Lecture 30: Mutualism

Reading:  None.

Reminder:  Examination III (on lectures 23-30).

Mutualisms (more examples)

Ants and aphids
        Aphids are plant fluid sucking insects that puncture the plant phloem. The phloem has an excess of carbohydrates compared to proteins, so carbohydrate rich fluid continually flows through the gut of a feeding aphid as waste (honey dew). Ants harvest the honey dew from aphids and provide protection to the aphids from predatory insects. Ants “herd” aphids to make a group more easily defended. The ants get food, and the aphids get protection.

Ants and swollen thorn acacia trees
        Swollen thorn acacias provide ants with large thorns which are used as nest sites, Beltian bodies, extra floral nectaries, and foliage all year. Acacia ants provide a 24-hour patrol that repels both vertebrate and invertebrate herbivores. The ants also remove plants from the soil around the base of the acacia tree. The acacia gets protection from herbivores and competitors. The ants get shelter and food.

Humans and gut bacteria
        The human stomach provides bacteria with shelter and carbohydrates. The normal gut fauna bacteria aid in our digestion of foods.

Ruminants, bacteria, and protozoa
        Ruminant animals, cows, goats, elephants, deer, antelope, are unable to digest high cellulose foods such as grass and tree leaves because eukaryotes do not produce cellulase. These animals are able to consume high cellulose foods because they have organisms living in their stomach that produce cellulase. Ciliates living in the stomach of these animals can digest cellulose because they contain bacterial cells that are the source of the cellulase. This is a case of a triple mutualism. Bacteria get a ready supply of cellulose and protection inside the ciliates. The ciliates get simple carbohydrates from their bacteria. Ruminants get simple carbohydrates from their ciliates and ciliates get shelter and a ready supply of cellulose from the ruminant.

        The relationship between cockroaches and termites, stomach flagellates, and bacteria is similar to the triple mutualism described for ruminants.

Pollination and seed dispersal

        Interactions between plants and animals that result in plant pollination or seed dispersal need not be mutualistic interactions, but such interactions are often highly specialized and co-evolved.

Hummingbirds, bees, flies, butterflies, moths, and flowering plants
        Many animals visit flowers to collect nectar and incidentally carry pollen from one flower to another. The animals get food and the plant get a pollination service.

Animals, seeds, and fruits
        Many birds and mammals consume fruits and incidentally disperse the seeds contained in those fruits. The animals get food and the plant gets seed dispersal (often with fecal fertilizer).

Origin of Mutualisms
        Many, maybe most, mutualistic interactions began as predator-prey interactions.

        The participants in mutualisms provide aid to another species for the purpose of aiding their own survival and reproduction

Section III Review Problems and Sample Examination Questions

1. What are the characteristics of apparent and unapparent plants? How might these life history types differ in their tactics for minimizing the risk of predation?

2. Animals with aposematic (warning) coloration put themselves at risk of attracting predators and death for the sake of teaching predators about their toxicity. How could such an adaptation evolve by natural selection?

3. On Figure I, what is represented by the line labelled A? Draw the vectors for change in the predator population to the right and left of line A.

4. In Figure II, what is indicated by the label C? What would happen if the predator and prey densities were both at the point where the lines cross at point B?

5. On Isle Royale in Lake Superior, moose are a common herbivore and these moose are hunted by gray wolves. Define the terms for Lotka-Volterra Models for this interaction and state the population growth equations for each species.

6. What is a mast crop and how could it function as an anti-consumer defense?

7. Indiscriminate pesticides (most chemical insecticides) kill both herbivorous insects and their natural insect predators. Use of indiscriminate pesticides to remove insect herbivores (such as the cotton boll weevil) often results in the the return of an herbivore species in even greater numbers than before pesticide spraying. Explain this paradox with reference to the Lotka-Volterra Models of predator-prey interactions.

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