Plant defenses against herbivores
• Plants can’t run away from herbivores
• Plants can’t hide – leaves must be exposed
too catch light for photosynthesis.
• Plants are usually too abundant to be
cryptically colored!
• Therefore, plant defenses operate
in situ, either directly or indirectly
Types of defense
• Mechanical
• Chemical
• Biotic
• Phenological
A species may use more than one of these
lines of defense, and may use different ones
at different stages of its life
Mechanical defenses
• Hairs on leaf surface
• Spines – modified leaves
• Thorns – modified shoots
• Toughness
Other modifications – e.g. fake eggs on
Passiflora tendrils fool ovipositing
Heliconius butterflies
Hairiness - multipurpose
spiny thistles and Eeyore
Thorns to prevent biting, climbing
Chemical defenses
1. Palatability/acceptability influencers:
feeding inhibitors
2. Digestibility reducing compounds
3. Toxins
Palatability influencers
Pre-chomping: volatile compounds, may
repel herbivores, but specialized herbivores
may use them as a cue
Post-chomping:
Tannins (astringency) – oaks; Oxalic acid –
sorrels; Calcium oxalate – Araceae;
photosensitization; cyanogenic glycosides
Digestibility reducers
e.g. Tannins – reduce assimilation of plant
proteins by herbivores by interfering with
proteolytic enzymes
Herbivores eat a lot, assimilate little
Grow more slowly, may fail to develop in
suitable season
Longer in larval stage, vulnerable to enemies
Toxins – Cabbages and glucosinolates
Peter J. Bryant
Cabbage
white
Common
green-eyed
white
Giant
swallowtail and
Rutaceae
hostplants
Fireflyforest.net
Toxins
e.g. cardiac glycosides in Asclepias
Cows and sheep eating plants get sick, have
abortions, etc…
Most insects avoid foliage except monarch
butterflies. Their caterpillars are toxic due
to sequestration of the milkweed toxins!
Certain specialists are adapted to feed despite
toxins.
Milkweed and monarch caterpillar
Monarch
butterfly life
cycle
Adults benefit
from
sequestered
cardenolides
too!
Naïve birds will remember…
c/o Tatyana Livshulz
Fieldwork in Taiwan
Idea leuconoe (Lepidoptera subfamily Danainae)
on Parsonsia alboflavescens (Apocynaceae)
Tithorea pinthias Nymphalidae
on Prestonia portobellensis
Dan Janzen – Santa Rosa National Park
Plumeria and Pseudosphinx tetrio
http://biological-diversity.info
Belize wildlife site
Squirrel
cuckoos
whack and
snack
Biotic protection
• Ants tending extrafloral nectaries and/or
residing in plant body protect plants
• Parasitoids visiting nectaries may also
provide plant protection
• Plants can benefit from ants tending certain
herbivores as well, but not always
• Beneficial mites can eat herbivorous mites –
plants may have domatia to house them
Senna mexicana foliar nectaries and ant
Turnera ulmifolia in greenhouse
Vicia sativa with
stipular nectaries
Phenological defense
• Timing of production of parts susceptible to
herbivory
• Leaf flushing
• Masting – flowering/fruiting every 2-5
years synchronously with others (predator
satiation)
New leaf flushing – phenological defense?
Mast fruiting
mast = beech
A “mast
year” occurs
when the number
of acorns or other
nuts (like Beech
nuts Hickory
nuts or Hazel
nuts) produced
by a tree or shrub
in a single season
is much higher
than usual.
Jargon of antiherbivore defense
Plant types
Feeny
Short-lived (rare “hard to find”
&/or ephemeral) Qualitative
defenses
Long-lived
“bound to be
(abundant &/or found”
persistent)
Quantitative
Rhoades &
Cates
“Unapparent”
“Apparent”
Qualitative vs. Quantitative
Qualitative defenses (e.g. toxins) – very
effective against non-adapted species,
effective in small amounts on all but
specialist herbivores
Quantitative defenses – more effective in
larger doses, general-purpose, e.g.
toughness, low nutrients, digestibilityreducers
Plants and their parts
Unapparent
Apparent
Annual species
Woody perennial spp
Early successional spp
Climax spp
Rare spp
Common spp
Young leaves
Mature leaves
Leaves
Bark, stem
Deciduous leaves
Evergreen leaves
Optimal defense theory
• Considers costs to plants of antiherbivore
defenses
• Qualitative defenses cheaper than
quantitative defenses?
• Maybe just more appropriate – act quickly
against specific herbivores
• Lots of debate over the years…
Physiological ecologists to the rescue…
• The cost of defense are one of the
constraints on leaf form and function
• Photosynthetic capacity, nitrogen,
longevity, and susceptibility to herbivores
are all related
• Quantify these things for a cost-benefit
analysis
Benefit from a leaf =
[rate of carbon gain * carbon gain period]
minus
[carbon cost of growth and maintenance +
losses to herbivory]
• CO2 exchange can take care of all except
herbivory losses
• Amount of nitrogen in leaf correlates with
photosynthetic capacity
Effects of herbivores can be
dramatic!
Hawaiian landscape
protected from cattle
grazing on Mauna Loa
Odocoileus
virginianus
ssp. clavium
Earlier deer exclosures
on Key Deer Refuge –
at least 25 years old.
No fire in that time (note
wooden posts).
Woody plant cover
substantially greater
inside fenced areas
Long term protection from
deer browsing?
Fire and Key Deer Herbivory
• Fire clears dense understory
• Promotes resprouting and seed
germination
• New foliage more palatable to
deer
• Preferential grazing may affect
understory plants:
– Growth
– Reproduction
– Recruitment and/or persistence