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Population and Community Ecology

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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)

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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
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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
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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

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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

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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 – N t
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= N
0 e rt (j curve) e=natural log, t=time, N t r=intrinsic growth rate
=future population, N
0
=current population,
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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.

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K-selected species population grows slowly until the carrying capacity
R selected species have a high intrinsic growth rate (reproduce early and often)
Trait
Life span
K-selected species
Long
Time to reproductive maturity Long
Number of reproductive events Few
Number of offspring Few
Size of offspring
Parental care
Large
Present
Population growth rate
Population regulation
Population dynamics
Slow
Density dependent
Stable, near carrying capacity
R-selected species
Short
Short
Many
Many
Small
Absent
Fast
Density independent
Highly variable

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Boom and bust cycle (common in r-strategists)
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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
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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

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Competition – when individuals struggle for the same limiting resource
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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
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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

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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

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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)

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Ecological succession is the gradual replacement of species over time
Primary succession occurs on abandoned or new land masses where there is no soil
 Rock is covered by lichens and mosses (don’t need soil)
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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

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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
Time – more time allows for more species to evolve
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Habitat size – larger habitats typically means more species because dispersing species land here, can support more species, and a wider range of environmental conditions
Distance from other communities

 Theory of island biogeography- the theory that explains that both habitat size and distance determine species richness.