Principles of Biology - Biology 102
Lake Tahoe Community College
spring Quarter
Instructor: Ralph Sinibaldi
_ Written by: Sue Kloss
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Chapter 52 - Population Dynamics
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Intro A. Population Ecology is concerned with changes in population size and the environmental factors that regulate
populations over time
1. fur seal population was reduced by overhunting to low numbers in the early 20 century, but has
rebounded under human protection (Fig. 52.1)
B. Populations are defined in several ways
1. Populations are generally confined geographically
a. for instance, a population of coyotes in the S. Lake Tahoe Basin.
2. Ecologists usually define a population as an interacting group of individuals of a species that use
common resources and are regulated by the same natural phenomena,
a. such as temperature,
b. food and water supply,
c. and predation.
3. Humans construct “population boundaries”, when a population is continuously distributed
4. 2 important characteristics of any population are
a. density and
b. distribution or dispersal patterns.
I. Population structure and dynamics:
A. Density and dispersion patterns are important population variables
1. Density - number of individs of a particular species per unit area or volume - eg.
a. number of oak trees per km2, or
b. number of earthworms/m3 in soil.
2. Usually, you can’t count every individual so sampling allows estimates;
a. f or example, instead of counting all the individuals of alligators in the Everglades, plots.
b. the more plots you have, the more closely you approximate true numbers.
c. sometimes you don’t count actual individuals, but indirect methodsd. scat or tracks, or bird songs, for example.
e. nests, or burrows.
3. mark and recapture - N =marked # in first catch x total catch 2nd time
recaptured marked individuals
a. you might use this method for voles in a meadow
b. catch particular number in first attempt, then wait 2 weeks and try again
c. eg 50 voles caught and marked in 1st attempt, 100 second time, 5 recaptured
1. 50x100 = 5000; 5000/5 = 1000 voles estimated to be in meadow
d. this method assumes marked individuals have the same chance of being caught as
unmarked.
e. Problems with this assumption? animal may be more interested in food or wary of trap
5. Density is not static
a. immigration
b. emigration
c. birth
d. death (fig. 52.2)
6. Dispersion pattern - spacing of individuals in an area. (Fig. 52.3)
a. uniform - even pattern of dispersion - nesting birds, territoriality of animals
b. random - spaced in patternless, unpredictable way - clams in a mudflat when resources are
plentiful, or when conflicting needs for diff’t resources creates chaotic pattern, windblown seeds
c. clumped - individuals aggregated in patches - most common dispersal pattern,
often results from uneven distribution of resources; eg salamanders clumping under a log
B. Demography -study of vital statistics of a population
1. esp. birth rates and how they vary among individuals and death rates
2. life tables are useful models for summarizing statistics
a. life tables follow fate of a cohort - group of individuals of the same age (table 52.1)
b. males in first year, for example, have a higher death rate than females in squirrels listed
3. survivorship curves constructed from life tables (Fig. 52.5)
a. type I - whales, elephants and humans - have a few young, care for them, increase survival
to maturity
b. type II - some rodents and hydras - constant death ratec. type III - oysters, lots of marine animals - high death rates for the very young, then low
death rates for those few individuals that survive to adulthood
4. reproductive rates- demographers focus on females
a. will differ wildly for species- eg. oak trees vs. squirrels vs. mussels etc.
II. Life history traits are the products of natural selection
A. Evolution shapes life history1. Life history includes events from birth thru reproduction to death in a species
2. include age at first reproduction
a. frequency of reproduction
b. number of offspring
c. amount of parental care given
d. energy cost of reproduction
e. age at first reproduction
3. reproduction patterns
a. semelparity
1. salmon
2. century plant
b. iteroparity - repeated reproduction
1. most other plants and animals
c. critical factor is survival of offspring
4. Natural selection will favor combination of life history traits that maximizes and individuals output of
viable offspring, just as it shapes physical features (fig 52.7)
a. opportunistic life history - small animals, reproduce when young, many offspring
b. equilibrial life history - larger animals, reproduce later, few offspring
III. exponential growth - population growth in idealized, unlimited environment
A. Growth Models help us understand population growth
1. idealized models help us understand population growth in unlimited environment
a. Exponential growth - unrestricted growth
1) dN/dt = rN
a) dN/dt = change in population over change in time
b) N = size of population
c) r = per capita rate of increase- organisms inherent capacity to reproduce
birth rate - mortality rate
1. zero population growth - when birth rate = death rate
d) J curve
B. logistic growth includes concept of carrying capacity
1. the closer the population size is to the limit that can be supported by the environment, the
slower the growth rate is of that population (Fig. 52.11)
a. Logistic model - limits on growth (Fig. 52.12)
1) dN/dt = rN(K - N)
K
a) K is Carrying Capacity
b) overshooting carrying capacity may cause population crash, drop in K
c) K doesn't influence the equation very much until N starts getting large
relative to K.
d) when K is reached, (K-N)/K = 0 and growth rate is zero
e) growth rate is small when population is very small and very large;
f) growth rate is largest when N is intermediate level of K
g) S - shaped curve
2. early in population growth, fewer individuals may cause slow growth
a. Allee effect - individs can’t find each other to mate, etc.
3. life history and logistic model
a. r - selection - life history traits that maximize success in uncrowded, unpredictable
surroundings
1. mature early
2. large numbers of offspring
3. weeds, insects, many invertebrates
b. K - selection - organisms w/ pops close to K of environment
1. maturity and reproduction at later age
2. few, large, well cared for offspring
3. polar bears, giraffes, humans
IV. Multiple factors may regulate population growth
1. logistic model - as pop density increases, population growth slows
2. Density Dependent factors - decrease growth rate or increase death rate
a. food - Fig. 52.15 - song sparrows clutch size decrease as pop increases; plantain seeds (log)
b. shelter/space/territories - thin seedlings to achieve best productivity of plants (fig. 52.16,17)
c. build up of toxins
d. disease
e. intrinsic factors- high pops of mice, woodchucks other rodents decrease growth rate even when
more food, shelter supplied a) stress hormones increase
b) shrinks repro organs,
c) delays sexual maturation,
d) depresses immune system
f. predator interest in large pops
3. Density independent factors
a. climate
b. weather - some pops, especially insects, boom until change in conditions, then
bust dramatically, with no leveling off.
c. natural catastrophes
4. Most pops have regular fluctuation in numbers (Fig. 52.18-20) - Population dynamics focuses on
complex interactions of biotic and abiotic factors in determining population size
a. stability in moose population to some extent bc. lake never froze over (no immigration and
emigration) but lots of other factors control it too
b. dungeness crab - lots of cannibalism when high birth rates, or if currents take larvae offshore,
can’t settle on bottom (note log scale in graph)
c. song sparrow pops increase due to immigration
D. Some pops have boom bust cycles 1. predator prey relationships - Canadian lynx and snowshoe hare every 10 yrs. (Fig. 52.21)
2. food supply of prey affects predator pops, but why does hare pop boom and bust?
3. food supply of prey, over exploitation by predators, both predation and food supply
4. hormones and stress also may affect prey pops, coincidentally keeping predator pops from
getting too large
5. time lag in response to prey increase by predator populations - reproduce more slowly
V. Human Population
A. continuous growth for over 1000 yrs. (Fig 52.22)
1. at ~ 8000 bc - hunter gatherer to ag
2. plague
3. industrialization at 1700
4. falling death rates largely causing pop growth
5. technological vs. biological changes
6. fluctuations around carrying capacity - exceed by a lot?
a. human population no longer fits comfortably on earth
b. 7.3-8.4 billion people expected on earth by 2025 - but rate of growth is slowing
c. all arable land is currently being farmed
d. oceans currently being overfished
e. 2/3 of all the available water on earth will have to be in use
7. demographic transition - drop in death rate will eventually bring about drop in birth rate (fig. 52.24) death rate drops before birth rate falls - occurs w/ increase in living standards, education, social security,
a. increase in health care, sanitation
b. education of women
8. Age structure diagrams and birth and death rates affect how a pop will grow in the future(Fig. 52.25)
imply different social issues
a. Some developed countries have stable population rates; eg. Sweden
b. most undeveloped countries high pop growth rates - 80% of world pop is in LDC, most of
current population growth is occurring in these nations
c. in 48 of LDC, pop growth is expected to triple by 2050; current models suggest that US pop will
hit 390 from 270 currently by 2050
d. China’s pop growth rate - in 1970, 5.9 kids/family; 1999 1.85; still, it will take 30 yrs for pop
growth to show effects - why?
e. US, Japan, Canada, Europe, pop growth nearing equilibrium - birth rates at or below
replacement level, though pops are still increasing in some countries, (including US).
f. about a 30 yr. time lag from when birth rates drop to population growth stops
g. Human control over reproduction thru contraception/family planning
1. delaying reproduction
2. decreasing pop thru decrease in birth rate, not increase in death rate
9. infant mortality and life expectancy - (Fig. 52.26)
B. Global Carrying Capacity
1. estimates of carrying capacity - van Leeuwenhoek in 1679 - 13.4 billion
2. lots since, most 10-15 billion
a. ecological footprints - amt of land in ha (hectare - 2.47 ac) per person to produce all rescources
being consumed, current demand on resources (Fig 52.27)
1. US uses 8.4 ha/person but has only 6.2 available; India - .8; China, ~ 1
2. red means you are using more land than your country has, black means you are
within your country’s limits
3. Earth isn’t big enough to support whole human pop at US standards; world is already
slightly above carrying capacity using this info
Ch 52 objectives
1. Distinguish between density and dispersion of a population.
2. Explain how ecologists may estimate the density of a species.
3. Describe conditions that may result in clumped dispersion, uniform dispersion, and random dispersion of individuals in a
population.
4. Explain how a life table is constructed.
5. Distinguish between a life table and a reproductive table.
6. Describe the characteristics of populations that exhibit Type I, Type II, and Type III survivorship curves.
7. Define and distinguish between semelparity and iteroparity. Explain what factors may favor the evolution of each life
history strategy.
8. Explain, with examples, how limited resources and trade-offs may affect life histories.
9. Compare the exponential model of population growth with the logistic model.
10. Explain how an environment’s carrying capacity affects the per capita rate of increase of a population.
11. Explain the meaning of each of the following terms in the logistic model of population growth:
a. rmax
b. K 2 N
c. (K 2 N)/K
12. Distinguish between r-selected populations and K-selected populations.
Population-Limiting Factors
13. Explain how density-dependent factors affect population growth.
14. Explain, with examples, how biotic and abiotic factors may work together to control a population’s growth.
15. Describe boom-and-bust population cycles, explaining possible causes of lynx/hare fluctuations.
16. Describe the history of human population growth.
17. Define the demographic transition.
18. Compare the age structures of Italy, Afghanistan, and the United States. Describe the possible consequences for each
country.
19. Describe the problems associated with estimating Earth’s carrying capacity for the human species.