Community: Interspecific Relationships (HL IB Biology)

• 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

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 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

• 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 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

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

• A species that has moved into an ecosystem where it was previously unknown is an invasive species
  • 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

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