include: Scarcity of resources Competition Predation Disease

Biotic Potential

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Population - all the members of a single species living in a specific area at the same time

Exponential Growth - growth at a constant rate of increase per unit time (geometric) ; has no limit

dN/dt = rN The change in the number of individuals (dN) per

change in time (dt) equals the rate of growth (r) times the number of individuals in the population (N). r is often called the intrinsic capacity for increase.

Dynamics of Population Growth

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Number of individuals added to a population at the beginning of exponential growth is relatively small. But numbers increase quickly because a % increase leads to a much larger increase as the population grows.

J curve when the equation is graphed

Exponential growth is a simple, idealized model. In the real world there are limits to growth.

Exponential Growth

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Carrying capacity - limit of sustainability that an environment has in relation to the size of a species population

Overshoot - population exceeds the carrying capacity of the environment and death rates rise as resources become scarce

Population crash - growth becomes negative and the population decreases suddenly

Boom and bust - population undergoes repeated cycles of overshooting followed by crashing

Carrying Capacity

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Logistic Growth - growth rates regulated by internal and external factors until coming into equilibrium with environmental resources dN/dt = r N (1 - N/K) Terms have the same definitions as previous

slide, with K added to indicate carrying capacity.

Growth rate slows as population approaches carrying capacity.

S curve when the equation is graphed

Growth to a Stable Population

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Logistic Growth Curve or S Curve

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External factors include habitat quality, food availability and interaction with other organisms.

Internal factors include physiological stress due to overcrowding, maturity, body size, and hormonal status.

These factors are density-dependent, meaning as population size increases the effect intensifies.

Density independent effects (drought, an early frost, flooding, landslides, etc.) also may decrease population size.

Factors Affecting Population Growth

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r selected species rely upon a high reproductive rate to overcome the high mortality of offspring with little or no parental care. Example: A clam releases a million eggs in a lifetime.

K selected species have few offspring but more parental care. Example: An elephant reproduces every 4 or 5 years.

r and K Selected Species

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Reproductive Strategies

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Natality - production of new individuals Fecundity - physical ability to reproduce Fertility - measure of actual number of

offspring produced

Immigration - organisms introduced into new ecosystems Dispersal of organisms by wind or water

currents over long distances. Sometimes carried by animals or on rafts of drifting vegetation.

Factors that Increase Population

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Mortality - death rate Survivorship - percentage of cohort surviving

to a certain age Life expectancy - probable number of years

of survival for an individual of a given age Increases as humans age. By older age,

most individuals destined to die early have already done so.

Has risen in nations/areas with good nutrition, sanitation and medical care

Women live longer than men.

Factors that Decrease Population

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Life span - longest period of life reached by a given type of organism Bristlecone pine lives 4,600 years. Human maximum lifespan is 120 years. Microbes may live a few hours.

Differences in relative longevity among species are shown as survivorship curves.

Life Span

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Four general patterns:a. Full physiological life span if organism

survives childhoodExample: Humans in the U.S.

b. Probability of death unrelated to ageExample: Sea gull

c. Mortality peaks both early and late in life.Example: Deer

d. Mortality peaks early in life. Example: Tree

Survivorship Curves

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Survivorship Curves

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The last factor in our list of factors that decrease population is emigration, the movement of members out of a population. Many organisms have specific mechanisms

to facilitate migration into new areas.

Factors that Decrease Population

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Intrinsic factors - operate within or between individual organisms in the same species

Extrinsic factors - imposed from outside the population

Biotic factors - Caused by living organisms. Tend to be density dependent.

Abiotic factors - Caused by non-living environmental components. Tend to be density independent, and do not really regulate population although they may be important in increasing or decreasing numbers. Example: Rainfall, storms

Factors that Regulate Population Growth

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Density Dependent Factors Reduce population size by decreasing natality

or increasing mortality. Interspecific Interactions (between species)

Predator-Prey oscillations

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Intraspecific Interactions - competition for resources by individuals within a population As population density approaches the

carrying capacity, one or more resources becomes limiting.

Control of access to resources by territoriality; owners of territory defend it and its resources against rivals.

Stress-related diseases occur in some species when conditions become overcrowded.

Density Dependent Factors Continued

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Critical question in conservation biology is the minimum population size of a species required for long term viability.

Special case of islands Island biogeography - small islands far

from a mainland have fewer terrestrial species than larger, closer islands

MacArthur and Wilson proposed that species diversity is a balance between colonization and extinction rates.

Conservation Biology

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In a large population, genetic diversity tends to be preserved. A loss/gain of a few individuals has little effect on the total gene pool.

However, in small populations small events can have large effects on the gene pool.

Genetic Drift Change in gene frequency due to a random

event Founder Effect

Few individuals start a new population.

Conservation Genetics

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Demographic bottleneck - just a few members of a species survive a catastrophic event such as a natural disaster

Founder effects and demographic bottlenecks reduce genetic diversity. There also may be inbreeding due to small population size. Inbreeding may lead to the expression of recessive genes that have a negative effect on the population.

Conservation Genetics

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Genetic Drift

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Minimum Viable Population is the minimum population size required for long-term survival of a species. The number of grizzly bears in North

America dropped from 100,000 in 1800 to 1,200 now. The animal’s range is just 1% of what is once was and the population is fragmented into 6 separate groups.

Biologists need to know how small the bear groups can be and still be viable in order to save the grizzly.

Population Viability Analysis

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Metapopulation - a collection of populations that have regular or intermittent gene flow between geographically separate units Source habitat - Birth rates are higher

than death rates. Surplus individuals can migrate to new locations.

Sink habitat - Birth rates are less than death rates and the species would disappear if not replenished from a source.

Metapopulations

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Metapopulation