Transcript lecture on seed dispersal

Seed Dispersal
Thinking like a plant….
• Coast redwood
– Live 2,000 years
– Make 1-10 billion seeds
(lifetime)
• Replacement?
• “Sweepstakes
reproduction”
Seed dispersal
• Move to “safe site:” suitable for germination
& establishment
• Recent importance: track “climate
envelopes” during global climate change?
Why disperse?
Why disperse?
• 1) Escape competition
(parent/siblings)
• 2) Escape predation/disease (if
density-dependent)
Why disperse?
• 3) Discover new habitats
• “Nothing lasts forever, even cold November
rain”
Axl!
Why disperse?
• 4) Gene flow/genetic diversity
• 5) Maintain “sink” populations
(metapopulations: source vs sink
populations)
Dispersal in seed plants
• What’s a seed?
Dispersal in seed plants
• Seed: mature ovule in ovary (of pistil)
• Seed: “baby plant (embryo) in box (seed coat)
with lunch (endosperm in flowering plants)”
Fruits
• What’s a fruit?
Fruits
• Fruit: Mature ovary (contains one or more seeds)
• Ovary wall becomes pericarp in fruit.
Fruits
• Pericarp may develop layers.
• At most:
– exocarp: outer layer
– mesocarp: middle layer
– endocarp: inner layer
Fruits vs seeds
• Seed or fruit?
Fruits vs seeds
• Dry indehiscent fruits
– achene: one seed, fused to pericarp at one point
– Ex, dandelion, sunflower.
Fruits vs seeds
• Dry indehiscent fruits
– grain (caryopsis): one seed, fused entirely to pericarp
– Ex, corn, rice, wheat.
rice
Resolution
• What’s dispersed: fruit or seed?
• Solution. Diaspore: Single dispersal unit
(seed or fruit)
Fruit/Seed coat functions
Fruit/Seed coat functions
• 1) Protect against:
– Seed predators
– Environmental conditions
Fruit/Seed coat functions
2) Seed dormancy
– Hard endocarp/coat seals out water/oxygen
– Breaking layer: scarification
– Fruit/seed coat can contain germination
inhibitors
Fruit/Seed coat functions
• 3) Aid seed dispersal
Maple samaras
Milkweed
Seed Dispersal
• May be mutualism
Not!
Ballistic dispersal
• NOT mutualism
– Ballistic dispersal=Ballistochory: fruit throws or
squirts seeds
– Ex, touch-me-not (Impatiens, right), sorrel (Oxalis,
left)
Wind/water dispersal
• Other non-mutualist (physical forces)
• Anemochory=Wind dispersal (dandelion)
• Hydrochory=Water dispersal (coconut)*
Dispersing dandelion fruits (achenes)
*controversy!
Ectozoochory
• NOT mutualism
– Most ectozoochory (or epizoochory):
seed/fruit carried by animal outside body
(hooks, barbs, glue)
Fruits of sandspur (Cenchrus)
Ectozoochory
– Many plant species
– 17 top “hitchhiker plants”: ranked by SRDUs
(Sock Removal Difficulty Units)
Uncarina #1!!
Dispersal Mutualisms
• Ectozoochory mutualisms:
• Ex, scatter hoarding animals (squirrels,
nutcrackers)
Dispersal Mutualisms
– Pinus albicaulis (white bark pine) and Clark’s
nutcracker
– 1 bird can hide 90,000 seeds in 1 season
Dispersal Mutualisms
• Endozoochory: fruit eaten & seeds thru gut
– Fleshy fruits (soft, sweet fruits)
– Fruit pulp nutritive reward
Dispersal Mutualisms
• Endozoochory: sometimes seed scarified
– Ex., dodo & Calvaria tree
Myrmecochory
• Seed dispersal by ants (ectozoochory mutualism)
– Elaiosome: food body on outside seed coat/fruit
– Only major mode using insect!
Stylophorum seeds
Fremontodendron seeds
Myrmecochory
• Common Eastern forest spring flowering plants.
Trillium
Bloodroot
Hexastylis
Trout lily
Viola
Biotic pollination vs biotic seed dispersal
• Trait
• Animals
Pollen dispersal
Often insects
Seed dispersal
Mammals/birds
• “Target”
Stigma
Safe site
• Motivation
to target
Floral reward
None
• Cues to target
Floral traits
None
Biotic pollination vs biotic seed dispersal
• Seed “directed dispersal” (concept: seed carried directly
to safe site) unlikely
Syndromes?
Syndromes
– Protein rare
reward
– Bird fruits no
odor
– Ants only major
insect group
– Less taxonomic
specificity
– Guild: group
species with
similar
ecological
function
The (Plant) View
• How measure dispersal? Metric?
• 1) Single species study: absolute distance
The Plant View
•
•
•
•
•
How measure dispersal? Metric?
1) Single species study: absolute distance
2) Comparative study: canopy diameters
Ex, grasses vs oaks
3) Self-incompatible clonal plants: genetic
neighborhood
Plant “choices”
• As seed size increases:
– dispersal ___________
– chance establishment ____________
Plant “choices”
• Seeds yummy! High protein, high lipid
• How protect?
Plant “choices”
• Bribery!
• Large number seeds
(scatter hoarders)
Plant “choices”
• Bribery!
• Non-seed food reward. Fruit pulp, etc
Aril on
nutmeg seed: aril
is spice “mace”
Aril: outgrowth
attachment ovule
to ovary
(funiculus)
Mace Windu…..
Plant “choices”
• Poison!
– Defend seeds with toxins
– Ex, castor beans (ricin)
– Lethal dose: 1/5000 gram (twice as
deadly as cobra venom!)
Castor beans
Plant “choices”
• Armor! Mechanical protection (stony seed coat,
endocarp)
Plant “choices”
• Advertise: disperser gets message before
seed predators
– Ripeness cues (color, smell, etc)
– Pre-ripening flags: ripening begun
– Foliar flags: leaf color indicates fruits ripening
(ex, poison ivy)