seed dispersal. thinking like a plant…. coast redwood –live 2,000 years –make 1-10 billion...
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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
Trout lily
Bloodroot
Viola
Hexastylis
Biotic pollination vs biotic seed dispersal• Trait Pollen dispersal Seed dispersal• Animals Often insects Mammals/birds
• “Target” Stigma Safe site
• Motivation Floral reward None
to target
• 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 onnutmeg 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)