DP Biology HL B4.2 Ecological Niches

DP Biology HL B4.2 Ecological Niches - Full notes

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DP Biology HL | B4.2 Ecological Niches

DP BIOLOGY · THEME B: FORM AND FUNCTION

B4.2

Ecological Niches

Full notes for HL · IB-style

Contents

1	The ecological niche
2	Obligate and facultative anaerobes and aerobes
3	Modes of nutrition: autotrophs
4	Modes of nutrition: heterotrophs
5	Adaptations for feeding in animals
6	Adaptations of herbivores, predators and plants
7	Fundamental and realized niches and competitive exclusion (HL only)
8	The ecological niche
9	Fundamental and realized niches
10	Competitive exclusion and resource partitioning
11	Modes of nutrition: autotrophs
12	Modes of nutrition: heterotrophs
13	Mixotrophs
14	Adaptations of herbivores
15	Adaptations of predators and prey
16	Adaptations of teeth and diet
17	Plant adaptations for harvesting light
18	Fundamental niche and climate change
19	Mixed review: worked problems
20	Additional exam-style practice
21	Obligate anaerobes, facultative anaerobes and aerobes
22	Archaea and diverse metabolism
23	Practical: investigating competition
24	Symbiosis
25	Common misconceptions and links
26	Exam-style question with markscheme
27	Check your understanding
28	Summary and topic map

These are full teaching notes. Read each section in order, try the Think about it question before you read on, and do every Quick check before moving to the next section.

Before you start

Rate yourself on each statement now, and again after you've worked through the notes. Shade R (not yet), A (getting there) or G (confident).

I can…	R	A	G
define ecological niche
distinguish obligate and facultative anaerobes and obligate aerobes
distinguish photoautotrophs and chemoautotrophs
distinguish holozoic nutrition, saprotrophs and parasites; and mixotrophs
explain adaptations of hominin teeth to diet
explain adaptations of herbivores, predators and plants for feeding and defence
distinguish fundamental and realized niches
explain competitive exclusion and resource partitioning

Sections marked HL only (Section 7) are Additional Higher Level content. Every other section is common to SL and HL.

Guiding questions

How does each species obtain the energy and resources it needs? Why can two species not share exactly the same niche?

How this topic is examined

B4.2 is examined through definitions (niche, modes of nutrition), classification of organisms by nutrition, and adaptations for feeding, often using data about diets or tooth structure. HL questions frequently use competition experiments (such as Paramecium) to illustrate competitive exclusion.

Give named examples and link each adaptation to how it helps obtain food or avoid being eaten.

Command term	What the examiner wants
State / Identify	a brief answer with no explanation needed
Outline	a brief account or summary
Describe	a detailed account, including trends or features, without reasons
Explain	a detailed account including causes, reasons or mechanisms
Distinguish / Compare and contrast	give the differences (and similarities), point by point
Calculate	a numerical answer with working and units
Deduce / Suggest	reach a conclusion from data; propose a reasonable explanation
Evaluate / Discuss	weigh strengths and limitations and reach a judgement

How to use these notes

Each section teaches one part of the syllabus in full. You'll see these boxes throughout; each has one job:

Box	What it's for
Think about it	a question to get you thinking before a section; the answer comes in the section
Key point	the one idea you must not forget
Common mistake	an error examiners see every year, and how to avoid it
In the real world	where the biology shows up outside the classroom
Try this	something quick you can do yourself to test the idea
Worked example	full solutions with IB-style marks (M1 method, A1 answer, R1 reasoning)
Quick check	short questions to test yourself at the end of each section

1 The ecological niche

Not just where a species lives, but how it lives.

Think about it

Two bird species live in the same tree. Can they both survive there forever? What would need to be different about them?

A species' ecological niche is its role in the ecosystem: the particular combination of biotic and abiotic conditions it needs and the way it uses resources. It includes where the species lives (habitat), what it feeds on, what feeds on it, when it is active, its tolerance of abiotic conditions, and its interactions with other species.

The habitat is the 'address' of a species; the niche is its 'profession'. Each species has a unique niche. Two species with very similar niches in the same place compete strongly for resources (Section 7).

Quick check

1. Define ecological niche.

2. Distinguish habitat and niche.

Answers: 1. the role of a species, including its use of resources and interactions 2. habitat: where it lives; niche: how it lives and its role

2 Obligate and facultative anaerobes and aerobes

Oxygen requirements define part of a niche.

Obligate aerobes need oxygen for respiration and cannot survive without it: most animals and plants. Obligate anaerobes cannot use oxygen and are poisoned by it; they live in oxygen-free environments such as deep mud, the gut and waterlogged soil. Examples include Clostridium bacteria and methanogenic archaea. Facultative anaerobes use oxygen for aerobic respiration when it is available but can switch to anaerobic respiration when it is not; yeast and E. coli are examples.

Type	Oxygen	Example
Obligate aerobe	required	humans, most plants
Obligate anaerobe	toxic; cannot use it	Clostridium, methanogens
Facultative anaerobe	used if present; not required	yeast, E. coli

3 Modes of nutrition: autotrophs

Making organic compounds from inorganic carbon.

Autotrophs make their own organic carbon compounds from inorganic carbon (CO₂), using an external energy source.

Photoautotrophs use light energy in photosynthesis: plants, algae and cyanobacteria. They are the main producers in most ecosystems.

Chemoautotrophs use energy from oxidizing inorganic chemicals, such as hydrogen sulfide, ammonia or iron(II) ions. Examples are bacteria around deep-sea hydrothermal vents that oxidize hydrogen sulfide and support entire communities without sunlight, and nitrifying bacteria in soil that oxidize ammonia to nitrite and nitrate. Acidithiobacillus ferrooxidans oxidizes iron(II) to iron(III).

Quick check

1. Distinguish photoautotrophs and chemoautotrophs.

2. Give an example of a chemoautotroph.

Answers: 1. energy from light vs from oxidizing inorganic chemicals 2. hydrothermal vent bacteria; nitrifying bacteria; Acidithiobacillus

4 Modes of nutrition: heterotrophs

Obtaining organic compounds from other organisms.

Heterotrophs obtain organic carbon compounds from other organisms. They cannot make their own food from inorganic carbon. There are several modes of heterotrophic nutrition.

Holozoic nutrition: ingesting food, then digesting it internally, absorbing the products and egesting undigested material. Most animals, including humans, feed this way. Some unicellular organisms, such as amoeba and Paramecium, ingest food by phagocytosis.

Saprotrophic nutrition: secreting digestive enzymes onto dead organic matter and absorbing the products of external digestion. Many fungi and bacteria are saprotrophs (decomposers); they recycle nutrients in ecosystems.

Parasitic nutrition: obtaining nutrients from a living host, harming it. Examples: tapeworms, head lice, and the protist Plasmodium (malaria).

Mixotrophs can use both autotrophic and heterotrophic nutrition. Euglena photosynthesizes in light but feeds on organic matter in the dark. Some mixotrophs, such as certain carnivorous plants, photosynthesize but also obtain nitrogen from trapped insects. Detritivores (earthworms, woodlice) ingest dead organic matter and digest it internally.

Mode	Carbon source	Energy source	Example
Photoautotroph	CO₂	light	plants
Chemoautotroph	CO₂	inorganic chemicals	vent bacteria
Holozoic heterotroph	organic (ingested)	organic	humans
Saprotroph	dead organic (external digestion)	organic	fungi
Parasite	living host	organic	tapeworm
Mixotroph	both	light and organic	Euglena

Common mistake

Calling saprotrophs and detritivores the same. Saprotrophs digest externally and absorb; detritivores ingest dead matter and digest internally.

Quick check

1. Define holozoic nutrition.

2. How do saprotrophs feed?

3. Why is Euglena a mixotroph?

Answers: 1. ingestion, internal digestion, absorption, egestion 2. secrete enzymes onto dead matter and absorb products 3. photosynthesizes in light, feeds heterotrophically in dark

5 Adaptations for feeding in animals

Teeth reveal diet: the hominin example.

The structure of teeth and jaws is adapted to diet, so fossil teeth reveal what extinct species ate. Comparing hominins (members of the human lineage) shows changes in diet over evolution.

Paranthropus robustus (about 2 million years ago) had very large, thick-enamelled molars and premolars, small incisors and canines, powerful jaw muscles attached to a bony ridge on the skull (sagittal crest). These suggest a diet of tough, fibrous plant material: roots, nuts and seeds that required heavy grinding.

Homo floresiensis (about 60 000 to 100 000 years ago) had smaller molars, thinner enamel and smaller jaw muscles, suggesting a more varied diet including meat and perhaps cooked food, which is softer and needs less chewing.

Modern humans have small teeth and jaws compared with earlier hominins, consistent with a diet of softer, often cooked and processed food.

Feature	Paranthropus robustus	Homo floresiensis
Molars	very large, thick enamel	smaller, thinner enamel
Jaw muscles	very large; sagittal crest	smaller
Suggested diet	tough plant material	omnivorous, including meat

6 Adaptations of herbivores, predators and plants

An evolutionary arms race between eaters and eaten.

Herbivores are adapted to eat plants, which can be tough and have defences. Leaf-eating insects such as caterpillars have mandibles for biting and chewing; aphids have piercing mouthparts (stylets) to feed on phloem sap. Mammalian herbivores have broad, flat molars for grinding, and many have specialized guts with microorganisms that digest cellulose.

Plants defend themselves against herbivores with physical defences (thorns, spines, hairs, tough leaves) and chemical defences: toxic secondary metabolites such as alkaloids, tannins and cyanogenic compounds that deter or poison herbivores. Some herbivores have in turn evolved to tolerate these toxins: monarch butterfly caterpillars feed on toxic milkweed and store the toxins, making themselves poisonous to birds.

Predators have adaptations for catching prey: speed (cheetahs), sharp claws and teeth, stealth and camouflage, venom (snakes), and pack hunting (wolves). Prey have adaptations to avoid predators: camouflage, warning colours (aposematism), mimicry, speed, protective spines (porcupines), and chemical defences (skunks).

These adaptations of predators and prey, and of herbivores and plants, evolve in response to each other: an evolutionary 'arms race'.

Quick check

1. Give two physical and one chemical plant defence.

2. How are aphids adapted to feed on plants?

3. Give two adaptations of predators.

Answers: 1. thorns, hairs; alkaloids/tannins 2. piercing stylets reach phloem 3. speed, claws, venom, camouflage, pack hunting

7 Fundamental and realized niches and competitive exclusion (HL only)

Competition shrinks the niche a species actually occupies.

HL only

This section is Additional Higher Level content. SL students do not need it.

The fundamental niche is the full range of conditions and resources a species could potentially use in the absence of competitors. The realized niche is the part of the fundamental niche that the species actually occupies, because of competition (and predation) from other species. The realized niche is usually smaller.

The competitive exclusion principle states that two species with identical niches cannot coexist indefinitely in the same place: one will outcompete the other, which will be excluded (become locally extinct), or they will diverge in their resource use.

Gause's experiments (1934) with two species of Paramecium showed this. Grown separately, both species thrived. Grown together in the same culture, P. aurelia outcompeted P. caudatum, which died out.

Species with similar niches can coexist through resource partitioning: each specializes on a different part of the resource. Robert MacArthur showed that five species of warbler feed in the same spruce trees, but each forages in a different part of the tree (top, outer branches, inner branches), reducing competition.

Barnacles on Scottish shores (Connell's study): Chthamalus could live in both upper and lower zones (fundamental niche), but was excluded from the lower zone by competition with Balanus, so its realized niche was only the upper zone.

Figure 1. Gause's experiment: when grown together, P. aurelia excludes P. caudatum.

Quick check

1. Distinguish fundamental and realized niche.

2. State the competitive exclusion principle.

3. What is resource partitioning?

Answers: 1. fundamental: potential; realized: actual, after competition 2. two species with identical niches cannot coexist indefinitely 3. species divide a resource to reduce competition

8 The ecological niche

Role and requirements.

A species' niche is its role in an ecosystem: its habitat, its interactions with other species (what it eats, what eats it, competitors), and its tolerance of abiotic factors. No two species can occupy exactly the same niche indefinitely (competitive exclusion).

WORKED EXAMPLE 1 Niche (standard)

Describe the niche of a honeybee.

Solution

Feeds on nectar and pollen; pollinates flowersR1

Lives in colonies; prey for some birds; active in warm conditionsR1

9 Fundamental and realized niches

Potential versus actual.

The fundamental niche is the full range of conditions and resources a species could use; the realized niche is the part it actually occupies because of competition and other interactions. Connell's barnacles: Chthamalus could live lower on the shore but is excluded by Balanus.

WORKED EXAMPLE 2 Barnacles (exam level)

Explain why Chthamalus is found only on the upper shore.

Solution

Its fundamental niche includes the lower shoreR1

Balanus outcompetes it lower down, restricting its realized nicheR1

10 Competitive exclusion and resource partitioning

Avoiding competition.

When two species compete for the same limited resource, one usually outcompetes the other (Gause's experiments with Paramecium). Species can coexist by resource partitioning: MacArthur's warblers feed in different parts of the same trees.

WORKED EXAMPLE 3 Paramecium (standard)

Describe Gause's results when P. aurelia and P. caudatum were grown together.

Solution

P. aurelia survived; P. caudatum died outR1

Supports competitive exclusionR1

11 Modes of nutrition: autotrophs

Making their own food.

Autotrophs make organic compounds from inorganic carbon (CO2). Photoautotrophs use light (plants, algae, cyanobacteria); chemoautotrophs use energy from oxidizing inorganic chemicals (such as iron-oxidizing bacteria or bacteria at hydrothermal vents).

WORKED EXAMPLE 4 Chemoautotrophs (standard)

Explain how chemoautotrophs obtain energy.

Solution

Oxidize inorganic substances (e.g. H2S, Fe2+, NH3)R1

Use energy to fix CO2 into organic compoundsR1

12 Modes of nutrition: heterotrophs

Using organic compounds made by others.

Heterotrophs obtain organic compounds from other organisms: holozoic nutrition (ingestion and internal digestion, most animals), saprotrophic nutrition (external digestion of dead matter, fungi and many bacteria), and parasitism.

Type	Method	Example
holozoic	ingest, digest internally	humans, cats
saprotroph	secrete enzymes onto dead matter, absorb products	fungi, decomposer bacteria
parasite	feed on a living host	tapeworm, mistletoe
mixotroph	both autotrophic and heterotrophic	Euglena

13 Mixotrophs

Flexible nutrition.

Mixotrophs can switch between autotrophic and heterotrophic nutrition. Euglena photosynthesizes in light but can ingest food in darkness. Some carnivorous plants photosynthesize and also digest insects for nitrogen.

Quick check

1. Why do carnivorous plants digest insects?

Answers: 1. to obtain nitrogen in nutrient-poor soils

14 Adaptations of herbivores

Eating plants.

Herbivores have adaptations for plant diets: grinding teeth, long guts and symbiotic microbes for digesting cellulose (ruminants), and tolerance of plant toxins. Plants defend themselves with spines, tough leaves and toxic chemicals.

WORKED EXAMPLE 5 Ruminants (standard)

Explain why cows can digest cellulose.

Solution

Microorganisms in the rumen produce cellulaseR1

Animals themselves do not produce cellulaseR1

15 Adaptations of predators and prey

An evolutionary arms race.

Predators have adaptations for finding and catching prey (keen senses, speed, camouflage, venom). Prey have adaptations for avoiding predation (camouflage, warning colours, mimicry, toxins, speed, vigilance in groups).

WORKED EXAMPLE 6 Warning colours (standard)

Explain why poison dart frogs are brightly coloured.

Solution

Bright colours warn predators that they are toxic (aposematism)R1

Predators learn to avoid themR1

16 Adaptations of teeth and diet

Hominin dentition.

Teeth reflect diet: herbivores have broad molars for grinding; carnivores have sharp canines and carnassials for tearing. Hominin fossils show changes in jaw and tooth size that reflect diet changes, including cooking and meat-eating.

Quick check

1. What can fossil teeth reveal about the diet of extinct species?

Answers: 1. shape and wear patterns indicate plant or meat diet

17 Plant adaptations for harvesting light

Competing for sunlight.

In forests, plants adapt to light levels: tall trees, climbing plants (lianas), epiphytes growing on branches, and shade-tolerant understory plants with large, thin leaves and more chlorophyll.

WORKED EXAMPLE 7 Shade leaves (standard)

Explain why shade leaves are larger and thinner than sun leaves.

Solution

Large area captures more of the limited lightR1

Thin: less tissue to support with limited photosynthesisR1

18 Fundamental niche and climate change

Shifting ranges.

As climate changes, the area where conditions fall within a species' fundamental niche shifts. Species may move, adapt or decline. Ecological niche models use climate data to predict future ranges.

Quick check

1. How can niche models help conservation?

Answers: 1. predict where species can survive in future to plan protected areas

19 Mixed review: worked problems

Bringing the ideas together.

WORKED EXAMPLE 8 Mixed problem (exam level)

Distinguish between the modes of nutrition of a fungus, a plant and a lion.

Solution

Fungus: saprotrophic (external digestion)R1

Plant: photoautotrophicR1

Lion: holozoic heterotrophR1

20 Additional exam-style practice

Paper 1 and Paper 2.

WORKED EXAMPLE 9 Paper 1 question (standard)

Which organism is a saprotroph? A. mushroom B. fern C. deer D. tapeworm

Solution

AA1

WORKED EXAMPLE 10 Paper 2 question (exam level)

Explain the difference between fundamental and realized niches using an example.

Solution

Fundamental: full potential rangeR1

Realized: actual range limited by competition (e.g. barnacles)R1

21 Obligate anaerobes, facultative anaerobes and aerobes

Oxygen requirements.

Obligate aerobes need oxygen; obligate anaerobes are killed by it (Clostridium); facultative anaerobes can respire aerobically or anaerobically (yeast, E. coli). Oxygen availability is an important niche factor in soils, sediments and guts.

WORKED EXAMPLE 11 Classifying (standard)

Classify yeast by its oxygen requirement.

Solution

Facultative anaerobeA1

22 Archaea and diverse metabolism

Life in extreme niches.

Archaea occupy extreme niches: methanogens produce methane in anaerobic environments (wetlands, guts), halophiles live in salt lakes, thermophiles in hot springs. Their diverse metabolism expands the range of niches occupied by life.

Quick check

1. Where are methanogens found?

Answers: 1. anaerobic environments: wetlands, rice paddies, ruminant guts

23 Practical: investigating competition

Plants and microbes.

Competition can be investigated by growing seedlings at different densities and measuring biomass, or by culturing two microorganisms together and separately.

WORKED EXAMPLE 12 Control variables (standard)

Name two variables to control in a seed-density experiment.

Solution

Soil type and volume; light; water; temperatureA2

Try this

Sow cress seeds at different densities in identical pots. After two weeks, compare mean seedling height and dry mass.

24 Symbiosis

Close relationships.

Symbiosis is a close, long-term relationship between species: mutualism (both benefit: lichens, mycorrhizae, zooxanthellae in corals), commensalism (one benefits, the other unaffected) and parasitism (one benefits at the other's expense).

Quick check

1. Give an example of mutualism.

Answers: 1. mycorrhizal fungi and plant roots (or lichens)

25 Common misconceptions and links

Students often think…	Actually…
habitat and niche mean the same	the niche includes the role and resource use
all heterotrophs ingest food	saprotrophs digest externally
anaerobes cannot survive with oxygen	facultative anaerobes can
the realized niche is larger	it is smaller than the fundamental niche

How B4.2 connects to the rest of the course

Topic	Connection
B4.1 Adaptation	abiotic tolerance
C1.2 Respiration	aerobic and anaerobic
C1.3 Photosynthesis	photoautotrophs
C4.1 Populations	competition, predation
C4.2 Energy	trophic levels

26 Exam-style question with markscheme

Try this under exam conditions (about 15 minutes) before reading the markscheme.

Figure 2. Two Paramecium species grown together.

(a) Define ecological niche. [2]

(b) Distinguish photoautotrophs and chemoautotrophs. [2]

(d) Outline how the teeth of Paranthropus robustus are adapted to its diet. [2]

(e) Using the graph, explain the outcome of growing the two species together. [3]

Markscheme

Part	Answer	Marks
(a)	role of a species in an ecosystem	A1
	includes resource use, interactions and conditions	A1
(b)	photo: energy from light	A1
	chemo: energy from oxidizing inorganic chemicals	A1
(c)	secrete enzymes onto dead matter	R1
	absorb products of external digestion	R1
(d)	large molars with thick enamel	R1
	grinding tough plant material	R1
(e)	P. caudatum declines to zero	A1
	competition for the same resources (identical niches)	R1
	competitive exclusion	R1

Examiner's comments. In (a), 'where it lives' alone scored no marks: that is the habitat.

27 Check your understanding

Section A: multiple choice

1. Yeast is an example of

A. a facultative anaerobe B. an obligate aerobe C. an obligate anaerobe D. a chemoautotroph

2. Chemoautotrophs obtain energy from

A. light B. dead organic matter C. oxidizing inorganic chemicals D. living hosts

3. Saprotrophs

A. ingest food B. digest food externally C. photosynthesize D. parasitize hosts

4. Euglena is a

A. chemoautotroph B. saprotroph C. parasite D. mixotroph

5. Holozoic nutrition involves

A. photosynthesis B. ingestion and internal digestion C. external digestion D. absorption only

6. Alkaloids in plants are

A. chemical defences B. physical defences C. hormones D. enzymes

7. The realized niche is

A. larger than the fundamental niche B. the habitat C. the food web D. the part of the fundamental niche actually occupied

Section B: structured

8. Distinguish four modes of heterotrophic nutrition. [4]

9. Explain two adaptations of plants against herbivores. [4]

10. Explain why oxygen requirements are part of a species' niche. [2]

11. Explain competitive exclusion with an example. [4]

Answers

1. A 2. C 3. B 4. D 5. B 6. A 7. D

8. Holozoic, saprotrophic, parasitic, detritivores, with definitions.

9. Thorns/hairs deter feeding; secondary metabolites are toxic.

10. Determines which habitats a species can occupy, e.g. obligate anaerobes in anoxic mud.

11. Identical niches cannot coexist; Gause's Paramecium; one species excluded.

28 Summary and topic map

Idea	Remember
Niche	role of a species; resource use and interactions
Oxygen	obligate aerobes, obligate anaerobes, facultative anaerobes
Autotrophs	photoautotrophs, chemoautotrophs
Heterotrophs	holozoic, saprotrophs, parasites, mixotrophs
Hominin teeth	Paranthropus vs Homo floresiensis
Feeding adaptations	herbivores, plant defences, predators, prey
HL	fundamental vs realized; competitive exclusion; resource partitioning

Before the exam, make sure you…

•define niche beyond habitat
•classify organisms by nutrition with examples
•link teeth to diet
•explain arms races between species

Ecological Niches on one page

Cover this page with a sheet of paper and try to rebuild it from memory. Whatever you can't recall is what to revise next.

Figure 3. Topic map for Ecological Niches.

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