# Population Biology.

## Presentation on theme: "Population Biology."— Presentation transcript:

Population Biology

Ostriches are nomadic, wandering in small groups.
Populations Population: a group of organisms of the same species that live within a given area Key characteristics: Growth Rate Population density Dispersal Patterns Life-History Patterns Ostriches are nomadic, wandering in small groups. Populations A population is a group of individuals of the same species living within a designated area at one time. The boundary of the population may be physical—such as a mountain range—or defined by a scientist for purposes of study. Demography is the statistical study of populations. Three important aspects of population structure are: dispersion patterns or spacing, population density, and growth rate. References Campbell, N.E. & Reece, J.B. (2002). Biology, (6th ed.). San Francisco: Benjamin Cummings. Image Reference NOVA Development Corp. (1995) Birds #2289. Art Explosion, Volume 2 Clip Art NOVA Development Corp. (1995) New England #57. Art Explosion, Volume 2 Clip Art Aspen trees are quick to pioneer areas that have been disturbed by fire.

Populations Populations—large and small—are DYNAMIC Meaning, they change over time Humans face the same problems as large and small populations in nature

Population Growth Exponential Growth J-Curve
Unchecked growth due to unlimited resources As a population gets larger it grows faster Birth Rate is greater than Death Rate

Population Growth Logistic Growth S-Curve Carrying Capacity
Number of organisms the environment can support Carrying capacity limited by resources Birth Rate is equal to Death Rate at carrying capacity

 Self Check  A. population increases exponentially
Characteristics of Population Growth Exponential growth J curve S curve Population Time Carrying capacity  Self Check  Which of the following would you expect to observe after a population exceeds its carrying capacity? A. population increases exponentially B. births exceed deaths C. deaths exceed births D. population growth rate is unaffected by limiting factors The answer is C.

Population Size Perils of Small Populations
low genetic diversity subject to inbreeding less likely to adapt to environmental changes Problems being a Large population Increase food shortages & diseases Decrease in space, clean water Live at carrying capacity so can experience huge crash

Factors That Influence Population Size
Population growth rate is determined by: Natality or Birth rate Death rate It is also influenced by the number of individuals that enter and leave a population. Immigration – move into population Emigration – move out of population

Limits on Population Growth
Limiting Factors- any factor that causes a population to decrease Density Dependent Limiting Factors Depends on the size of the population Ex. Food, Water, Shelter, Disease, Competition Density Independent Limiting Factors Can affect populations regardless of their density Ex. Weather, Climate Floods, Drought, Tornadoes, Fire, Volcanoes Water and shelter are critical limiting factors in the desert. Limits on Population Growth Carrying capacity (K) is the maximum number of organisms of a population that can be supported by a particular habitat. As population numbers approach the carrying capacity of an environment, in other words as density increases, competition for resources is amplified. Density-dependent factors in an environment include available food, nutrients in the soil, water, and shelter, among many others. The buildup of metabolic wastes also increases with density and adversely affects many populations as well. Weather, climate, and human activities can be density-independent factors which affect the environment. In the case of catastrophic events or the pressure of toxins, populations are affected regardless of size. Populations recover at different rates, some even experiencing a permanent decline after a major change in the environment. References Campbell, N. E. & Reece, J. B. (2002). Biology (6th ed.). San Francisco: Benjamin Cummings. Raven, P. H. & Johnson, G. B. (2002). Biology (6th ed.). McGraw-Hill. Image Reference NOVA Development Corp. (1995) Birds #2516. Art Explosion, Volume 2 Clip Art NOVA Development Corp. (1995) Wilderness #319. Art Explosion, Volume 2 Clip Art Fire is an example of a Density independent Limiting factor.

Limiting Factors Density-dependent factors Density-independent factors
Disease Competition Predators Parasites Food Density-independent factors Volcanoes Temperature Storms Floods Drought Habitat disruption

Other population factors
Predation Competition

Predator & Prey Predator: Organism that eats all or part of another organism Prey: an organism that gets eaten by another organism

Competition: two or more organisms using the same resources
Between different species and the same species

Population Density Population density is total population size per unit of area. Population densities depend on: Interactions within the environment Quality of habitat Density dependent factors Density independent factors Birth rate and death rate Population Density Population density is a measure of the number of individuals of the same species living in a designated unit of space. It is influenced by relationships among organisms, movement of individuals in and out of the habitat, resources, and abiotic environmental factors (such as climate). Fluctuations in population density can be indications of changes in the environment. Carrying capacity is the maximum number of organisms in a population that can be supported by a particular habitat. Many factors determine carrying capacity, some of which are influenced by the density of the population, while others are not. Density-dependent factors in an environment might be influenced by available food, water, and shelter. Density-independent factors include all facets of weather and climate, such as droughts, storms, and volcanic eruptions. It often is difficult to determine the size of a population because of the wide range of the habitat or mobility of the organisms. In such cases, ecologists use a variety of sampling methods. For instance, a designated area of study might be sectioned into grids or plots. Numbers of organisms counted in selected grids are extrapolated to estimate the total population size. Mark-and-recapture is another method used to estimate population size in large geographic areas. Traps are set in the study area. Trapped organisms are tagged and released. After a period of time, traps are set again, and calculations are made based on the number of marked organisms that are recaptured. Total population = total size of 2nd sample X marked # in 1st catch marked # recaptured in 2nd catch References: Campbell, N. E. & Reece, J. B. (2002). Biology (6th ed.). San Francisco: Benjamin Cummings. Ricklefs, R. E. & G. L. Miller. (2000). Ecology (4th ed.). New York: W.H. Freeman and Co.

Dispersal Patterns Within Populations
Three common patterns of distribution are: Dispersion Patterns Within Populations The arrangement, or dispersion, of individuals in relation to one another within a given area is one key characteristic of population study, as it reflects interactions among the population and the environment. Three patterns of population dispersion are clumped, evenly spaced, and random. The most frequent pattern of distribution in a population is clumped. Individuals are clustered together in groups in response to uneven distribution of resources, tendency of offspring to remain with parents, or some type of social order. Clumping also may be linked with defense (safety in numbers) or mating behavior. In plants, soil type, availability of water or the manner in which the plant reproduces may favor clumped distribution patterns. Evenly spaced distributions, in which members of the population maintain a minimum distance from one another, generally indicates strong intraspecific competition. In plant populations, this could result from competition for water, sunlight, or available nutrients, while among animals, even spacing indicates strong territoriality. Random spacing is the least common pattern of distribution found in populations. It usually occurs because members of a species do not frequently interact with one another or are not heavily influenced by the microenvironments within their habitat. References Campbell, N. E. & Reece, J. B. (2002). Biology (6th ed.). San Francisco: Benjamin Cummings. Raven, P. H. & Johnson, G. B. (2002). Biology (6th ed.). McGraw-Hill. Image Reference Young, M. (2004). Dispersion patterns within populations. Houston, TX: Baylor College of Medicine, Center For Educational Outreach.

Dispersal Patterns Clumped Random Uniform

When Studying Populations…
The best way to determine population size is to collect an absolute number. Count up all the individuals in the population. More frequently used is population density. The number of individuals per unit area. Can be measured using a variety of sampling techniques.

Random Sampling A method of selecting a sample from a statistical population in such a way that every possible sample that could be selected has a predetermined probability of being selected (random sample). BEST FOR: Stationary Populations Ex. Plants Even Dispersal Patterns

mathematical formula estimates the pop size
Mark and Recapture A method of sampling an animal population where animals are caught alive and tagged, and then returned (unharmed) to their habitat over time animals from the pop are trapped and those with marks/tags are counted mathematical formula estimates the pop size

Patterns in Populations
Reproductive pattern = life-history pattern Variety of patterns, but TWO extremes

Patterns Rapid life-history patterns
Changing or unpredictable environment Small Mature rapidly Reproduce early Short life span

Patterns Slow life-history pattern Large species Stable environments
Reproduce slowly Matures slowly Long life span Stay at or near carrying capacity

Reproductive Strategies
Rapid (maximum growth rate, below carrying capacity) Early reproduction Short life span High mortality rate Little or no parental care Large investment in producing large numbers of offspring Below carrying capacity Examples: Bony fish Grasshoppers Slow (maximizes population size near carrying capacity) Late reproduction Long life span Low mortality rate Extensive parental care Greater investment in maintenance and survival of adults At or near carrying capacity Examples: Sharks Elephants Reproductive Strategies In an uncrowded environment, such as a recently abandoned crop field, natural selection pressure tends to favor populations that invest heavily in offspring, have shorter life spans, capacity for widespread dispersion, and usually provide little or no parental care for offspring (for example, mosquitoes, ragweed, or mice). These populations tend to increase exponentially and often are referred to as r-strategist, where r refers to the intrinsic rate of growth of the population. In contrast, crowed conditions favor organisms with lower rates of population growth, but improved capabilities to utilize and compete for resources. These populations maintain themselves at levels close to carrying capacity (K) and are referred to as K-strategist. Biologist refer to the types of selection pressure placed on populations as r-selection, if individuals that reproduce rapidly and abundantly are favored, and as K-selection, if individuals that compete well in crowded conditions are favored over time. References Campbell, N. E. & Reece, J. B. (2002). Biology (6th ed.). San Francisco: Benjamin Cummings. Odum, E. (1997). Ecology: A Bridge Between Science and Society. Sunderland, MA: Sinauer Associates, Inc. Raven, P. H. & Johnson, G. B. (2002). Biology (6th ed.). McGraw-Hill.

Survivorship in Populations
Survivorship curves are graphic representations of the age structure of a given population. They are used to predict the future growth of the population. Type I curves reflect relatively low death rates early in life and through midlife, with a sharp increase in death rate among older-age groups (e.g., humans). Type II curves illustrate a fairly even mortality rate throughout the life span of the organism (e.g., birds). Populations with high death rates early in life followed by a sharp decline of death rates for the survivors are represented by Type III survivorship curves (e.g., fish and many insect populations). References Campbell, N. E. & Reece, J. B. (2002). Biology (6th ed.). San Francisco: Benjamin Cummings. Ricklefs, R. E. & G. L. Miller. (2000). Ecology (4th ed.). New York: W.H. Freeman and Co. Image Reference Young, M. (2004). Survivorship graph. Houston, TX: Baylor College of Medicine, Center For Educational Outreach.

Survivorship Curves Patterns of Mortality
Populations show three patterns of mortality or survivorship curves: Type I (low mortality until late in life) Type II (constant mortality throughout life) Type III (high mortality early in life followed by low mortality for the remaining life span)

Survivorship in Populations
Survivorship curves are graphic representations of the age structure of a given population. They are used to predict the future growth of the population. Type I curves reflect relatively low death rates early in life and through midlife, with a sharp increase in death rate among older-age groups (e.g., humans). Type II curves illustrate a fairly even mortality rate throughout the life span of the organism (e.g., birds). Populations with high death rates early in life followed by a sharp decline of death rates for the survivors are represented by Type III survivorship curves (e.g., fish and many insect populations). References Campbell, N. E. & Reece, J. B. (2002). Biology (6th ed.). San Francisco: Benjamin Cummings. Ricklefs, R. E. & G. L. Miller. (2000). Ecology (4th ed.). New York: W.H. Freeman and Co. Image Reference Young, M. (2004). Survivorship graph. Houston, TX: Baylor College of Medicine, Center For Educational Outreach.

Rapid Life History Pattern Type III Survivorship
Type III Species: have high reproductive rates tend to occur in unpredictable environments Ex. Fish, Plants r-selected Reproductive Strategy For r-selected species, “r” refers to the growth rate term in the logistic population growth model. For these species, population sizes and mortality tend to be variable and unpredictable. Since populations frequently are far from carrying capacity (“K”), intraspecific competition often is weak. Selection tends to favor individuals with rapid development, high and early reproduction that is not repeated, small body sizes, high resource requirements, and short lives. The potential for populations of r-selected species to grow is large. Reference: Pianka, E. (1970). On r- and K selection. American Naturalist, 104, Image Reference: Baylor College of Medicine, Center For Educational Outreach. (2004). Martha Young, Senior Graphic Designer

Slow Life History Pattern Type I Survivorship
Type I Species: occur near carrying capacity experience effects of population density have low reproductive rates, high parental care Ex. Humans, Elephants K-selected Reproductive Strategy In contrast, k-selected species have more constant mortality and population sizes that often are close to carrying capacity. Intraspecific competition tends to be strong. Selection favors slower development, late, repeated reproduction, long lives, and efficient use of resources. Reference: Pianka, E. (1970). On r- and K selection. American Naturalist, 104, Image Reference: Baylor College of Medicine, Center For Educational Outreach. (2004). Martha Young, Senior Graphic Designer

Age Structures and Human Growth
Human Populations Age Structures and Human Growth

Age Structure A population’s age structure indicates the percentage of individuals at each age. The right side shows females; the left, males The x-axis is number is populations size Usually in millions The y-axis is age ranges usually 0-4, 5-9, 10-14, etc…

Population Age Structure
Differences in environmental conditions and past history may cause populations to differ in their age distributions. The future growth of a population depends on its current age distribution. Population Age Structure Differences in environmental conditions and past history may cause populations to differ in their age distributions. The future growth of a population will depend on its current age distribution if birth and death rates vary with age. Reference: Ricklefs, R.E. & Miller, G.L. (2000). Ecology ,(4th ed.). NY: WH Freeman and Co. U.S. Census Bureau. (2003). International Data Base. Retrieved from Image Reference: Baylor College of Medicine, Center For Educational Outreach. (2004). Martha Young, Senior Graphic Designer.

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History of Human Population Growth
The Development of Agriculture About 10,000 to 12,000 years ago, the development of agriculture increased the growth rate of the human population.

What happened in the 1600s? The Population Explosion
Around 1650, improvements in hygiene, diet, and economic conditions further accelerated population growth. After World War II, the human population grew at the fastest rate in history, largely because of better sanitation and medical care in poorer countries.

Advances in Human Technology = Growth

Human Population Growth
Human population growth rate has been growing more than exponentially. Limited resources eventually will cause human population growth to slow, but global human carrying capacity is not known.

1. What is the difference between linear growth and exponential growth as plotted on a graph?

2. Why don’t populations of organisms grow indefinitely. 3
2. Why don’t populations of organisms grow indefinitely? 3. What is the relation ship of births to deaths in a population before the population reaches the environment’s carrying capacity? 4. What happens when the population exceeds the carrying capacity? 5. What are some limiting factors that can curb population growth?