Transcript Herbivory & Parasitism

Plant Ecology - Chapter 11
Herbivory & Pathogens
Herbivory
The consumption of
all or part of a living
plant
A predator when it
kills and eats an
individual
Herbivory
Granivores - eat
seeds or grains
Grazers - eat grasses,
low-growing plants
Browsers - eat leaves
from trees, shrubs
Frugivores - eat fruits
Herbivory
How much do they
eat?
Estimate - 10% of
leaves of forest trees
eaten each year
Least in temperate
forest, most in dry
tropical forest
Is herbivory good for a plant?
Reduce self-shading
Remove leaves in excess of optimum LAI
Reduce respiratory “drag” on plant
Herbivory can cause death
Girdling (ring-barking) of young trees by
rabbits, squirrels, and rodents
Herbivory can cause death
Introduction of disease
into plant by grazer
Dutch elm disease
Fungus carried by elm
bark beetle
Clogs “circulatory”
system of American elm
trees
Herbivory can cause death
Grazing on one species may be
sufficient to sway competitive interaction
in favor of another species
Herbivory can cause death
Large populations of fluid-suckers (e.g.,
aphids) can virtually stop growth and/or
kill a plant
Herbivory can affect survival
Repeated defoliation often required to
kill mature plant
Large proportion of seedlings killed by
single “attack”
But some seedling plants have high
tolerance - e.g., 75% survival after 5
defoliations
Herbivory can affect growth
Effects range from none to total
cessation of growth
Depends on:
Timing of defoliation
Type of plant involved (grasses most
tolerant because of basal meristem rather
than apical meristem)
Herbivory can affect fecundity
Grazed plants tend to be smaller and
bear fewer seeds
Herbivory can delay flowering (move it
into inhospitable season), reduce, or
totally inhibit flowering
Some eat flowers, fruits, and seeds and
reduce fecundity
Good herbivores
Some pollen-eaters help pollinate
Some fruit-eaters help distribute seeds
Some seed-eaters store seeds in
ground and forget them
Mutualistic relationships
Compensation for herbivory
Temporarily mobilize stored
carbohydrates until regrowth returns
photosynthesis to normal
Compensation for herbivory
Reroute photosynthetic products to
damaged areas to enhance regrowth
To roots, or shoot, or leaves
Compensation for herbivory
Increase rate of photosynthesis in
remaining leaf surface area
Compensation for herbivory
Stimulate dormant buds to grow, or
reduce death rate among surviving
parts
Despite all these possible mechanisms,
compensation is rarely perfect, so
plants are harmed in the long-term
Compensation for herbivory
Defensive responses to
grazers
Grow bigger, sharper spines
Defensive responses to
grazers
Produce more or new defensive
chemicals
Defensive responses to
grazers
Reduce palatability
Tougher
More fiber
Lower nitrogen content
Effect of grazing on whole
population of plants
Do they only prey on the weak?
Reduction in intraspecific competition
Can reduce high LAI to more optimal levels
and improve plant productivity
Typically only works in high-density
populations; little or no compensation in
low-density populations
Effect of grazing on whole
population of plants
Controversial and unresolved
Two explanations on why herbivores
are NOT important regulators of plant
populations
Top-down
Bottom-up
Effect of grazing on whole
population of plants
Top-down - herbivores usually at such
low densities because of their
predators, cannot have negative effects
on entire plant population
Effect of grazing on whole
population of plants
Bottom-up - plant populations are
limited by abiotic factors (light, water,
nutrients), not by herbivores
Effect of grazing on whole
population of plants
On the other hand, there are some
various obvious examples of population
control by herbivores - e.g., gypsy
moths and oaks
Effect of grazing on whole
population of plants
Another example - bark beetles and
conifers - widespread mortality in N.
Amer.
Effect of grazing on plant
distribution
Eating can limit distribution in some
areas, or rodent/squirrel caches can
enhance abundance
Biological control
Moth introduced into Australia to kill
invasive prickly pear cactus - good
there, but problems elsewhere
Biological control
Beetles introduced to control purple
loosestrife
Herbivory & communities
Vertebrate, invertebrate grazers can
have dramatic effects on plant
communities
Herbivory & communities
E.g., rabbits and grasslands of southern
England
Herbivory & communities
Native and introduced grazers can have
significant profound effects
Herbivory & communities
Large herbivores in Yellowstone
Parasitic Plants
Obligate parasitic
plants - obtain
energy, nutrients,
water from host
plant
E.g., mistletoes
Parasitic Plants
Hemiparasites independent and
photosynthetic, or
parasite on other
plants (e.g., roots)
Plant Pathogens
Fungi, water molds,
bacteria, viruses
cause diseases in
plants
Individual,
population, and
community effects
Plant Pathogens
Soybean rust fungus from Asia,
infects leaves
Survives only on
green tissue
(eliminated each fall
here, but kudzu in
south is infested)
Plant Pathogens
Citrus canker bacterium causes
premature leaf, fruit
drop
Plant Pathogens
Smuts - affect
flowers, are caused
by fungi
Sexually transmitted
Plant Pathogens
Chestnut blight fungal canker
disease, kills
cambium under bark
American chestnut
formerly dominated
plant communities
Plant Pathogens - people
Irish potato famine
resulted from potato
blight caused by
water mold
Destroyed Irish
potato crop in 1840s
Plant Pathogens - people
1 million people died
from famine and
disease
1 million emigrated
to U.S., Canada
(especially New
York, Boston)
Plant Pathogens - people
Population of Ireland
has not recovered
Remnants of former
potato farms remain
today
Plant defense against
pathogens
Phytoalexins - secondary chemicals
produced at site of infection to kill microbes
Phloem plugging - phloem clogs in response
to damage, prevents spread of infection
through vascular system
Localized tissue death - barrier to infection