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First teaching 2023

First exams 2025

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Community: Interspecific Relationships (HL IB Biology)

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Naomi H

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Naomi H

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Community in an Ecosystem

  • Species do not exist by themselves in their own isolated environment; they interact with other species, forming communities
  • A community can be defined as:

Multiple populations of different species living and interacting in the same area

    • For example, a garden pond community is made up of populations of fish, frogs, newts, pond snails, damselflies and dragonflies and their larvae, pondweed, water lilies, and all other populations living in the pond
  • Communities include populations from all groups of living organisms, e.g. plants, animals, fungi, and bacteria
  • Living communities interact with their abiotic environment to form an ecosystem

Levels of organisation within ecosystems diagram

Levels of Organisation in an Ecosystem

A community is all of the populations of living organisms interacting in an area. Communities interact with their non-living environment to form an ecosystem.

Interspecific Relationships within Communities

  • Interspecific relationships are the interactions between different species that occur within a community
    • 'Inter' = between
  • There are several types of interaction that can occur between different species:

Types of interactions between species table

Type of interaction Description Example
Herbivory An organism feeding on a plant

Cattle graze on grass

Sea turtles feed on sea grass

Honeybees consume nectar and pollen
Predation An organism catching and consuming an animal, or consuming a recently dead animal

Dolphins catch and eat fish

Lions hunt and eat zebra

Red kites eating roadkill
Interspecific competition Organisms of different species compete for the same resources

Oak and beech trees compete for light and minerals

Lions and hyenas compete for prey

Red and grey squirrels compete for food and territory
Mutualism Organisms of different species work together for the benefit of both

Pistol shrimp share their burrows with goby fish, which provide a warning when predators are near

Oxpecker birds remove parasites from large mammals, providing the birds with food
Parasitism A parasite organism lives in or on a host organism, causing its host harm

Mistletoe plants grow in the branches of trees, taking water and nutrients from their hosts

Fleas live on the bodies of mammals, feeding on their blood
Pathogenicity An infectious microorganism (pathogen) lives inside a host organism, causing disease

Mycobacterium tuberculosis bacteria cause the disease tuberculosis in human hosts

Dutch elm disease is caused by a fungal pathogen that causes elm trees to lose their leaves and die

Mutualism

  • Mutualism occurs between members of different species, and is an example of cooperation; both members of a mutualistic relationship benefit from the interaction
  • Examples of mutualism include
    • Bacteria living in the root nodules of plants
    • Mycorrhizal relationships between fungi and plants
    • Coral polyps and algae

Root nodules in Fabaceae

  • The Fabaceae, or legume, family of plants includes species of peas, beans, and clover
  • Many legumes have a symbiotic relationship with bacteria, which live in nodules attached to the plant roots
    • Nodules are small, sphere-like structures
  • The bacteria convert nitrogen gas in the air into ammonia (NH3) which can then be converted again into nitrates
    • The conversion of nitrogen gas into a form that is useful to plants is known as nitrogen fixation
    • Rhizobium is an example of a genus that lives in root nodules and fixes nitrogen
    • Nitrates can be used by plants to build essential biological molecules such as proteins and nucleic acids
  • The bacteria also benefit from this relationship, as they gain carbohydrates that are produced by the plant in photosynthesis

Mycorrhizae in Orchidaceae

  • Many plants have evolved symbiotic relationships with fungi
  • The fungi form long, thin filaments known as hyphae, which interact with the roots of the plants
  • These hyphae greatly increase the surface area of the root systems of the plants, increasing the amount of water and mineral ions that can be absorbed by the plant roots
  • In return the fungi receive organic compounds, e.g. glucose, from the plant
  • These relationships between plant roots and fungi are known as mycorrhizae (singular mycorrhiza)

Mycorrhizae diagram

Mycorrhizae interacting with a plant root

Mycorrhizae are interactions between fungal hyphae and the roots of plants

  • The orchidaceae, or orchid family, are known to form many mycorrhizal relationships, e.g.
    • Orchid seeds may gain the nutrients needed for germination from mycorrhizae
    • Some unusual orchids are unable to photosynthesis, relying on their mycorrhizal fungi to break down dead matter in the soil and provide them with all of their nutrients
      • The orchid in this relationship is a heterotroph and not an autotroph
      • The orchid does not benefit from this relationship until the orchid dies, at which point it can access the biological molecules in the orchid's tissues by decomposition

Zooxanthellae in hard corals

  • The coral reefs associated with the hard corals are produced by tiny animals known as coral polyps, which live in a symbiotic relationship with zooxanthellae algae
    • Polyps are in the phylum cnidaria, along with jellyfish and sea anemones
    • Their soft bodies have tentacles which contain stinging cells called nematocysts, and which also contain the zooxanthellae cells
    • The polyps secrete calcium carbonate which forms the hard structure of the coral skeleton
  • The polyp's body provides shelter and protection for the algae
  • The algae carry out photosynthesis and produce carbon compounds, such as carbohydrates, which can be used by the polyp

Coral polyp diagram

coral polyp

The tentacles of coral polyps contain algae cells with which they have a mutualistic relationship

Examiner Tip

Note that Latin names, e.g. Fabaceae, are not essential when answering exam questions; you will be credited for either Latin or common names of organisms.

Resource Competition: Endemic & Invasive Species

  • An invasive species is a non-native species that causes harm to the environment to which it has been introduced
    • Invasive species are sometimes referred to as alien species
  • An invasive species can occur naturally as a result of a species migrating or expanding their habitat, but most recorded incidents of invasive species have been caused by humans, e.g. humans have:
    • Knowingly collected and traded species between countries
      • E.g. bringing attractive plant species into gardens, or animals into zoos or as pets, which then escape into the wild
    • Unknowingly provided transport for invasive species to a new ecosystem, e.g. rats on board ships
    • Introduced alien species deliberately as biological control for pests
  • In a new ecosystem invasive species will have little or none of the natural population controls that existed in their previous ecosystem:
    • They will have no natural predators or competitors
  • As a result they are able to increase in number at a rapid rate
  • This can affect the processes within an ecosystem
    • Competition may occur between invasive species and native species that occupy a similar niche, with the native species getting displaced or pushed to extinction
    • Many invasive species can be successful predators, causing a massive decline in their prey species
    • Invasive species can introduce new diseases, to which the native species have no natural immunity

Red and grey squirrel population diagram

Interspecific competition (grey and red squirrels), IGCSE & GCSE Biology revision notes

The relationship between a grey squirrel population and a red squirrel population over time graph

Grey squirrels were introduced into the UK from North America in the 1800s. They have been highly successful and have largely outcompeted the native UK red squirrels in interspecific competition. Note that red squirrel populations in the UK are not at zero, and are currently recovering in some locations.

Invasive species & endemic species

  • Endemic species are found in a particular place and in no other location in the world, e.g. the Scottish crossbill is found in the conifer forests of Scotland and nowhere else, so is said to be endemic to Scotland
  • Endemic species are especially vulnerable to the effects of invasive species, as local extinction will mean that the species has gone entirely extinct
  • E.g. Australia is home to many endemic species that have evolved over many years of isolation from other continents
    • The introduction of invasive predators, such as the European red fox and the domestic cat, has caused huge declines in native Australian species
      • The red fox was introduced in the 1800s by European settlers for the purposes of fox hunting
      • Cats are likely to have travelled on ships and been introduced by accident
    • Evidence suggests that more than 10 % of Australia's endemic mammal species have already gone extinct since the arrival of European settlers
    • Small mammals are at the highest risk due to being the best food source for the invasive predators
    • Some species now only survive on the islands around mainland Australia where foxes and cats have not yet arrived

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Naomi H

Author: Naomi H

Expertise: Biology

Naomi graduated from the University of Oxford with a degree in Biological Sciences. She has 8 years of classroom experience teaching Key Stage 3 up to A-Level biology, and is currently a tutor and A-Level examiner. Naomi especially enjoys creating resources that enable students to build a solid understanding of subject content, while also connecting their knowledge with biology’s exciting, real-world applications.