topic 8 Plant-herbivore interactions

Question 1

define herbivory. who benefits?

Answer 1

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)

Question 2

how many animals are insects?

Answer 2

• Most terrestrial animals (60-
	95%) are insects
	• Most insects (50-80%) feed on
	plants
	• Most plant-herbivore work
	focuses on insects

Question 3

coevolution of plants and herbivores?

Answer 3

• Coevolution – reciprocal evolutionary interaction between 2 or more species

Question 4

herbivore classification?

Answer 4

belong to almost every taxonomic group
fungi, viruses, bacteria - broadly defined as plant diseases
animals

Question 5

animal herbivores - how can their difference in scale impact the relationship

Answer 5

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

Question 6

• Herbivore classification: diet breadth

* Bernays & chapman 1994 - define monophagus

Answer 6

• Monophagous: feed on plants in 1 species (or genus); also called specialists
○ e.g., Koala & eucalyptus
○ Brown spruce longhorn beetles & spruce trees

Question 7

define oligaphous

Answer 7

• 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)

Question 8

define polyphagous

Answer 8

• Polyphagous: can feed on plants from many families (also called generalists)
○ e.g., Gypsy moth consume leaves of >300 species of plants

Question 9

classification based on diet or mode of feeding categories?

Answer 9

• 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)

Question 10

prickily pair example

Answer 10

• 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

Question 11

canadian thistle example?

Answer 11

• Canadian example:
○ Nodding thistle (Eurasian): major pest of grazing land in Prairies
Control by a specialist seed-eating weevile
• Herbivores can impact plant populations

Question 12

is death common in herbivory? exceptions?

Answer 12

• 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)

Question 13

reduced birth rates (subleathal) as a mechanism of pop impact of herbivory? (4 types)

Answer 13

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

Question 14

two plant responses to herbiovory

Answer 14

olerate or defend

Question 15

how can herbivory be tolerated

Answer 15

-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

Question 16

how can herbivory be resisted or defended against

Answer 16

-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

Question 17

when to tolerate?

Answer 17

• Tolerate if fitness cost to defend > expected damage from

herbivory (& associated fitness costs)

Question 18

when to defend

Answer 18

• Invest in defense if expected damage from herbivory (& associated
fitness cost) > fitness cost to defend

Question 19

what is a mechanism for tolerance?

Answer 19

• Mechanism for tolerance – compensatory growth

*Tolerant plants re-grow to compensate for herbivore damage

Question 20

look at graphs for 3 possible impacts of herbivory on plant fitness

Answer 20

OK

Question 21

no tolerence -impacts of herbivory on plant fitness

Answer 21

no compensatory growth
any lost tissue reduces fitness
pop dynamics tightly coupled
similar to predator-prey

Question 22

tolerence of moderate herbivory -impacts of herbivory on plant fitness

Answer 22

plant adjusts development to compensate (complete compensation)
mobilize resources and regrow lost tissue
no impact on fitness
increasing levels of damage reduce fitness

Question 23

overcompensation -impacts of herbivory on plant fitness

Answer 23

low or moderate amounts of herbivory increases plant fitness
mechanisms unclear

Question 24

look at overcomp example

Answer 24

grazed plants exactly comp for damage

Question 25

describe plant defense - escape in time

Answer 25

1. 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

Question 26

plant defense - escape in space

Answer 26

• 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

Question 27

plant defense: REDUCE PREFERENCE (ANTIXENOSIS) OR PERFORMANCE (ANTIBIOSIS) OF HERBIVORES

Answer 27

antixenosis or antibiosis

Question 28

describe antixenosis

Answer 28

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

Question 29

describe antibiosis

Answer 29

• 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)

Question 30

describe constitutive defense

Answer 30

• Constitutive defenses
Always expressed, regardless of attack history
• Background level of defence
• e.g. length of thorns, amount of toxin

Question 31

describe induced defenses

Answer 31

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

Question 32

trade offs / costs of defense in plants?

Answer 32

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)