Module 8 Lesson 7 - 3

Defence Mechanisms

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Read pages 722 - 723

Plants and animals have adapted to defend themselves against their predators. Many plants use chemical defenses against consumers. Some plants produce chemical toxins that taste bad or can poison herbivores who eat them. The roots of some plants secrete toxins that inhibit growth of their neighbours, reducing interspecific or intraspecific competition.

1. Many animals use their body shapes to protect themselves. Such behavioural defenses are fascinating. The arched back of a cat is similar to the enlarged, threatening silhouette as a bear standing on its hind legs or a dog with its hair on end.


2. Markings and colouration usually provide distinct selective advantages.
   
   
   • Cryptic colouration allows prey organisms to evade predators, and it allows predators to sneak up on prey. Cryptic colouration includes colours that help organisms blend into the environment. For example, the caterpillars of peppered moths camouflage as broken twigs to hide from predators.
     
     
     
     

     Peppered moth caterpillars.
     Credit:  Noor MAF, Parnell RS, Grant BS. A Reversible Color Polyphenism in American Peppered Moth (Biston betularia cognataria) Caterpillars. PLoS ONE 3(9): e3142. doi:10.1371/journal.pone.0003142
     
     
   
   
   • Some species evolved to use bright protective colouration to warn the predators that they are poisonous or bitter tasting. For example, the strawberry poison dart frog uses bright red colour to signal clear warnings to potential predators. Many predators learn to avoid prey with bright colouration after eating the organism and experiencing the unpleasant taste.
     
     
     
     

     Strawberry poison dart frog. Credit: Marshal Hedin from San Diego.
     
     


3. Other species use the fact that some predators avoid organisms with certain colouration. These harmless species mimic the colouration to avoid being eaten. For example, harmless hoverflies mimic the yellow and black colouration pattern of wasps.
   
   
   
   

   Hoverfly. Credit: Thomas Bresson.

Self-Check

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1. We can argue that the relationship is positive for the predator and negative for the prey. Can you support the argument that predator-prey relationships are mutualistic relationships when considering populations rather than individual organisms?
   


2. Complete the following table.
   
   

   Example	Relationship	Population that increases?	Population that decreases?	Population that remains stable?
   Remora fish waiting near the mouth of the shark for scraps
   Tapeworm and human
   Plant species that produces root toxins that kill plants of the same species nearby
   Coyote and hawk
   Coyote and mouse
   Acacia tree and stinging ants of Latin America
   Viceroy and monarch butterflies

   
   


3. Read the following two statements. Although both indicate chemical defenses, what is the difference between them?
   
   
   1. Some species of plants produce toxins that prevent them from being eaten by herbivores.
   2. Some species of plants secrete chemicals into the ground that prevent other plants of its own species from growing near it.


4. Describe the following as (A) chemical defense, (B) cryptic colouration, (C) protective colouration, or (D) mimicry.
   
   
   1. When elephants begin to eat the leaves of the thorn tree, the tree releases a bitter tasting substance into the leaves that prevents elephants from consuming the whole tree.
   2. The beautiful red, blue, and yellow tree frogs of Costa Rica produce some of the most poisonous venoms of the natural world.
   3. The dusty, golden-brown coyote is difficult to see against the dry grasses of the prairie.
   4. A harmless sea snake has bright markings almost identical to those of the highly venomous coral snake.