A2 Biology - populations and ecosystems

THE NICHE A SPECIES OCCUPIES DEPENDS ON - 

• BIOTIC INTEERACTIONS E.G. ALL THE ORGANISMS IT EATS 
• ABIOTIC INTERACTIONS E.G. OXYGEN AN ORGANISM BREATHES IN 

EVERY SPECIES OCCUPIES OWN UNIQUE NICHE 

IF THEY TRY TO OCCUPY SAME NICHE, 2 SPECIES WILL COMPETE 

ONE SPECIES WILL BE MORE SUCCESSFUL THAN THE OTHER AND WIPE THE OTHER OUT 

FEATURES THAT INCREASE AN ORGANISMS CHANCE OF SURVIVAL AND REPRODUCTION 

THEY CAN BE PHYSIOLOGICAL, BEHAVIOURAL OR ANATOMICAL CHANGES 

ORGANISMS WITH BETTER ADAPTIONS - MORE LIKELY TO SURVIVE - PASS ON ALLELES FOR THEIR ADAPTIONS - SO THE ADAPTIONS BECOME MORE COMMON - NATURAL SELECTION 

EVERY SPECIES IS ADAPTED TO USE AN ECOSYSTEM IN  A WAY THAT NO OTHER SPECIES CAN

ORGANISMS ADAPT TO ABIOTIC AND BIOTIC CONDITIONS 

E.G. ABIOTIC - OTTERS HAVE WEBBED PAWS - WALK ON LAND AND SWIM EFFECTIVELY - INCREASES CHANCE OF SURVIVAL BECAUSE THEY CAN LIVE AND HUNT IN WATER AND ON LAND 

BIOTIC - SCORPIONS DANCE BEFORE MATING - ATTRACTING MATE OF SAME SPECIES - INCREASES CHANCE OF REPRODUCTION BY MAKING SUCCESSFUL MATING MORE LIKELY.

INVESTIGATING POPULATIONS INVOLVES LOOKING AT ABUNDANCE AND DISTRIBUTION OF SPECIES IN AN AREA 

ABUNDANCE - THE NUMBER OF ONE SPECIES IN ONE AREA - impossible to count every organism - would be time consuming and would cause damage to habitat being studied-

SMALL SAMPLES ARE TAKEN IN AN AREA - REPRESENTATIVE OF WHOLE AREA

RANDOM SAMPLE TO AVOID BIAS - PICK RANDOM SAMPLE SITES BY DIVIDING THE FIELD INTO GRID AND USE RANDOM NO. GENERATOR TO SELECT COORDINATES 

REPEAT - AS MANY SAMPLES AS POSSIBLE - MORE RELIABLE RESULTS 

3 FACTORS TO CONSIDER WHEN USING QUADRATS 

1. SIZE OF THE QUADRAT - depends on size of animal plant being measured, larger species need larger quadrats.
2. NUMBER OF SAMPLE QUADRATS TO RECORD WITHIN THE STUDY AREA - the larger the sample of quadrats the more reliable the results - greater the number of different species in an the area being studied the greater the number of quadrats required to produced valid results.
3. THE POSITION OF EACH QUADRAT - to produce statistically signifcant results a technique know as random sampling must be used.

SAMPLING RANDOM - TO AVOID BIAS 

AVOIDING BIAS ENSURES DATA IS VALID 

CANT JUST THROUGH QUADRAT RANDOMLY STILL BIA (likely to stand in dry area rather than wet one when doing this)

METHOD

1. LAY OUT TWO TAPE MEASURES AT RIGHT ANGLES ALONG TWO SIDES OF SAMPLE AREA
2. COLLECT COORDINATES BY USING RANDOM NO. GENERATOR 
3. PLACE QUADRAT AT EACH CORDINATE 

SOMETIMES MORE INFORMATIVE TO MEASURE ABUNDANCE AND DISTRIBUTION IN SYSTEMATIC MANOR RATHER THAN RANDOM 

E.G. WHEN THERE IS A TRANSITION IN THE COMMUNITIES OF PLANTS AND ANIMALS TAKES PLACE E.G. A TIDAL SEA SHORE, DISTRIBUTION IS DETERMINED BY TIME SPENT UNDER WATER.

A LINE TRANSECT IS USED FOR THIS 

It is a tape or string laid along the ground in a straight line between two poles as a guide to a sampling method used to measure the distribution of organisms. Sampling is rigorously confined to organisms that are actually touching the line get recorded 

A BELT TRANSECT IS ALSO USED 

A *****, typically 1 m wide, marked by putting one ***** parallel to the the other, that is marked out across a habitat and within which species are then recorded to determine their distribution in the habitat. 

ABUNDANCE - THE NUMBER OF INDIVIDUALS OF A SPECIES WITHIN A GIVEN A SPECIES

IT CAN BE MEAUSRED IN SEVERAL WAYS 

1. FREQUENCY - LIKELIHOOD OF SPECIES OCCURING INA QUADRAT E.G. IF IT OCCURS IN 15 OUT OF 30 THEN THERE IS A 50% FREQUENCY OCCURANCE 

POSITIVES - USEFUL WHEN SPECIES E.G. GRASS HARD TO COUNT, GIVES QUICK IDEA OF SPECIES PRESENT 

NEGATIVES - DOES NOT PROVIDE INFO ON DENSITY AND DETAILED DISTRIBUTION 

1. PERCENTAGE COVER - ESTIMATE OF THE AREA WITHIN A QUADRAT THAT A PLANT SPECIES COVERS 

POSTIVE - USEFUL WHEN A SPECIES IS PARTICULARY ABUNDANT OR HARD TO COUNT, DATA CAN BE COLLECTED RAPIDLY - INDVIDUAL PLANT DONT NEED TO BE COUNTED 

NEGATIVES - WHERE ORGANISMS OCCUR IN SEVERAL OVERLAPPING LAYERS 

THE LARGER THE NUMBER OF SAMPLES THE MORE REPRESENTATIVE OF THE COMMUNITY THE RESULTS WILL BE.

METHODS OF MEASURING ABUNDANCE - OFTEN WORK WELL WITH PLANTS NOT ANIMALS 

ANIMALS ARE MOBILE AND MOVE AWAY WHEN APPROACHED 

DIFFERENT TECHNIQUE NEEDED - MARK RELEASE RECAPTURE 

A portion of the population is captured,marked, and released back to community. Later, another portion is captured and the number of marked individuals within the sample is recorded.

calculation = total number of individuals cuaght in 1st sample x total number caught in the 2nd sample

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number of marked individuals recaptured in 2nd sample 

relies on assumptions - marked individuals from 1st sample evenly distrubuted themselves amongst population and have time to do so, no emmigration or immigration, few deaths and births, marking does not harm individual and the marking is not rubbed off.

QUANTATIVE DATA COLLECTED - PRESENTED IN TABLE OR GRAPH - MAKES IT EASIER TO COMPARE DATA E.G. FROM DIFFERENT LOCATIONS

FURTHER COMPARE USING STATISTICAL ANALYSIS E.G FINDING THE MEAN

ONLY TENTATIVE (NOT CERTAIN) CONCLUSIONS MAY BE BASED FROM COMPARING 2 SETS OF DATA - AS THERE ARE MANY FACTORS THAT CAN CONTRIBUTE TOWARDS THESE RESULTS E.G. CHANCE 

CHECK STATISTICALLY USING T- TEST TO SEE IF RESULTS ARE DOWN TO CHANCE OR DOWN TO A PARTICULAR FACOTR 

POSITIVE CORRELATION -   is a relationship between two variables such that their values increase or decrease together.

NEGATIVE CORRELATION - is a relationship between two variables such that as the value of one variable increases, the other decreases.

A POPUATION IS A GROUP OF INTERBREEDING INDIVIDUALS OF THE SAME SPECIES IN A HABITAT

NUMBER OF INDIVIDUALS IN A POPULATION IS THE POPULATION SIZE

USUAL PATTERN OF GROWTH FOR A NATURAL POPULATION NORMALLY HAS 3 PHASES;

• PERIOD OF SMALL GROWTH AS THE SMALL NUMBERS REPRODUCE TO SLOWLY BUILD UP NUMBERS
• PERIOD OF RAPID GROWTH WHERE EVER INCREASING INDIVIDUALS COTINUE TO REPRODUCE 
• PERIOD OF POPULATION GROWTH DECLINE WHEN THE SIZE BECOMES STABLE - decline due to food supply

POPULATION SIZE VARIES BECAUSE OF ABIOTIC FACTORS... E.G. AMOUNT OF LIGHT, WATER OR SPACE 

WHEN ABIOTIC CONDITIONS ARE IDEAL FOR A SPECIES, ORGANISMS CAN GROW FAST AND REPRODUCE E.G WHEN TEMP OF MAMMALS SURROUNDING IS IDEAL FOR METABOLIC REACTIONS - NOT AS MUCH ENERGY USED - MORE FOR GROWTH AND REPRODCUTION 

WHEN ABIOTIC CONDITIONS ARNET IDEAL ORGANISMS CANT GROW FAST AND REPRODUCE 

E.G. MAMMAL WILL HAVE TO USE ALOT OF ENERGY TO MAINTAIN BODY TEMP 

OTHER ABIOTIC LIMITING FACTORS

• LIGHT - E.G. MORE PHOTOSYNTHESIS  - MORE GROWTH 
• PH - ENZYME ACTION INCREASES (OPTIMUM PH) 
• WATER AND HUMIDTY - WHERE WATER IS SCARCE POPULATIONS ARE SMALL

COMPETITION IS A BIOTIC LIMITING FACTOR ON POPULATION SIZE 

WHERE 2 OR MORE INDIVIDUALS SHARE A RESOURCE THEN COMPETITION OCCURS. 

2 TYPES OF COMPETITION 

• INTRASPECIFIC COMPETITION - WHEN INDIVIDUALS OF THE SAME SPECIES COMPETER WITH ONE ANOTHER FOR RESOURCES LIKE FOOD AND WATER, THE AVALIABILITY OF THESE RESOURCES DETERMINES POPULATION SIZE

• INTERSPECIFIC COMPETITION - WHEN INDIVIDUALS OF A DIFFERENT SPECIES COMPETE FOR RESOURCES, WHERE POPULATIONS OF 2 SPECIES OCCUPY THE SAME NICHE ONE WILL HAVE COMPETITIVE NICHE AND ONE WILL START TO DIMINISH - COMPETITIVE EXCLUSION PRINCIPLE 

PREDATION IS ANOTHER TYPE OF BIOTIC LIMITING FACTOR ON POPULATION SIZE 

A PREDATOR IS AN ORGANISM THAT FEEDS ON ANOTHER ORGANISM KNOW AS PREY 

AS PREDATORS HAVE EVOLVED THEY HAVE BECOME BETTER ADAPTED FOR CAPTURING PREY 

AS PREY HAVE EVOLVED THEY HAVE BECOME BETTER ADAPTED FOR AVOIDING PREDATORS

EFFECT OF PREY-PREDATOR RELATIONSHIP ON POPULATION SIZE.....

• PREDATORS EAT THEIR PREY - POP OF PREY DECREASES 
• FEWER PREY AVALIABLE, PREDATORS HAVE GREATER COMPETIION FOR PREY
• PREDATOR POPULATION DECREASES AS INDIVIDUALS DONT GET ENOUGH PREY 
• FEW PREDATORS, FEWER PREY EATEN, PREY POPULATION INCREASES
• WITH MORE PREY, PREDATOR POPULATION INCREASES

organinms often eat more than one food so fluctuations are often less severe

DISEASE AND CLIMATIC FACTORS ALSO PLAY A PART 

THIS IS KEY IN EVOLUTION - E.G. PREY THAT IS ABLE TO ESCAPE PREDATORS, SURVIVE DISEASE AND ADVERSE CLIMATE WILL REPRODUCE TO PRODUCE BETTER ADAPTED OFFSPRING.

HUMAN POPULATION CONTROLLED BY FOOD AVALIABILITY, DISEASE, PREDATORS AND CLIMATE (like that of other organisms) 

2 RECENT EVENTS RAPIDLY INCREASED HUMAN POPULATION 

• AGRICULTURE 
• DEVELOPMENT OF MANAFACTURE AND TRADE 

WARS, DISEASE AND FAMINE, HAVE ONLY CAUSED TEMPORARY REVERSALS IN UPWARD TREND

SIGMOID (S SHAPE CURVE FOR OTHER ORGANIMS DOESNT APPLY)

THE EXPONENTIAL PHASE (GROWING) CONTINUNES RATHER THAN REACHING A STATIONARY PHASE

2 MAIN THINGS THAT AFFECT THE GROWTH AND SIE OF HUMAN POPULATIONS 

• BIRTH RATE 
• DEATH RATE 

BALANCE BETWEEN THESE THAT AFFECTS WHETHER POP IS INCREASING OR DECREASING

ANOTHER FACTOR IS MIGRATION (INDVIDUALS MOVING FROM ONE POP TO ANOTHER)

THIS CAN BE SPLIT INTO.....

• IMMIGRATION - JOINIG
• EMMIGRATION - LEAVING

POPULATION GROWTH = (births + immigration) - (deaths + emmigration)

PERCENTAGE POP GROWTH = (pop change during period) / (pop at start of period) x 100

• ECONOMIC CONDITIONS - countries with low gdp have high birth rate 
• CULTURAL AND RELIGIOUS BACKGROUNDS - countries/ religions encourage larger families 
• SOCIAL PRESSURES AND CONDITIONS - in some countries larger family improves social standin
• BIRTH CONTROL 
• POLITICAL FACTORS - governments influence birth rate through education and tax 

BIRTH RATE = NUMBER OF DEATHS PER YEAR / TOTAL POPULATION IN SAME YEAR X 1000

• AGE PROFILE - the more elderly people the higher death rate 
• LIFE EXPEXTANCY 
• FOOD SUPPLY - adequate and balanced diet reduces death rate 
• SAFE DRINKING WATER - less disease e.g cholera
• MEDICAL CARE 
• NATURAL DISASTERS 
• WAR 

DEATH RATE = NUMBER OF DEATHS PER YEAR / TOTAL POPULATION IN THE SAME YEAR X 1000