2. 2.1 Species and Populations
A population is a group of organisms of the same species living
in the same area at the same time, and which are capable of
interbreeding.
• S and J population curves describe a generalized response of
populations to a particular set of conditions (abiotic and biotic
factors).
• Limiting factors will slow population growth as it approaches
the carrying capacity of the system
Organisms in an ecosystem can be identified using a variety of
tools including keys, comparison to herbarium or specimen
collections, technologies and scientific expertise
Methods for estimating the abundance of non-motile organisms
include the usecof quadrats for making actual counts, measuring
population density, percentage cover and percentage frequency.
3. Direct and indirect methods for estimating the abundance of
motile organisms can be described and evaluated. Direct
methods include actual counts and sampling. Indirect methods
include the use of capture–mark–recapture with the application
of the Lincoln index.
Lincoln Index = [(n1) (n2)] / nm
– n1 is the number caught in the first sample
– n2 is the number caught in the second sample
– nm is the number caught in the second sample that were
marked
Species richness is the number of species in a community and is
a useful comparative measure.
4. Species diversity is a function of the number of species
and their relative abundance and can be compared using
an index. There are many versions of diversity indices,
but students are only expected to be able to apply and
evaluate the result of the Simpson diversity index as
shown below. Using this formula, the higher the result
(D), the greater the species diversity. This indication of
diversity is only useful when comparing two similar
habitats, or the same habitat over time.
D = N(N-1) / Σn(n-1)
– D is the Simpson diversity index
– N is the total number of organisms of all species found
– n is the number of individuals of a particular species
7. Population
A group of individuals of the same
species found in the same area
(habitat) at the same time
The gopher tortoises in scrub
habitats in Volusia county
The bottlenose dolphins of the Indian
River Lagoon
8. Sea Otters: A case study
Sea otters keystone species in Pacific kelp forests
Daily consume 25% body weight in urchins &
molluscs
Population > 1 million before settlers arrived
1700’s hunted to near extinction – 1000 in the
Aleutians, AK only 20 off California
In 1971 A-bomb test in AK used sea otter
population to assess bomb’s power 1000’s died
1973 Endangered Species Act passes, 1976
Marine Mammal Conservation Act
1989 1000’s died in Exxon Valdez Oil spill
Otters recovering in most places after 1970’s
The spring 2008 survey found 2760 sea otters,
down 8.8-percent from the record 2007 spring
survey.
9. New Threats?
Pollution Effects
- Shellfish magnify
toxins
- Reduce disease
resistance
- Reduce fertility
Increased Predation
- Killer Whales
- Switch to otters
when other food
is scarce
10. Population characteristics
Populations are dynamic – change in
response to environment
• Size (# of individuals)
• Density (# of individuals in a certain space)
• Dispersion (spatial pattern of individuals)
Random, Uniform, Clumped based on food
• Age distribution (proportion of each age)
Changes called Population dynamics
• Respond to environmental stress & change
14. Capacity for Growth
Capacity for growth = Biotic potential
Rate at which a population grows with
unlimited resources is intrinsic rate of
increase (r)
High (r) (1)reproduce early in life,
(2)short generation time, (3)multiple
reproductive events, (4)many offspring each
time
BUT – no population can grow indefinitely
Always limits on population growth in nature
15. Carrying Capacity
Environmental resistance = all
factors which limit the growth of
populations
Population size depends on
interaction between biotic potential
and environmental resistance
Carrying capacity (K) = # of
individuals of a given population
which can be sustained infinitely in a
given area
16. Limiting Factors
Carrying capacity established by limited
resources in the environment
Only one resource needs to be limiting
even if there is an over abundance of
everything else
Ex. Space, food, water, soil nutrients,
sunlight, predators, competition, disease
A desert plant is limited by…
Birds nesting on an island are limited by…
17. Minimum Values
(r) depends on having a certain
minimum population size MVP –
minimum viable pop.
Below MVP
• 1 – some individuals may not find mates
• 2 – genetically related individuals reproduce
producing weak or deformed offspring
• 3 – genetic diversity may drop too low to
enable adaptation to environmental changes
–bottleneck effect
18.
19. Forms of Growth
Exponential growth starts slow and
proceeds with increasing speed
• J curve results
• Occurs with few or no resource limitations
Logistic growth (1) exponential
growth, (2) slower growth (3) then
plateau at carrying capacity
• S curve results
• Population will fluctuate around carrying
capacity
22. Carrying capacity alterations
In rapid growth population may
overshoot carrying capacity
• Consumes resource base
• Reproduction must slow, Death must
increase
• Leads to crash or dieback
Carrying capacity is not fixed, affected
by:
• Seasonal changes, natural & human
catastrophes, immigration & emigration
23. Density Effects
Density Independent Factors: effects
regardless of population density
Mostly regulates r-strategists
• Floods, fires, weather, habitat destruction,
pollution
• Weather is most important factor
24. Density Effects
Density dependent Factors: effects based on
amount of individuals in an area
Operate as negative feedback mechanisms
leading to stability or regulation of population
External Factors
• Competition, predation, parasitism
• Disease – most epidemics spread in cramped
conditions
Internal Factors
• Reproductive effects Density dependent fertility,
Breeding territory size
25.
26. Natural Cycles: Predation
Over longer time spans populations
cycle
Canadian lynx & Snowshoe hare - 10
year cycles
Once thought that predators controlled
prey #’s Top down control
Now see a negative feedback
mechanism in place community
equilibrium
29. Reproduction Strategies effect
Survival
Asexual reproduction
• Produce clones of parents
• Common in constant environments
Sexual reproduction
• Mating has costs – time, injury, parental
investment, genetic errors
• Improves genetic diversity survive
environmental change
• Different male & female roles in
parental care
30. MacArthur – Wilson Models
Two idealized categories for reproductive patterns
but really it’s a continuum
r-selected & K-selected species depending on
position on sigmoid population curve
r-selected species: (opportunists) reproduce early,
many young few survive
• Common after disturbance, but poor competitors
K-selected species: (competitors) reproduce late,
few young most survive
• Common in stable areas, strong competitors
32. r-Selected Species
cockroach dandelion
Many small offspring
Little or no parental care and protection of offspring
Early reproductive age
Most offspring die before reaching reproductive age
Small adults
Adapted to unstable climate and environmental
conditions
High population growth rate (r)
Population size fluctuates wildly above and below
carrying capacity (K)
Generalist niche
Low ability to compete
Early successional species
33. Fewer, larger offspring
High parental care and protection of offspring
Later reproductive age
Most offspring survive to reproductive age
Larger adults
Adapted to stable climate and environmental
conditions
Lower population growth rate (r)
Population size fairly stable and usually close
to carrying capacity (K)
Specialist niche
High ability to compete
Late successional species
elephant saguaro
K-Selected Species
34. r versus K
Most organisms somewhere in the
middle
Agriculture crops = r-selected,
livestock = K-selected
Reproductive patterns give
temporary advantage
Resource availability determines
ultimate population size
35. Survivorship curves
Different life expectancies for different
species
Survivorship curve: shows age structure
of population
1. Late loss curve: K-selected species with
few young cared for until reproductive
age
2. Early loss curve: r-selected species
many die early but high survivorship
after certain age
3. Constant loss curve: intermediate
steady mortality
37. Humans Impact Natural
Populations
1. Fragmenting & degrading habitats
2. Simplifying natural ecosystems
3. Using or destroying world primary
productivity which supports all consumers
4. Strengthening pest and disease populations
5. Eliminating predators
6. Introducing exotic species
7. Overharvesting renewable resources
8. Interfering with natural chemical cycling
and energy flow
40. Step 1: Identify the organism
Use dichotomous keys, field guides,
observe a museum collection, or consult
an expert
http://www.earthlife.net/insects/orders-
key.html#key
Sample key for insect ID
http://people.virginia.edu/~sos-
iwla/Stream-Study/Key/Key1.HTML
Macroinvertebrate key
42. Mark & Recapture Method
Used for fish & wildlife populations
Traps placed within boundaries of study area
Captured animals are marked with tags, collars,
bands or spots of dye & then immediately released
After a few days or weeks, enough time for the
marked animals to mix randomly with the others in
the population, traps are set again
The proportion of marked (recaptured) animals in
the second trapping is assumed equal to the
proportion of marked animals in the whole
population
Repeat the recapture as many times as possible to
ensure accuracy of results
Marking method should not affect the survival or
fitness of the organism
43.
44.
45.
46.
47.
48. Mark & Recapture Calculation
# of recaptures in second catch = # marked in the first catch
Total # in second catch Total population (N)
Assuming no births, deaths, immigration, or emigration
population size is estimated as follows (Lincoln
Index)
N = (# marked in first catch) (Total # in second catch)
# of Recaptures in second catch
MEMORIZE THIS EQUATION
49. Example
50 snowshoe hares are captured in box
traps, marked with ear tags and released.
Two weeks later, 100 hares are captured
and checked for ear tags. If 10 hares in
the second catch are already marked
(10%), provide an estimate of N
N = (50 hares x 100 hares) / 10 = 5000 /
10
= 500 hares
**Realize for accuracy that you would
recapture multiple times and take an
average**
50. Quadrat Method
Used for plants or sessile organisms
1. Mark out a gridline along two edges of an area
2. Use a calculator or tables to generate two random
numbers to use as coordinates and place a
quadrat on the ground with its corner at these
coordinates
3. Count how many individuals of your study
population are inside the quadrat
4. Repeat steps 2 & 3 as many times as possible
5. Measure the total size of the area occupied by the
population in square meters
6. Calculate the mean number of plants per quadrat.
Then calculate the population size with the
following equation
51. Quadrat Method
N = (Mean # per quadrat) (total area)
Area of each quadrat
This estimates the population size in an
area
Ex. If you count an average of 10 live oak trees per
square hectare in a given area, and there are 100
square hectares in your area, then
N = (10 X 100 hectare2) / 1 hectare2 = 1000 trees in
the 100 hectare2
52. In addition to population size we can
measure…
Density = # of individuals per unit area
• Good measure of overall numbers
Frequency = the proportion of quadrats sampled
that contain your species
• Assessment of patchiness of distribution
% Cover = space within the quadrat occupied by
each species
• Distinguishes the larger and smaller species
53.
54.
55. How can changes in these
populations be measured?
Necessary because populations may
change over time through processes
like succession
But also because human activities
may impact a population and we
want to know how
• Impacts include toxins from mining,
landfills, eutrophication, effluent, oil
spills, overexploitation
56. Measuring changes cont.
Can still use CMR or quadrat method
Just do it repeatedly over time
Also could use satellite images taken over
time
1. Do pre and post impact assessments
in one area
2. Measure comparable areas – one
impacted, one not at a given time
60. In a mark – recapture study of lake
trout populations, 40 fish were
captured, marked and released. In a
second capture 45 fish were caught;
9 of these were marked. What is the
estimated number of individuals in
the lake trout population
Question 1
61. Question 2
Woodlice are terrestrial crustaceans
that live under logs and stones in damp
soils. To assess the population of
woodlice in an area, students collected
as many of the animals as they could
find, and marked each with a drop of
fluorescent paint. A total of 303 were
marked. 24 hours later, woodlice were
collected again in the same place. This
time 297 were found, of which 99 were
seen to be already marked from the
first time. What approximately, is the
estimated population of woodlice in this
62. Review points
1. Dispersion patterns
2. Carrying capacity and limiting
factors
3. r and K selection
4. Natural population cycles
5. Human effects
