topic 8 Plant-herbivore interactions Flashcards
define herbivory. who benefits?
onsumption of living plant tissue – Usually benefits herbivore to the detriment of the host plant (+ -) – Some plant-herbivore relationships mutualistic (+ +) • Distinct from detritivores – Consume dead plant (or animal tissue)
how many animals are insects?
• Most terrestrial animals (60- 95%) are insects • Most insects (50-80%) feed on plants • Most plant-herbivore work focuses on insects
coevolution of plants and herbivores?
• Coevolution – reciprocal evolutionary interaction between 2 or more species
herbivore classification?
belong to almost every taxonomic group
fungi, viruses, bacteria - broadly defined as plant diseases
animals
animal herbivores - how can their difference in scale impact the relationship
nvertebrates (insects, mites, nematodes etc.) & vertebrates
Important differences in scale (& biology)
• Small invertebrates rarely eat whole plants
Large herbivores may kill host – acting like a predator
- Herbivore classification: diet breadth
* Bernays & chapman 1994 - define monophagus
• Monophagous: feed on plants in 1 species (or genus); also called specialists
○ e.g., Koala & eucalyptus
○ Brown spruce longhorn beetles & spruce trees
define oligaphous
• Oligaphous: feed on many plant genera, but from only 1 plant family
Polyphagous: can feed on plants from many families (also called generalists)
○ e.g., western spruce budworm (Pinaceae: firs, larch, spruce)
define polyphagous
• Polyphagous: can feed on plants from many families (also called generalists)
○ e.g., Gypsy moth consume leaves of >300 species of plants
classification based on diet or mode of feeding categories?
• plant part consumed or mode of feeding: Different Diets • Granivory (seeds) • Frugivory (fruit; often mutualistic) • Folivory • Browsing (woody plants): moose • Leaf chewing • Leaf mining ie caterpillars • Grazing (herbaceous plants) ie sheep stem/root boring gall making sap sucking (phloem or xylem)
prickily pair example
• Prickly pear introduced to Australia (1880)
• Quickly spread – invasive species
• Cactus moth introduced for biocontrol
(specialist)
• Controlled prickly pear at small fraction of
former density
canadian thistle example?
• Canadian example:
○ Nodding thistle (Eurasian): major pest of grazing land in Prairies
Control by a specialist seed-eating weevile
• Herbivores can impact plant populations
is death common in herbivory? exceptions?
• Herbivores may kill small juveniles (seed predation or browsing of seedlings)
• Killing of adult plants uncommon
(few herbivores consume whole plants)
Girdling: mammals, insects (bark beetles)
• Defoliated conifers: lack ability to regrow foliage quickly
• Herbivore vectored diseases (indirect)
reduced birth rates (subleathal) as a mechanism of pop impact of herbivory? (4 types)
1.. Consume reproductive structures (flowers/fruit)
2. Remove resources needed to make reproductive structures
(sap-suckers remove sugars/nutrients)
3. Consume leaf/root tissue, reducing the plant’s ability to gather
resources & make reproductive structures
4. Consume leaf/root tissue, forcing plant to divert resources
from reproduction to repair/re-growth
5. Reduce plant growth rate, reducing its ability to compete with neighbours
two plant responses to herbiovory
olerate or defend
how can herbivory be tolerated
-Invest few resources defending against herbivory
– -Focus resources on re-growing damaged tissues (reduce harm associated with being eaten)
– -A more tolerant plant is one for which a given level of herbivore damage
-imposes a smaller reduction in survival/reproduction
how can herbivory be resisted or defended against
-Invest many resources in reducing herbivory (toxins, thorns)
– -Reduce herbivore preference or performance (reduce likelihood of being
eaten)
– -A more resistant or defended plant is one for which a given level of exposure
to herbivores results in less damage
when to tolerate?
• Tolerate if fitness cost to defend > expected damage from
herbivory (& associated fitness costs)
when to defend
• Invest in defense if expected damage from herbivory (& associated
fitness cost) > fitness cost to defend
what is a mechanism for tolerance?
• Mechanism for tolerance – compensatory growth
*Tolerant plants re-grow to compensate for herbivore damage
look at graphs for 3 possible impacts of herbivory on plant fitness
OK
no tolerence -impacts of herbivory on plant fitness
no compensatory growth
any lost tissue reduces fitness
pop dynamics tightly coupled
similar to predator-prey
tolerence of moderate herbivory -impacts of herbivory on plant fitness
plant adjusts development to compensate (complete compensation)
mobilize resources and regrow lost tissue
no impact on fitness
increasing levels of damage reduce fitness
overcompensation -impacts of herbivory on plant fitness
low or moderate amounts of herbivory increases plant fitness
mechanisms unclear
look at overcomp example
grazed plants exactly comp for damage
describe plant defense - escape in time
- Escape in time
• fast growth rate (reduce length of vulnerable stages)
– - Altering phenology
• -Change timing of bud burst
• -Delayed budburst -> herbivore starvation
• -Early budburst ->some herbivores can only consume new tender foliage
plant defense - escape in space
• Disperse to enemy free space
– -Alter distribution (become less apparent, i.e., more difficult to adapt to)
– -Invest in vertical growth; reduce predation of ground-level herbivores
plant defense: REDUCE PREFERENCE (ANTIXENOSIS) OR PERFORMANCE (ANTIBIOSIS) OF HERBIVORES
antixenosis or antibiosis
describe antixenosis
Antixenosis:
○ Characteristics of plants that reduce herbivore preference
§ Trichomes, thorns, surface wax,
○ chemical repellents, etc.
§ Plants less likely to be attacked
§ Affects herbivore populations (b & d) indirectly (via resource availability
describe antibiosis
• Antibiosis
○ Plant toxins
• Plant compounds that reduce digestibility
(e.g., tannins in leaves) = low nutritional value
• Indirect: plant cues that attract natural enemies of herbivores
- Often olfactory cues
- E.g., “smell” of stressed tree attracts parasitoids of insect herbivores
§ Affects herbivore populations directly (alters b & d)
describe constitutive defense
• Constitutive defenses
Always expressed, regardless of attack history
• Background level of defence
• e.g. length of thorns, amount of toxin
describe induced defenses
Expressed (or increased) after herbivore attack
• e.g., new toxins, increased thorn length
• Time scale: from mins to yrs
Delayed-induced defence: time-lagged feedback (can generate
plant-herbivore population oscillations
trade offs / costs of defense in plants?
Trade-off:
Defence, growth,
reproduction, maintenance
• Metabolic cost
– - Synthesis of defensive compounds & structures requires energy & nutrients
• Storage cost
– - To protect other cells from toxins used for defense, they are often stored in
vacuoles (need to be produced & maintained)
