BIOGEOGRAPHY

Theme 5:

CHARACTERISTICS OF ECOSYSTEMS

Lectures 13-15

NET PRIMARY PRODUCTIVITY

(NPP)

GROSS PRODUCTIVITY REFERS TO THE TEMPO AT WHICH

GREEN PLANTS PRODUCE CHEMICAL ENERGY BY MEANS OF

PHOTOSYNTHESIS AND THEN STORE THIS ENERGY IN THE

PLANT CELLS WHERE IT THEN PROVIDES FOOD FOR OTHER

ORGANISMS. THIS IS KNOWN AS GPP.

WHEN OTHER ORGANISMS EAT THE GREEN PLANTS A CERTAIN

AMOUNT OF ENERGY IS LOST BECAUSE OF RESPIRATION . THIS

IS KNOWN AS R

NET PRIMARY PRODUCTIVITY IS THEREFORE THE ENERGY

THAT IS LEFT OVER AFTER RESPIRATION HAS TAKEN PLACE

AND IS STORED IN PLANT TISSSUE THAT KEEPS THE PLANT

ALIVE.

NPP = GPP - R

NB: Natural systems vs man-made systems: e.g. agriculture

BIOMASS

BIOMASS CANNOT BE SEPARATED FROM THE CONCEPT OF

PRODUCTIVITY

BIOMASS IS THE DRY MASS OF LIVING ORGANISMS (PLANTS)

PER UNIT AREA USUALLY EXPRESSED AS KG.

IT IS INFLUENCED BY PRODUCTIVITY AND IS A FUNCTION OF

PHOTOSYNTHESIS AND DIFFERS FROM ECOSYSTEM TO

ECOSYSTEM

IN THE BIOSPHERE THE FOLLOWING APPLIES TO BIOMASS:

ANIMALS MAKE UP 3% OF THE TOTAL BIOMASS

PLANTS MAKE UP 97% OF THE TOTAL BIOMASS

THIS INDICATES THE IMPORTANCE OF GREEN PLANTS

POPULATION NUMBERS

ALL ORGANISMS HAVE THE ABILITY TO REPRODUCE AND

THEREFORE TO PRODUCE OFFSPRINGS

Birds produce 1-40 offspring per year

Amphibians produce 100 offspring per year

Fish produce 1000 offspring per year

Insects produce 1 million offspring per year

IN VIEW OF THIS IT WOULD APPEAR AS IF ORGANISMS

JUST GO ON INCREASING IN NUMBERS BUT IN REALITY

THIS IS NOT SO AS CERTAIN ENVIRONMENTAL FACTORS

ACT AS CHECKS AND BALANCES ON THESE POPULATION

NUMBERS

FACTORS CONTROLLING

POPULATION NUMBERS

FOUR MAIN FACTORS CONTROL POPULATION NUMBERS:

1. NATALITY I.E.THE NUMBER OF OFFSPRING BORN

2. MORTALITY I.E. THE NUMBER OF OFFSPRING THAT DIE

3. IMMIGRATION I.E. THE NUMBER OF ORGANISMS THAT

COME TO A REGION

4. EMIGRATION I.E. THE NUMBER OF ORGANISMS THAT

LEAVE A REGION.

THIS TIES UP WITH THE QUESTION OF SURVIVAL RATES. FOR

EXAMPLE IF THE MORTALITY RATE IS 15% THEN THE SURVIVAL

RATE WOULD BE 85% AND THE POPULATION NUMBERS WOULD

DECREASE. THE INVERSE WOULD THEN ALSO APPLY

AGE OF A POPULATION

MORTALITY AND NATALITY INFLUENCE THE

AGE/ NUMBERS OF A POPULATION

THREE GENERAL AGES CAN BE RECOGNIZED:

1. GROWING POPULATION - USUALLY HAS A

HIGH PERCENTAGE OF YOUNG ORGANISMS

2. DECREASING POPULATION - USUALLY HAS A

HIGH PERCENTAGE OF OLD PEOPLE

3. STABLE POPULATION - USUALLY HAS A HIGH

PERCENTAGE OF BOTH OLD AND YOUNG

ORGANISMS

POPULATION GROWTH

THE GROWTH OF A POPULATION CAN SHOW THREE

TENDENCIES:

1. POSITIVE GROWTH - MORE BIRTHS THAN DEATHS

2. NEGATIVE GROWTH - MORE DEATHS THAN BIRTHS

3. STABLE - EQUAL NUMBER OF BIRTHS AND DEATHS

THE POPULATION GROWTH IS USUALLY INDICATED BY TWO

TYPES OF GRAPHS:

S-SHAPED J-SHAPED

CARRYING CAPACITY

THE CARRYING CAPACITY OF AN ECOSYSTEM REFERS

TO THE TOTAL NUMBER OF ORGANISMS THAT CAN BE

MAINTAINED BY THE RESOURCES PRESENT IN THE

ECOSYSTEM

BOTH PHYSICAL AND BIOLOGICAL FACTORS CAN AFFECT

THE CARRYING CAPACITY

PHYSICAL E.G. MOISTURE , LIGHT, SHORTAGE OF FOOD,

SHORTAGE OF LIVING SPACE, UNSUITABLE SHELTER, FIRE ETC

BIOLOGICAL E.G. COMPETITION, PREDATION ETC.

THIS MAY EXHIBIT TEMPORAL AND SPATIAL VARIATIONS

BIOLOGICAL CONTROL MECHANISMS

FOR POPULATION NUMBERS

THIS RELATES TO INTERACTIONS BETWEEN ORGANISMS.

INTERACTIONS CAN BE:

1. POSITIVE - ONE SPECIES GAINS ADVANTAGE OVER

ANOTHER SPECIES BUT DOES NOT REALLY INFLUENCE

THE SPECIES OR BOTH SPECIES MAY GAIN AN

ADVANTAGE

2. NEGATIVE - BOTH SPECIES SUFFER OR ONE SPECIES

GAINS ADVANTAGE OVER ANOTHER SPECIES AND SO

HAS A DETRIMENTAL EFFECT ON THE OTHER SPECIES

THERE ARE A NUMBER OF MECHANISMS:

CONTROL MECHANISMS

PREDATISM/ PREDATION:

- ONLY OCCURS IN THE ANIMAL WORLD

- ONE ANIMAL EATS ANOTHER ANIMAL

- NEGATIVE EFFECT

- WEAKEST ANIMALS THINNED OUT

PARASITISM:

- ORGANISM THAT LIVES ON ANOTHER PLANT OR ANIMAL

- OCCURS IN BOTH PLANT AND ANIMAL WORLDS

- DOES NOT KILL BUT WEAKENS

- NEGATIVE EFFECT ON HOST

- CAN OCCUR OUTSIDE OR INSIDE BODY OF HOST

CONTROL MECHANISMS

CANNIBALISM:

- ONE ORGANISM EATS ANOTHER ORGANISM OF THE SAME

SPECIES

- USUALLY OCCURS IN THE ANIMAL WORLD

MUTUALISM / SYMBIOSIS:

- BOTH ORGANISMS GAIN ADVANGE BY ASSOCIATING WITH

EACH OTHER

- ORGANISMS DIFFER FROM EACH OTHER

COMMENSUALISM:

- ONE SPECIES GAINS AN ADVANTAGE BY LIVING IN

ASSOCIATION WITH ANOTHER

COMPETITION

COMPETITION USUALLY REFERS TO THE BATTLE FOR THE

SAME RESOURCE

IT CAN BE INTRA-SPECIFIC I.E. BETWEEN ORGANISMS OF THE

SAME SPECIES

IT CAN ALSO BE INTER-SPECIFIC I.E. BETWEEN ORGANISMS OF

DIFFERENT SPECIES

Plants usually compete for: - sunlight

- moisture

- nutrients

- may make adaptation to compete more

successfully

animals usually compete for: - space

- food

- breeding

STABILITY

THE STABILITY OF AN ECOSYSTEM REFERS TO THE

ABILITY TO MAINTAIN A CONDITION OF EQUILIBRIUM IN

SPECIES COMPOSITION

HOMEOSTASIS: THIS IS THE ABILITY OF AN ORGANISM

TO RESIST CHANGE AND TO REMAIN IN A STATE OF

EQUILIBRIUM KNOWN AS HOMEOSTASIS

DYNAMIC EQUILIBRIUM: NO ECOSYSTEM CAN REMAIN

ABSOLUTELY STABLE BECAUSE THE SYSTEM IS

CONTINUALLY AFFECTED BY FACTORS FROM OUTSIDE. AS SOON AS AN EXTERNAL FACTOR DISTURBS THE STABILITY

OF THE ECOSYSTEM A PROCESS OF RESTRUCTURING

IMMEDIATELY OCCURS. THIS MAINTAINS THE DYNAMIC

EQUILBRIUM OF THE ECOSYSTEM

ECOLOGICAL SUCCESSION

ECOLOGICAL SUCCESSION REFERS TO THE

PROCESS IN WHICH COMMUNITIES OF PLANTS

AND ANIMALS IN A PARTICULAR AREA ARE

REPLACED OVER TIME BY A SERIES OF

DIFFERENT AND USUALLY MORE COMPLEX

COMMUNITIES

SUCCESSION CAUSES AN ECOSYSTEM TO

PASS THROUGH A NUMBER OF STAGES OF

DEVELOPMENT CALLED SERAL CHANGES.

THESE ARE - THE PIONEER STATE, THE

INTERMEDIATE STAGE AND THE CLIMAX STAGE

STAGES IN SUCCESSION

PIONEER STATE:

THIS IS THE ESTABLISHMENT OF A PIONEER SPECIES ON

OR IN A NEWLY FORMED HABITAT E.G. A LAVA FLOW.

THE SPECIES ARE USUALLY VERY HARDY WITH A SIMPLE

LIFE CYCLE AND A SMALL NUMBER OF SPECIES WHERE

ONE OR ANOTHER OF THE SPECIES DOMINATES THE

OTHERS. THEY ALSO HAVE A SMALL BIOMASS, A FAST

REPRODUCTION TEMPO AND A SHORT FOOD CHAIN

PIONEER SPECIES USUALLY OCCUR BECAUSE OF A LACK

OF COMPETITION

STAGES IN SUCCESSION

INTERMEDIATE STAGE:

THE INTERACTION BETWEEN PLANSTS IS

DOMINANT, MORE ORGANIC MATTER

ACCUMULATES IN THE SOIL AND MORE HUMUS

IS PRESENT. AS A RESULT THE SOIL IS MORE

FERTILE. PLANT SPECIES THEREFORE

BECOME LARGER AND MORE ADVANCED. THE

PLANT COMMUNITIES MAY CHANGE TO MORE

COMPLEX COMMUNITIES

STAGES IN SUCCESSION

CLIMAX STAGE:

THIS IS THE FINAL OR ADULT STAGE OF PLANT

DEVELOPMENT. THE MAXIMUM AMOUNT OF

BIOMASS OCCURS UNTIL A CONDITION OF

DYNAMIC EQUILIBRIUM RESULTS

ECOLOGICAL DOMINANCE

SOME ORGANISMS COMPETE MORE SUCCESSFULLY

THAN OTHERS IN A PARTICULAR HABITAT FOR

ESSENTAIL REQUIREMENTS SUCH AS WATER, LIGHT AND

NUTRIENTS.

ECOLOGICALLY DOMINANT ORGANISMS ARE ORGANISMS

WHICH HAVE A GREATER INFLUENCE THAN OTHERS ON

THEIR HABITAT AND ON OTHER ORGANISMS WITH WHICH

THEY LIVE.

THE DOMINANT PLANT SPECIES ARE THE LARGEST OR

MOST NUMEROUS FORMS, FIXING THE MOST ENERGY

AND MAKING UP THE LARGEST PROPORTION OF

BIOMASS IN A COMMUNITY.