Population Ecology

Biosphere

Earth's zone of air, soil, and water that is capable of supporting life

Biome

A large area with a similar climate and geography, characterised by its vegetation and animals

Ecosystem

A complex system of interactions between living organisms and their non-living environment

Community

A group of different species interacting and sharing an environment

Population

A group of organisms of the same species that occupy the same area at the same time and can breed freely with each other

Individual

A single organism that forms part of a species

Niche

The specific role/position an organism has in an ecosystem, or the specific conditions needed for it to survive

Levels of organisation (largest to smallest)

Biosphere → Biome → Ecosystem → Community → Population → Individual

4 factors affecting population size

Natality, Mortality, Immigration, Emigration

Natality

Birth rate

Mortality

Death rate

Immigration

Movement of individuals INTO a population

Emigration

Movement of individuals OUT of a population

When does a population increase?

When births + immigration exceed deaths + emigration

When does a population decrease?

When deaths + emigration exceed births + immigration

Carrying capacity

The maximum number of individuals a specific environment can sustain without negative impact to the organism or environment

Environmental resistance

The total number of factors that stop a population from reproducing at its maximum rate

Biotic potential

The maximum possible growth rate of a population under ideal conditions with unlimited resources

Stable population

A population that fluctuates around the carrying capacity — decreases when it exceeds K, increases when below K

Unstable population

Develops when a population far exceeds carrying capacity, causing rapid habitat deterioration and population crash

Limiting factors

Resources or conditions that lower the population growth rate and regulate how many organisms live in an ecosystem

Density dependent factors (definition)

Factors that restrict population size based on its density

Density dependent factors (6 examples)

Food supply, space availability, lack of shelter, disease/parasitism, predation, competition

Density independent factors (definition)

Factors that regulate population size but are NOT related to density; affect all populations similarly regardless of size

Density independent factors (examples)

Climate change, environmental disasters (drought, flood, fire, earthquake, volcano, tornado), pollutants, human activities (deforestation, damming)

Exponential growth curve shape

J-shaped

Exponential growth

Growth rate accelerates rapidly and is unrestricted

Logistic (sigmoid) growth curve shape

S-shaped

Logistic growth

Growth rate goes through noticeable stages as the population approaches carrying capacity

Census

The procedure of counting and recording information about every member of a given population

Direct methods (population size)

Methods that count every individual; only suitable for stationary/sessile, slow-moving, or very large organisms

Direct method techniques for counting animals

Helicopters flying over an area to count individuals; aerial photographs of an area

Indirect methods (population size)

Methods that count a representative sample and use statistics to estimate total population size

Two indirect sampling methods studied

Mark-recapture; quadrat sampling

Random sampling

Every member of the population has an equal and independent chance of being selected; the only factor is chance

Mark-recapture formula

P = M × C ÷ R

Mark-recapture: P =

Estimated population size

Mark-recapture: M =

Total number of animals captured and marked in the first sample

Mark-recapture: C =

Total number of animals caught in the second sample

Mark-recapture: R =

Number of marked (recaptured) animals in the second sample

Mark-recapture formula name

Lincoln-Peterson Index

Mark-recapture Step 1

Initial capture/sample done in a well-defined area

Mark-recapture Step 2

Individuals in the initial capture are marked (e.g. tag or paint spot)

Mark-recapture Step 3

Marked individuals released and given time to mix thoroughly with the population

Mark-recapture Step 4

A second capture/sample is done after some time in the same area

Mark-recapture Step 5

Individuals and marked individuals in the second capture are counted, then the formula P=MC/R is applied

Mark-recapture precautions

Mark must last the whole investigation; mark must not harm the organism or affect its behaviour/survival; capturing must not affect movement/behaviour; population must be closed (no migration); time between samples must allow mixing but limit births/deaths

Quadrat method used for

Sampling sessile or slow-moving organisms

Quadrat formula

N = (number in sample) × (area of whole habitat) ÷ (area of quadrats used)

Quadrat method steps

1. Work out area of whole habitat 2. Work out area of quadrat 3. Count individuals in quadrats 4. Apply the formula

Quadrat method precautions

Size/area/shape of quadrats must be known; quadrats must be randomly selected; exact number of organisms per quadrat must be counted

How to take random samples

Map the area into a numbered grid, then select squares by drawing numbers from a hat or using a random-number table

Predation

A feeding interaction where one organism (the predator) hunts, kills and eats another organism (the prey)

Roles predators play in ecosystems

Transfer energy to higher trophic levels; keep prey populations under control; maintain/increase biodiversity by preventing dominance of one species; keep prey population genetically fit by removing weak individuals

Predator-prey population graph relationship

The predator population lags slightly behind the prey population

Predator-prey cycle order

Prey increases → predators increase (more food) → prey decreases (more predation) → predators decrease (less food) → cycle repeats

SA predator-prey example: lion & zebra

Lions decrease zebra numbers; fewer zebra means less food for lions so lion numbers drop, letting zebra recover — an ongoing oscillation

SA predator-prey example: great white shark & seals/fish

Sharks multiply when prey is plentiful, then decimate the food supply and crash from starvation; prey then rebuilds and the cycle begins again

Food web

Linked food chains showing the feeding relationships and energy flow among organisms in an ecosystem

Trophic level

Each step/level in a food web

1st trophic level

Producer — makes its own food (e.g. plants)

2nd trophic level

Primary consumer — consumes producers (e.g. mice eating seeds)

3rd trophic level

Secondary consumer — consumes primary consumers (e.g. snakes eating mice)

4th trophic level

Tertiary consumer — consumes secondary consumers (e.g. hawks eating snakes)

% of energy transferred between trophic levels

About 10%

Where does the other 90% of energy go?

Used for metabolic processes or given off to the environment as heat

Why are there rarely more than 4 trophic levels?

Energy loss (~90%) at each level leaves too little energy to support additional levels

Ways social organisation helps a population

Avoid predator attack; hunt collectively for food; divide labour among individuals; find mates; protect resources; regulate population size

Herd

A social group formed by herbivores (e.g. zebra) mainly to avoid predator attacks

Confusion and distraction effect

When attacked, a herd scatters in all directions; zebra stripes create visual motion illusions that confuse the predator

Flock

A social group formed by bird species

Pack

A social hunting group, e.g. African wild dogs, typically 10-15 individuals

Wild dog hunting success rate

8 out of 10 attempts

Wild dog hunting speed

About 45 km/h

Dominant breeding pair (wild dogs)

The only pack members that normally mate; monogamous for life; identified by increased urine marking

Southern ground hornbill breeding group

A monogamous dominant pair (mating every 2½ years) plus 0-9 helpers, usually adult males or juveniles

Eusocial animals

Species with an extreme dominance hierarchy — some individuals reproduce a lot, others little/none; overlapping generations; cooperative care of young

Example of a eusocial animal

Termites

Termite colony characteristics

Cooperation in rearing young; sharing of resources; overlapping generations; division of labour into castes

Termite castes

King, Queen, Worker, Soldier, Alate (winged reproductive)

Worker termite role

Blind and wingless; gather food, build/repair the nest, feed and groom others, care for young; live 2-4 years

Soldier termite role

Have large mandibles for defence; attack enemies and block tunnels; cannot feed themselves, so workers feed them

Alate

A winged reproductive termite that leaves the colony to mate; sheds wings afterward to become a king or queen

Queen termite reproduction

Can lay over 1000 eggs/day; grows an extra pair of ovaries with each moult; body lengthens until she can't move freely

Intraspecific competition

Competition between individuals of the SAME species; the most intense type

Interspecific competition

Competition between individuals of DIFFERENT species with similar niches

Ecological niche

All conditions necessary for an organism to survive and reproduce: tolerance of the physical environment, obtaining energy/nutrients, avoiding predators, coping with competition

3 outcomes of niche overlap

Specialization; competitive exclusion; competitive co-existence

Specialization (niche overlap outcome)

Structural/behavioural adaptations that let different species co-exist, e.g. Galapagos finches with different beak types

Competitive exclusion

Competition results in one species surviving while the other disappears or is excluded from the habitat

Competitive co-existence

Species use resources differently (resource partitioning), creating separate niches to reduce competition

Resource partitioning

Species co-exist by using the same resources differently (e.g. at different times or in different parts of the habitat)

Plant example of resource partitioning

Forest stratification — different vertical layers (canopy, understorey, floor) have adaptations to different light intensities

Animal example of resource partitioning

Giraffes eat upper acacia leaves, kudu eat lower branches, zebra/wildebeest graze grasses — reducing competition in the savanna

Ecological succession

A predictable pattern of gradual change in community structure and species composition over time

Primary succession

Begins on sites with no previous community, e.g. bare rock, lava flow, sand dune

Secondary succession

Begins where a disturbance removes species but soil remains, e.g. fire, logging, overgrazing

Succession stages order

Pioneer species → Intermediate species → Climax species

Pioneer species examples

Lichens and mosses

Pioneer species role

Hardy colonizers of bare rock; build/stabilize/enrich soil as they decompose; alter light levels via shade