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HERBIVORY
I. Introduction.
A. Herbivory is plant eating and is widespread among organisms.
B. About ½ the insect species in temperate regions are herbivorous.
C. Many mammals and a few birds are also herbivorous.
II. Types of herbivores.
A. Grazers, such as bison and grasshoppers, eat herbaceous plants ("graze" and "grass" have
the same Anglo-Saxon root).
B. Browsers, such as deer and rabbits, eat leaves and twigs of woody plants.
C. The distinction between grazing and browsing is handy, but it just scratches the surface
of the way animals make use of plants as food. Some other modes of feeding include:
1. Fruit, seed, nectar, and pollen eating.
2. Leaf mining.
a. Larvae of several species of small beetles, flies, sawflies, and especially moths eat
away the tissue between the upper and lower epidermis of leaves.
b. More than 50 species of leaf miners attack oak leaves.
3. Boring.
a. A large number of beetles, moths, and flies obtain their nourishment by boring
into plant tissues, including roots, stems, trunks, buds, seeds, and fruits.
b. The entire organism bores into the plant tissue in some species or a specialized
organ enters the tissue in others.
4. Root eating.
a. Some invertebrates such as nematodes and cicada larvae concentrate on the
below-ground parts of plants.
b. Such burrowing mammals as pocket gophers and the naked mole rat of Africa
also eat roots, bulbs, and tubers.
5. Sap sucking.
a. Many insects in the orders Hemiptera and Homoptera are adapted to living on
plant juices.
b. Well-known examples include aphids, spittlebugs, and leafhoopers.
c. Only a few vertebrates have adopted sap as a diet.
1) The yellow-bellied and other sapsuckers lap sap from holes they make in tree
trunks, also eating the insects attracted to the sap.
2) At least 1 mammal, the marmoset, sucks sap. They use specialized incisors
that point forward to gouge holes in tree bark and then lick up the gummy
sap.
6. Gall forming.
a. Certain insects (mostly flies and wasps) develop within structures called galls.
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b. Galls are produced by exaggerated growth of the plant, resulting from the
activities of the animal.
c. The developing animal lives within the gall and feeds on the abundant tissue
being produced.
III. Plant defenses against herbivory.
A. Introduction.
1. At first glance, it may seem that for most herbivores, plants are free for the picking.
2. There are some obvious protective devices such as the thorns and prickles that
discourage browsers from eating certain savanna and forest-border trees and shrubs.
3. But the world is green; isn't food unlimited for many plant eaters.
4. It is now clear that plant defenses against herbivory are numerous and subtle.
B. Plant defenses can be organized into 3 categories: 1) morphological, 2) chemical, and
associational.’
1. Morphological plant defenses.
a. Besides obvious morphological defenses, such as spines and stinging hairs, there
are a variety of less obvious ones.
b. The heavy calcification of certain algae protects them from many aquatic grazers.
2. Chemical plant defenses fall into 2 general categories.
a. Disruptive chemical defense.
1) The plant may accumulate compounds that make tissues hard to eat, difficult
to digest, or unpalatable.
2) Tannins are an example.
a) It appears that 1 group of tannins (hydrolyzable tannins) serves a
protective function against many insects by inactivating digestive
enzymes.
b) Another group of tannins (the condensed tannins) seem to defend plants
against microbial and fungal attack. This results in poor digestion of
leaves high in condensed tannins by ruminants (which, by the way,
depend on microbial fermentation of food in their digestive tract). In
African savanna, ruminants tend to avoid leaves with more than 5%
condensed tannins (Cooper and Owen-Smith 1985).
b. Protective/defensive chemical defense.
1) The plant may produce chemicals that are strongly aversive or even toxic.
2) Examples include the mustard oils (glucosinolates) found in several families
but most notably in the mustards, alkaloids in the nightshade family, and
cardiac glycosides (cardenolides) in the milkweeds and dogbanes.
3) In general, defensive chemicals tend to occur in plants of disturbed areas and
early successional stages.
4) Protective chemicals are generally referred to as secondary compounds,
implying that they are not essential compounds of the plant's basic cellular
metabolism but by-products instead.
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5) The realization that the production of defensive chemicals in plants can be
induced by browsing is fairly new (Rhoades 1979).
a) The plant may, for example, begin to turn sugars into tannins rather that
translocating them to storage.
b) Chemical responses occur in damaged leaves but also in undamaged ones
on the same plant.
c) In large doses, protective substances may kill herbivores. In practice, the
main effects may be that odor or taste repel the animal or deter feeding.
d) Some protective substances seem to have subtler effects than simple
toxicity.
c. Many plants are now known to produce various animal hormones or mimics of
them.
1) Potentially, these could reduce or halt feeding on the plant by causing the
animal to go into diapause, metamorphose prematurely, or produce fewer
young.
2) Examples are the ecdysone (molting hormone) present in many ferns,
among other plants, and estrogen-like substances in many legumes.
3) There are estrogen-like compounds in whitebark pine seeds.
3. Associational plant defenses.
a. Associational defense means the protection gained by a plant by living in
association with another species.
b. It may be the simple matter of unpalatable plants happening to form refuge
zones for palatable one growing nearby.
c. One of the most often cited cases of associational defense is the attraction of
ants by certain species of plants, usually having extrafloral nectarines—that is,
nectar-producing organs somewhere on the plant other than the flowers.
1) This ant-plant coaction is a type of mutualism in which the ant receives
energy and provides defense against herbivores and seed predators by
attacking invading herbivores, especially insects, or by removing insect eggs
laid on the plant.
2) Several studies have shown that the ants are effective in cutting the loss of
plant tissue to herbivores, but an increase in fitness for the plant has not
always been demonstrated.
IV. Effects of herbivory on plant distribution and abundance.
A. The role that selective herbivory plays in determining the species composition of
vegetation are substantial but poorly known.
B. Many studies have focused on the effects of herbivory through the use of exclosures—
plots from which various herbivores are excluded.
1. The effects of protecting the vegetation range from dramatic to nil.
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2. In certain areas where deer are abundant, their browsing can alter the composition of
the forest that regenerates after clearcutting or even prevent woody regeneration,
maintaining herbaceous openings in the forest.
3. Exclosure studies in clearcuts in the Allegheny plateau region show the selective
impact of browsing.
a. Although most woody species show reduced densities outside exclosures, the
effect is especially strong on pin cherry, sugar maple, and beech.
b. Species less favored by the deer, such as black cherry, birch, and striped maple,
show proportional increases.
4. How often deer reached densities high enough to produce similar effects under
primeval conditions when large predators were present is not known.
5. Exclosures in Rocky Mountain National Park, Colorado, show dramatic evidence of
selective browsing by the Park's burgeoning elk population.
V. Resources and herbivory.
A. There are 3 major influences o the dynamics of an animal population.
1. The dynamics of its limiting resources.
2. The functional response of an animal to the level of the resource.
3. The numerical response, in terms of rate of increase, to the level of the resource.
B. Some definitions.
1. A resource is something that an animal needs.
a. The most obvious example is food, and to that we may add shelter, water, nesting
sites, and a particular range of temperature.
b. By definition, a resource is beneficial.
2. Habitat comprises all the physical attributes of the environment that make an area
hospitable for a species.
a. It is not in itself a specific resource but the sum of all physical resources for that
species.
b. Habitat is what wildlife managers see when they declare an area suitable for a
particular species.
C. Kinds of resources.
1. It is necessary at this stage to give a classification of resources because the interaction
between the resource and the animals that depend upon them can take several
forms. These in turn influence the dynamics of the population in several ways.
2. A resource may be”
a. Nonconsumable.
1) A resource may be used by an animal in a way that does not subsequently
reduce the level of that resource available to itself or other individuals.
2) A reptile needs a certain level of ambient temperature before it is active
enough to seek food and digest that food. Its use of that heat has no effect
on the heat available to other reptiles, neither then nor subsequently.
b. Consumable.
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1) A resource whose level is reduced by the individual utilizing it.
2) Food provides the obvious example.
3. The use of a resource may be:
a. Preemptive.
1) The use of cavities by birds.
2) Individuals are either winners or losers.
b. Consumptive.
1) All individuals have access to the resource and each individual’s use of it
reduces the level of the resource available to others.
2) An example is the use of plants by herbivores.
4. The relationship between a resource and a population may be:
a. Interactive
1) The level of the resource influences the rate of increase of the population, and
reciprocally the level of the population’s density influences the rate of
increase of the resource.
2) The dynamics of the animals interact with the dynamics of the resource.
b. Reactive.
1) The rate of increase of the animal population reacts to the level of the
resource but the density of the animals has no reciprocal influence of the rate
of renewal of the resource.
2) The relationship between a scavenger and its food supply or between an
herbivore and salt licks are examples of reactive relationships.
D. The dynamics of the resource.
1. A resource may be inert, as with suitable soil for rodents to burrow into, or it may
be growing.
2. If the latter, the intrinsic pattern of that growth will profoundly affect the dynamics
and the density of the population utilizing it.
3. In general, the higher the rate of renewal of the resource the higher will be the mean
density of the animal population using it.
E. The functional response of a consumer to a resource: the trend of intake per individual
against the level of the resource.
F. The numerical response of a consumer to a resource: the effect of the resource on the
animals.