Herbivory
 What
is herbivory?
 What kinds of impacts do
herbivores have in
nature?
 Why is the Earth green?
 Control
of herbivores by
carnivores
 Plants fight back: plant
defenses
 Importance
of herbivores
economically
What is herbivory?

Herbivory is the eating or consumption of plants (primary
producers) or plant material by animals known as
primary consumers
 Heterotrophs
 Autotrophs


Herbivory, like parasitism &
predation, is a +,- interaction
Herbivory is important because plants are the ultimate
source of energy for food webs in most environments.
Are all herbivores equal?
Feeding Specialization
 Specialists:



Generalists:




Koalas
Monarch caterpillars
Deer
Cows
Leaf-cutter ants
The degree of
feeding
specialization is
more like a
continuum
What are the various ecological
effects of herbivores?

Herbivores can affect plant fitness
Reduce plant growth rate
 Reduce plant reproductive output

Directly as seed predators
 Indirectly by reducing plant biomass

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
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Herbivores can control plant distribution and abundance
Through alterations of plant distribution patterns and
relative abundances, can alter plant community structure
and composition
The effects of herbivores on plants often depend on the
degree of feeding specialization.
Herbivores can control their “prey”
(plants) under some circumstances

Example: Prickly pear cactus (Opuntia stricta) and cactus moth
Introduced to Australia in 1839, came to cover large areas by 1925,
destroying rangeland.
 Several herbivores were introduced to try to control the plant
 In 1925, cactus moth (Cactoblastis cactorum--moth larva) was
introduced from South America, with great success controlling the
prickly pear

Klamath weed (St. John’s wort (Hypericum
perforatum) became a pest as an invasive species
in California; it was finally controlled by an
introduced (biological control) leaf beetle
(Chrysomelid: Chrysolina).
Lessons from prickly pear and
Klamath weed examples?




Invasive plant such as prickly pear, introduced to Australia
without its co-evolved herbivore control agents, was able to
spread rapidly and become a pest
A specialized herbivore was able to bring the plant totally
under control
However, this example involved introduced organisms, and
thus a potentially artificial situation... how general is this
phenomenon of herbivores controlling plant populations?
In a 1991 review of efforts to control plant weeds biologically,
using herbivores (N = 701 cases), noticeable control by
herbivore occurred in only 26% of cases, so herbivores
certainly are not always effective!
Third example: Eucalyptus
Native Eucalyptus
trees of Australia
were 100% taller
when native
herbivorous insects
were experimentally
removed compared
with controls
Cattle grazing can radically alter
plant communities
Effects of cattle grazing
on plant communities,
Mauna Loa Volcano,
Hawaii
Again, this example is
artificial, involving a
domesticated
herbivore.
Herbivores can alter plant
species dominance patterns by
feeding selectively on dominant
interspecific competitors:
E.g., Bison feeding in Great
Plains feed selectively on
C4 plants (tall grasses),
allowing more light at
ground level and a greater
proportion of C3 plants to
thrive (see text).
“Why is the Earth Green?”

Why don’t herbivores eat more of the terrestrial plant
biomass?
What controls abundance of herbivores?

Two basic answers involve trophic interactions:

 Predators
and parasites control herbivore abundance
 Plants defend themselves (particularly terrestrial plants)

We’ll look at these answers one at a time…
Top-down control of herbivores


Theory put forth in basic form by Hairston, Smith, &
Slobodkin (HSS) in 1960 paper
Assumptions & assertions of HSS study:
 Most
terrestrial ecosystems have just three trophic levels
 Plants generally limited by resource availability (not by
herbivores)
 Herbivores generally limited by their predators and
parasites
 Carnivores generally food-limited, since their predators
are rare or absent
 Any system with an odd number of trophic levels follows
this scheme
Consequences of this HSS theory?


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Top-down trophic cascade of
effects from carnivore to plant
Carnivores tend to compete with
each other for food, and plants
also, but herbivores tend to
compete less (controlled by
predators)
This theory was based on much
empirical support, but few if any
direct tests at the time of indirect
effects such as benefit by
carnivores to plants by controlling
herbivores.
Better tested in aquatic systems
Predator
-
+
Herbivore
+
+
-
Plant
Empirical support from experiments:
Marquis & Whelan (1994) show cascading topdown effects


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Three treatments on white oak saplings in Illinois: (1)
spray saplings with insecticide to remove insect
herbivores (caterpillars), (2) cages to exclude birds &
thus protect caterpillars, (3) controls (untreated trees)
Predict that insect herbivore densities on plants would be
in following order: cages > control > insecticide
Predict, furthermore, that + indirect effects (plant
benefits) would be observed in following order:
insecticide > control > cages
Results of Marquis & Whelan study
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Greatest leaf damage & herbivore density in cages where protected from
carnivorous birds
Leaf biomass greatest where herbivores least abundant: spray & control
(fewer caterpillars) had more biomass than cages (with most caterpillars)
Different letters over bar indicate statistically significant differences
Results of Marquis & Whelan study, continued:
Indirect effects of birds on plants
Indirect effects of birds
shown by larger leaf
sizes in controls
compared to cages, and
in spray treatment
(simulates “super
carnivores”) compared
to controls.
Sea otter-sea urchin-giant kelp
community shows similar results

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When sea otters present, they control
sea urchins, allowing huge communities
of giant kelp
When sea otters removed (e.g., by over
hunting), sea urchins released from
control, and themselves devour giant
kelp beds.
Many experimental studies in aquatic
communities show same top-down
effects
(phytoplanktonzooplanktonzooplan
ktivorous fish; fish control zooplankton,
allowing phytoplankton to flourish)
Why is the Earth green?
Plants fight back!

Defenses come in many
forms:
 Mechanical
 Phenological
 Hormonal
 Biotic
 Biochemical
Mechanical defenses against browsers, grazers, etc.:

Leaf toughness

Spined leaf margins

Thorns, spines
Honey locust tree


Common in Jean Lafitte
Park
Presumably the very
sharp thorns on trunks &
branches evolved to
defend against large
climbing herbivores that
are now extinct in N.
America (e.g., giant sloth)
Some generalizations about
situations where mechanical
defenses have evolved (Peter Grubb)

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Open sites, such as deserts, where plants are close to
ground & thus vulnerable to grazers (also benefit of
producing thorns, etc., outweighs costs because of
expense of replacing lost tissue in desert)
Plants with only one meristem (such as palms) often
spiny, to protect apical meristem
Evergreens such as American holly may be spiny
(common in Louisiana upland forests) because they face
severe herbivore pressure in winter when other plants
have lost their leaves
Phenological Defense
Mast fruiting to swamp seed predatorse.g., oaks, bamboos
Hormonal Defense
Reproductive inhibition
(e.g., firs, genus Abies,
produce insect hormone
derivatives that inhibit
metamorphosis of
insect herbivores)
Biotic Defense
Mutualisms for defense--e.g., ant
acacias use ants to protect foliage
Biochemical Defense:
Green Desert


Chemical plant compounds—these are
often referred to as secondary
chemicals (i.e., 2º metabolites)
Plants produce a huge variety of
compounds, in widely varying
amounts
Chemical defensive compounds
can be classified into broad classes

Quantitative defenses = substances eaten in large amounts by
herbivores that impede digestion. Tend to be carbon-based.
E.g., tannins (phenolics) in oak trees, resins (terpenoids) in long-leaf
pine that fight off attacks of southern pine beetle
 In some cases these tannins affect herbivores’ feeding preferences:
e.g., a variety of browsers (goats, kudus, impalas) rejected a variety
of plants with > 5% condensed tannins
 In general condensed tannins tend to attach to cellulose and fiberbound proteins, acting against microbial and fungal attack, thus
affecting also some vertebrates; hydrolyzable tannins tend to
inactivate digestive enzymes of herbivores
 Slow development times, prolonging exposure to predators and
parasitoids

Chemical defensive compounds
can be classified into broad
classes


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Qualitative defenses – often toxic compounds that can
deter feeding or affect herbivores after ingestion of small
doses. Often nitrogen-based
Hypericum (Klamath Weed--see example earlier in lecture)
contains alkaloid hypericin, evolved to protect it from many
insect herbivores…and it’s also toxic to cattle.
What are some other examples of qualitative defenses?
Examples of secondary
compounds used in
defense against
herbivores

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THC in marijuana is an alkaloid, a
nitrogenous secondary compound
Mustards in the family Cruciferae contain
glucosinolates, another nitrogenous
class of defensive compounds
Queen Anne’s lace in family Umbelliferae
contains furanocoumarins, belonging to
the flavanoids (phenolic, carbon-based
compounds)
Tobacco contains nicotine, another
alkaloid
Generalizations about these
two kinds of chemical defense?
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Quantitative defenses tend to be found in plants that are highly
apparent to herbivores (long-lived, large), especially K-selected
species like oak trees
Qualitative defenses tend to be most important in species that are
un-apparent, especially herbaceous and other r-selected (weedy)
species
Note that many exceptions exist to the above assertions
Induced defenses (both physical, and chemical) are facultative
defenses that are strengthened in proportion to consumption of
plant material by herbivore (e.g., oaks increase production of
tannins in response to herbivory; browsed Acacia depranolobium
trees in Kenya have longer thorns than unbrowsed trees)
Evolutionary “Arms Race” Between
Plants and Herbivores

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Complex plant-herbivore interactions may have resulted from a
long evolutionary history
Some herbivores have evolved to benefit from, even depend on
chemicals originally evolved as defense
These herbivores, often specialists on a particular plant group,
have evolved the ability to use secondary chemicals as
protection against the herbivores’ natural enemies (e.g., wasps,
birds)

An example of this is Monarch butterflies, which feed on
milkweeds and use the cardenolides (terpenoids) from the plants
as defensive compound against birds which find the monarchs to
be toxic (famous series of experiments done by Browers using
blue jays as predators)
Plants often produce a
combination of several chemical
defenses

Why?



Synergism
Many herbivores
But, this strategy may be
expensive
Piper cenocladum
Beetles
Ants
Herbivores
Plants
Herbivory is extremely important
economically, and huge scientific
effort goes into controlling the
negative impacts of herbivores in
agricultural systems

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Pesticides
Biological control
Integrated pest management (use of biological control,
coupled with judicious use of pesticides)
Conclusions:

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Herbivores can in unusual situations completely
control the plants on which they feed, consuming
essentially all the available leaf tissue biomass
Normally herbivores in terrestrial environments, at
least, do not consume much of the green
(photosynthetic) biomass…why?
Plants fight back with diverse weapons (part of a coevolutionary arms race involving plants and their
herbivores).
Herbivores are a major factor influencing agricultural
productivity, and scientists have developed a variety
of strategies, such as integrated pest management,
to try to control herbivores.