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Biol 121- 2010.04.07- Ecology- Demograpy and Population Growth (Ch. 52).docx

Course Code
BIOL 121
Carol Pollock

of 4
Biol 121 225
Freeman 1173-1183 Apr 7, 10
Population Ecology (how to study?)
-population ecology is the study of how and why the number of individuals
in a population change over time
-look at distribution patterns:
-measure changes in abundance and distribution
-identify causes of these changes
What is a population?
-a group of individuals of the same species that live in the same area at the
same time
Characteristics of populations
-individuals in one population tend to rely on the same resources (ie. Food,
water, nesting sites)
-they are influenced by similar environmental factors
-they behave in a similar way
-they have a high likelihood of interbreeding
How (what terms used) to describe
-distribution: what areas they are in and how many?
-(population) density (number per unit area)
-pattern of spacing in habitat (3):
a. clumped (e.g. a few schools of fish)
b. Uniform (e.g. penguins)
c. Random (e.g. trees)
-is the study of vital statistics that affect population size
-the number of individuals depends on (4):
-births, deaths, immigration and emigration (emigration is leaving)
How to measure population density?
-count the whole population (most accurate but time consuming)
-sample (e.g. quadrats)
-index of population size (indirect counting: e.g. count nests, burrows,
-marked recapture (method):
-traps placed in study area, trapped animals are marked with tags bands or
tracking devices
-then allow some time for them to mix randomly with the unmarked
-recapture (set traps a second time) and look at the proportion of
recaptured animals
N(total population) = 
1. Individuals leaving or entering the population
2. Probability of catching individuals might not have been the same on both
the 1st and 2nd times (can be due to different setup, or environmental
change, or perhaps intelligence/learning behaviour of the species)
3. Experimenter bias
4. Loss of eggs
5. Location
What affects change in population size?
1) Survivorship
2) Fecundity
3) Role of life history
1) Survivorship
-how many individuals of each age come out alive (after each year)?
-how likely is it that individuals survive to the next year?
Biol 121 225
Freeman 1173-1183 Apr 7, 10
-how many offspring does each female have?
-note: a cohort is a group of individuals of same age that can be followed
through time
-survivorship can be shown by a life table showing year (0, 1, 2...) and
survivorship and fecundity as the columns
-survivorship x fecundity = average number of offspring produced per
female born
-survivorship can be shown by a survivorship curve of number of survivors
vs. age (three types of survivorship)
-1st type is high survivorship at first and then very low survivorship at a later
age (humans)
-2nd is a steady survivorship (so constant negative slope of the survivorship
curve) (e.g. rodents)
-3rd is low survivorship at a very young age but afterwards a high
survivorship (e.g. fish, sea turtles)
2) Fecundity
-the number of female offspring produced by each female in the population
a. female offspring per female
-calculate age-specific fecundity average # female offspring produced by a
female in age class x, where age class is a group of individuals at a specific
age (e.g. lizards between 4 and 5 years old)
b. Fertility rate
-in humans, the fertility rate is highest near 20 and then decreases at a
constantly increasing rate (concavity is negative)
3) Role of Life History
-time to mature sexually?
-how many offspring?
-care of young
-there is a trade-off between survival and reproduction (e.g. in birds, as the
probability of survival to the next year decreases, the average number of
eggs increases)
-life history trade-offs are universal
high fecundity/low survivorship moderate fecundity/moderate
survivorship low fecundity/high survivorship
Population Growth
-analyzing and predicting changes in population size is fundamental to
managing threatened species
-four processes affect population size and rate of chance of population size:
birth, death, immigration, emigration
-a population’s growth rate is change in the number of individuals (dN) per
unit time (dt) (N = # individuals)
-b = birth date per individual, d = death rate per individual
     (given no immigration/emigration)
-r is the growth rate or per-capita rate of increase (per capita = for each
Biol 121 225
Freeman 1173-1183 Apr 7, 10
-if per-capita birth rate b is greater than per-capita death rate d, r is positive
and population grows
-if d > b, then r is negative and population declines to zero extinction
-if d = b, then population does not grow/decline and r = 0
-r will vary with environmental conditions such as predation, resources
(food, habitat), etc.
dN = r*N
-r changes over time; therefore, a specific r is instantaneous
-rmax is the intrinsic rate of increase
-it is the maximal r which occurs when conditions are optimal for a particular
species (b is maximized, d is minimized)
-each spp has a characteristic rmax that does not change
   
   (for optimal conditions)
Exponential growth
-occurs when r does not change over time
-growth rate does not depend on number of individuals (density
-exponential growth adds an increasing number of individuals as the total
number of individuals N increases
-exponential growth in nature occurs when a few individuals found a new
population in a new habitat or when a population has been devastated but
is recovering with few surviving individuals
-eventually, growth will become density dependent exponential growth
cannot occur indefinitely
Logistic model of population growth
(density dependence)
-resources are limited, and population growth is density dependent
-k is the carrying capacity defined to analyze what happens when growth is
density dependent
-k is the maximum number of individuals in a population that can be
supported in a particular habitat over a sustained period of time
-carrying capacity K depends on food, space, water, soil quality,
resting/nesting sites, intensity of disease and predation, etc
-K changes from year to year because it depends on conditions
-if N < K (population size is below carrying cap) then it will grow
Logistic growth equation
  
 
-this equation shows that a population’s growth rate is proportional to
- (K N) is the number of additional individuals that can be accommodated
in a habitat with capacity K, and dividing by K gives the proportion of
“unused resources and space” in the habitat
-when N small, (K N)/K is close to 1, and growth rate high
-as N increases, (K N)/K decreases