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Chapter 35 Population Dynamics
1. Population Ecology
a. study of changes in pop. size and the factors that regulate it
2. Populations are defined in several ways (35.1)
a. Population – group of individuals of a single species that occupy the same
general area
i. rely on same resources
ii. influenced by same environment (abiotic and biotic) factors
iii. high likelihood of interbreeding
3. Idealized models help us understand population growth (35.3)
a. single bacterium
i. under ideal lab condition (no growth limits) – binary fission every
20 minutes
ii. Continue for 36 hours
1. entire surface of Earth covered one foot deep
b. Elephants
i. six young in 100 years
ii. Continue for 750 years
1. 19 million elephants
c. The exponential growth model (ideal conditions – nothing limiting
growth)
i. rate of population expansion under ideal conditions
ii. The exponent increases every generation
1. 22  24  26 ….
2. G = rN (J-shaped curve)
a. G = growth rate (change in number of individuals
over time)
b. N = population size
c. r = intrinsic rate of increase
i. depends on the type of organism
ii. an organisms maximum capacity to
reproduce
iii. constant
3. So growth rate (G) at any given time depends only on
number of individuals in population (N)
iii. Obviously no population grows without limits and bacteria will not
cover the Earth one foot deep by tomorrow (Malthus)
4. Population limiting factors and the logistic growth model
a. populations may grow exponentially for a while
i. introduced to new environment
ii. rebounding from a catastrophic decline
b. population size will eventually stop increasing or even crash
c. population limiting factors
i. environmental factors that restrict population growth (Fig. 35.3B)
1. fur seal population on St. Paul island
2. until 1925 – uncontrolled hunting
3. then population jumped until 1935 (why did it stabilize?)
a. restricted by some hunting and space suitable for
breeding.
d. Logistic growth model (S-shaped curve)
i. accounts for limiting factors in the environment (Fig. 35.3C)
ii. G = rN(K-N)/K
iii. exp. equation modified by (K-N)/K
iv. (K-N)/K represents overall effect of limiting factors
1. K = carrying capacity
a. max. populations environment can support with no
degradation to habitat
b. What is K for the fur seal population?
i. ~10,000
c. What will determine K?
i. the species itself
ii. the resources available
2. Consider a new, small population (N << K)
a. G = rN(1) = exp growth model
3. Consider when N gets close to K (carrying capacity)
a. (K – N)/N approaches 0
b. growth rate slows dramatically
4. When N = K, growth is 0
a. density dependent rate
v. What does this model tell us about real populations?
1. When is growth rate low?
a. Growth rate small for small (N is small and so not
producing many offspring)
b. Growth rate small for large (N approached K and
growth slows) population – limiting factors
i. birth rates decrease or death rates increase or
both
ii. when birth rate = death rate, N = K
2. When is growth rate at highest?
a. when pop. is at intermediate level relative to
carrying capacity
e. Both the exponential and logistic models are mathematical ideals
i. no population fits either perfectly
ii. Good for making predictions
5. Multiple factors may limit population growth (35.4)
a. What causes density- dependent rates - declining birth rates or rising death
rates or both as population increases?
i. Biotic Factors
1. Competition among members for limited resources
a. food supply
i. song sparrow field study – island of British
columbia (Fig. 35.4A)
ii. Give extra food and you do not see decline
b. space
i. many vertebrates are territorial and compete
for space
1. ex. # of possible nesting sites limit
certain birds
2. Health
a. plants grown under crowded conditions
i. smaller
ii. less likely to survive
iii. less flowers
b. animals too
c. increased disease spreading
d. accumulation of waste products
3. Predation
4. Physiological factors
a. white-footed mouse experiment
i. enclose in a small field
ii. they reproduce quickly to 30 or 40
individuals
iii. reproduction then declines
iv. adding food and shelter does not alter
population
v. high pop. induces a stress syndrome
resulting in hormonal changes
1. delay sexual maturation
2. reproductive organs shrink
3. depress immune system
vi. result: increased mortality and decreased
birth rate
vii. Other organisms have been observed to do
this
ii. abiotic factors
1. climate and weather (35.4B)
a. aphids – insects that feed on phloem sap of plants
i. exp growth in spring
ii. rapid die out in hot, dry summer
iii. few individuals remain to begin population
if favorable conditions return
1. genetic drift
b. Some insects – mosquitoes and grasshoppers – die
out all together leaving only eggs.
2. Fire, flood storms, habitat disruption by humans
b. Most populations regulated by a mixture of these factors (35.4C)