Population density formula:
Dp = N / A
N = sum of
individuals in the quadrats
A = total
area of the quadrats
Estimating population size:
Use the population density of several quadrats.
Example: Density population of daisies in 4 different 1.00 m2 quadrats. The section of the field is 10 000 m2
Quadrat #1: 5
Quadrat #2: 3
Quadrat #3: 2
Quadrat #4: 1
Dp = N / A
= 5 +3 +2 +1 / (4 X 1.00 m2)
= 2. 75 daisies / m2
If the entire study area is similar to the sample area, this density can then be extrapolated to determine the population size
Estimated population size = Dp X total study area
= 2.75 daisies/ m2 X 10 000 m2
= 27 500 daisies
Mark recapture
Animals are recaptured, marked with a tag or other, released and then recaptured later.
Example: 20 sparrows are captured and marked and
released. One week later, 50 sparrows are captured, and 10 of these were the marked sparrows
Population size (N) = (number original marked) X ( total individuals in recapture)
(marked individuals in recapture)
= (20 X 50)/10
= 100
Population distribution
- Populations are rarely evenly distributed throughout their
habitat. - There are 3 distribution patterns for populations: Uniform, random, and clumped
Clumped distribution
- Organisms tend to clump around resources, such as a water source
- Also common among species in which individuals gather into groups for positive interactions
Uniform distribution
- Where resources are evenly distributed but scarce, population exhibit uniform distribution
- It is usually a consequence of competition between two individuals
Random distrubution
- If resources are plentiful and uniformly distributed across an area, populations exhibit random distribution
- There is no need for individuals to defend their share and interactions between individuals are neutral
- These conditions are rarely met
in nature
Distribution patterns are fluid
Example: moose may cluster in small groups near food in the winter, but randomly disperse in the summer
Fecundy
- The average number of offspring produced by a female member of a population over her lifetime
- This number varies greatly depending on the organism. Some organisms only reproduce once in the their lifetime, others will reproduce many times. Some organisms produce one baby at a time, others hundreds.
- Fecundity is also affected by the age at which an organism becomes sexually mature. The faster the animal sexually matures, the more offspring they can have
Survivorship
-The number or percentage of organisms that typically live to a given age in a given population
- To study this, ecologists study a large group of individuals born at the same time, called a cohort.
- They will monitor this group over its lifetime and record the age of death for each organism.
Type 1 survivorship
This pattern is a curve and shows a high rate of juvenile survival and individuals that live until sexual maturity and beyond
Example: Humans. They have a high parental care for their young and even though we produce
fewer young, we have a high level of survivorship
Type 2 survivorship
The risk of mortality is constant throughout an individual’s lifetime.
Example: birds
Type 3 suvivorship
Most individuals will die as juveniles and only a few members will live long enough to produce offspring, and only a small few will live to old age
Example: Oysters
Female oysters release many eggs into the water, many of which will not be fertilized, and if they do they will most likely be consumed by predators.
Measuring population
Δ N = (B + I ) – (D + E)
Δ N = population change
B= birth
I= immigration
D= death
E= emigration
Calculating population growth rate
gr = ΔN/Δt
gr= growth rate
ΔN = change in population size
Δt = change in time
Calculating per capita growth rate
cgr = Δ N/N
cgr = per capita growth
Δ N= change in the number of individuals
N= original number of individuals
Population growth in unlimited
environments
- Conditions: ideal, no predators and resources are unlimited
- Biotic potential: Species have the highest possible growth rate.
Determined by 5 factors
- The number of offspring per reproductive cycle
- The number of offspring that survive long enough to reproduce
- The age of maturity
- The number of times the individuals reproduce in a life span
- The life span of the individual
Exponential growth pattern
- A population growing at its biotic potential grows exponentially.
- There is a brief lag phase, followed by a steep increase in the growth curve
- A pattern of a population where organisms reproduce continuously at a constant rate
Carrying capacity
- Carrying capacity is not a static condition
- It can change from season to season as conditions change
- Over time the population size fluctuates around the carrying capacity of the habitat in a stable equilibrium
- Populations can crash if resources are depleted faster than they can replenish
Life strategies
- Life strategies are strategies employed by organisms that can be correlated to the type of environment in which the organism lives
- Organisms have to make trade-offs to maximize the number of offspring that survive
R and K species
Species that have an r-selected strategy live close to their biotic potential (r). In general, these organisms
- Have a short life span
- Become sexually mature at a young age
- Produce large broods of offspring
- Provide little or no parental care to their offspring
Example: insects. They reproduce quickly when resources are favourable, but die in large numbers at the end of the season
Organisms with a K-selected strategy live close to the carrying capacity (K) of their habitats. In general, these organisms:
- Have a relatively long life span
- Become sexually mature later in life
- Produce few offspring per reproduction cycle
- Provide a high level of reproductive care
Example: mammals
Density independent factors
An abiotic factors that affects population growth in the same way, regardless of population density.
Examples:
- Natural disaters
- Temp change
Density dependent factors
A biotic interaction that varies in its effect on population growth, depending on the density of the populations involved.
- Disease
- Predators
- Water
- Mating
Competition
Intraspecific competition: a situation in which
members of the same population complete for resources.
Interspecific competition:
A situations in which two or more populations compete for limited resources
Often one species will out-compete the other
Competitive exclusion principle: two species with overlapping niches cannot coexist, but if their niches are different they can live in the same area, but affecting their population
