Chapter 52: Population Ecology
Population - members of the same species occupying the same area
- compete for resources
Natural vs. Artificial boundaries
Population density – the number of individuals per unit area or volume
Counting/Estimating/Indicators (nests)/Mark-Recapture Method
Dispersion – the pattern of spacing among individuals of a population
Patterns of Dispersion Fig. 52.2
- Clumped (schools of fish)
- Uniform (penguins – from social interactions)
- Random (rare)
Demography – the study of the vital statistics that affect population size
Birthrate (fecundity), Death rates, Age structure, Sex ratio, Generation time
Represent data from a life table – number of members alive at each age
Type I – few offspring, good care (humans, other large mammals)
Type II – constant mortality (Hydra, some lizards)
Type III – many offspring, high death rate for young, high survivorship later
Diverse life histories
Annual flowers germinate, grow, produce seeds, die
Salmon hatch in stream, migrate to
ocean, return to stream to spawn, die –
Oaks don’t reproduce until 20 years old, then prolific for a century
Investments in Reproduction
Mathematical models as alternatives to experiments
- Used to study how various factors might affect a population’s growth rates
Population growth – an exponential model
- Describes an idealized population in an unlimited environment
- Bacteria would form a layer a foot deep (over entire planet) in 36 hours!
- Obviously, growth is regulated by limited resources and other factors
Ideal environment, no external limitations on population growth
DN/Dt = B - D where N = population numbers
t = time (generations) B = births D = deaths
Zero population growth (ZPG) B = D
K (carrying capacity) = maximum stable population that a particular environment can support over an extended period of time
· varies over space and time as a function of limiting resources
· energy resources most frequently limit
· also: specialized nesting, refuge sites
· population growth rates affected by crowding, resource limitations (density-dependent factors)
Logistic population growth – Model showing that population growth levels off as population size approaches carrying capacity (K) – incorporates the effect of population density on the per capita rate of increase
r-selected species are defined as those opportunistic spp. having:
· short generation times
· high reproductive potential
· numbers mostly limited externally (density independent factors)--Fig. 52.18
K-selected species are equilibrial, more influenced by the carrying capacity (K) due to:
· long generation times
· low reproductive potential
· intraspecific competion
· territoriality, defense of resources as space becomes limiting
Logistic model one of intraspecific competition – the reliance of individuals of the same species on the same limited resources
Density-dependent factors are those that intensify as population increases:
Food supply
Health
Predation
Accumulation of waste products (wine-making)
Crowding/Stress
Density-independent factors are unrelated to population size; they affect the same percentage of individuals regardless of population density:
Freeze
Fire
Flood
Near exponential growth cannot continue
OLD
NOTES:
use per
capita B
and D rates (#’s per 1000 over a specific time period)
B =
bN ( eg. 34/1000
births = b= 0.034 X 500 = 17)
D = dN
DN/DT = bN -dN
r = b - d = per capita population growth rate
therefore: DN/ Dt = rN
in
conditions of zero population growth (ZPG), r = 0
intrinsic
rate of increase = rmax = max. growth rate possible (leads to exponential
population growth): DN/ Dt = rmaxN (Fig. 53.12)
2. Logistic
population growth model--model of intraspecific competition (Fig. 52.14)
dN/dT =
rmaxN{K-N/K}
·
incorporates
population density effects on r, varying from rmax to r = 0 as carrying capacity is
approached ( as N nears K, growth slows, r decreases)
·
ZPG
occurs when b = d, and when N = K
as carrying capacity reached, growth levels off, yielding sigmoidal growth curve