1. CHAPTER 56 POPULATION ECOLOGY Brenda Leady, University of Toledo
Prepared by
Brenda Leady, University of Toledo
Copyright (c) The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2. Population – group of interbreeding individuals occupying the same habitat at the same time
Water lilies in a particular lake
Humans in New York City
Population ecology – study of how populations grow and what promotes and limits growth
Uses the tools of demography – birth rates, death rates, age distributions, and sizes of populations

3. Understanding populations
Density – number of organisms in a given unit area
Population growth affects population density
Knowledge can help answer resource management questions

4. Quantifying population density
Simple visual count
Sampling methods to extrapolate captured organism number to size of population
Mark-recapture method
Captured animals may learn to avoid traps or seek out food baited traps

5. Dispersion patterns Clumped Uniform Random Most common
Resources tend to be clustered in nature
Social behavior may promote this pattern
Uniform
Competition may cause this pattern
May also result from social interactions
Random
Rarest
Resources are rarely randomly spaced
May occur where resources are common and abundant

7. Reproductive strategies
Semelparity – produce all offspring in single reproductive event, individuals reproduce once and die
Iteroparity – reproduce in successive years or breeding seasons
Seasonal iteroparity – distinct breeding seasons
Continuous iteroparity – reproduce repeatedly at any time of the year

9. Age classes
Reproductive strategy has a strong effect on subsequent age classes of a population
Semelparous organisms with same-aged young called cohorts
Iteroparous organisms have young of different ages
Expect a population increasing in size to have many young and a decreasing population to have few young

11. Life tables
Data on the number of individuals alive in a particular age class
Males are usually not included
North American beaver example
Trappers provided mandibles
Teeth extracted for age classification

13. Survivorship curve – plots numbers of surviving individuals at each age
Use logs to make it easier to examine wide range of population sizes
Beavers have a fairly uniform rate of death over the life span

15. 3 patterns of survivorship curves
Type I – rate of loss of juveniles low and most individuals lost later in life
Type II – fairly uniform death rate
Beaver example
Type III – rate of loss for juveniles high and then loss low for survivors

17. Murie’s Collections of Dall Mountain Sheep Skulls Permitted Accurate Life Tables to Be Constructed
Live in mountainous regions in Alaska
Thought wolves responsible for population decline
Collected sheep skulls using annual growth rings on horns to determine age at death
Slight initial decline in survivorship for young, flattens out, and sharp decline in old age
Wolves prey on youngest and oldest
Cold winters killed many sheep and weakened others making them easy prey
The wolves were not at fault

19. Age-specific fertility rate, mx
Proportion of female offspring born to females of reproductive age
100 females produce 75 female offspring mx=0.75
Age-specific survivorship rate, lx
Use survivorship data to find proportion of individuals alive at the start of any given age class
lxmx = contribution of each age class to overall population growth

20. R0 = net reproductive rate
Overall growth rate per generation
Number of offspring born to females of all ages

21. If R0>1, population growing If R0<1, population declining
To calculate future size of population, multiply number of individuals in the population by the net reproductive rate
For beaver example,
Nt+1 = NtR0
= 1,000 x 1.1
= 1,100
If R0>1, population growing
If R0<1, population declining
If R0 = 1, population is at equilibrium

22. How populations grow
Life tables can provide accurate information about how populations grow from generation to generation
Simpler models can give insight to shorter time periods
Exponential growth – resources not limiting, prodigious growth
Logistic growth – resources limiting, limits to growth

23. Per capita growth rate Change in population size over any time period
Often births and deaths expressed per individual
100 births to 1000 deer = 0.10
50 deaths in 1000 deer = 0.50

24. Exponential growth When r>0, population increase is rapid
Characteristic J-shaped curve
Intrinsic rate of increase, rmax = r at maximum
Because population growth depends on the value of N as well as the value of r, the population increase is even greater as time passes
Reintroduction of a population to a habitat, growth of introduced exotic species, and global human population

27. Logistic growth
For most species, resources become limiting as populations grow
Carrying capacity (K) or upper boundary for population
Logistic equation

28. Growth is small at high and low values of N
Growth is greatest when N=K/2
Logistic growth – pattern where growth slows down as it approaches K
Model fits some populations but not others
Variations in nature change resource levels that cause changes in carrying capacity

33. Density-dependent factors
Mortality factor whose influence varies with the density of the population
Parasitism, predation, and competition
Predators kill few prey when the prey population is low, they kill more prey when the population is higher
Detected by plotting mortality against population density and finding positive slope
Density-independent factor
Mortality factor whose influence is not affected by changes in population size or density
Generally physical factors – weather, drought, flood, fire

35. Life history strategies
Continuum
r-selected species – high rate of per capita population growth, r, but poor competitive ability (weeds)
K-selected species – more or less stable populations adapted to exist at or near carrying capacity, K
Lower reproductive rate but better competitors (trees)

38. Hexaploidy Increases the Growth of Coast Redwood Trees
California home to world’s most massive tree, the giant sequoia (Sequoiadendron giganteum), and world’s tallest tree, the coast redwood (Sequoia sempervirens)
As of today, over 95% of old growth coast redwoods gone due to logging
Hexaploid- each cell contains 6 sets of chromosomes for a total of 66
Unusual in trees, only hexaploid conifer
Means very genetically diverse – each tree may have several alleles for each gene
Living redwoods have no known killing diseases and pests do no significant damage

40. Human population growth
In 2005, the world’s population was estimated to be increasing at the rate of 153 people every minute
2 in developed nations and 151 in less developed nations
Human growth fits an exponential pattern
Low until agriculture and animal domestication
Between 1750 and 1998, population surged from 800 million to 6 billion

42. High birth and high death rates
Human populations can exist at equilibrium densities in one of two ways
High birth and high death rates
Before 1750, this was often the case, with high birth rates offset by deaths from wars, famines, and epidemics
Low birth and low death rates
In Europe, beginning in the 18th century, better health and living conditions reduced the death rate
Eventually, social changes such as increasing education for women and marriage at a later age reduced the birth rate

43. Demographic transition
Shift in birth and death rates with development
First stage - birth and death rates are both high, and the population remains in equilibrium
Second stage - death rate declines first, while the birth rate remains high - high rates of population growth result
Third stage - birth rates drop and death rates stabilize, so that low population growth ensues
Fourth stage - both birth and death rates are low, and the population is again at equilibrium

45. Exact pace varies between countries depending on culture, economics, politics, and religion

46. Age structure
Relative numbers of individuals in each defined age group
Commonly displayed as population pyramid
Helps predict future population growth

48. Earth’s carrying capacity
Many and varied estimates
Lifestyle has a huge influence
Total fertility rate – average number of live births a woman has during her lifetime
Declining from 5.0 in 1960s to 2.9 in 1990s
2.1 needed for zero population growth
Differs considerably between geographic areas
In developed nations, population has stabilized
In developing countries, population is still increasing dramatically

51. Ecological footprint
Aggregate of land needed for survival in a sustainable world
Average footprint size is about 2 hectares or 5 acres (1ha=10,000 m2)
Wide variation is found around the globe
7.5 for Canadians, 10 for Americans