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Plant Fruit/Seed Dispersal
First, some defini6ons
• Propagules, refer to the dispersing
structures, which may be a
seed, fruit, or something else.
• Seed dispersal: usually refers to
propagule dispersal, and may mean
both yearly seed movements, as
well as migra6on or range
expansion. Default is seed leaving
a parent; context for migra6on
or range expansion.
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Primary Dispersal
Secondary Dispersal
Seed Dispersal refers to movement
of a single individual’s seed
or a single species.
Seed Rain refers to all seed
arriving in a loca6on
by any means.
Seed Rain
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Seed Shadow
Seed Shadows refer to a
density distribu6on of seeds aEer
dispersal from a single plant.
Two dimensions are illustrated,
but remember this is a
three dimensional process.
Reminder of Plant Dynamics
emphasizing role of seed rain.
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How do plants disperse?
• Wind dispersal • Ballis6c dispersal
• Water dispersal • Gravity dispersal
• Animal dispersal
endozoochory
epizoochory
Seed Dispersal Syndromes
General rules of thumb.
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Seed shadows for several herbs.
ant
wind
ballis6c
Seed shadows for miscellaneous trees.
wind w/ high terminal velocity
rodents ballis6c
wind then rodents
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Wind Dispersal Examples
Aerodynamics of Wind Dispersal
Size and Weight modify the Shape
and Distance of seed dispersal.
Mass Surface area
Shape Real fruit
Weight decreases distance.
Wing area increases distance
Shape or weight loca6on
cri6cal.
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Animal Dispersed Fruit
Animal Dispersed Fruit (secondary)
Peromyscus boylii
Peromyscus californicus
Tamais
Scurius
Dipodomys
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MORE
Animal Dispersed Fruit
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Mammalian
Frugivores
Variety of possums and rodents
Many monkeys and primates, Possoms,
rodents, many others
Lots of other mammals like the
Spectacled Bat
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Frugivorous Birds
Cassowary
Parrots Pigeons
Hornbills
Issues for animal dispersal
• AXrac6ng frugivores • Surviving the
animal handling, especially if the
fruit is consumed.
• Diversity among frugivores leads to
some6mes conflic6ng selec6on (guts of
birds and mammals quite different).
• Cost of fruits • Poten6al size
of seed depends on vector
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The specifics of the vectors are
interes6ng, but what drives the
ecological and evolu6onary
selec6on?
Why Disperse?
• Intraspecific Issues like
density-‐dependent compe66on, sibling
compe66on, and inbreeding.
• Escape from interspecific constraints,
like seed predators or pathogens
• Colonize new or beXer habitats
• Metapopula6on dynamics depends on
dispersal.
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Ul6mately, all habitats are transient.
Varia6on in scale and intensity of
transience.
Large Temporal Scale
• Post-‐glacial shiEs in climate
resulted in rapid migra6on among
forest trees.
• Rates of migra6on suggest phenomenal
dispersal rates, even by species
you might not expect.
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Post-‐glacial migra6on rates.
Quercus exhibited one of the
faster rates of migra6on,
equivalent to several kilometers
per genera6on.
Numbers refer to when, in
thousands of years ago, pollen
appears in soil cores at that
loca6on.
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Three hypotheses are common in
plant studies of dispersal
• Escape Hypothesis: Dispropor6onate
mortality risks near parent.
• Coloniza6on hypothesis: seeds disperse
to sites different from parents;
perhaps shade-‐intolerant species
dispersing to patches.
• Directed Dispersal hypothesis: vectors
take seeds to special sites
that must be predictable and
rela6vely stable. Supported
by mistletoes and chili peppers.
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Escape and Coloniza6on hypotheses depend
on the shape and density of
seeds in the seed shadow.
Long-‐distance dispersal may be the
most important aspect, the ‘tail’
of the seed shadow. (think
post-‐glacial migra6ons).
A ‘fat tail’ is important for
long-‐distance dispersal.
The shape of the dispersal
curve
at the tail is believed to
be the most important
character.
Fat tail
Shape of seed shadows
This has led to detailed work
trying to determine the best
‘fit’ for seed shadows because
models differ in their ‘tails’.
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Seed traps are a labor intensive
way of
collec6ng sufficient dispersal data;
either seed rain or seed
shadows, but it can be done
right.
Empirical evidence oEen doesn’t help
dis6nguish among a variety of
models, too many data points
are required.
Connie Clark worked on this
ques6on using isolated trees in
a highly diverse African forest,
comparing wind-‐, monkey-‐, and large
bird-‐
dispersed trees.
For traps beneath trees: Note
that during the frui6ng season,
only bird-‐dispersed seeds
differen6ally ended up beneath
bird-‐dispersed trees.
Clark et al. 2004. Oecologia 139: 66-75.
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Clark et al. 2004 Oecologia 139: 66-75
Here, birds differen6ally disperse
“bird-‐fruit” to other frui6ng trees
bearing ‘bird-‐fruit’. Monkeys don’t
do that.
During the non-‐frui6ng season, it’s
all chance dispersal.
Summary of Dispersal PaXerns.
Both bird and monkey dispersal
paXerns yielded fa1er tails
(especially the bird dispersed
species) –
which means farther overall distances.
Clark et al. 2005 Ecology 86: 2684-2694.
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The Janzen-‐Connell Model predicts
recruitment at a distance from
the parent plant.
Based on mortality due to
predators or pathogens associated
with the parent.
Recruitment
No mortality associated
with parent.
Mortality associated
with parent.
Some evidence supports the
Janzen-‐Connell Model
but other studies do not.
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May be more complicated than
simple Janzen-‐Connell models
Hardesty et al. 2006. Ecol.
LeXers 9:516-‐525.
For many tropical trees, seedlings
have a lower rate of survival
beneath parent, but fairly uniform
distribu6on for the next 1000 m
(this species has a seed shadow
to 39 m).
Long distance vertebrate dispersal is
differen6ally successful.
Summary Dispersal theory has developed
several hypotheses based on poten6al
selec6ve forces, inbreeding, compe66on,
seed predators, coloniza6on, patch
dynamics.
Many studies focus on natural
history or on mathema6cal modeling
of seed shadows.
While s6ll know very liXle about
shapes of seed shadows, how
plants aXract frugivores, linkages
between dispersal and subsequent life
history stages, clearly long-‐distance
dispersal is a crucial aspect
of long-‐term plant fitness.
