Population Ecology

Population Ecology

What is population? • Group of individuals of the same species that live in the same area

Does population change? • Sea otters, kelp, and sea urchins

•Kelp forests offer a habitat for sea otters •Sea urchins feed on kelp •Sea otters feed on sea urchins

Then come the hunters...

• What do you think happens when sea otters are hunted? – Sea urchins increase or decrease? – Kelp forest increase or decrease?

• Sea otters are then placed on the endangered species list • So now what happens to the population of sea otter? – Starts to increase

• How does this affect the kelp and the sea urchins? – Sea urchins start to get eaten again=decrease in # – Kelp increases b/c less sea urchins to eat them

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But now we have a new hunter….

What happens to the sea otter, kelp, and sea urchins?

What does this tell us about population? • Population changes • There are many factors that influence a population – Natural – Unnatural

• Population density has a great impact on ecosystems 

3 important characteristics of Population

• Geographic Distribution • Density • Growth Rate –Birthrate, death rate, individuals entering/leaving 

***Population Age structure is also an important characteristic

Geographic Distribution • AKA Range

• Describes an area inhabited by a population • Can vary –Few cubic centimeters –Kilometers of the ocean

Density • # of individuals per unit area • Low density –Cactus in desert

• High density –Other desert plants and succulents 

Math Time • Formula for calculating population density • Population density=Number of individuals  units area • Problem: Suppose there are 150 bullfrogs living in a pond that covers an area of 3 square kilometers. What is the density of the bullfrog population? • 50 bullfrogs per square kilometer •

Growth Rate • Many factors affect growth rate 

Sampling Techniques • How would we measure the population of a species? • Impractical to count each and every one • Variety of sampling techniques – Quadrants – Indirect counting – Mark-Recapture

Quadrants • Involves marking off specific area, boundary • Count specific species within the boundary • Repeat in several locations within desired ecosystem • Average the results to determine population density • More quadrants sampled=more accurate

Indirect Counting • Used for species that are too difficult to see or move around too quickly • Does not involve counting organisms themselves – Count nests, burrows, tracks

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Mark-Recapture • • • •

Most common Choose a study area Trap/capture animals Mark the captured animals and release back into habitat – Markings are not to disturb organism

• After a period of time, recapture animals in the same study area • Count marked and unmarked organisms •

How to Estimate Population from Mark-Recapture Method 

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(# in first capture) x (# in second capture) number of marked animals RECAPTURED

Total population=

Limits to Accuracy • Involve making assumptions about populations – Assumptions not valid=estimate not accurate

• Quadrant – Assumption: • Organisms distributed evenly in study area

– Problems • “Clumps” • Quadrant with clump vs quadrant without clump

– Minimize problem • Analyze how study population is distributed in order to choose appropriate quadrant size

• Mark-Recapture – Assumption: • Both marked and unmarked animals have same chance of surviving and being recaptured in second trial

– Problem • After being captured once, how do you think animals will behave? • Leads to overestimating population size

– Minimize problem • Minimize effects of trapping on organisms

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3 Factors that affect population size • # of births • # of deaths • # of individuals that enter or leave population

Population will increase or decrease depending on # of individuals added or removed

What happens to the population when we…. • Have more births than deaths? – Population increases

• Have more deaths than births? – Population decreases

• Have equal amounts of births and deaths? – Population remains constant

Immigration • “im”= in • Migrate= to move from one place to another • Immigration is the individual movement into an area • Animals in search of mates and food in new areas

Emigration • “E” means ‘out’ • Migrate means to move from one place to another • Emigrate means individuals moving out of one place and into another • Young wolves and bears leaving as they mature • Shortage of food

Two types of growth • Exponential growth – Individuals reproduce at a constant rate – Population multiplies by constant FACTOR over constant time interval

• Logistic growth – Occurs when a populations growth slows or stops after a period of exponential growth – As population approaches carrying capacity, BR may decrease, DR may increase or both, until equal

Exponential Growth • Occurs under ideal conditions with unlimited resources • Think about exponents in math…. • Starts slowly then sky rockets to infinity • Our graph will look like a J • Bacteria

Lets look at bacteria… • Bacteria reproduce by splitting in half • Bacteria have a doubling time of 30 minutes • If you start will one bacterium, how many bacteria will there be after the first 30 minutes? – 2

• After an hour? – 4

• After an hour and a half? – 8

• After two hours? – 16

• After 15 hours? – Over a billion

Logistic Growth • As resources become less available, the growth of the population slows or stops • S-shape curve • No net increase or decrease in population • What we usually see in nature

Carrying Capacity • The largest number of individuals that a given environment can support • The part of the logistic graph after the exponential growth…the flattening out • The point at which this flat line reaches the yaxis is the size of the population when the growth rate reaches zero • This doesn’t mean the population stops growing • Many factors slow the growth of plants and animals… •

Limiting Factor • Condition that can restrict a population’s growth • Could be: – Space – Disease – Availability of food 

Factors Affecting Population Growth •Density-dependent Factors •Factor that limits population as population density increases

–Competition –Predation –Disease –Parasitism –Crowding and Stress

•Density-Independent Factors •Factors that affect population but are unrelated to population density; affect population regardless of size •Insects vulnerable to this

–Weather /Natural disasters –Human activities –Fires –deforestation

Boom and Bust Growth Cycles • Increase rapidly for a period of time followed by a sharp decrease in population for a brief period of time • Still not completely understood • Hypothesis: – Changes in food supply – Stress from overcrowding – Other organisms influence on population

Boom-and Bust Population Cycles -Involves more than one population -Rapid increase and decrease in populations