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Chapter 6
Population and Community Ecology
Natures Exists in Several Levels
 Individual, population, community, ecosystem, biosphere
 Population is all the same members of the same species in a given area.
This is the unit of evolution
 Community is all the populations. Looks at species interactions.
Communities can be grouped together to form biomes
 Ecosystem looks ate energy and matter flow, biotic/abiotic
 Biosphere is all earth’s ecosystems (anywhere life occurs)
Population Characteristics
 Population size is the total number of individuals within a defined area at a
given time.
 Population density is the number of individuals per unit area at a given time.
Used to set hunting/fishing laws, wildlife boundaries, etc
 Larger organisms usually have smaller density due to less resources
 High density = easy to find mates, but more competition and disease
 Population distribution is how individuals are distributed in respect to one
another
 Random – no pattern (solitary animals with large territories) (least common)
 Uniform – individuals evenly spaced out (territorial animals, competition)
 Clumped – large groups of organisms (fish, birds). Enhances feeding opportunities
and protection (most common)
 Population sex ratio is males vs. females. Usually close to 50:50. Helps
predict future populations
 Population age structure is dispersion of how many individuals fall in certain
age groups. Most important is how many individuals fall in reproduction age
Factors That Influence Population Size
 Biotic potential is how a population would grow if there was unlimited
resources (reproductive characteristics)
 Density dependent factors influence an individual’s probability of
survival and reproduction in a manner that depends on population size
 Limiting resources – a resource a population cannot live without and occurs
in quantities lower than the population would require to increase in size
(food, water, shelter, nutrients, competition)
 Carrying capacity is the max population an ecosystem can support. It can
overshoot, then birth rates decrease and death rates increase
 Density independent factors have the same effect on an individuals
probability of survival and reproduction at any population size
 Natural disaster, weather, temp, habitat destruction
Growth Models
 Natural population growth rate  growth rate = birth rate – death rate
 Intrinsic growth rate is maximum possible growth rate
 Actual population growth rate  (Crude birth rate + immigration rate) –
(Crude death rate + emigration rate)
 Exponential growth model – Nt = N0ert (j curve)
 e=natural log, t=time, Nt=future population, N0=current population,
r=intrinsic growth rate
 Population grows very rapidly (lots of food/space and little comp)
 Density independent
 Logistic growth model starts off exponential, but slows as population
approaches carrying capacity (s curve, density dependent)
 Variations of the logistic say a population can overshoot the carrying
capacity
 You have just been offered a job that will last one month. You
have 2-salary options. You can either receive $10 a week with a
$5 per week raise every week, or you can receive one penny for
your first day on the job, and then double the previous day’s pay
for each of the remaining days. Calculate which option would
be better. How does this compare to the two different types of
population growth?
 $70 vs 10,000,000
You decide to invest $1000 in a savings account.Your investment
will grow at a rate of 10% each year. Assuming that you reinvest
the interest each year, how much money will you have in 30 years?
Doubling Time and the Rule of 70
The doubling time or Rule of 70 is a useful tool for calculating the time it will take for a
population (or money) to double. The rule of 70 explains the time periods involved in
exponential growth at a constant rate. To find the approximate doubling time of a quantity
growing at a given annual percentage, such as 10%, divide 70 by the percentage growth rate.
Remember, the Rule of 70 is an approximation, the actual Rule is 69.3.
So the doubling time for the $1000 investment with an annual percentage rate of 10% is
70/10 = 7 years
The actual Rule of 69.3 is
69.3/10 = 6.93 years
Here is an example of a similar AP multiple-choice question that asks student to calculate
doubling time using the Rule of 70.
Example: If the population of rabbits in an ecosystem grows at a rate of
approximately 4 percent per year, the number of years required for the rabbit
population to double is closest to
a. 4 years b. 8 years c. 12 years d. 17 years e. 25 years
Solution: 70/4 = 17.5 years, the closest answer to 17.5 would be “d” 17 years.
Reproductive Strategies and Survivorship Curves
 K-selected species population grows slowly until the carrying capacity
 R selected species have a high intrinsic growth rate (reproduce early and often)
Trait
K-selected species
R-selected species
Life span
Long
Short
Time to reproductive maturity
Long
Short
Number of reproductive events
Few
Many
Number of offspring
Few
Many
Size of offspring
Large
Small
Parental care
Present
Absent
Population growth rate
Slow
Fast
Population regulation
Density dependent
Density independent
Population dynamics
Stable, near carrying capacity
Highly variable
Population cycles
 Boom and bust cycle (common in r-strategists)
 Rapid increase in a population, then a rapid drop off
 More predictable (temp or nutrient changes)
 Strategy is “get it while it is good”
 Predator prey cycle
 If the prey reproduces successfully due to ideal conditions, the predator will
soon have success as well
 Predator’s population shortly trails preys population change
Species Interactions
 Competition – when individuals struggle for the same limiting resource
 Members of same species compete of niches overlap
 Direct vs. indirect
 Competitive exclusion principle – two species competing for the same
limited resource cannot coexist (one will be driven out)
 Reduce competition by hunting at different times, using different habitats,
and evolution of body shape/size (finches)
 Predation – the use of one species as a resource by another species.
Does not always result in death
 True predators consume and kill their pray
 Herbivores consume plants, but don’t usually kill them
 Parasites live in or on host, but only consume a small piece without usually
killing host. Pathogens make host sick
 Parasitoids lay eggs in host, and they eat their way out killing the host
Continued
 Mutualism – both species benefit from one another (birds/pollination,
humans/bacteria, coral/algae, lichens)
 Commensalism – one species benefits, and the other is neither helped
nor harmed (fish/sharks, tree branches as perches for birds)
 Commensalism, mutualism, and parasitism are all examples of symbiotic
relationships, ones where two species live in close association with one
another
Keystone and Indicator Species
 Most ecosystems can exist without the presence of one of its species
 A keystone species is one that is disproportionately important to the
community
 Typically occur in small numbers (sea star example)
 Ecosystem engineers are create or maintain the habitat for other species
(grizzly bear)
 Indicator species are used as the standard to evaluate the health of an
ecosystem
 Typically sensitive to change, so they can give warning signs (trout and poll)
 Indigenous species are ones that naturally live in an area
 Invasive species are ones that are introduces to a new ecosystem (zebra
musssles)
Primary Succession
 Ecological succession is the gradual replacement of species over time
 Primary succession occurs on abandoned or new land masses where
there is no soil
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Rock is covered by lichens and mosses (don’t need soil)
They secrete an acid that breaks down rock to create soil
Lichen and mosses die and add organic matter to soil
Soil gets deeper so grasses move in
If climate favors, trees will follow
 Secondary succession occurs in areas that have been disturbed, but still
have soil and nutrients
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Often after natural disasters
Grasses/flowers usually arrive within a year (pioneer species)
Seeds come by wind, and trees quickly follow and compete for sun
When everything is in balance, called climax community
Aquatic Succession
Factors That Influence Species Richness
 Four majors factors
Latitude – angular distance north or south of the equator. Further
away from equator, less variety of animals because cold and little sun
2. Time – more time allows for more species to evolve
3. Habitat size – larger habitats typically means more species because
dispersing species land here, can support more species, and a wider
range of environmental conditions
4. Distance from other communities
1.
Theory of Island Biogeography
 Theory of island biogeography- the theory
that explains that both habitat size and
distance determine species richness.