Herbivory and
Mutualism
– Chapter 14
How Can The World Be Green?
• Some herbivorous populations may evolve selfregulatory mechanisms that prevent them from
destroying their food supply
• Predation may hold herbivore populations in
check
• Plants have evolved an array of defenses against
herbivores
– Has led to a coevolutionary contest betweens plants and
herbivores in evolutionary time
Plant Defense
• Plants may discourage herbivores by structural
mechanisms
– Rose thorns
• Some plants produce secondary plant
substance’s for defense
– Not found in all plants
– Are by-products of the primary metabolic pathways in
plants
Secondary Plant Substance Evolution
1) Compounds are primarily waste products
of metabolism
– Adaptation to avoid autointoxication
– Plant excretion is different than animal
excretion
– Plants can volatize, leach, or chemically alter
toxic chemicals
•
Two possible benefits
– Releasing chemicals into the environment
may have an allelopathic effect
– Plant may become toxic or distasteful to
herbivores
Secondary Plant Substance Evolution
2) Chemicals were specifically evolved by
plants to deter herbivores
– Assumes that few secondary plant
substances are really useful to the plant as
excretory products, and that most are actively
produced at a metabolic cost to the plant.
– Plants that produce secondary plant
substances are at an advantage
– If all herbivores were removed, plants would
not produce secondary chemicals
-The plant defense theory argues that herbivores have
a strong effect on plant fitness.
-Then well-defended plants are fitter
-If plant defense characteristics are inherited, then all
the elements needed for natural selection are present!
If plant defense has a cost in terms of
fitness then:
• Plants evolve more defenses if they are exposed
to much damage, and fewer defenses if the cost
of defense is too high.
• Plants allocate more defense to valuable tissues
that are at risk.
• Defense mechanisms are reduced when enemies
are absent, and increase when plants are
attacked.
• Defense mechanisms are costly (energy and
nutrients are diverted from other needs) and
cannot be maintained if plants are severely
stressed by environmental factors.
Plant Defense And Life History
• Long-lived plants are more likely than short-lived
plants to be found by herbivores.
– Long-lived (easy to locate) plants evolve a different
chemical defense than short-lived plants (not easy to
locate).
• Quantitative defense – the amount of chemical in
the plant can vary.
– Tannins and resins in leaves may make up to 60% of the
dry weight of a leaf
• Qualitative defense – the chemical is either
present (usually in low quantities) or absent.
– Poisons that protect plants from generalized herbivores.
Fast-growers vs. Slow-growers
Variable
Fast-growers
Slow-growers
Maximum Growth Rates
High
Low
Maximum Synthetic Rate
High
Low
Dark Respiration Rates
High
Low
Leaf Protein Content
High
Low
Flexible
Inflexible
Short
Long
Often Early
Often Late
Expected Rates of Herbivory
High
Low
Amount of Defense Metabolites
Low
High
Qualitative
Quantitative
High
Low
More Flexible
Less Flexible
Growth Characteristics:
Responses to Pulses in Resources
Leaf Lifetimes
Successional status
Antiherbivore Characteristics:
Type of Defense
Turnover of Defense
Flexibility of Defense Expression
• Herbivores tend to prefer fast growing plants
– Slow-growing plants tend to lose more
investment/leaf, so they invest a lot in chemical
defense (resource availability hypothesis)
Arrows indicate
levels of defense
that maximize
growth rate.
•
Inducible defense – plants can avoid the cost
of producing defense chemicals when they are
not needed
–
•
Once a plant is attacked, it will activate its defenses.
Not a heavily studied area, but if induced
responses are occurring then:
1) Do the induced changes affect herbivore foraging or
herbivore distributions?
2) Do plants suffer less damage and have greater
fitness as a result of induced changes in leaf
chemistry?
•
Herbivores can either detoxify plant chemicals
or alter their life cycle to avoid the noxious
chemicals of the plants
Tannins in Oak Trees
• Oaks are eaten by winter moth larvae early in
spring, but not in summer.
– Leaves become less suitable as they age?
Diet
Mean Peak Wt
May 16: ‘young leaves’
45
May 28 – June 8: ‘old leaves’
18
No adults emerged from the ‘old leaves’ feed
Older leaves are tougher
than younger leaves.
• If the larvae are fed ground up leaves, then age
doesn’t seem to matter
Diet (ground up leaves)
Mean Peak Wt
May 16: ‘young leaves’
37
May 28 – June 8: ‘old leaves’
35
• Why not evolve mouthparts (Some have)
• Tannins increase and proteins decrease in
summer – so less nutrition
• Those that feed on oak leaves late in summer
usually have low growth rates
Spines and Thorns
• Actually little evidence that spines and
thorns are used for defense
• Resource availability hypothesis – desert
plants
Herbivore Response to Plants
• Irruption – The increase and subsequent
decrease of herbivore (usually ungulates)
populations.
– Has been observed in natural and introduced
populations
– Caused by overpopulation / over grazing
Introduced reindeer
populations on a
subantarctic island.
Four Stages of Irruption
1)
2)
3)
4)
Population Increases and
food resources are reduced.
Population exceeds
carrying capacity, food
plants are over utilized and
damaged.
Population collapses due to
food shortage (often
correlated with severe
weather).
Population either collapses
to near extinction or
stabilizes at a lower
population level.
Noninteractive Grazing
• European finches feed on seeds of trees and
herbs and do not affect the subsequent
production of their food plants.
• Controls operate in one direction:
– Food Plant Production  Herbivore Density
• One group feeds on herb seeds and their
populations are stable.
• Another group feeds on tree seeds and their
population fluctuates.
– Herbs tend to produce the same amount of seeds each
year, trees do not.
– Populations are strongly influenced by year to year seed
production
• Finches that depend on tree seeds can
experience great irruptions in population density.
Seed Dispersal
• Plants benefit from herbivores via seed
dispersal.
• Coevolution between plants and their seed
predators may help to explain some
features of plant reproductive biology and
animal feeding behavior.
– Coevolution is most clear in the evolution of
fruits
– Plants advertise their fruits by a ripening
process in which the fruit change color, taste,
and odor.