1. Biology Chapter 4- Population Biology
Limiting factor
Exponential growth
Carrying capacity
Life-history patterns
Density
R strategy
K strategy
Competition

2. More Vocabulary Stress Crowding Demography Birthrate Deathrate
Doubling time
Age structure

3. Population Growth J Curve
Limits to growth (Limiting Factors y’all)
Biotic and abiotic
Carrying Capacity
Exponential growth
Page 94 growth graph and explaination

4. More graphing Add carrying capacity

5. Life History patterns Rapid Life history Slow life history
K reproduction strategy (but intrinsic, not conscious)
R reproduction strategy

6. Population Density Patterns Limiting Factors Density-dependent
Random
Clumped
Uniform
Limiting Factors
Density-dependent
Disease
Competition
Density-independent
Usually abiotic

7. Organism interactions
Predation
Graph p 98
Interspecies competition
Intraspecies competition
Effects of crowding

8. Addendum An S shaped curve is logistic growth
This is the more common representation of population growth

9. Basic Characteristics of Populations
The suitability of habitats influences the geographic distribution of a species.
Insights can be gained by studying the spatial distributions of populations within habitats.
Basic Characteristics of Populations
Geographic Ranges: The geographic range of a species depends on the suitability of the habitat, interactions with other species, and opportunities to colonize. The presence of a species shows that the habitat is suitable. However, the absence of a species from a habitat does not necessarily mean that the habitat is unsuitable for that species.
Spatial Distributions: Random distributions of individuals are those similar to Poisson distribution (probability distribution of discrete random variables), in which the locations of individuals are determined independently of each other. The clumping of populations may reflect locally suitable environmental conditions, clustering of offspring near parents, or social interactions. Uniformly spaced distributions may indicate strong competition within individual species.
Reference:
Ricklefs, R.E. & Miller, G.L. (2000). Ecology ,(4th ed.). NY: WH Freeman and Co.
Image Reference:
Baylor College of Medicine, Center For Educational Outreach. (2004). Martha Young, Senior Graphic Designer.

10. 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.

11. Density-Independent Population Growth
Simple models describe how idealized populations would grow in an infinite environment.
In these models, populations increase to infinity or decrease to zero.
Continuous Model
Reproduction occurs in the population at all times.
Discrete Model
Populations reproduce only at certain times.
Density-Independent Population Growth
Continuous population growth models use differential equations (population growth rate is represented as dN/dt=rN0). Discrete models employ difference equations (time at some number of time steps in the future is a function of the current population size - Nt+1=λNt). In these simple equations, r and λ depend only on immigration, emigration, births and deaths.
The dynamics of these models also are simple. If births exceed deaths (λ>1 or r>0), the population increases exponentially to infinity. If births are the same as deaths, the population stays constant. If deaths exceed births (λ<1 or r<0), the population decreases asymptotically to zero. The only difference in the dynamics of these two models is the shape of the population growth curve: smooth with continuous reproduction and stair-step when reproduction is discrete.
Reference:
Alstad, D. (2001). Basic Populus models of ecology. Upper Saddle River, NJ: Prentice Hall.
Image Reference:
Baylor College of Medicine, Center For Educational Outreach. (2004). Martha Young, Senior Graphic Designer.

12. Density-Dependent Population Growth
In density dependent population growth, the per capita growth rate decreases as the population approaches a carrying capacity.
When population growth rate depends on current population size, the population smoothly approaches carrying capacity.
When there is a delay such that population growth depends on past population sizes, the population may cycle or have chaotic dynamics.
Density-Dependent Population Growth
To represent an environment that is not unlimited, models incorporate a carrying capacity (“logistic growth models”). When the population is far from carrying capacity (“K”), density effects are minor. As the population approaches carrying capacity, using the continuous time model, per capita population growth rate decreases towards zero. When the population is greater than carrying capacity, per capita population growth is negative. This causes populations to approach carrying capacity smoothly.
Although per capita population growth rate is highest when the population is small, overall population growth rate is highest at half of carrying capacity (“maximum sustained yield”). At low population sizes, per capita growth is high but few individuals are able to contribute to population growth. At population sizes close to K, per capita growth is close to zero.
Complex dynamics can occur when there is a delay in the response of populations in relation to their densities. Delays between population growth and population density may be caused by fat storage, gestation or incubation, seed banks, or litter build-up. Discrete time models treat time as occurring in clear steps. As population growth rate increases, populations first overshoot carrying capacity but eventually reach carrying capacity. With increasingly high population growth rates, populations may cycle (boom and bust cycles). Populations with extremely high growth rates have chaotic dynamics. Models with reproduction occurring only at certain times (discrete models) show the same progression of dynamic behaviors as the time delay models.
Reference:
Alstad, D. (2001). Basic Populus models of ecology. Upper Saddle River, NJ: Prentice Hall.
Image Reference:
Baylor College of Medicine, Center For Educational Outreach. (2004). Martha Young, Senior Graphic Designer.

13. Dynamics of Lagged Logistic Growth Models
As growth rate increases, populations overshoot carrying capacity (K).
Further increases cause the population to cycle.
Dynamics of Lagged Logistic Growth Models
Under the discrete-time model, populations often overshoot carrying capacity. These models help predict what happens when the effects of density dependence are not instantaneous.
If there is a delay in feedback, a series of predictable behaviors occur as population growth increases. With a short delay in feedback, the population growth rate will smoothly approach the carrying capacity with small adjustments as shown in the top series of graphs. As population growth accelerates (shown in the second series of graphs), populations will begin to cycle in various periods such as 2 point, 4 point, 8 point, or 16 point cycle. As population growth rates cycle faster and faster, the population can enter into apparent chaos. However, at this point, even though the changes seem random, there is some regularity to the oscillations.
References:
Alstad D. (2001). Basic Populus models of ecology. Upper Saddle River, NJ: Prentice Hall.
Image Reference:
Baylor College of Medicine, Center For Educational Outreach. (2004). Martha Young, Senior Graphic Designer.

14. Human Population Growth
Human population growth does not currently show density effects that typically characterize natural populations.
In natural populations, per capita population growth rate decreases with population size, whereas global human population growth rate has a positive relationship.
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.
Human Population Growth
Current population projections are that there will be between 7 and 11 billion people on Earth by However, the carrying capacity of the earth may be smaller than this.
Estimates of global carrying capacity vary widely from several to hundreds of billions of people. The dominant standards of living (especially diet) have a large influence on estimates of global carrying capacity. Many scientists believe that the current world population already exceeds the carrying capacity if everyone were to have the current standards of living found in the United States.
Reference:
Towsend, C.R., Harper, J.L., & Begon, M. (2000). Essentials of ecology. Malden, MA: Blackwell Scientific.

15. Density-Dependent and Density-Independent Effects on Populations
In many habitats, the forces that limit population sizes are independent of population density. For example, extreme weather events may decrease populations.
For most species, density-dependent factors limit birth rates or increase death rates at least some of the time. This type of population determination often is referred to as “regulation.”
Disease outbreaks and starvation are two factors that may increase with population density.
Density-Dependent and Density-Independent
Density-independent factors are events and influences that affect the growth of a population, independent of the population’s size. Often, these are environmental factors, such as extreme cold, drought, tornados, or volcanic eruptions. For many organisms, harsh abiotic conditions keep populations far from carrying capacity much of the time. Such species often show little density-dependent regulation.
Density-dependent effects of specialist herbivores and diseases are thought to be important in promoting the high diversity of tropical rain forests. Seedlings growing in the vicinity of their parents experience higher losses than those growing distant from conspecifics. In general, plants with a diverse range of other plants growing in their vicinity experience lower herbivory and disease than those growing in a monoculture. If plants are sown at high densities, the number of plants surviving decreases as they grow in stature (self-thinning).
Animals also show density-dependent regulation. As density increases, mortality decreases from limited food, higher disease frequencies, and other factors. Animals in crowded populations are less likely to breed, and their success in food-limited, crowded conditions is lower than in less crowded populations.
Reference:
Ricklefs, R.E. (2000). The Economy of Nature (6th ed.). NY: WH Freeman and Co.

16. r-selected Reproductive Strategy
r-selected Species:
have high reproductive rates
tend to occur in unpredictable environments
typically have type III survivorship curves
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

17. K-selected Reproductive Strategy
K-selected Species:
occur near carrying capacity
experience effects of population density
have low reproductive rates, high parental care
have type I survivorship curves.
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

18. Ostriches are nomadic, wandering in small groups.
Populations
Groups of organisms of the same species that live within a given area
Key characteristics:
Dispersion patterns
Population density
Growth rate
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.
BioEd Online

19. Dispersion Patterns Within Populations
Three common patterns of population 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.
BioEd Online

20. 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
Carrying capacity is the maximum number of organisms that can be supported in a given habitat.
Population size can be measured by several sampling techniques.
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.
BioEd Online

21. Population Growth Exponential vs. Logistical Growth BioEd Online
Birth, death, immigration, and emigration rates factor into the growth rate of a population. Two simple models of population growth are the exponential model and the logistical model.
The growth pattern for a population with unlimited resources is exponential and represented by a “J” shaped growth curve. A population that is growing exponentially increases in a geometric pattern (for example, 2, 4, 8, 16, 32, etc.). In the formula dN/dt = riN, dN/dt is the rate of change in the number of individuals at any instant in time and ri represents the innate capacity for growth of the population (biotic potential) when in an unlimited environment. Populations that are introduced to a new environment or are recovering from a catastrophic event (such as a fire) usually exhibit “J” shaped growth curves.
Population growth eventually reaches a limit imposed by factors such as light, space, nutrients, or water. Carrying capacity (K) is the maximum number of individuals a particular habitat can support. Growth in a logistical model slows as it approaches the carrying capacity of the environment and forms an “S” shaped growth curve. In reality, populations sometimes will overshoot K, followed by a rapid decline, until conditions for growth are restored.
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). Exponential vs. logistical growth graph. Houston, TX: Baylor College of Medicine, Center For Educational Outreach.
BioEd Online

22. 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.
BioEd Online

23. Reproductive Strategies
r- Selected (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
K-Selected (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.
BioEd Online

24. Limits on Population Growth
Density Dependent Limits
Food
Water
Shelter
Disease
Density Independent Limits
Weather
Climate
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.
BioEd Online