Zoology 369 Gilbert's Lecture 6.

Comments about Herbivory

Evidence of the selective impact of herbivores on plants is found in aspects of plant chemistry and morphology which serve defensive functions. I discussed several examples.

1. Passiflora, the host plants of Heliconius butterflies contain

a. toxic alkaloids (strychnine, caffeine, nicotine, cocaine, and morphine are all alkaloids which we extract from plants for pharmaceutical, recreational and agricultural uses. Nicotine sulfate is a potent pesticide) and
b. cyanogenic glycosides (These compounds are essentially sugars attached to a CN or cyanide group . As glycosides, these compounds are non-toxic and contained in an intracellular compartment. A specific glycosidase enzyme resides in a separate compartment. When a herbivore munches on green tissues of a passion vine, the enzyme is brought into contact with the glycoside and cyanide is released.

Many plants like Passiflora produce nitrogen containing toxins which can, at relatively low concentrations, effectively deter all but the most metabolically specialized herbivores . Toxins containing nitrogen represent a significant diversion of this valuble and limited resource from growth and reproduction budget of the plant to defense. In crop species, when we select for higher productivity with respect to a particular harvestible plant part, we are inadvertently selecting genotypes which allocate N to growth rather than defense. This is certainly one reason that we find it necessary to protect crop plants with pesticides.

I showed two morphological defenses which function against Passiflora-specialist Heliconius butterflies. Not only do these butterflies manage to deal with Passiflora toxins, some species actually sequester Passiflora toxins to use in their own defense against birds and lizards. It does not matter if a plant species has protected itself against 99.99% of all of its potential herbivores if one good specialist like Heliconius is still in the game. A single species of dedicated herbivore can defoliate a plant. Thatıs why many plants, including Passiflora, are under selection to develop other modes of defense which target specialized herbivores like Heliconius butterflies. The two modes of morphological defense were:

a. Hooked hairs or trichomes which impale caterpillars.
b. Fake Heliconius eggs which discourage female butterflies from oviposition. Note that females of some species avoid already-laid eggs. The new larvae will eat the competition!

2. I showed the host plant of an ithomiine butterfly. This relative of the Petunia and tomato possesses inducible defense against the caterpillar. As soon as a herbivore damages a leaf, increases synthesis of the leaf's chemical defenses begin. In the case shown, the first stage caterpillar cuts a circular trench in the leaf's mesophyll thereby reserving a small patch of edible tissue. When that is consumed the larva must move to a new leaf which has not previously been damaged. Large mature larvae can eat most of a leaf before it can mobilize increased defenses.

Why have an inducible system? Aside from being a drain on the plantıs energy and nitrogen budget, some defenses pose danger to the plant itself. For example freezing can disrupt cells and lead to CN release in the leaves of cyanogenic glycoside-containing plants.

3. Thorns defend plants against large vertebrate browsers. Optunia without spines is very edible to deer and cattle. In drought Texas ranchers burn the spines off with a flame thrower called a "pear burner." Mentioned also was Acacia ridigula , a Texas Acacia which exhibits an induction of much larger thorns after deer browse branch tips (subsequent branches have bigger thorns). This means that when the Acacia grows above the browse line and becomes a small tree it will no longer be wasting energy on big thorns. Mesquite does the same thing.

4. I repeated a description of the mutualistic defense syndrome illustrated by ant acacia. The new point made had to do with this plantıs lack of chemical defenses. Apparently constrained by a limited material and energy budget , all of this Acacia's "defense allocation" goes into keeping the Pseudomyrmex ant colony big, mean and nasty. By contrast, closely related Acacia species possess bitter, alkaloid-rich leaves.

5. Tannins and other phenolic compounds represent a different mode of chemical defense. These are large molecules based on carbon, and lacking nitrogen, they are less costly in terms of limited nutrients. These compounds reduce the digestibility of proteins in the gut of herbivores by cross-linking the protein molecules (this is why tannins from oak leaves can be used to "tan" leather). Phenolics may be most effective against large vertebrate herbivores (deer, monkeys etc.) and it has been noted that the concentration of these compounds goes up plants living and evolving in low nutrient (low nitrogen) soils presumably because

a. carbon-based defensive compounds are less costly to make and
b. the value of each leaf is relatively greater to the plant given the lack of nutrients to make new ones.

Nutrient-poor tropical white sand forests in Africa and South America are drained by streams and rivers (e.g., Brazil's Rio Negro) that look like strong tea. This is caused by the tannins leaching out of the decaying leaves which compose the litter of these forests. One appealing theory of plant chemical defense predicts such ecosystem-level phenomena.

Some kinds of herbivores

1. Most vertebrate herbivores are relatively general in diet. Even Bison, which subsist largely on grasses, not only use a wide variety of those but also feed on herbaceous dicots. In neotropical rainforests peccaries, howler monkeys and tapir feed on very broad arrays (hundreds of species) of woody plants. These animals rely on gut microbes for detoxification and digestion of the plants they eat. They tend to switch between plant species during feeding, a behavior that reduces the chances of eating too much of any one plant's defensive compounds.

2. Leaf-cutter ants (Atta) are really much more like large vertebrates than like other insects. Their fungus gardens inside the nest contain fungi which convert leaves to fungal products which the ants eat. Cows feed grass to their gut bacteria and digest the bacteria and their products. What's the big difference?

3. Many insects specialize on both plant species and parts of those species (such as new growth). Flea beetles and some caterpillars simply eat holes in young leaves and are minor parasites as long as their own biological controls are operating. Avoided old leavesLeaf mining moths and beetles feed between the cuticles of the leaf and create highly distinctive feeding damage. Aphids are sucking insects of new shoots and can damage new leaves and reproductive parts.

4. Some insects, regardless of size are effective predators of seeds. These include sucking bugs such as these coreids feeding on thistle seeds, and beetles such as weevils and bruchids which develop inside seeds. Seed pods of plants like mesquite often show exit holes of emerging beetles. Caterpillars such as this queen butterfly occasionally eat the seed pods of host plants and so are occasionally predaceous.

5. Some insects feed in groups, use the plant's own toxins against the next trophic level and create conspicuous warning displays as they defoliate hosts. Some large grasshoppers do this (including the African locust, Shistocerca, in its source populations), as do some butterfly and moth larvae. A good example is the butterfly Eumaeus whose gaudy larvae contain the potent toxins of host cycads.

6. Some vertebrates, e.g., sloth and koala, are rather specialized in terms of diet. Are they more like caterpillars in the general scheme of herbivory?

Evidence that herbivores affect the dynamics and density of plant populations comes from

1. the observations that

a. after accidental introductions of plants to new continents they, freed from herbivory, become weedy pests and

b. followed by intentional introductions of their natural enemies from home exotic plants are brought under control.

I gave the example of Opuntia in Australia.

2. after severe droughts reduce parasitoid controls of snout butterflies (Libytheana), these insects undergo tremendous outbreaks in which snout larvae repeatedly defoliate the host desert hackberry (Celtis pallida) leading to the death of some of these shrubs in wetter habitats. During big population explosions these insects migrate and snouts origination from south of San Antonio may fill the skies of Austin on rare occasions (read the paper on reserve on this system which we will see at Chaparral WMA on the field trip). Reserve items relative to this lecture include Science and Scientific American articles about Heliconius influening traits of Passiflora and a natural history of snout butterfly population outbreaks in South Texas.

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