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Population Biology:

IB TOPIC 5.3

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Biotic Potential

• 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

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Dynamics of Population Growth

• 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.

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Exponential Growth

• 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.

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Carrying Capacity

• Carrying capacity (K) - 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

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Growth to a Stable Population

• 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. Sigmoid (S-shaped) growth curve when the

equation is graphed

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Sigmoid Growth Curve or S Curve

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Factors Affecting Population Growth

• 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.

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r and K Selected Species

• 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.

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Reproductive Strategies

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Factors that Increase Population

• 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.

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Factors that Decrease Population

• 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.

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Life Span

• 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.

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Survivorship Curves

• Four general patterns: Full physiological life span if organism survives

childhood- Example: Humans in the U.S.

Probability of death unrelated to age- Example: Sea gull

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

Mortality peaks early in life. Example: Tree

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Survivorship Curves

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Factors that Decrease Population

• 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.

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Factors that Regulate Population Growth

• 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

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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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Predator-Prey oscillations

• Canada lynx vs. snowshoe hare

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Density Dependent Factors Continued

• 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. become overcrowded.

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Density Dependent Factors: A Case Study

• Desert Locust (Schistocerca gregarius) – Usually solitary creatures, like grasshoppers

• Desert storms (~once every 10 years) cause vegetation to flourish – increases locust reproduction

• Stress and high population causes physiological and behavioral changes Stop reproducing Grow longer wings Group together in swarms

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Density Dependent Factors: A Case Study

• Desert Locust (Schistocerca gregarius)

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Density Dependent Factors: A Case Study

• Desert Locust (Schistocerca gregarius) Clouds of locusts travel as much as 100km per

day A single locust can consume its own body weight

in a day Swarm can cover 1,200 km2 and contain 50-100

billion individuals- Strip pastures, denude trees, and destroy

crops- Consume as much food in a day as 500,000

people need in a year

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Density Dependent Factors: A Case Study

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Conservation Biology

• 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.

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Conservation Genetics

• 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.

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Conservation Genetics

• 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 deleterious effect on the population.

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Genetic Drift

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Population Viability Analysis

• 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.

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Metapopulations

• 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.

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Metapopulation