C4.1 Population & Communities

Define population

Interacting groups of organisms of the same species living in one area

Compare the interactions within & between populations [3]

• Members of a population share a common gene pool & typically breed w/ each other 

• Different populations are often separated by geographical or other barriers 

  • No gene flow = reproductive isolation

Explain why sampling is used to measure population size [4]

• Counting every individual is an impractical way of measuring population size due to the following reasons:

  • Populations may be very large & vastly spread

  • Animals are mobile & many are camouflaged

• As a result, a representative sample is used as an estimate

Describe and evaluate the use of random sampling [3]

• Sampling procedures require randomness, meaning subjects have an equal chance of selection 

  • This is to reduce bias, both conscious & unconscious

  • However, this method inevitably results in sampling error where the estimated population size differs from the true size

What is a quadrat and its use?

• A quadrat is a square frame used to mark a small area in a habitat 

• It is used to count how many sessile (non-moving) organisms are inside

• It is placed randomly to avoid bias

Describe the method of random quadrat sampling [5]

Step	Method
1. Define the area	Measure the total site to be estimated
2. Select positions	Generate random x- and y-coordinates & place quadrats at those positions
3. Count consistently	Identify the species correctly & use a fixed rule for individuals touching the quadrat boundary
4. Repeat	Use many quadrats to represent spatial variation
5. Estimate population size

What is standard deviation & its use in population measurement? [4]

• Standard deviation is a statistic that measures variability of data 

• In context of population, it can measure how evenly distributed organisms are:

  • Low standard deviation: Similar number in each quadrat = population evenly spread

  • High standard deviation: More clumped in some quadrats = population unevenly spread

What is the use of Capture-Mark-Release-Recapture?

Estimate the population size of motile (moving) animals

Describe the method of Capture-Mark-Release-Recapture [4]

1. Capture a sample of individuals (M) & mark them

2. Release these individuals & allow them to mix 

3. After some time, capture another sample (N)

4. Count how many marked individuals are recaptured (R)

What are the assumptions of Capture-Mark-Release-Recapture? [6]

1) No immigration or emigration during study

2) No births or deaths during study

3) Marked individuals mix evenly back into the population

4) Marked & unmarked individuals have equal chance of capture 

5) Marks stay identifiable  

6) Marks do not affect behaviour or survival

What is the Lincoln Index, and its use?

• The Lincoln Index uses values collected from CMRR to estimate population size

Define carrying capacity

The maximum number of individuals an environment can support

Explain the principle of carrying capacity [2]

• As resources are limited, once a population exceeds its carrying capacity, competition will begin 

• Examples: Food/mineral nutrients, water, space, & light 

What are the two types of factors that impact population growth?

1) Density-dependent 

2) Density-independent

Describe the role of density-dependent factors and give examples [5]

• Have stronger effects on greater population density

  • Examples: competition for resources, increased predation, diseases, pests

• They create a negative feedback effect:

  • When population grows too big → effects strengthen to reduce it back towards carrying capacity (death rate ⬆, birth rate ⬇) 

  • When population drops too low → effects lessen to allow growth again

Describe the role of density-independent factors and give examples

• Impacts populations regardless of density (fluctuations only)

  • Examples: Natural disasters, weather changes, fires

What is a sigmoid curve?

A sigmoid (S-shape) curve is a model that illustrates an ideal population growth with three phases 

Describe the phases of sigmoid curve of population growth

Phase	Main trend	Features
1. Exponential phase	Rapid population growth	Resources are abundant & limiting factors are few Birth rate greatly exceeds death rate
2. Transitional phase	Growth slows	Resources become limited, so competition, predation, disease increase Death rate rises toward birth rate
3. Plateau phase	Population levels off at carrying capacity	Birth rate & death rate equalise  Resources cannot support any further growth

[CS] Population growth - St Matthew Island reindeer [3]

• A herd of reindeer were introduced to the island, with population increasing rapidly

• Then, demand exceeded food supply (e.g. lichens) & harsh winter conditions led to severe drop in population 

  • It shows why real growth may not follow a stable sigmoid curve

Define intraspecific relationship

A relationship that exists between individuals of the same species, and typically within the same population

Explain intraspecific competition [3]

• Some members of a population have the same ecological niche = require same resources

• Since resources are limited, individuals will compete for them

  • Successful individuals will survive & reproduce → natural selection that pass down traits advantageous for competition

Explain intraspecific cooperation [2]

• Members of the same species also work together for mutual benefit 

  • This helps improve survival through boosting the efficiency of hunting, defence, or care for offspring

Define community

All populations of different species living and interacting together in the same area

Define ecosystem

A community of organisms and its abiotic environment, along with interactions between them

Define interspecific relationship

A relationship that exists between individuals of different species

Describe the categories of interspecific relationship & give examples for each [6×3]

Category	Description	Example
Herbivory	When primary consumers feed on producers	Caterpillar feeding on leaves
Predation	When one consumer species (predator) kills & eats another consumer species (prey)	Anteaters feeding on ants
Interspecific competition	Two or more species using the same resource, where the amount taken by one species reduces the availability for another	Ivy competing with oaks for water & nutrients
Mutualism	Two species live in close association, with both species benefiting from it	Coral & zooxanthellae exchange resources
Pathogenicity	One species (pathogen) lives inside another species (host) & causes a disease in it	Anthrax bacteria in kudu (African antelope)
Parasitism	One species (parasite) lives in/on another species (host) & obstains food from them → parasite benefits but host is harmed	Black-legged ticks on white-tailed deer

[CS] Describe the mutualistic relationship between Fabaceae & Rhizobium bacteria [3]

• Plants in the pea & bean family, e.g. Fabaceae, form a mutualistic relationship w/ Rhizobium bacteria 

Benefit to Bacteria	Benefit to Fabaceae
Fabaceae grows a root nodule which provides sugars & a low oxygen environment for bacteria to live	Bacteria can convert nitrogen gas into ammonium ions (NH4) → some are supplied to the plant which promotes growth in low-nitrogen soils

[CS] Describe the mutualistic relationship between Orchid & Mycorrhizal fungi [3]

• The hyphae of mycorrhizal fungi, e.g. Russula, grow & penetrate orchid’s root cell wall 

Benefit to Orchid	Benefit to Fungus
The hyphae serves as extension to the root system, passing water & mineral nutrients to orchid	Orchid supplies fungus w/ sugars & other carbon compounds made by photosynthesis

[CS] Describe the mutualistic relationship between Zooxanthellae & Coral [3]

• The cells of most reef-building hard corals contain zooxanthellae 

Benefit to Zooxanthellae	Benefit to Coral
Coral provides shelter & nutrients (e.g. CO2 from aerobic respiration)	Zooxanthellae perform photosynthesis which supplies sugar to the coral

Distinguish an endemic & alien species

Endemic: A species that occurs naturally in an area

Alien: while species that are introduced by humans

How does a species become invasive?

When an alien species increases in number, spreads rapidly, & causes ecological harm, they become an invasive species

Explain the competitive advantage of invasive species & the consequence [5]

• Invasive species often have a competitive advantage over endemic species in acquiring resources due to:

  • Lack of natural enemies (e.g. predators)

  • Rapid reproduction & spread

  • Genetic traits (e.g. better resource use/tolerance to tough conditions) 

Consequence: As a result, endemic species may occupy a smaller realised niche, decline in population, or eventually become extinct  

Invasive species example - Red lionfish [3]

• Invaded Caribbean & Atlantic waters 

• Due to lack of predators, they multiplied & spread on coral reefs 

• They outcompeted endemic fish like Groupers for food & space, establishing territories exclusive to their species

Describe the different tests for interspecific competition

1) Observation - Use random sampling to compare abundance/distribution of both species 

2) Laboratory experiments - Growing species together & apart while controlling abiotic conditions

3) Field manipulation experiments - Remove or reduce one species from selected plots & study response of the other species

• Competition is evidenced if the other species increases in number or biomass

Describe the steps to a chi-squared test for association between two species [7]

Describe how predator-prey interactions allow density-dependent population control

• The size of one population influences the size of another

  • High prey density supports larger predator populations, while low prey density reduces predator numbers 

• The numbers of predators & prey often show a regular cyclical pattern over time

[CS] Describe the density-dependent control of Red fox and mountain hare in Sweden [4]

Change	Consequence
1. Hare population grows	Due to prey availability, fox population increases
2. Fox population grows	Due to intensified predation pressure, hare population decreases
3. Hare population falls	Due to lower food supply, fox population decreases
4. Fox population falls	The system stabilises until hare numbers recover, restarting the cycle

Describe the two types of population control in communities [3]

1) Top-down control - Higher trophic levels (predators) regulate the population sizes of lower trophic levels (prey)

2) Bottom-up control - Lower trophic levels (producers) regulate the population sizes of higher trophic levels (consumers)  

• Both controls can operate in the same community, but one may have a dominant effect

What is allelopathy & its role? [6]

Definition: A process where plants secrete chemicals (allelochemicals) into the soil or air to that negatively affect the growth of other plants

• This process will inhibit: 

  • Seed germination

  • Root growth

  • Nutrient absorption

• This allows allelopathic plants to secure more resources for survival (e.g. sunlight, water, nutrients), giving them a competitive advantage 

[Example] Describe the allelopathic nature of the Tree of Heaven [2]

• It releases the secondary metabolite alianthone into the soil, inhibiting germination, growth, & survival of other tree species for up to 5m from its trunk 

• This reduces competition for nutrients & space 

What is the secretion of antibiotics, & its role?

Definition: A process where microorganisms produce & secrete chemical substances (antibiotics) that kill or inhibit the growth of other microorganisms

• Antibiotics target critical cellular processes in bacteria, such as:

  • Inhibiting cell wall synthesis

  • Blocking protein production

  • Disrupting DNA replication 

• This gives microbes a competitive advantage 

[Example] Describe how Penicillium uses antibiotics to reduce competition [3]

• Penicillium is a genus of fungi secrete penicillin, an antibiotic that is toxic to bacteria

  • It interferes with the cross-linking of peptidoglycan molecules in the cell walls of Gram-positive bacteria → weakening walls so bacteria burst & die

• This reduces competition for habitat & food