1. Introduction
A population is a group of individuals of a single species that simultaneously occupy the same general area.
The characteristics of populations are shaped by the interactions between individuals and their environment.
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2. 1. Two important characteristics of any population are density and the spacing of individuals.
Populations have size and geographical boundaries.
The density of a population is measured as the number of individuals per unit area.
The dispersion of a population is the pattern of spacing among individuals within the geographic boundaries.
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3. Measuring density of populations is a difficult task.
We can count individuals; we can estimate population numbers.
Fig. 52.1
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4. This information allows estimates of population changes to be made.
Unfortunately, it is usually impractical to attempt to count individuals in a population.
One sampling technique that researchers use is known as the mark-recapture method.
Individuals are trapped in an area and captured, marked with a tag, recorded, and then released.
After a period of time has elapsed, traps are set again, and individuals are captured and identified.
This information allows estimates of population changes to be made.
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5. Mark & Recapture Practice Problem
R / C = M / N
R (marked recaptures) / C (total in second sample)  =  M (marked initially) / N (total pop. size)

6. Mark & Recapture Practice Problem
Problem: Suppose that you capture 10 individuals of a rare subspecies of brook trout from an impounded watershed. You place a tag in the body cavity of each individual and then release these fish.
You come back a month later and capture 20 fish and find that four of these are individuals that you had previously captured and released. What is your best estimate of the population size, N?

7. Clumped dispersion is when individuals aggregate in patches.
Fig. 52.2a
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8. By contrast, uniform dispersion is when individuals are evenly spaced.
Fig. 52.2b
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9. Overall, dispersion depends on resource distribution.
In random dispersion, the position of each individual is independent of the others.
Overall, dispersion depends on resource distribution.
Fig. 52.2c
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10. Chi Square Break!
An area has previously had a fairly uniform dispersal of a certain type of plant. An industry moved in and wants to check to see if their factory is adversely affecting species dispersal. They do not believe that they are, but they want to appease nearby neighbors who are expressing concerns.

11. Chi Square Break!
They select 10 sample plots and count the species in the plots. The results are on the next slide.
What is the null?
Degrees of freedom?
Would you recommend that they reject or fail to reject the null?

12. Plot Number Number of Plants

13. 1. Life histories are highly diverse, but they exhibit patterns in their variability
Life histories-the traits that affect
an organism’s schedule of
reproduction and survival
Some organisms, such as the agave plant, exhibit what is known as big-bang reproduction, where large numbers of offspring are produced in each reproduction, after which the individual often dies.
This is also known as semelparity.
Fig. 52.4
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14. By contrast, some organisms produce only a few eggs during repeated reproductive episodes.
This is also known as iteroparity.
What factors contribute to the evolution of semelparity and iteroparity?
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16. Variations also occur in seed crop size in plants.
The number of offspring produced at each reproductive episode exhibits a trade-off between number and quality of offspring.
Fig. 52.7
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17. Populations have the potential for tremendous growth
If N represents population size, and t represents time, then N is the change is population size and t represents the change in time, then:
N/t = B-D
Where B is the number of births and D is the number of deaths
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18. Expressing births and deaths as an average number of births and deaths per capita during the specified time interval
B = bN
D = dN
So if 34 births per year in a population of 1,000 individuals, per capita birth rate is Now we can calculate expected number of births per year in a population of any size! Try with a population of 500.

19. Population ecologists are most interested in the difference between the per capita birth rate and per capita death rate.
Per capita rate of increase, r
r = b-d
r>0 it is growing
r<0 it is declining

20. If r = 0 then there is zero population growth (ZPG).
We can simplify the equation and use r to represent the difference in per capita birth and death rates.
N/t = rN OR dN/dt = rN
If r = 0 then there is zero population growth (ZPG).
Under ideal conditions, a population grows rapidly.
Exponential population growth is said to be happening
Under these conditions, we may assume the maximum growth rate for the population (rmax) to give us the following exponential growth
dN/dt = rmaxN
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21. Figure 52.8 Population growth predicted by the exponential model

22. The J-curve – exponential growth
Characteristic of some populations that are introduced into a new environment
Can be a populations whose numbers have been reduced by a catastrophic event and are rebounding

23. 2. The logistic model of population growth incorporates the concept of carrying capacity
Typically, unlimited resources are rare.
Population growth is therefore regulated by carrying capacity (K), which is the maximum stable population size a particular environment can support.
Energy, shelter, refuge from preds, nutrient availability, water, and suitable nesting sites can all be limiting factors
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24. The logistic growth equation
We can modify our model of population growth to incorporate changes in growth rate as population size reaches a carrying capacity.
The logistic population growth model incorporates the effect of population density on the rate of increase.
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25. dN/dt = rmaxN((K-N)/K)
The graph of this equation shows an S-shaped curve.
Fig
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26. The s - curve When N is small compared to K, the term (K-N)/K is large
Per capita rate of increase, rmax (K-N)/K is close to the maximum rate of increase

27. Table 52.3
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28. How well does the logistic model fit the growth of real populations?
The growth of laboratory populations of some animals fits the S-shaped curves fairly well.
Fig
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29. The logistic population growth model and life histories.
This model predicts different growth rates for different populations, relative to carrying capacity.
The life history traits that natural selection favors may vary with population density and environmental conditions.
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30. Natural Selection Favors Certain Life History Traits Depending on the Circumstances at the Time
K-selection: selection for life history traits that are sensitive to population density
r-selection: selection for traits that maximize reproductive success in uncrowded environments

31. K-selection characteristics:
Maximizes population size (moves towards “K”)
Operates in populations living at or near carrying capacity
In other words, they stay near the maximum level supported by the available resources
Species that can survive and reproduce with few resources are favored
Competitive ability and maximum efficiency of resource utilization are favored
Also called density dependent selection

32. r-selection characteristics:
Maximizes “r”—the intrinsic rate of increase
Favored in an “empty” environment (like an area recently cleared by fire)
Very little competition for resources
Adaptations that favor rapid reproduction are favored
Increased fecundity (being fruitful and multiplying!) and rapid maturation are favored
Also called density-independent selection

33. What environmental factors keep populations from growing indefinitely?

34. This is a type of negative feedback.
Density-dependent factors increase their affect on a population as population density increases.
This is a type of negative feedback.
Density-independent factors are unrelated to population density, and there is no feedback to slow population growth.
Fig
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35. Reproductive Competition ex: fruit flies living in crowded conditions will lay fewer eggs; honeybee queen regulates laying based on availability of food; lab rats will eat their young when space is restricted even if food and water is plentiful. These are limits on the number of offspring
Territoriality, defense of a space, may set a limit on density. This limits the number of parents.
Interspecific (between species) competition for food and resources
Migration: member of the population emigrate as numbers grow.
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36. Predation: as the numbers of prey increase, predators can harvest it more easily. Ex: vole population goes through 3 year boom and bust due to predator regulation
Parasitism and Disease: as the numbers increase, disease and parasites spread more easily. Ex: “black death” in Europe killed ¼ of population between 1348 and 1350.