B4.2 Ecological Niches

Define ecological niche

A unique role that a species plays in its ecosystem

What aspects of a species does an ecological niche include? [6]

• Where it lives 

• What it eats & how it gets food

• How it interacts with other species

• When it is active (day/night, seasonal patterns) 

• How it reproduces

• Its tolerance to abiotic factors

Define autotroph

An organism that uses external energy to synthesise carbon compounds from simple inorganic compounds

Define heterotroph

An organism that obtains energy and carbon by consuming other organisms or matter

Describe biotic interactions included in an ecological niche [4]

1) Food supply - What the organism eats & how it obtains it 

• Autotrophic

• Heterotrophic

2) Predators - Who eats the organism

3) Competitors - Species competing for the same resources

4) Symbiosis - Mutually beneficial relationships 

Describe differences & examples between organisms with different oxygen tolerances [3×3]

	Obligate Aerobes	Obligate Anaerobes	Facultative Anaerobes
Oxygen requirement	Must have for survival	Cannot survive in presence of oxygen → damages enzymes & DNA	Can live with or without oxygen
Type of respiration	Aerobic respiration to generate ATP	Anaerobic respiration or fermentation	Aerobic when oxygen available, switches to anaerobic/fermentation in low or no oxygen
Examples	Humans, most animals	Minority of bacteria  (e.g. Clostridium botulinum, some gut bacteria)	E. coli, some yeasts

How is photosynthesis used as a mode of nutrition?

It involves using light energy to fix carbon dioxide into organic compounds, e.g. amino acids & sugars

What are the groups of organisms that use photosynthesis? [5]

• Three groups of organisms use photosynthesis: 

  • Plants - Mosses, ferns, conifers, & flowering plants 

  • Eukaryotic algae - Seaweeds, unicellular algae

  • Photosynthetic bacteria - Cyanobacteria, purple bacteria 

• Hence, photosynthesis only occurs in two of three domains: eukaryotes & bacteria 

Define holozoic nutrition

The process by which animals obtain energy and nutrients by consuming other organisms or matter

Describe the stages of holozoic nutrition [5]

1. Ingestion - Food is taken in through the mouth

2. Digestion - Food is broken down mechanically (chewing) & chemically (by enzymes) into smaller, soluble molecules 

3. Absorption - Nutrients are absorbed into the bloodstream from the intestines

4. Assimilation - Absorbed nutrients are used by cells for growth, repair, & energy

5. Egestion - Undigested waste is eliminated from the body as faeces

Describe the exception to holozoic nutrition [3]

• Some animals digest externally & are not holozoic

• They absorb products of external digestion into their gut & assimilate them 

  • Example: Spiders inject digestive enzymes into prey & suck out liquids produced

Define mixotroph

Organisms that can obtain energy and carbon through both autotrophic and heterotrophic modes of nutrition

What are the two types of mixotrophs?

1) Facultative

2) Obligate

Describe the features of a facultative mixotroph [2]

• Can use entirely autotrophic/heterotrophic or both modes of nutrition

• Switches based on conditions (e.g. light availability)

Example of facultative mixotroph - Euglena gracilis

• Has chloroplasts & carries out photosynthesis when there is sufficient light 

• In the dark, it can feed on detritus or smaller organisms by endocytosis

Describe the features of an obligate mixotroph [3]

• Cannot grow unless they use both autotrophic & heterotrophic modes

  • This is because they eat food to obtain carbon compounds that they cannot synthesise

  • Some go through kleptoplasty, where mixotrophs that lack their own chloroplasts take chloroplasts from algae they consume 

Define saprotrophic nutrition

A type of heterotrophic nutrition where organisms (saprotrophs) feed on dead or decaying organic matter

How do organisms go through saprotrophic nutrition?

Saprotrophs digest food externally by secreting enzymes onto the material then absorb the nutrients

What are the two organism types that undergo saprotrophic nutrition and their roles?

Organism type	Role
Bacteria	Break down leaf litter, wood, dead organisms
Fungi	Decompose organic matter in soil & water