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

Common green-eyed white

Cabbage 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…

Idea leuconoe (Lepidoptera subfamily Danainae)on Parsonsia alboflavescens (Apocynaceae)

c/o

Tat

yana

Liv

shul

zF

ield

wor

k in

Tai

wan

Tithorea pinthias Nymphalidaeon 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 fruitingmast = 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 Rhoades & Cates

Short-lived (rare &/or ephemeral)

“hard to find”

Qualitative defenses

“Unapparent”

Long-lived (abundant &/or persistent)

“bound to be found” Quantitative

“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, digestibility-reducers

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 landscapeprotected from cattlegrazing on Mauna Loa

Odocoileusvirginianusssp. clavium

Earlier deer exclosures on Key Deer Refuge –at least 25 years old.

No fire in that time (note wooden posts).

Woody plant coversubstantially greater inside fenced areas

Long term protection fromdeer 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