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Symbiosis

Learning outcomesTo know the following interspecies relationships andexamples of each:1. parasitism

2. mutualism.

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Detailed learning outcomes

Symbiotic relationships are co-evolved and specific intimaterelationships between members of two different species.

1. Parasitism

A parasite benefits in terms of energy or nutrients, whereas ahost is harmed by their loss. Parasites often have a limitedmetabolism so cannot survive out of contact with a host.

2. MutualismBoth mutualistic partner species benefit from the relationship.One example is that of cellulose-digesting protozoa/bacterialiving in the guts of many herbivores.

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Symbiotic relationships

Symbiosis

MutualismParasitism

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Symbiosis

Symbiosis is the close relationship betweentwo different species, eg the relationshipbetween nitrogen-fixing soil bacteria ( Rhizobia )and leguminous plant roots (peas, beans andclover).

There are two types of symbiosis:

parasitism

mutualism.

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Parasitism

Harmed

A BBenefits

A parasite lives on another organism(the host) from which it obtains food.This: benefits the parasite harms (or kills) the host. Some parasites ( ectoparasites ) live on the outer skinof the host:

ticks fleasleech

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Parasitism

Some parasites ( endoparasites ) live inside thehost (usually in vertebrates):

tapeworm

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Living with parasites

Only poor parasites kill their host as this is notto their advantage, especially for obligateparasites.

There is a balance between parasitic damageand host defence, so the relationship isrelatively stable.Diseases often occur in cycles as parasite andhost co-evolve.

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Secondary host species or vectors

Vectors are organisms that transmit parasites,usually insects and ticks. These vectors areessential for the development of the infectious

stage of the lifecycle of some parasites beforethey are transmitted to the next host (egmosquitoes transmit the malarial parasite).Some of these vectors therefore have a doubleaspect of parasitism.

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Types of parasite

Parasites may be animals or plants.Viruses are parasites.

Unicellular organisms, such as somebacteria, are parasites.Multicellular organisms, such as worms

and insects, may be parasites.

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Leguminous plants andR h i z o b i u m bacteria

These bacteria live freely insoil.They are able to infect the

roots of leguminous plantsand form nodules.Inside the nodules bacteriaobtain energy from theirhost and supply nitrogen

compounds in return.Picture: www.apsnet.org

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Fungi and algae (lichens)

Lichens are able to coloniseareas where other plants areunable to grow.The fungal part of the lichenproduces tough cellulose wallsand secretes mineral-digestingchemicals while receiving

carbohydrate.The algal part of the lichenphotosynthesises and receivesprotection and essentialminerals.

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Herbivores and gut bacteria

Herbivores cannotproduce cellulose-

digesting enzymes.Bacteria which can dothis are found in thealimentary tract ofherbivores.These bacteria receivefood and shelter, andaid plant digestion.

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Ants and aphids

Some species of ants farm

aphids. They move them fromplant to plant and protect themfrom predators.In return the aphids secrete asugary liquid that the antsdrink.

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Summary

Parasitism Mutualism

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Species A

Species B

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Acknowledgments

Many thanks to R. Purdie from Sanquhar Academy for the use of some materials.